Author: Emily Gutowski

Many in our industry can misunderstand the differences between temperature and heat, although these are related. 

Substances are composed of many moving molecules that change their speed as they release or gain heat. When we heat a glass of water in a microwave, its molecules start to move more quickly. The velocity increase results in a temperature increase. Molecules can have different velocities, but the temperature is the average value of all their speeds.

Two substances have the same temperature if their average molecular speed is equal. If we dip a 50°F glass of water into a 50°F pool, we do not transfer heat between them because there is thermal equilibrium. In other words, the substances have the same temperature. However, the glass of water has a much lower volume than the swimming pool. That means that the glass has much less heat content, even though they have the same temperatures. 

Usually, in HVAC systems, the amount of heat is measured in BTUs, which corresponds to the amount of energy it takes to change 1 pound of water by 1°F. In air conditioners, BTUs measure the quantity of heat that an air conditioning unit can remove from a space per hour. So, if you want to cool down a room where many people work or where equipment heats up, you need to consider this heat when selecting the system in addition to the heat entering through the walls, floor, ceiling windows, etc…Those are heat sources, so they will affect the quantity of heat to remove from the environment. We often call these heat gains, and these gains always occur based on a temperature differential. 

Most air conditioning and refrigeration systems use compression refrigeration to manipulate temperatures and therefore move heat. The refrigeration cycle consists of four stages: compression, condensation, expansion, and evaporation, as shown in Figure 1. All of these stages are essential to accomplish the job of moving heat. However, in this article, the spotlight is the compression stage. That stage is considered the heart of the refrigeration circuit.

Figure 1 – Refrigeration Circuit

There are many different types of compressors used in HVAC systems. Reciprocating, rotary, scroll, screw, and centrifugal compressors are the five standard types technicians see. Regardless of their different sizes and applications, their function is the same: to reduce vapor refrigerant’s volume resulting in a pressure increase. This process makes the molecules get closer to each other, increasing their velocity and raising the vapor temperature. Imagine this process as a room full of ping-pong balls bouncing around, and the balls represent molecules. If the walls of the room start moving in, and the ping-pong balls begin to speed up, bouncing against one another and the room walls. This example illustrates an increase in the average molecular velocity, temperature. The same thing essentially happens inside a compressor. Referring to physics, Boyle’s and Gay Lussacs’s Laws explain the effect of compression on temperature. 

In the refrigeration cycle, the refrigerant vapor flows through the suction line from the evaporator to the compressor. At this point, the vapor temperature is often about 55°F, and its pressure is low. When it enters the compressor, the vapor’s volume rapidly decreases. The piston, powered by an electrical motor, compresses the vapor within the cylinder. The reduction in volume results in an increase in pressure and temperature. In this process, similar to our room full of ping-pong balls, the velocity of refrigerant molecules increases, which results in a high-temperature rise, usually from around 55°F to about 165°F in typical comfort cooling systems under typical operating conditions. There is an additional heat gain due to the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. That heat gain occurs because the refrigerant cools the compressor down when it flows over it.

It’s essential to keep the compressor lubricated and ensure that the refrigerant flowing in the suction line is completely vapor to have a stable operation. Liquid refrigerant can damage a compressor quickly. Also, if a liquid refrigerant dilutes the oil in the compressor crankcase and creates foam, it can significantly reduce the compressor’s lifespan. We call this problem bearing washout or “flooding.”

To better understand vapor compression in HVAC systems, it’s also important to look at the Pressure vs. Enthalpy diagram (P-H) of the refrigerant. Figure 2 shows an example of this diagram.

Figure 2 – Pressure vs. Enthalpy Graph.

In this diagram, the y-axis shows the pressure, which is non-linear, and the x-axis shows the enthalpy. The bottom part of the graph is low pressures, while the top is high pressures, and from left to right, the enthalpy increases. Enthalpy is the total heat content of a system, represented here in BTUs per lb. Thinking of the compressor, when it increases the vapor temperature, it also increases the system’s enthalpy because the heat content in the refrigerant now is much higher than before. From Figure 2, it’s possible to notice that when the vapor refrigerant is compressed, its enthalpy increases. Another example of enthalpy is when we boil water on the stove. In this case, the water molecules gain heat from the stovetop, so the energy (heat content) in this system increases, which results in a boost in enthalpy.

You can also see the effects of vapor compression on temperature in the Temperature vs. Entropy (T-S) diagram presented in Figure 3. Entropy is how much energy has flowed from being localized to becoming more widely spread out. It also measures the level of order in a system. For example, ice is in a more ordered state because the atoms are locked in an ordered form. On the other hand, liquid water is more disordered because the molecules have spread out. Therefore, when we put an ice cube into a glass of water at ambient temperature, the disorder level increases, so the entropy also increases. When you melt ice, dissolve salt or sugar, or boil water in your kitchen, you increase entropy. The molecules spread out in all those processes. However, compressing a vapor is an isentropic process, which means constant entropy. Since the compressor does not allow for any heat exchange with the surroundings, the entropy level is unaffected.

Figure 3 – Temperature vs. Entropy (T-S) 

After being compressed and reaching a high-temperature level, the refrigerant now flows through the discharge line towards the condenser. In this stage, the superheat will drop off until the refrigerant reaches its saturation temperature. The refrigerant then condenses, subcools, expands, and evaporates. This refrigeration cycle continues until the system turns off.

Therefore, it’s essential to understand basic thermodynamic concepts such as heat, temperature, pressure, and the impacts of vapor compression on temperature. These concepts are fundamental when working with HVAC. 




[1] “Refrigerant Compression and Temperature,” HVAC School, 22 July 2019. [Online]. Available:
[2] “The Basic Refrigeration Circuit, Pressure & Enthalpy w/ Carter Stanfield,” HVAC School, 04 September 2017. [Online]. Available:
[3] “Pressure / Enthalpy Diagram Example,” HVAC School, 13 December 2018. [Online]. Available:
[4] “What is Temperature?,” HVAC School, 28 February 2019. [Online]. Available:
[5] “HVAC/R Refrigerant Cycle Basics,” HVAC School, 01 March 2019. [Online]. Available:
[6] “Air Conditioning Compressor Basics,” HVAC School, 20 February 2019. [Online]. Available:

We just wrote about rejecting heat to the atmosphere via radiant cooling. That’s one example of cooling without refrigerants, but there are quite a few others out there. 

In this article, we’ll look at some other cooling methods that don’t use refrigerants.

Vortex tubes

Vortex tubes swirl gas in a chamber, separating it into hot and cold streams. 

After hot gas gets deposited into the vortex tube at an angle, it spins along the tube’s sides and travels up it in a wide spiral. At the end of the tube, the hot air outlet is interrupted by a conical nozzle. This nozzle limits the hot gas that passes through, and it sends the cooler gas backward through a countercurrent in the center of the tube. The countercurrent deposits the cooler gas out of the other end, and that’s where cold air flows out. No refrigerants or moving parts are used in this process.

Georges Ranque invented the vortex tube in 1931, but it didn’t become popular until a German physicist named Rudolf Hilsch published a paper on it in 1947. An engineer named Charles Darby Fulton acquired patents to develop the vortex tube from 1952 to 1963. In 1961, he founded Fulton Cryogenics and began manufacturing the vortex tube. Fulton Cryogenics became Vortec Corporation in 1968 and focused almost exclusively on the development and manufacturing of vortex tubes. Vortec continues to exist and produce vortex tubes today.

Vortex tubes have also been repurposed for a few different separation purposes. For example, English physicist Paul Dirac found out that it can separate isotopes and gas mixtures. (This process is called Helikon vortex separation and isn’t relevant to HVAC, but this example shows that vortex tubes have uses beyond our field.)


What do we use vortex tubes for?

It would be great if vortex tubes could cool houses, but they’re far too small to cool an entire building. I spoke with Ellen Chittester at Vortec, and she told me a little bit more about vortex tube applications, benefits, and limitations.

She said that vortex tubes are less efficient than typical compression-refrigeration A/C units, but they have a unique set of benefits. For example, vortex tubes don’t rely on ambient air temperature, and they can cool effectively in ambient conditions up to 200° Fahrenheit. Vortex tubes are also inexpensive to purchase, easy to install, have a small footprint, and are easy to maintain.

Vortex tubes are impractical to use for applications that require more than 5,000 BTUs. However, they can cool small and enclosed spaces, such as cabinets. The most common applications for vortex tubes are sensor and product testing, CNC machine controls, gas sampling, injection molding, and saw blade cooling. 

Many of the applications I just listed are for “spot cooling,” which entails cooling over a small area and may require some degree of portability. That’s why vortex tubes are perfect for cooling machine controls (such as for 3D printers). Vortex tubes are also small enough to be effective in a personal air conditioning system, such as a diffuse-air vest. Vortex tube technology is incorporated into vests that you wear, and cold air diffuses through the vest to cool your body.

When it comes to technological advances, the vortex tube has pretty much reached its full potential as an individual unit. However, it still has the potential to improve lots of other new technologies. It will always provide easy, efficient, and controlled cooling.



Turboexpanders are essentially centrifugal or axial turbines. They rely on high-pressure gas expansion to power a generator or compressor. Turboexpanders can extract liquid from natural gas, generate power, or work with a compressor and electric motor in a refrigeration system.

A vortex tube is a simple form of a turboexpander. They don’t have the rotors that drive many turboexpanders, but the gas movement and energy conservation obey the same rules. Compressed gas enters the vortex tube at an angle, which drives the gas movement without any mechanical help.

Thermoelectric cooling

Apart from vortex tubes, thermoelectric cooling is another means of refrigeration without refrigerant. 

You’re already familiar with thermocouples. These are devices that form an electrical junction between two different types of metal. They generate a small voltage from temperature differences between the metals.

Thermoelectric cooling occurs when heat is removed at a junction between two dissimilar metals. The interaction between metals of two different temperatures is called the thermoelectric effect, and it has a few different extensions.

I’ll touch on the historical significance of the Seebeck effect and the Peltier effect, as they are most relevant to the topic of refrigeration without refrigerant. 

The Seebeck effect is the earliest extension of the thermoelectric effect, discovered by Italian physicist Alessandro Volta in 1794. However, it was named after the person who rediscovered it in 1821, German physicist Thomas Johann Seebeck. The Seebeck effect merely acknowledged the buildup of an electric potential across a temperature gradient. When two metals of different temperatures get connected at a junction, the temperature difference can generate some electrical energy.

We’re mostly interested in a later discovery, the Peltier effect. The Peltier effect is an extension of the Seebeck effect. 

Discovered by French physicist Jean Charles Peltier in 1834, the Peltier effect describes the heating or cooling at a junction of two metals at different temperatures. Instead of merely describing the electric potential, the Peltier effect describes the heat transfer between two different metals of varying temperatures. 

When two metals have different temperatures, one must evolve heat and the other must absorb heat. When this happens, heating or cooling occurs at the electrical junction that connects the heat sources. For this reason, the Peltier effect can be used for refrigeration and heat pumps. Refrigeration is the more common use, so we are going to focus on those applications.


What do we use Peltier cooling for?

Imagine using a thermocouple that generates enough electricity to cool PC towers or lab incubators. That’s essentially what the Peltier effect does.

Like vortex tubes, Peltier cooling is limited to smaller applications. One of its most common uses, as I said, is to regulate temperatures in lab incubators. These are the containers that store lab cultures at controlled temperatures. For example, if certain fungi must grow at a constant temperature, Peltier systems can provide constant conditions for that.

Peltier cooling is not practical for cooling large areas.  In the case of lab incubators, Peltier systems have difficulty maintaining temperatures below 50° Fahrenheit (10° Celsius). Despite that, Peltier coolers can dip well below sweltering ambient temperatures. Many portable camping or car coolers use Peltier cooling for that reason.


Vortex tubes and Peltier cooling rely on natural physics to cool small applications and aren’t affected by high ambient temperatures. Still, they have their practical and efficiency limitations, so they likely won’t ever be used to cool entire buildings. Despite that, they will continue to be useful for cooling small enclosures and overheat-prone technology.

Net refrigeration effect (NRE) is the quantity of heat that each pound of the refrigerant absorbs in the refrigerated space to produce useful cooling.

That’s a pretty vague definition. We know that it’s an amount of heat in processes that take place within the evaporator. Still, the phrase “useful cooling” seems rather broad. Even though it may seem a bit undefined right now, “useful cooling” is the key to understanding what NRE is and how it applies to HVAC techs in their everyday operations.


What is “useful cooling,” anyway?

Useful cooling occurs in the evaporator when the refrigerant absorbs heat from the conditioned environment, cooling the space. The total quantity of heat absorbed in the evaporator is the NRE. In essence, the NRE is all the difference between the total energy absorbed in the evaporator and the cooling that occurred anywhere outside of the evaporator (in BTUs/lb). 

When a refrigerant evaporates or condenses, it undergoes a phase change. When it undergoes a phase change from liquid to vapor or vice versa, the temperature stays the same. All of the heat added or removed (in BTUs/lb) contributes to the phase change. The heat energy required to complete the phase change is called the latent heat of vaporization.

In almost all cases, the condensation temperature is higher than the evaporation temperature. That’s because the vapor’s temperature rapidly increases in the compressor before going to the condenser where the heat is rejected to another medium, usually outdoors. 

Some heat must be removed to reduce the refrigerant from the condensing temperature to the evaporation temperature. However, that heat removal occurs in the liquid line and the end of the condenser and does not immediately contribute to refrigeration. We must drop the liquid line temperature to achieve subcooling and limit flash gas in the evaporator inlet. However, the refrigerant doesn’t absorb heat from the conditioned environment in the liquid line, so this process is not useful cooling.  


Enthalpy and NRE

Enthalpy is the total internal energy contained in the refrigerant, including sensible and latent energy.

Changes in enthalpy correspond with the phases of the refrigeration cycle. Enthalpy significantly increases in the evaporator. This occurs because the refrigerant absorbs heat in the evaporator. 

On a pressure-enthalpy diagram of the ideal refrigeration cycle, you’ll notice that the bottom edge of the figure will have a horizontal line. This line represents the evaporation phase. The pressure remains low and constant while the enthalpy rises. The refrigerant’s enthalpy at the end of evaporation is significantly higher than its enthalpy when it entered the evaporator.


How do we determine NRE?

To find the NRE, you subtract the enthalpy of the liquid entering the evaporator (He) from the vapor leaving the evaporator (Hl). All units in the equation will be BTUs/lb.

NRE = Hl – He 

Enthalpy heavily increases during the evaporation phase, so you will have a positive answer. Your answer will tell you how many BTUs per pound are being used to absorb external heat. Thus, the NRE is a system’s ability to cool its environment by extracting heat from it.


Why should HVAC/R techs care about NRE?

A unit’s NRE is its measure of cooling performance in both the HVAC and refrigeration industries. The NRE of an A/C unit will give you an idea of its comfort cooling capabilities. The same idea applies to product cooling in the refrigeration industry. 

How can we improve a cooling system’s NRE?

You can improve a system’s NRE by facilitating greater subcooling in the liquid line. 

Flash gas is reduced at the evaporator inlet when the subcooled refrigerant enters the evaporator at a lower temperature. Flash gas is the percentage of refrigerant that immediately boils upon entering the evaporator. Boiling is necessary for cooling to occur, and some flashing is required to drop the temperature from the liquid line to the evaporator inlet. Too much flash gas in the liquid line hinders the cooling process in the evaporator coil, so you can improve NRE by limiting flash gas at the evaporator inlet and liquid line.

Liquid line/suction line heat exchangers and mechanical subcoolers help promote subcooling when inserted into an HVAC system. You can also reduce flash gas in the evaporator inlet by keeping the liquid line insulated and inspecting it for kinks or inappropriate length.

When you optimize your NRE, you aim to get the most out of your evaporator. A lot of this comes down to keeping your system well-maintained and looking after the suction and liquid lines that surround the evaporator. Another thing you can do to optimize your NRE is keeping your evaporator outlet superheat as low as safely possible. A lower superheat indicates that the saturated refrigerant feeds more of the evaporator coil. Coils that are better-fed with saturated refrigerant are more efficient. 

Superheat can also indicate problems with your metering device at the evaporator inlet or the evaporator load. When you safely limit your superheat by checking and fixing all the causes of high superheat, you can make your evaporator more efficient and increase the system’s NRE. Just don’t overdo the low superheat… You can easily lose control of the superheat and run liquid into the compressor, resulting in flooding or even slugging.

You likely already know a bit about radiant heating from your education and work in the HVAC industry. Science textbooks like to use the sun and campfires as classic examples of radiant heat transfer. We also have plenty of heating devices with radiation right in the name, such as water radiator heaters and radiant space heaters.

It turns out that before compression refrigeration, ancient people were experts at using the cooling and heating methods at their disposal. Radiant heating/cooling was one of the methods they used. Their knowledge holds some valuable clues that can help us envision and develop modern radiant cooling technologies.

We will take a bit of a meandering path to answer the question posed in the title, but we will get there.


Heat transfer types

Radiation is one of three means of heat transfer. The other two are conduction and convection.

Conduction is the transfer of heat through touch. When an object with more heat touches an object with less heat, it transfers some of its heat energy. Think about heating a pan on the stovetop. The pan gets hotter because the flame directly touches it and transfers its heat. 

Convection is the transfer of heat through the air, where molecules float around freely. Have you ever held your hand above a pot of boiling water? Even though you didn’t directly touch the water, your hand still got warmer. That’s because your hand was heated by the free-moving hot gas molecules in the vapor above the water.

Radiation is a bit more complicated than conduction and convection. Heat transfer occurs through electromagnetic waves as they move through air or a vacuum. An example of this would be your microwave oven. The electromagnetic waves are generated, and your food gets warmed up because the water molecules in the food absorb the radiant heat and vibrate faster.

But what about radiant cooling?

Radiant cooling is the opposite of radiant heating. Radiant cooling occurs when matter loses heat by emitting thermal radiation. 

Every physical body spontaneously emits some degree of thermal radiation. We constantly give off heat to the environment around us, and we absorb the heat given off by the people and objects around us.

The earth absorbs solar radiation during the day and gives off heat at night. When the sun doesn’t heat a part of Earth’s surface, Earth emits some of the heat it had absorbed during the day. Clear, cloudless nights are best for emissions, as clouds reflect electromagnetic waves back to Earth’s surface.

Have you ever noticed that the ground in the morning has less dew on it when it’s under an awning or even tree branches? This occurs because the covering blocks radiant heat transfer from the ground to the night sky, which keeps the ground warmer and sometimes keeps it completely above the dew point.  

Certain materials emit more heat than others. Some of the white paints used on commercial building roofs have a high thermal emittance rate and don’t absorb heat easily, which helps passively cool the buildings. That’s one way to cool buildings without entirely relying on electricity.

Engineers have developed radiant cooling panels for indoor usage, too. These panels absorb heat emitted from people and objects in the room to maintain a cool temperature. 

Some companies, such as MESSANA, have developed dual-purpose indoor radiant cooling panels. These absorb heat in the summer and emit heat in the winter, keeping buildings comfortable year-round.

Uponor Radiant Cooling Panels

The indoor radiant cooling caveat

The idea of radiant cooling sounds great, but it has its limitations when used indoors.

When temperatures drop below the dew point, condensation can form. Radiant cooling panels cannot be allowed to drop below the dew point, or they would condensate. That’s why indoor radiant cooling panels aren’t generally used in homes in humid climates. The soil is charged with moisture in those humid areas, making the dew point high and easy to reach. 

If you choose to install radiant cooling panels indoors, a secondary dehumidifier is almost always needed.

Space blankets

Although space blankets aren’t very fashionable, they’re great for keeping you warm.

Space blankets work by trapping your body’s thermal radiation. The heat leaves your body, but the blanket reflects that heat off its surface. With nowhere else for the heat to go, your body reabsorbs it.

The blankets are made of highly reflective material (or sometimes coated in a highly reflective metallic agent) that doesn’t absorb heat well. Heat must be either reflected or absorbed by objects, so space blankets reflect around 95% of the heat your body gives off. Your emitted heat goes straight back to you.

These blankets were initially designed to help protect astronauts from the cold of outer space. The deep-space temperature is just barely above absolute zero (0 kelvins, -460° Fahrenheit), clocking in at approximately -455° Fahrenheit. That’s less than 3 kelvins! You know it’s cold when you see single-digit kelvins! (At least that’s what my grandma used to always say) 

Banishing heat

In the summer, especially here in Florida, the heat can be downright brutal. Many of us want to send it to a faraway place where it can never come back.

But what if we could banish the sweltering summer heat to a lonely frontier where it won’t cause trouble? Like, I don’t know, banishing it to outer space?

Well, it’s a real thing…

It’s possible to reject heat from an entire building to outer space… or at least the upper atmosphere. Scientists are still working on ways to maximize rejected heat, but it’s definitely possible. Many planners of new buildings have harnessed the power to reject heat and send it back to space. But did you know that radiative cooling was first harnessed by the people of India and Iran long ago?

Space is cold, but what about our stratosphere?

Earth’s stratosphere is quite a bit warmer than deep space. Just to keep everyone on the same page, the stratosphere is the atmospheric layer that lies between the troposphere (where we are) and the mesosphere on top of it. 

The stratosphere is unique because it has the ozone layer, which increases the temperature with higher altitude. Inversely, temperature decreases with increasing altitude in the troposphere and mesosphere.

The stratosphere’s lowest temperature is around -65° Fahrenheit at the tropopause, and the temperature tops out around -5° Fahrenheit near the top of the stratosphere. It’s still cold, but it isn’t anywhere near as frigid as deep space.

Theoretically, if we could reject heat from Earth and bypass the lower atmospheric layers, the cold of outer space is a perfect place to dump excess heat.

Ancient ice-making

Long before modern refrigeration technology, the people of India and Iran used to make ice at night. I’m going to focus mostly on ice-making in Kolkata, India.

India only gets cold in the northern mountain ranges, so it wasn’t like people in tropical Kolkata could march to the rivers and cut ice like in Disney’s Frozen. No, they had to rely on radiative cooling to do it. 

At dusk, they would fill pits with unglazed ceramic trays. Those trays held some pre-boiled water. The people would then line the bottoms of the holes with sugarcane or other plant material to insulate the trays. 

The water would freeze overnight because its heat would leave the earth via radiation, thus cooling what was left behind. This method worked only on clear nights, as there were no clouds to reflect the heat back to Earth’s surface. The heat would escape to the upper atmosphere, which is very cold and can accept our heat. (I’ll talk more about that later.)

People would collect the ice in the morning. They would move it, beat it down, and store it as a single mass in another large pit. That collection pit was insulated with straw and covered by a thatched roof.


Modern radiant cooling technologies

Space blankets are good for using radiant heating to keep bodies warm, but what about radiant cooling technologies?

According to the MIT Technology Review, SkyCool Systems has developed radiative cooling panels that can increase the efficiency of heating, cooling, and ventilation in buildings. These panels aren’t fully efficient as they stand, but they take some load off the electricity grid by rejecting indoor heat and reflecting incoming heat from the sun’s rays. For business owners and building managers, the most exciting part about this technology is the savings potential. 

For me, the most exciting part about it is the science. SkyCool Systems’ panels work because they reject heat into space by taking advantage of the electromagnetic spectrum.


The electromagnetic spectrum’s role

As you know, various types of electromagnetic waves exist. We have radio waves, microwaves, X-rays, and so on. They all have a place on the electromagnetic spectrum and are arranged by wavelength. Microwaves and radio waves have long wavelengths, while X-rays have short wavelengths.

Humans are capable of seeing visible light, which is in the middle of the electromagnetic spectrum. The color spectrum of visible light is based on each color’s wavelength. Red waves are the longest and have the lowest energy. Violet waves are the shortest and have the highest energy. That’s why the longer waves just below visible light are called infrared (beneath red), and the shorter waves just above visible light are called ultraviolet (beyond violet). Some animals can see limited infrared or ultraviolet light, but humans can’t see those with the naked eye.

Still, we’re interested in the infrared rays here. All objects radiate some heat via infrared waves. As I said earlier, our bodies are a major source of infrared rays. That’s how mosquitoes can find their victims. They’re one of those animals that can see limited infrared light, so they see the heat emitted by our bodies. 

Our atmosphere reflects some of our emitted heat back to Earth’s surface. However, there is a range of infrared wave frequencies that can bypass the atmosphere. The MIT Technology Review says that this range is between 8 and 13 micrometers. 

If radiant barriers can reject infrared rays and send them upward at a wave frequency between 8 and 13 micrometers, the heat would theoretically be cast to outer space (or at least the upper atmosphere). The heat wouldn’t be sent back to the lower atmosphere because it would be within the appropriate range to slip past it.


How can we manipulate those waves in direct sunlight?

A lot of the information I just described is highly conceptual. Without a practical application for all this science, sending heat to outer space is nothing more than a pipe dream. So, where does the practicality come in? 

Luckily for us, Stanford engineers have been on the case. In 2014, Dr. Shanhui Fan led an engineering team to invent a new material that reflects visible and invisible light. The invention is a coated photonic panel that rejects heat and reflects it from incoming sunlight during the day. 

The coating is multilayered silicon dioxide and hafnium dioxide over a layer of silver, and it’s only 1.8 microns thick. The panel’s coating serves as a mirror to the direct sunlight. Dr. Fan’s findings reported that the photonic panel reflects up to 97% of sunlight away from the building during the day. 

Outer space already takes in our rejected heat at night, as we saw from ancient India and Iran’s ice-making technologies. However, the Stanford engineers’ panels can use outer space as a heat sink during the day.

According to MIT’s Device Research Laboratory, photonic radiative cooling panels can cool a structure about 10.8° Fahrenheit below ambient temperature during the day. However, they are confident that these panels will eventually cool a structure by up to 36° Fahrenheit. Theoretically, commercial buildings would be able to maintain OSHA’s recommended 68°-76° temperatures without traditional air conditioning, even on those 95°+ Florida summer days!


Future applications

Obviously, anytime we can transfer BTUs in the direction we want without spending money on watts, we want to take advantage of it. 

There may come a time where we utilize energy “banking.” We could use panels that continuously reject energy from a bank of fluid or gel, even when that cooling capacity isn’t required. That “banked” energy could then be deployed during high load periods to balance out the grid or capitalize on smaller capacity systems.

There could be applications like this to make better use of CO2 in warm climates to prevent transcritical operation. It could be used in grocery and industrial cold storage applications to decrease load variability and so on.

All of these ideas have costs and barriers to overcome. Still, as we look toward better and more efficient technology, we may do well to look at what our ancient forbearers already knew and send some of our heat to space.

Simply stated, a compressor’s volumetric efficiency (VE) is a compressor’s ability to pump the most pounds of refrigerant over time. The compressor’s function is right in the name: it compresses gaseous refrigerant. After compression, the gas moves on to the condenser and continues the refrigeration cycle.

However, some of the refrigerant vapor that enters the compressor may not leave it. The gas that remains re-expands and occupies space within the compression chamber. When this happens, the compressor cannot operate at its full efficiency. 

The VE measures that inefficiency by comparing the amount of refrigerant that enters the compressor to the amount that leaves it.


Why should we care about the VE?

We should care about the VE for the same reasons that we care about other types of efficiency.

When the VE is closer to 100%, you can maximize your compressor’s mass flow rate. The compressor can pump more pounds of refrigerant over a given time. In other words, it takes less work for the compressor to make more progress at its job.

You will likely also maximize your compressor’s lifespan if you prioritize its efficiency. This effect isn’t limited to compressors, either. The more efficient a system or part is, the less work it has to do. When a system or part has less work to do, it doesn’t wear out as quickly as it would otherwise.


How do we determine the VE?

Let’s start by defining the variables that we compare when we measure VE.

Relatively cool, low-pressure vapor enters the compressor through the suction line. The compressor raises the vapor’s temperature and reduces volume by applying a massive amount of pressure. The hot, high-pressure vapor leaves the compressor through the discharge line. 

The compressor’s VE is a ratio of the discharge vapor to the suction vapor. You can express this as a percentage to get an idea of the total efficiency.


What affects a compressor’s VE?

The compression ratio mainly affects the compressor’s VE. The compression ratio deals with the pressures of the evaporator and condenser.

It may seem a bit strange that the compression ratio has little to do with the compressor. However, the evaporator and condenser surround the compressor within the circuit. Those two parts affect the compressor’s function. The compression ratio is the ratio of condenser pressure to evaporator pressure. 

The higher the compression ratio, the harder it is for the compressor to perform its job efficiently. When you run against the wind, you waste a lot of your stamina resisting the environmental conditions. A similar principle applies to a compressor with a high compression ratio, where it has a lot to overcome.

We mostly use reciprocating compressors, which are usually among the more efficient types of compressors. However, their structure makes them prone to developing poor volumetric efficiency.

Why are reciprocating compressors challenging?

The reciprocating compressor relies on a piston to compress the gas. When the piston moves down, it increases the cylinder volume creating a low-pressure area that draws the refrigerant into the compressor. When the piston moves up, it compresses the gas by reducing the volume in the cylinder. 

Pistons have room for clearance at the top, allowing gas to get trapped after most of the gas moves to the condenser. Gas left in the compressor or discharge port will re-expand, and this decreases the VE.

The higher the compression ratio, the less efficient the compressor becomes. More matter is left in the cylinder when the discharge pressure is high. When the suction pressure is low, the piston must drop the pressure further before suction gas can enter.


How can we reduce the compression ratio?

As you know, low evaporator pressure and high condenser pressure negatively impact the compressor’s VE. You’ll want to do whatever you can to keep the evaporator pressure high and the condenser pressure low.

The best thing you can do to keep the evaporator pressure as high as effectively possible to ensure that the pressure doesn’t drop below what is needed to do the job.

There are a few more things you can do to keep the condenser pressure low. Cleanliness is a vital element of condenser maintenance. When you keep your condenser clean, you reduce the likelihood of pressure increase. You’ll also want to keep the condenser cool. 

You may also consider checking out alternative compressors. There are opportunities for gas to get left behind and re-expand because of a reciprocating compressor’s piston and valves. Each type of compressor has a unique structure, and ones without valves and pistons will not have the clearance and re-expansion issues of reciprocating compressors.

What is a more efficient compressor type?

The scroll compressor is widely used in the HVAC industry because of its efficiency. A scroll compressor doesn’t require the valves and pistons of a reciprocating compressor. Instead, it has two coils (also known as scrolls): a fixed coil stays in the center while another coil oscillates around the fixed coil. As the moving scroll oscillates, it moves the refrigerant towards the center and compresses the gas before sending it to the condenser.

Scroll compressors do not have the same clearance or valve concerns that reciprocating compressors have. Gas re-expansion is not a concern and will not significantly impact the compressor’s efficiency.

Heat is a byproduct of mechanical energy that presents itself as a temperature increase. French physicist James Prescott Joule aimed to equate heat to mechanical energy. Early experiments used paddle-like structures to stir water within a container and raise its temperature. The paddles would move and agitate the water whenever a mass on a pulley system would fall from a fixed height. Joule evaluated the relationship between the energy he put in and the temperature increase. 

He created an equation that multiplies all the variables of potential energy to determine the heat produced (in Joules, J). These variables are mass in kilograms (m), the force of gravity (g = 9.8 m/s2), and height in meters (h). 

mgh = J

If a 10 kg mass were to fall 1 meter, then the equation would look like this:

10 kg(9.8 m/s2)(1 m) = J

The resulting number is 98 Joules, but this is not sufficient to raise the temperature of water significantly. Another equation equates heat (Q) to the mass (m) of water multiplied by temperature change (ΔT) and the amount of energy required to raise the temperature of a given substance (coefficient of heat, c). 

Q = cmΔT

After further experimentation, Joule determined that it takes 4186 Joules to raise the temperature of a kilogram of water by 1° Celsius. This is the coefficient of heat (c) for water.

Units of Heat

There are three main units for measuring heat: calories, Joules, and British Thermal Units (BTU). 

A calorie (cal) is the amount of heat required to raise the temperature of 1 gram of water from 14.5° Celsius to 15.5° Celsius (ΔT = 1° Celsius). The temperature change (ΔT) is only equivalent to 1° Celsius with a starting temperature (T0) of 14.5° Celsius and final temperature (TF) of 15.5° Celsius. However, these are the standard temperatures used to define a calorie. These calories are not the same as the calories we consume in our diets; the calories we consume are kilocalories (kcal), equivalent to 1000 calories.

A Joule (J) is the standard unit of energy named after the previously mentioned physicist, James Prescott Joule. A single calorie is equivalent to 4.186 Joules. Since we often use kilograms to measure the mass of water, it is worth noting that it takes 4186 Joules to increase a single kilogram of water by 1° Celsius.

A British Thermal Unit (BTU) is the amount of heat required to raise the temperature of 1 pound of water from 63° Fahrenheit to 64° Fahrenheit (ΔT = 1° Fahrenheit). Notice that BTUs use the imperial system while calories and Joules use the SI (or metric system). Even though BTUs are not very common internationally, many American heaters and air conditioning units are still rated in BTUs. One BTU per pound Fahrenheit is equal to one calorie per gram Celsius, but one BTU per pound Celsius is equivalent to about 1.8 calories per gram Celsius.


Heat Capacity

Heat capacity is the ability of an object to store heat. An object’s mass and specific heat primarily influence its heat capacity.

It takes more heat to raise the temperature of an object with a greater mass. Let’s say you have two objects of the same material, but one of them is ten times larger than the other. Intuitively, it would take ten times the amount of heat to raise the larger object’s temperature by the same amount as the smaller object. The larger object can store more heat without increasing the temperature, giving it a higher heat capacity than the smaller object. 

Specific heat is the amount of heat required to raise the temperature of the unit mass of a substance by 1° Celsius. Substances have different specific heats, which affects the heat capacity of each substance. For example, a pound of water has a greater heat capacity than a pound of copper. Even though you have the same amount of each substance, water’s specific heat is about ten times greater than copper’s. It takes ten times the amount of heat to cause water’s temperature to increase the same amount as copper’s. In the equation, Q = cmΔT, the coefficient of heat (c) represents the specific heat of the measured substance. Figure 1 shows a table of different substances and their specific heats.

Figure 1. The specific heats of water, aluminum, copper, glass, and lead. Each column represents the specific heats in different units: BTUs per pound Fahrenheit, BTUs per pound Celsius, calories per gram Celsius, and Joules per kilogram Celsius. Notice that the BTUs per pound Fahrenheit are equivalent to the calories per gram Celsius.


Heat and Temperature Increase

You can use the equation Q = cmΔT to measure a change in temperature if you already know the substance’s initial temperature, mass, and how much energy is being added. We would rewrite the equation as Q = cm(TF – T0), where TF represents the final temperature and T0 represents the initial temperature. 

Since we know every variable except TF, we are going to solve for TF. To isolate TF, divide both sides of the equation by cm, and add T0 to both sides. The new equation looks like Q/cm + T0 = TF. Divide Q by cm to get the temperature change, and add that quotient to the initial temperature (T0) to get the new temperature (TF). See Figure 2 for a step-by-step visualization of this process.

Figure 2. The step-by-step mathematical process for finding the final temperature after applying 50 BTUs of heat to 5 pounds of water at 15° Celsius.


Phase Changes and Latent Heat

Whether a solid becomes a liquid or a liquid changes to a gas, phase changes require energy. The added energy changes a substance’s molecular structure. The energy stimulates movement, which reconfigures the molecular bonds. When a solid melts, the vibrations force the molecules to fall away from each other and slide past one another. When a liquid turns into a gas, the molecules’ movements become so fast and powerful that they break free of the liquid and become a gas or vapor.

It takes more energy to cause a phase change than to heat a substance. It takes 1 BTU to heat 1 pound of water, but it takes about 144 BTUs to melt 1 pound of ice completely. Evaporating liquid water takes even more energy; boiling water transforms into water vapor after adding 970 BTUs. 

The temperature does not change during a phase change. An ice cube at 32° Fahrenheit (0° Celsius) will transform into water at 32° Fahrenheit. Because there is no temperature change, we do not use TF – T0 to account for the energy change. Instead, we use a fixed variable for latent heat. Latent heat is the heat required for a substance to undergo a phase change, not change temperature. There are two kinds of latent heat: latent heat of fusion and latent heat of vaporization. Figure 3 shows the heat energy applied to water and the corresponding temperatures.

Figure 3. The heat energy applied to water (in BTUs) and the corresponding temperatures. Notice that the temperature stays the same at the melting and boiling points of water. The energy added during these phase changes is latent heat.

Latent heat of fusion is the amount of energy required for a substance to change from a liquid to a solid or vice versa (via freezing or melting). Waters latent heat of fusion is roughly equivalent to 144 BTUs per pound. Figure 4 shows how to calculate the number of BTUs needed to melt an ice cube that weighs 20 pounds.

Figure 4. Solving for Q using the 20-pound ice cube and water’s latent heat of fusion.

Latent heat of vaporization is the amount of energy needed for a substance to change from a liquid to a gas or vice versa (via evaporation or condensation). Waters latent heat of vaporization at atmospheric pressure is equivalent to 970 BTUs per pound. Figure 5 shows how to calculate the number of BTUs needed to vaporize boiling water with a weight of 15 pounds.

Figure 5. Solving for Q using the 15-pound mass and water’s latent heat of vaporization.

Heat Transfer and Refrigerants

The HVAC industry relies on refrigerants to remove heat. Refrigerants are substances that move heat through an HVAC system’s cycle and produce a cooling effect while vaporizing. Refrigerants exist in liquid and gaseous states when they cycle through a unit, so a refrigerant’s specific heat and latent heat of vaporization affect its performance.

There are a few variables to consider when choosing a refrigerant in a cycle with changing temperatures and pressures. A good refrigerant has pressures that can be easily manipulated. Since gases are much more compressible than liquids, the boiling point of a good refrigerant should be below the temperature of the room or refrigerated box. Although the boiling point should be low when the refrigerant is in the evaporator, its condensation point should be above the outdoor temperature. The refrigerant must be able to reject the outdoor heat.

Since a refrigerant’s goal is to move heat, an ideal refrigerant should have a high latent heat of vaporization. When a refrigerant has a high latent heat of vaporization, it can effectively move more heat when it boils and condenses within the target ranges.

If you intend to use a refrigerant as a pump fluid in the liquid phase, it would be best for the refrigerant to have a relatively high specific heat. Water and carbon dioxide are common refrigerants that we use as pump fluids in their liquid phases. If you look at Figure 6, you can see that water and carbon dioxide have much higher specific heats than other common refrigerants. Keep in mind that we do not commonly use the other refrigerants as pump fluids in the liquid phase.

*Water is listed at 212 degrees F at 14.7 PSIA rather than a 40-degree evaporator temperature.

Figure 6. The specific heats and latent heats of vaporization of common refrigerants compared to water. 

Water may seem like the ideal refrigerant based on the previous information about specific heat and latent heat of vaporization, but it has several drawbacks. Refrigerants come into contact with metals during the refrigeration cycle, so nonreactive refrigerants are best for HVAC systems. Water has oxygen in it, which reacts with many metals and will most likely harm an HVAC system. Water would also require evaporators to be in a carefully controlled deep vacuum. Since this is highly impractical and inefficient, we rely on other refrigerants that don’t need the evaporator to be under such demanding conditions constantly.

When it comes to other refrigerants, you have to consider toxicity and flammability. Toxicity refers to a refrigerant’s potential to cause injury via a corrosive attack on the bodily tissues. ASHRAE classifies refrigerants into two toxicity categories: Class A and Class B. Class A refrigerants are primarily non-toxic. Most of the refrigerants in Figure 6 are Class A refrigerants. Class B refrigerants are toxic and should be handled with care. ASHRAE also has a classification system for a refrigerant’s flammability, the ease with which a refrigerant combusts. There are three categories for flammability: No Flame Propagation (1), Lower Flammability (2 and subclass 2L, which represents the slow burn in the new HFOs), and Higher Flammability (3). R-32 and R-290 are part of Class 3, presenting a high fire risk and requiring extreme caution.

ASHRAE classifies each refrigerant with the letter for toxicity and the number for flammability. For example, R-22 is an A1 refrigerant because it is non-toxic and has no risk of flame propagation. A1 is the best category for a refrigerant because A1 refrigerants present a low risk for bodily harm or property damage.

One more thing to consider when selecting a refrigerant is how it will mix with oil. Since oil will have to circulate through the system, it will come into contact with the refrigerant at some point. The oil should be able to move and mix with the refrigerant well, meaning that the oil should have good miscibility.


This is all to say – moving heat can be a tricky business, but it’s one we are excited to tackle.

In the trade we talk a lot about changes in Enthalpy, especially when we are looking at total heat exchange over an evaporator. Sometimes you will bump into the word Entropy and I wanted to take a stab at making it more understandable. 

Many people understand entropy as the condition in which molecules become more disorganized and spread out. Some people would simply describe entropy as a state of disorder, and my favorite is that entropy is a mathematical relationship between heat and temperature. While these are correct, they are rather broad definitions of the term. They don’t precisely describe entropy’s role in refrigeration.

Refrigeration occurs in a cycle with temperature and pressure changes throughout. The concentration of refrigerant molecules responds to those changes in temperature and pressure. 

One way the molecules react is by undergoing a phase change. Refrigerants exist in gaseous and liquid forms at different points of the cycle, and the molecules of gases are much more sparse and disorganized than liquid molecules. That is one example of entropy at work during refrigeration.

But what does entropy actually do for us? How does the change in the molecules’ organization affect the way we make our HVAC systems work?


What does entropy indicate?

Before we answer any more complicated questions about entropy, we should establish what entropy means for system performance.

On a fundamental level, entropy indicates that the HVAC system has the capacity to perform work. Temperature, pressure, or phase changes wouldn’t occur within a unit if there weren’t enthalpy and entropy.


How does entropy fluctuate throughout the cycle?

There are four main phases in the refrigeration cycle: compression, condensation, expansion, and evaporation. 

Each stage of the cycle has a corresponding part within an HVAC unit. It’s pretty easy to remember those parts because they are named after the processes. For example, compression occurs in the compressor, while condensation occurs in the condenser. Expansion occurs after the expansion valve or other metering device, preceding the evaporator in the refrigeration circuit. (I’ll bet you can guess what happens in the evaporator.)

Entropy varies with each process, mainly where phase changes occur. Phase changes occur in the evaporator and the condenser. Entropy rises while the refrigerant is in the evaporator, and it falls while the refrigerant is in the condenser. Entropy slightly decreases and increases during the expansion phase, and it stays constant in the compressor.

A T-S diagram like the one shown below shows how entropy changes in the system along with the temperature. T represents temperature, and S represents entropy.

Why does entropy vary across those stages?

Remember when I said that changes in entropy are most noticeable between phase changes? 

That’s because the molecules move very differently in gases and liquids. I briefly mentioned that gas molecules are a lot more sparse than liquid ones. That’s true, but they also move a lot faster than liquid molecules. Liquid molecules slide past each other, but gas molecules zoom past each other at high speeds. 

Which situation sounds more chaotic? Driving at a slow to moderate pace on a high-traffic freeway or taking part in a street race on the interstate? The latter is much more disorderly, and a similar principle applies to the higher entropy of gases than liquids.

The refrigerant evaporates in the evaporator. It transforms from a liquid to a gas, meaning that its molecular structure becomes disorganized. The molecules begin moving so quickly that they break free from the liquid and vaporize. Entropy is the work performed during the phase change. It is the quickening and separation of the molecules as they adopt a gaseous form.

The opposite is true for the condenser. The compression phase is all about pressure and temperature increase (with almost no change in entropy), so the refrigerant enters the condenser as a hot, high-pressure gas. The refrigerant cools in the condenser, restoring some degree of order as the gas molecules tighten back into a liquid. Entropy appears to decrease. 

However, it’s worth noting that the second law of thermodynamics forbids entropy from decreasing over time. When entropy appears to reduce within the system, more entropy occurs outside the system, allowing entropy to drop within the HVAC unit.


Entropy’s relationship with temperature and pressure

Entropy doesn’t just correlate with phases of matter. Temperature and pressure also have a say in the entropy of a system.

We’ve already established that gases have more entropy than liquids. Because of that, it’s intuitive that a temperature increase will cause entropy to rise. After all, phase changes need a certain amount of heat in order to occur. In my research, I reached out to Carter Stanfield of the Fundamentals of HVACR blog, and I’ll paraphrase his good explanation of temperature’s relationship to entropy: 

If you increase temperature, you increase entropy. More energy in the system excites the molecules, leading to an increase in random activity. Rising temperatures also cause gases to expand, which increases the entropy because the molecules have more room to whiz about.

But what about pressure? 

Compression is the opposite of expansion, so it makes sense that compression would decrease entropy if expansion increases it. Since gases are compressible, the molecules can be squeezed closer together as high pressure reduces the volume. When molecules have less room to zoom past each other, they’re not as disorderly as they were before. 

Remember the street racing example? Imagine that the racers reach a mass of cars going about 10 mph over the speed limit. The racers have to slow down and stay in their own lanes because the highway is too crowded to weave in and out of traffic. It’s a bit more orderly without people driving in every direction, right? The same rule applies to gases when they have less room to zoom about due to high-pressure conditions.


Why should we care about entropy?

I know it sounds like this knowledge about entropy is only useful for nerds who love learning about the chemistry and physics of HVAC systems. Hear me out, though. When you know what entropy is and how it works within your HVAC units, that knowledge has quite a few applications.

Entropy is a good value to keep in mind when you try to boost a unit’s efficiency. When thinking about efficiency, it’s useful to think of entropy as [wasted] energy potential. Since there is no such thing as a 100% efficient unit, one way to get a more efficient unit is to aim to reduce entropy. 

This is where you should be aware of enthalpy. If you know your desired heat content within your system, you can focus on getting the desired amount of BTUs to move heat with the lowest possible entropy. 

Entropy will always occur, but you will get a more efficient system if you can get it to do its job without an excess of entropy. One way to do this is to reduce the temperature and pressure of your system. You obviously won’t want to minimize those variables enough to make your unit quit working, but you can save some energy by aiming to lower entropy.

In HVAC systems, liquid and vapor will exist at the same time and place, we call this saturation or at saturation. Phase changes occur in the evaporator and condenser, so these are spots where liquid and vapor coexist while the system is running.

Saturated conditions occur whenever liquid and vapor occupy the same closed space. Liquid and vapor obey pressure rules when they inhabit the same area in a closed system. These closed systems can be inside HVAC units or tanks and are static (still) when in tanks or when the system is off, and dynamic (moving) when the system is running.

When the liquid and vapor exist at the same place at a given temperature in a closed system, they have a known pressure. We call this the pressure-temperature (P-T) relationship. This relationship will exist as long as you have at least a droplet of liquid in a closed system.

However, the refrigerant must be at its saturation point. Saturation can be confusing, so this article will explain saturation and how a P-T chart fits into the concept. It’ll also teach you how to use your P-T chart to determine superheat and subcooling.



When something is saturated, it’s full of something else. For example, clothes become saturated with water in a washing machine. 

In physics, liquids at saturation are “full” of kinetic energy. When this happens, they’ve reached their boiling point. The “boiling point” can be a somewhat misleading term, though.

Liquids at saturation have reached their boiling point, but they don’t have to boil to evaporate. Temperature is only a measure of average molecular activity. Some individual molecules have a lot more kinetic energy than others. These molecules will escape into the air without boiling. That’s why puddles don’t have to boil for water to evaporate from them.

Boiling occurs only when the vapor pressure and atmospheric pressure are the same. Most refrigerants have a high vapor pressure and will boil easily. Whenever boiling occurs within a closed system, the gas molecules increase the pressure inside the vessel. Gas molecules are far apart from each other and move quickly, and boiling increases the amount of them. The pressure increases when more of those molecules zoom around the closed space.

At some point, the vessel’s pressure will exceed the liquid’s vapor pressure. Boiling will stop when this happens. When boiling stops within a closed system, the temperature and pressure stop increasing. 

The refrigerant will reach equilibrium. Molecules evaporate and condense at an even rate, at a constant temperature and pressure. When that happens, the refrigerant remains at its saturation point. At saturation, you can use the P-T relationship to predict temperature or pressure.


P-T charts

The P-T chart is a vital yet often overlooked tool. P-T charts use the pressure-temperature relationship to help you determine the refrigerant’s pressure at a given saturation temperature. 

The table’s top usually lists common refrigerants, and the left side lists saturation temperatures. The rest of the table has the saturation pressures for each refrigerant at the given saturation temperatures.

You can use this chart to determine the pressure when you read a temperature or vice versa. Refrigerants exist in vapor and liquid states simultaneously in the evaporator and condenser. The coils add or remove heat, which allows phase changes to occur. Before a phase change can take place, the refrigerant must reach saturation.

Remember, this chart is only accurate when liquid and vapor are present at the same time and place. The refrigerant has to be a certain temperature and pressure because it exists in both gas and liquid phases within a closed system.

Keep in mind many of us won’t use the chart itself very often. We will use apps like Danfoss RefTools or Measurequick to give us PT data or we will simply look at our gauge which will have a PT chart for various common refrigerants printed right onto the gauge face. If the gauge shown above was connected to an R410a system we would see the pressure is about 134 PSIG which points at about 46°F on the R410a (pink) temperature scale printed on the face. If it were R22 the green scale would show us 75°F for the very same pressure.


Superheat and subcooling

The temperature deviates from the P-T relationship outside the evaporator and condenser. In these cases, superheating or subcooling has occurred. 

Superheated vapor is hotter than the saturation temperature. The vapor/suction line should contain superheated vapor. Otherwise, vapor-liquid mixtures in that line may indicate flood back. Subcooled liquid is cooler than the saturation temperature, and it should be limited to the end of the condenser and the liquid line. 

You can determine the superheat or subcooling by finding the difference between the sensible and saturated temperatures at a given pressure. That’s where your P-T card or P-T app comes in handy.

You’ll find saturation temperatures inside the evaporator and condenser coils. You can take sensible temperatures anywhere in the liquid or vapor lines. 

To determine the superheat in the vapor / suction line, locate a specific point on the line. This point can be the coil outlet or anywhere else between the evaporator and the compressor depending on the purpose of the measrement. Take a sensible temperature measurement of the line and pressure reading. Find the pressure on the P-T card and look for the corresponding saturated temperature. Find the difference between the measured sensible and saturated temperatures. The tempertue increase from saturated to sensible is the superheat.

The same principle applies to subcooled liquid in the liquid line. Take a sensible temperature and pressure reading on the liquid line. Find the pressure on your P-T card and locate the corresponding saturated temperature. The difference is the subcooling amount and it will always be a lower measured line temperature than saturation when the refrigerant is fully liquid. 


The P-T relationship makes your job a lot easier. Still, it only exists under specific conditions. It’s a good idea to understand those conditions fully. That way, you can use the P-T chart to help you determine superheat and subcooling conditions as well as evaporator and condensing temperatures. It can also help us identify what type of refrigerant we have in a tank or if that refrigerant may be cross-contaminated. 

This knowledge is one of the basic building blocks of refrigerant circuit understanding and they start with understanding the P-T relationship and saturation. 

Compressibility is the ability of a substance to be squeezed into a smaller volume. It is the change in volume and increase in density that results from an increase in pressure. 

The subject of compression should be familiar to HVAC techs. After the return air passes over the boiling refrigerant in the evaporator coils, the refrigerant absorbs heat and goes to an A/C unit’s compressor. The compressor packs those low-pressure gas molecules a lot more tightly. When that happens, the refrigerant’s temperature and pressure increases enough to release the absorbed heat into the outdoor air via the condenser.


What about the liquids?

We’ve talked a bit about gases and are familiar with gases from working with them. This article is about the compressibility of liquids, though.

Yes, liquids are compressible. However, that doesn’t mean they are compressible enough to be relevant to the HVAC industry. 


Which properties affect liquid compressibility?

Unlike gases, liquids are heavily affected by the properties of cohesion and adhesion. 

Cohesion is the property that causes molecules of the same type to come together. It is what allows two different quantities of the same liquid to combine into a single large quantity. (Think about refilling a water glass. You will still have a single mass of water in your glass when you add more water to the glass.)

Adhesion promotes interactions between molecules of a like substance and an unlike one. Adhesion causes water and oil to separate when you attempt to mix them, and it causes visible water droplets to form on a sweating wax paper cup.

Both cohesion and adhesion come into play with surface tension. Surface tension is the property that causes molecules to force themselves together into the smallest possible surface area and create a barrier between themselves and unlike substances. Surface tension distinguishes a single body of water from the air above it, giving the water surface a filmy appearance to us. 

As you can see, liquid molecules naturally come together rather tightly and don’t require you to manipulate pressure to force them to do so. As a result, liquid molecules are already packed a lot more tightly than gas molecules. Because of that, liquids need a lot more pressure than gases to alter their volume, even by a negligible amount. 


How could we apply that to HVAC units?

Bluntly put, it’s impractical to think we can compress liquids in any measurable amount. 

Our units compress gases because gases are easily compressible. Compressors can manipulate the gases’ temperature and pressure comparitively easily. 

Even though there is no such thing as a truly incompressible fluid, liquids have properties that make them resist compression, even under high pressure. Compressing liquids is essentially irrelevant for any of our purposes within the industry. When we try to compress liquids, such as when a compressor tries to pump liquid due to a flooded start or overfed evaporator, the compressor gives and breaks rather than the liquid being compressed. 

While we may move liquids via pumping, we end up with a bucket of parts when we attempt to alter its volume via compression.


Ductless systems didn’t gain popularity in North America until relatively recently. However, it’s no surprise that they’re becoming quite common. They are quiet, efficient, and well worth the price in the long run.

Highwall ductless systems have their downsides, though. When they get dirty, they can spread gunk beneath them and make a room smell quite nasty. To help you solve those problems for your customers, we’ve put together a guide for cleaning the Mitsubishi Highwall Mini-Split in two different ways.

Disclaimer: Some of these steps are fairly universal and others are specific to certain models of Mitsubishi high-wall units


Method #1

The first method is going to be disassembly of the unit and removal of the blower wheel. You wash the blower wheel outside with a hose (and sometimes cleaner), let it dry out, and put the unit back together.


1. Locate the unit power source and turn it OFF

As with any job that requires you to handle parts that normally move, you will want to make sure the unit is powered off before you begin taking it apart to clean it.

You’ll remove the blower wheel later, and you could end up with some messy results if it starts spinning while you’re trying to take it out. Be safe, exercise common sense, and don’t take risks. Just turn the unit off and confirm with a meter. 


2. Remove the air filters

You’ll take apart the air handler in stages. The first thing you’ll do is pull out the air filters on both sides of the air handler.


3. Remove the horizontal vanes


The next step is to remove the horizontal vanes from the bottom of the mini-split. 

The horizontal vanes each have three tabs right on the inside. You can slide the tabs a little bit to the right, and the vanes should pop right off once you slide all three tabs. The Mitsubishi Mini-Split has two of these vanes, but other manufacturers may have a different number. You can still often remove them the same way.


4. Expose and remove the three Phillips-head screws on the bottom

Right below the place where you removed the vanes, there will be three tabs along the bottom of the unit. You can pull these tabs up to expose the screws underneath them.

These screws are somewhat large and require an appropriately-sized Phillips-head screwdriver for removal, so make sure you have one on hand. 


5. Remove the unit cover

 Once you have removed the screws, slide the bottom of the face out and gently pull up and back. 

There will likely be some clips or tabs on top of the air handler. The Mitsubishi Mini-Split has some clips along the top. If your unit has these, you will need to release them as you remove the face. These may vary by manufacturer and model, so be mindful of what’s on top of your unit and what you need to do to remove the unit’s face.

The unit’s face should pop off quite easily if you’ve removed everything that needs to be removed and pressed everything that needs to be pressed.


6. Release the drain pan

Most of the time, the drain pan should be relatively easy to remove. There may be a tab beneath the drain pan, so you’ll push that in. Move the drain pan assembly by pulling it towards you and lowering it gently. That way, you allow it to rest downward from the unit.

There may be water in the drain pan. If there is water, make sure it’s all dry before you release the pan. It’s also a good idea to make sure you have some drop towels around and suck it out with a wet/dry vacuum. You wouldn’t want to damage any of the customers’ property.

You may not be able to release the pan on all models. If you cannot release the drain pan, we recommend using the Mini-Split Bib® Kit to clean the mini-split in place and without further disassembly.


7. Loosen the set screw in the blower wheel

There is a Phillips-head screw set inside the blower wheel. It should be near one of the side edges of the wheel. Locate it.

Loosen the screw either completely or nearly all the way using a Phillips-head screwdriver. Otherwise, the blower wheel won’t release itself.

Be careful with this step. The fins on the blower wheel are a bit fragile and may crack or scratch easily. When you are using metal objects near the blower wheel, mind the sharp edges to avoid damaging the fins.


8. Move the coil to expose and remove the blower wheel

You have to loosen the coil before you can move it. There are a few screws that connect the coil to the unit. (These are typically on the opposite side of the blower wheel’s set screw. In our pictures, the set screw was towards the right side of the unit. The screws that connect the coil to the unit were on the left side, just above the man’s hand in the picture.) Remove the screws with a Phillips-head screwdriver.

After you remove the screws, you can move the coil by gently pulling it up and towards you. This part can be quite tricky for techs, especially on the first try. Hold the coil in place a bit above the unit. The blower wheel should be in plain view. Remove the blower wheel and return the coil to its original position.


9. Clean the blower wheel outside

Most of the time, a simple garden hose will suffice for cleaning the blower wheel. 

When you need to use a proper cleaner, we recommend Refrigeration Technologies Viper cleaning products. We like the Viper EVAP+ evaporator coil cleaner because it’s quite strong and deodorizes the blower wheel nicely.

Be mindful of where you clean the blower wheel. You don’t want to wash the grime all over a customer’s freshly pressure-washed driveway or near their prized flower garden. Wash the blower wheel in a place where the debris won’t be an eyesore.

Let the wheel completely dry before you attempt to reassemble anything.

11. Clean the Evaporator in Place

This step is still best done with the coil bib in place as shown in option #2 further down. If you don’t use the bib you will be left using a combination of a rag, some spray cleaner (such as Viper in the spray can), and possibly a shop vac and soft bristle brush. It can be a tedious process and a lot of water management is required to keep from making a mess. 

10. Reassemble the unit

Simply put, you will want to do everything you just did backward. We’ll go through the reassembly steps, but we’ll keep it brief.

Once the blower wheel has dried completely, reinstall it by lifting the coil and setting it in its usual place. Set the coil back down and re-screw it into place. 

Tighten the set screw on the blower wheel, so it fixes to the motor shaft. 

Raise the drain pan back up to its usual position and secure it in place.

Put the unit’s face back on. Once it’s back on, re-screw the three Phillips-head screws at the unit’s bottom and place the protective tabs back over them.

Add the horizontal vanes back, and finish the reassembly by re-adding the air filters.

Restore power and clean up the job site.


Method #2

The second method allows you to stay in one place by collecting the water indoors using a Mini-Split Bib® Kit to collect the water and cleaner without making a mess. Even if you decide to take the blower wheel out and rinse it outside, we recommend using the Mini-Split Bib to clean the coil and internal shroud if those need cleaning. With this method you can clean the blower wheel in place, it still just takes some time and care to get it completely clean.


1. Turn the power OFF and disassemble as necessary

As with any job, turn the unit’s power off. 

The Mini-Split Bib makes total disassembly unnecessary, but you may decide to take certain parts of the unit off and clean them separately. (For example, you might prefer to wash the face separately if it looks exceptionally filthy.) We explain all the disassembly steps in Method #1, so refer to that for the procedure for guidance.

Even though the bib goes around the entire air handler, it’s a good idea to remove the air filters.


2. Fasten the Mini-Split Bib around the remaining parts of the air handler.

The Mini-Split Bib® comes with brackets that hook to the upper corners of the unit. They fasten the bib to the mini-split. 

The bib comes with a splash guard to apply between the backside of the bib and the wall. For added coverage, you can cover parts of the wall in adhesive tape to protect against spills and water damage to the wall.

The brackets have some ropes attached to them. The ropes cross over the bib’s front, and you can tighten them to fit the bib snugly around the air handler.

The bib comes with a funnel at the bottom for the water and cleaner to run out. Place the funnel in a bucket.

For a video tutorial on how to fasten the Mini-Split Bib®, we recommend watching this Mini-Split Bib tutorial on SpeedClean’s official YouTube channel.


3. Spray the unit to clean it

We recommend using a SpeedClean CoilJet to clean the unit. (Be sure to use SpeedClean cleaner if you use their CoilJet.) This cleaning apparatus has adjustable wands to help you reach difficult crevices. It also provides an easy means of mixing water and chemicals into a cleaning solution.

The first blast of water will have the highest pressure, so we suggest spraying your first bit of water into the bib. This will prevent the high-pressure liquid from splashing back at you when you aim it at the unit.

Once you’ve got a steady flow of cleaning solution, you can clean all inside the unit. The soapy solution will run down the inside of the bib and into the bucket.

You can use various wands to get into hard-to-reach places, like the backside of the coil. You can use the flex wand to reach the back from the top of the unit.

Let the unit dry before reassembling it.


4. Reassemble the unit

Once everything has dried, you can replace the parts you removed from the unit. If you cleaned the unit with the blower wheel inside, keep the bib ON after putting everything back together.

When you’re ready, turn the power back on. The blower wheel may release some water and cause some splashing. That’s normal, and that’s why you want to make sure the bib is still on. 

When the blower wheel doesn’t appear to be spraying any more water, you can remove the bib and clean up your work area.


As a best practice for any cleaning job, make sure you have plenty of drop towels and maintain a clean workspace. Whether you clean indoors or outdoors, there are plenty of ways to make a mess with a mini-split cleaning job. The last thing you want to do is damage someone’s expensive hardwood flooring beneath their unit.

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