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.

Replacing a compressor is expensive, time-consuming, and physically taxing. If we are replacing a compressor I want us to be doggone sure we aren’t going to be dealing with the same thing again and this often includes a shiny new contactor and capacitor (on single-phase units).

We received a comment recently that called out the fact that we replaced a capacitor with a compressor even though it tested in range.

The commenter felt this was a slimy attempt to tack on more expense rather than a goodwill attempt to prevent future issues.

It is a fair question to ask, “What is appropriate to do when replacing a compressor?”

Why Replace the Capacitor?

For me, installing a new, properly sized capacitor with a new single phase motor or compressor is always cheap insurance. When I do this will always use higher quality, American-made capacitors just to give the customer the best chance at going quite sometime before another issue. In our market, run capacitors are among the most common failures we see due to the long run times, high temperatures, and high voltage transient events like surges. We have much better luck with higher quality capacitors so we use them as a standard operating procedure.

This also goes for hard start kits, I will always remove any old aftermarket hard starts and go back with a factory start capacitor and potential relay where it is called for by the application. Whenever I say something like this I get a lot of folks who love aftermarket hard starts who question it. My explanation is HERE.

If you are working on other motor types such as self-contained refrigeration with a current relay I would say the same, go ahead and replace it rather than run the risk of another issue.

Why Replace the Contactor? 

If a contactor is bright, shiny and brand new I’m probably not going to replace it. If it shows signs of wear it is a good practice to replace it with the compressor ESPECIALLY in three-phase units where single phasing can occur if one contact fails to connect.

This Tip from Emerson also confirms this policy as advised.

Anything Else? 

I recommend taking care of any contamination or burnout by using appropriate filter driers in both the liquid and suction lines and monitoring and/or replacing them depending on the application according to the recommendations by Emerson and Sporlan.

If the system contains an accumulator is advised to empty the old accumulator and measure the amount of oil it contains and its condition. I find it is often just easier to simply replace the accumulator rather than reinstalling especially if it has any signs of corrosion.

Take a close look at your pipework to make sure it is run properly without unnecessary oil traps, inverted traps at the coil where needed to prevent flooded starts, and good suction line insulation.

We also suggest using a virgin charge, cleaning the system condenser, evaporator, and condensate system, and making sure the proper airflow is present. It is also a good idea to make sure that all manufacturer-recommended accessories are installed especially if long lines are present. This could include things like a factory hard start, crankcase heater, pressure switches, or a liquid line solenoid.

Purge nitrogen, flow nitrogen while brazing, pull a proper vacuum, weigh in the charge and check everything… including suction temperature at the compressor and compressor discharge temperature to make sure it isn’t overheating.

All of this is in the service of the new compressor having a nice long life and the customer getting what they paid for… not as a way to drive up the invoice.

— Bryan

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.

Saturation

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.