Refrigeration Cycles

Another important concept for the potential PE Exam test taker is the refrigeration cycle. Understanding this cycle and how to navigate the cycle will definitely be a huge advantage to the test taker.

Refrigerants

Refrigerants are fluids used in the commercial HVAC field to transfer heat from one source to another. For example, in a water cooled chiller, refrigerant is used to remove heat from chilled water and transfer heat to condenser water. Or in a typical residential split air conditioner system, refrigerant is used to remove heat from the indoor air and transfer that heat to the outdoors through the use of a condenser.

The main requirement for a fluid to be classified as a refrigerant is the ability to transfer heat. Refrigerants must also be safe in order to be used for commercial and residential air conditioning purposes. Refrigerants are classified by the following information: (1) Flammability, (2) Toxicity, (3) Global Warming Potential (GWP), (4) Ozone Depleting Potential (ODP) and (5) Operating Pressure. The flammability and toxicity classifications are shown in ASHRAE 15. Be familiar with ASHRAE 15

Refrigerants can be split into four different types, (1) Hydrocarbons, (2) Chlorofluorocarbons, (3) Hydroclurofluorocarbons and (4) Hydrofluorocarbons.

  • Hydrocarbons consist of hydrogen and carbon. Some examples of hydrocarbons include methane, ethane, propane and butane. Hydrocarbons like propane and isobutene can be used in vapor compression cycles for refrigeration, but most commonly hydrocarbons are used in the combustion process.
  • CFCs consist of carbon, with the chemical addition of chlorine and fluorine. Common CFCs include R-12 and R-11, which were used heavily in air conditioning, vapor compression cycles. Unlike hydrocarbons, CFCs are non-flammable. However, CFCs when improperly handled and released into the atmosphere have been found to deplete the ozone layer. For this reason, CFCs have been scheduled to be phased out and in the United States. In fact, CFCs are no longer used in new air conditioning machines.
  • HCFCs consist of hydrogen and carbon, with the chemical addition of chlorine and fluorine. The most common HCFC is R-22, which was used heavily in air conditioning. HCFCs are non-flammable. They are also no longer used in new air conditioning machines in the United States, because they contain the ozone harmful element, chlorine. The Montreal Protocol requires that HCFC's be decreased in consumption and production, until HCFC's are completely phased out in 2030. Two specific HCFC’s, 22 and 142B, have been phased out of new equipment in 2010, with the complete phase out of these refrigerants in 2020 (existing and new equipment).
  • HFCs have been substituted for CFCs because they have an ozone depletion potential of zero and contain no chlorine. HFCs are also being substituted for HCFCs because they are currently the most efficient refrigerants that do not harm the ozone, since they do not contain chlorine. However, HFCs are also planned to be substituted in the future because of the greenhouse gases that are emitted.
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Ozone Depleting Potential

Ozone Depleting Potential [ODP]: The ODP is an index developed to identify how damaging a substance is to the ozone. The reference point from which all substances are compared is CFC-11. CFC-11 is assumed to have an ODP of 1, more damaging chemicals have a higher ODP and less damaging chemicals have a lower ODP. A summary of chemicals and their ODP is shown in the table below. Refrigerants with chlorine have a higher ODP. It is estimated that each chlorine atom destroys 100,000 ozone molecules.

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Global Warming Potential

Global Warming Potential [GWP]: The GWP is an index developed to identify the potential for a substance to prevent infrared radiation from leaving the earth's atmosphere. The reference point, from which all substances are compared, is carbon dioxide. CO2 is assumed to have a GWP of 1, chemicals with a higher potential to contribute to global warming have a higher GWP and those with a lower potential have a lower GWP. A summary of chemicals and their GWP is shown in the table below.

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Boiling Points

Low pressure refrigerants boil at a lower temperature, high pressure refrigerants condense at a higher temperature.

One key principle that must be understood for Refrigeration is the relationship between the boiling/condensing point of a fluid, in this case a refrigerant, and the temperature and pressure of the refrigerant. A refrigerant liquid’s boiling point is a function of the vapor pressure of the refrigerant vapor that is in equilibrium with the refrigerant liquid. If the pressure is low, then there is a smaller force acting upon the refrigerant liquid, thus it will take a lower temperature to boil the refrigerant liquid. For example, water at a pressure of 1 atmosphere or 14.696 PSI will boil at 212 F. However, if the water was at a pressure of 0.122 PSI, then the water will boil at 40 F.

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When a low pressure refrigerant changes from its liquid phase to a gas phase, it can absorb much more heat than if it were to simply increase in temperature. The same is also true when a high pressure refrigerant changes phase from its gas phase to a liquid phase, it release much more heat than if it were to decrease in temperature. The energy required to change the phase of a liquid from a liquid to a gas is called the latent heat of evaporation. The energy released to change the phase of a gas to a liquid is called the latent heat of condensation.

Step 1 - Evaporator

The vapor compression cycle is the primary cycle used in commercial refrigeration systems.

The vapor compression cycle starts at (Step 1) the evaporator, with cold, low-pressure, liquid refrigerant. It absorbs heat and evaporates to a low-pressure gas. Then the gas is (Step 2) Compressed to a high-pressure, high-temperature gas and (Step 3) condensed to a high pressure gas. Finally, the gas is condensed at the (Step 4) expansion device to a cold, low-pressure liquid refrigerant.

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Step 1: Evaporator. The first step in the vapor compression cycle is the evaporator, which can also be called a liquid cooler. The evaporator is simply a heat exchanger. Heat is exchanged from the warm medium (air or water) to the cold, liquid refrigerant. The heat gained by the liquid refrigerant causes it to change phases to a refrigerant gas. The refrigerant liquid gains the heat necessary to overcome the latent heat of evaporation, in order to change to a gas. There are two types of evaporators, (1) an air cooled evaporator and (2) a water cooled evaporator. Figure below shows the (1) air cooled evaporator which is most commonly referred to as a direct expansion system. In this evaporator, warm air from an air conditioned space is cooled and redistributed to the space. Also shown in the figure below is the water cooled system, where chilled water return is cooled and supplied to the chilled water distribution system.

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The most common system is the direct expansion system. This system is prevalent throughout smaller systems, like those serving residential systems. In this system, the hot air from the space is used to directly evaporate the refrigerant to a hot gas. Note that the hot air from the space is roughly ~75 °F and the refrigerant liquid is typically 40 °F. The 75 °F room air is cooled down to ~55 °F and then distributed back to the space. In a water-cooled system, which is more common for larger commercial systems, chilled water typically at 55 °F is cooled by the evaporator down to ~45 °F. The colder chilled water is then supplied to another heat exchanger, where air is cooled and then distributed to the space.

Besides the two different types of evaporator systems, there are also different types of heat exchangers used in refrigeration. The most common heat exchangers include: (1) Shell and Tube, (2) Tube in Tube and (3) Brazed Plate.

(1) Shell and Tube: This heat exchanger is the most common and consists of copper pipes arranged in a coil that is constructed in a cylindrical shell. One fluid is provided in the shell and contacts the outer surface of the inner tubes. Another fluid is contained inside of the tubes. Heat exchange occurs in the shell at the outer surface of the tubes. Often times aluminum fins are provided on the copper pipes. These fins provide more surface area for heat exchange to occur.

(2) Tube in Tube: A tube is constructed in a tube, sealed separately to keep the fluids in one tube from contaminating the other. Heat exchange is conducted at the outer surface of the inner tube and the inner surface of the outer tube.

(3) Brazed Plate: This type of heat exchanger consists of multiple thin plates separated by a small distance. Each plate either carries the hot or cold fluid. Heat exchange occurs between the surface areas of each plate.

As previously mentioned the evaporator acts as a heat exchanger with a cold side and a hot side. The cold side consists of a mixture of refrigerant gas and liquid. At this point, the partial liquid-gas refrigerant mixture moves through the evaporator, picking up heat from the hot side. But instead of heating the gas, the heat is used to boil the remaining liquid. It is important for the evaporator to boil all of the liquid, prior to the refrigerant entering the compressor in the following step. Once all the liquid has boiled, the liquid-gas mixture turns into a refrigerant gas (vapor), called a saturated vapor. Any additional heat will now increase the temperature of the refrigerant vapor, into a region called super heat. Any release in heat will cause some of the gas to condense back to a liquid.

It is important for the engineer to understand that the amount of cooling provided through the evaporation of the refrigerant liquid is much more than simply increasing the temperature of the refrigerant liquid. For example, R-134a takes 92.82 Btu of heat to change 1 lb of refrigerant from liquid to gas. While it takes 0.204 Btu of heat to increase 1 lb of refrigerant gas by 1°F.

Step 2 - Compressor

The vapor compression cycle is the primary cycle used in commercial refrigeration systems.

The vapor compression cycle starts at (Step 1) the evaporator, with cold, low-pressure, liquid refrigerant. It absorbs heat and evaporates to a low-pressure gas. Then the gas is (Step 2) Compressed to a high-pressure, high-temperature gas and (Step 3) condensed to a high pressure gas. Finally, the gas is condensed at the (Step 4) expansion device to a cold, low-pressure liquid refrigerant.

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Step 2: Compressor: The next step is where the refrigerant gas is compressed by the compressor, which raises the temperature and pressure of the gas. The compressor is where the work takes place. The compressor is also the driving force that moves the refrigerant through the vapor compression cycle and prepares the refrigerant before it enters the condenser. It is important that the refrigerant gas is raised to a temperature that is above the temperature of the fluid in the condenser. This will allow heat to be transferred from the refrigerant to the condenser fluid. The compression of the refrigerant gas occurs isentropically, meaning that there is no change in entropy. Since the compressor is not completely efficient there will be an increase in enthalpy as the heat generated by the compressor is transferred to the refrigerant gas.

Entropy - a measure of the amount of disorder in a thermodynamic system.

Enthalpy - a measure of the total energy in a thermodynamic system (sensible and latent energy).

The engineer should be knowledgeable of the 5 different types of compressors and their advantages and disadvantages, in order to determine when they should be used. The five types of compressors are centrifugal, scroll, reciprocating, screw and rotary. A brief overview of the different types of compressors is shown below.

  • Rotary: The rotary type compressor compresses refrigerant gas through positive displacement. Positive displacement simply means that the pressure of the gas is increased by reducing the volume.
  • Scroll: Similar to the rotary type compressor, the scroll compressor uses positive displacement to increase the pressure of the gas.
  • Screw: Similar to the rotary type compressor, the scroll compressor uses positive displacement to increase the pressure of the gas. The screw compressor consists of two interlocking screws. The gas moves through the screw from the beginning thread to the end thread, increasing the pressure as it moves to the discharge side.
  • Reciprocating: A reciprocating compressor compresses gas through positive displacement. A piston type movement compresses gas as it enters the cylinder.
  • Centrifugal Centrifugal compressors are not like positive displacement compressors, these compressors rely on a rotating impeller to use its centrifugal force to move the gas to the outside diameter of the rotating impeller, which increases the velocity of the gas. The increased velocity is then translated into increased pressure.

Another distinction between compressors is made between hermetic, semi-hermetic and open drive compressors.
Hermetic is most often recognized when used in the phrase “hermetic seal”, which means airtight.
Hermetic: A hermetic compressor is airtight. The compressor and motor are located in a welded container, so no refrigerant can escape. Since the motor is located in the same enclosure as the compressor, the compressor needs to account for the motor heat.
Open Drive: An open drive compressor indicates that the compressor and refrigerant are located in an enclosure and out of the enclosure is a shaft connecting it to a motor. The motor is outside of the enclosure and the heat is lost to the space and not to compressor.
Semi-Hermetic: A semi-hermetic is similar to a hermetic compressor, except the motor and compressor are located in a mechanically sealed container, which can be opened without cutting into the enclosure unlike the hermetic compressor.

Step 3 - Condenser

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Step 3: Condenser: The third step in the vapor compression cycle is the condenser. The condenser is the counterpart of the evaporator. Similar to the evaporator, the condenser is simply a heat exchanger. Except in this case, heat is exchanged from the warm refrigerant gas to the cold medium. The heat released by the warm refrigerant gas causes it to change phases. The refrigerant gas condenses to refrigerant liquid.

There are two types of condensers, similar to the two types of evaporators. Figure 5 shows a sample water cooled condenser, where cool condenser water at ~85 °F is used to remove heat from the refrigerant, causing it to increase in temperature to approximately ~95 °F. Figure 6 shows the air cooled system, where heat is removed from the refrigerant by blowing outside air over the coil. The location will determine the condenser water and outside air temperatures.

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The methods of heat exchange are similar to that of the evaporator. Refer to the evaporator section for the different types of heat exchangers.

Step 4 - Expansion Device

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Step 4: Expansion Device: The final step is the expansion device, which is the counterpart of the compressor. The expansion device reduces the pressure of the liquid, which causes not only the pressure to decrease but also the temperature to decrease. During this process, some of the liquid refrigerant is turned into a gas, this is called flash gas. The resultant of the expansion device is a cold partial liquid-vapor refrigerant mix. The cold refrigerant liquid-vapor mix then repeats the process at the evaporator.

The expansion device that is primarily used in air conditioning systems is called a thermostatic expansion valve (TXV. The TXV as its name describes, opens and closes, based on a thermal device. The adjustment of the opening/closing determines the amount of refrigerant that is passed through and evaporated. The TXV uses the temperature of the evaporator output as a basis for determining the amount of refrigerant.

For example, if the TXV senses that the evaporator is producing an output refrigerant temperature that is too cold, then there is too much refrigerant for the heat load (hot side of the evaporator) and the refrigerant sent to the evaporator needs to be throttled down (decrease cold side of the evaporator). If the TXV senses that the output of the evaporator is too high, then the amount of refrigerant cannot keep up with the heat load (hot side) then the TXV should allow more refrigerant to the evaporator (increase cold side).

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