Month: January 2021

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. 

 

References

 

[1] “Refrigerant Compression and Temperature,” HVAC School, 22 July 2019. [Online]. Available: https://www.youtube.com/watch?v=Y2ex2OxIXT0.
[2] “The Basic Refrigeration Circuit, Pressure & Enthalpy w/ Carter Stanfield,” HVAC School, 04 September 2017. [Online]. Available: https://www.youtube.com/watch?v=siV5xUPTRas.
[3] “Pressure / Enthalpy Diagram Example,” HVAC School, 13 December 2018. [Online]. Available: https://www.hvacrschool.com/pressure-enthalpy-diagram-example/.
[4] “What is Temperature?,” HVAC School, 28 February 2019. [Online]. Available: https://www.youtube.com/watch?v=RDIIpkVH_Jc.
[5] “HVAC/R Refrigerant Cycle Basics,” HVAC School, 01 March 2019. [Online]. Available: https://www.hvacrschool.com/hvacr-refrigerant-cycle-basics/.
[6] “Air Conditioning Compressor Basics,” HVAC School, 20 February 2019. [Online]. Available: https://www.youtube.com/watch?v=0lfa9rm8_x8&pbjreload=101.

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

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.

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