Impacts of Compression on Temperature

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.

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