HVAC Clinic. Refrigeration Cycle

Transcription

1 HVAC Clinic Refrigeration Cycle

2 Table Of Contents Introduction... 3 Fundamentals of Refrigeration... 3 Refrigerants... 4 Refrigeration Cycle... 9 Pressure Enthalpy Chart... 16

3 Introduction Refrigeration is a process in which work is done to remove heat from one location to another. In this clinic, we will discuss the principle of refrigeration as it applies to air conditioning. In commercial air conditioning, refrigeration is typically accomplished with the vapor refrigeration cycle or the absorption refrigeration cycle. For the purposes of this discussion, we will only focus on the vapor refrigeration cycle. Fundamentals of Refrigeration Refrigeration is generally associated with keeping something cold. A refrigerator, for example, keeps its contents cold (figure 1). It achieves this task by removing heat from the food. Therefore, refrigeration involves the removal of heat from a substance. Figure 1. Refrigeration To understand the process of refrigeration, we first need to understand what heat is and how it is removed from a substance. The three primary rules of heat transfer are: 1. Heat energy cannot be destroyed 2. Heat always flows from a higher temperature source to a lower temperature source 3. Heat can be transferred from one substance to another substance The first rule states that heat energy cannot be destroyed. Energy can be transferred from one substance to another substance, but the energy is maintained. This is the first law of thermodynamics and is known as the conservation of energy. The second rule is that heat always flows from a higher temperature source to a lower temperature source. Heat is a measure of the motion of (i.e. the kinetic energy of) molecules. When an object in motion impacts with an object that is stationary, it imparts some of its energy to the stationary object (via movement and plastic deformation). The same is true of heat. Thus heat always flow from a high temperature source to a low temperature source much the same way a moving object transfers energy to a stationary object. Finally, heat can be transferred to one substance to another. There are three methods of transferring heat; convection, conduction and radiation. Convection is the transfer of energy between two fluids. Convection cannot flow between solids. Conversely, conduction is the transfer of energy between adjacent molecules of a solid. Finally, radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. It represents a conversion of thermal energy into electromagnetic energy. Radiation does not require the presence of matter to propagate. WN Mechanical Systems Refrigeration Cycle Page 3 of 21

4 Refrigerants Refrigerants can be in the form of natural and chemical substances. An example of a natural refrigerant is ice. Ice is commonly used to preserve food. Because heat flows from a higher temperature substance to a lower temperature substance, ice can be used in a bucket to absorb heat from the relatively warm beer, thus cooling our precious beverage (figure 2). As the ice absorbs heat, it changes states and melts. Figure 2. Ice Used in this manner, ice is a refrigerant. It absorbs heat and transports the heat away from the food. Ice, however, does have a disadvantage. It absorbs heat and melts at 32 o F. If are trying to cool something with a melting point lower than ice, we cannot use ice. Ice cream, for example, melts at a temperature lower than 32 o F (figure 3). Thus ice cannot be used to refrigerate ice cream. We must use something with a lower melting point than the substance we are trying to cool. Figure 3. Ice Cream WN Mechanical Systems Refrigeration Cycle Page 4 of 21

5 Another type of natural cooling is dry ice, which is solid carbon dioxide (CO2). It evaporates directly from a solid phase to a vapor phase at F. Dry ice would keep the ice cream frozen because it evaporates at a lower temperature than the temperature at which ice cream melts (figure 4). Figure 4. Dry Ice Refrigerant R-410a is a chemical used in many small refrigeration systems. If an open container of liquid R-410a were placed in our beer cooler, when exposed to atmospheric pressure, it would absorb heat and boil violently at o F (figure 5). Figure 5. R-410a At atmospheric pressure, each of these three substances (common ice, dry ice, and R-410a) absorb heat and change phase at a fixed temperature. Common ice melts at 32 o F, dry ice evaporates at o F, and R-410a boils at o F. This then begs the question, why do we want a substance to change phase while producing refrigeration? The question is best answered by examining the effects of heat transfer on water. Consider 1 lb of 100 F water. By adding 1 Btu of heat energy, the water temperature is raised by 1 F (figure 6). WN Mechanical Systems Refrigeration Cycle Page 5 of 21

6 Figure 6. 1 BTU Therefore, adding 112 Btu to 1 lb of 100 F water raises its temperature to 212 F (figure 7). Figure BTU s While 212 F is the boiling temperature of water at atmospheric pressure, adding 1 more Btu will not cause all of the water to evaporate. It takes a much larger quantity of heat to completely boil 1 lb of water. In this example, 970 Btu s must be added to 1 lb of 212 F water to completely convert it to steam at the same temperature (figure 8). WN Mechanical Systems Refrigeration Cycle Page 6 of 21

7 Figure 8. Water to Steam Conversely, you must remove 970 Btu s to condense 1 lb of steam completely to water. That is, it gives off or releases 970 btu s of heat to convert a pound of steam to water (figure 9). Figure 9. Steam to Water Refrigerants can absorb a significant amount of heat when they change phase; far more than if they just change temperature. Different substances have distinctive specific temperatures at which these phase changes occur and different quantities of heat are required for this change to take place. This capacity of a substance to absorb or release heat is a property of the substance called specific heat. The specific heat of a substance is defined as the quantity of heat (btu s) required to raise the temperature of 1 lb of that substance 1 F. For example, two differing substances may be exposed to the exact same quantity of heat. However, substance A experiences an increase in temperature to a greater degree than substance B. Therefore, substance B would have a great heat capacity and thus a higher specific heat than substance A (figure 10). WN Mechanical Systems Refrigeration Cycle Page 7 of 21

8 Figure 10. Specific Heat In order for us to utilize refrigerants for building air conditioning, we must select refrigerants that change state at the appropriate temperatures and pressures. Generally speaking, the temperatures and pressures required do not lend themselves to natural refrigerants. It is for this reason we have created chemical refrigerants. These chemical refrigerants have evolved over the years in order to perform their task while maximizing efficiency and decreasing environmental impact (figure 11). Figure 11. Chemical Refrigerants WN Mechanical Systems Refrigeration Cycle Page 8 of 21

9 Refrigeration Cycle A very simple but rudimentary refrigeration system could theoretically be constructed using a container holding liquid refrigerant at atmospheric pressure, a coil, and a valve to regulate the flow of refrigerant into the coil. As you open the valve, liquid refrigerant would flow into the coil by gravity. As warm air is blown over the surface of the coil, the liquid refrigerant inside the coil will absorb heat from the air. This will cause the refrigerant to boil, thus cooling the air. By adjusting the valve, just enough refrigerant is provided to ensure complete evaporation before it exits the coil (figure 12). Of course, releasing refrigerants into the atmosphere is illegal and wasteful. Additionally, the boiling temperature of R-410a at atmospheric pressure is F. The coil surface is going to approach the refrigerant vapor temperature and will freeze any moisture contained within the airstream. This will eventually turn the surface of the coil into a block of ice. Figure 12. Open Loop Refrigeration System If we had an unlimited free source of refrigerant that was completely harmless to the atmosphere, this would make a very effective cooling system. The power requirements for the refrigeration system would be zero as the potential energy required to force the refrigerant through the coil is provided by gravity. Sadly, no such refrigerant is known to exist. To solve the problem, a method must be employed to return the refrigerant exiting the coil back to the valve in a liquid phase. Then the cycle can repeat and the refrigerant can be re-distributed back through the coil in a closed loop. This is precisely what happens in a typical mechanical refrigeration system. Liquid refrigerant absorbs heat and evaporates within a device called an evaporator. In the example in figure 12, air is cooled when it passes through the evaporator, while the heat is transferred to the refrigerant, causing it to boil and change into a vapor. As discussed earlier, a refrigerant can absorb a large amount of heat when it changes phase. Because of the refrigerant changing phase, the system requires a much lower mass flow rate of refrigerant than if the refrigerant was just increasing in temperature. WN Mechanical Systems Refrigeration Cycle Page 9 of 21

10 The refrigerant vapor must then be converted back into a liquid in order to return to the evaporator and repeat the process (figure 13). Figure 13. Closed Loop System As mentioned earlier in this clinic, when water vapor or steam condenses in to liquid water, it releases a large amount of energy (figure 14). Figure 14. Condensing Water The second rule of heat transfer dictated that heat always flow from a higher temperature source to a lower temperature source. Thus, if we want to remove heat from a vapor, we must expose that vapor to a substance with a lower temperature. In the case of steam, ambient air can easily be used as it is always less than 212 o F. However, recall that R-410a, the refrigerant most commonly used in small to moderate sized comfort cooling systems today, boils at -55 o F (figure 15). Clearly, ambient air cannot be used as it will virtually always be at a higher temperature (unless you live in the South Pole). WN Mechanical Systems Refrigeration Cycle Page 10 of 21

11 Figure 15. Condensing Atmospheric Pressure R-410a However, we know that the boiling temperature of different substances is a function of pressure. If we increase the pressure, we correspondingly increase the boiling temperature. The example in figure 16 depicts the boiling point of water at various temperatures. At atmospheric pressure or 14.7 psi, water boils at 212 o F. Note that as the pressure increases, the boiling temperature increases. Figure 16. Boiling Point of Water WN Mechanical Systems Refrigeration Cycle Page 11 of 21

12 As mentioned previously, R-410a boils at o F. However, if we pressurize R-410a to 145 psi, the boiling point becomes 45 o F (figure 17). Refrigeration coil discharge air temperatures are commonly in the neighborhood of 55 o F. With a 45 o F coil entering refrigeration temperature, we should be able to readily maintain a 55 o F coil leaving air temperature. Similarly, if we pressurize R-410a to 490 psi, the boiling point becomes 130 o F. At 130 o F, we should be able to reject the heat of a refrigeration system back to the atmosphere (assuming the ambient temperature at design is less than 130 o F). Figure 17. Boiling Point R-410a In order to achieve these pressures and corresponding temperatures, a compressor, condenser, and expansion device form the rest of the system. The compressor boosts the pressure, while the condenser reduces the temperature at a relatively constant pressure and finally the expansion device returns the refrigerant vapor to a low-temperature liquid, which can again be used to produce useful cooling. This cycle is called the vapor-compression refrigeration cycle (figure 18). WN Mechanical Systems Refrigeration Cycle Page 12 of 21

13 Figure 18. Refrigeration Cycle The purpose of the evaporator is to absorb heat from the conditioned media to a low pressure, low temperature refrigerant (figure 19). The low pressure, low temperature liquid refrigerant (A) absorbs heat from the conditioned media (air in this case) and boils, exiting the evaporator as a vapor (B). Figure 19. Evaporaor WN Mechanical Systems Refrigeration Cycle Page 13 of 21

14 Next, the compressor increases the pressure of the refrigerant to a temperature that will be above temperature of the substance that we are going to utilize as a source of heat rejection. In the case of an air cooled condenser, we will need to pressurize the refrigerant to a temperature above the design ambient air condition (figure 20). In the example, we take a low pressure, low temperature vapor (B) and pressurize it to a high pressure, high temperature vapor (C). Figure 20. Compressor The condenser rejects the heat from the high pressure, high temperature refrigerant to the heat rejection media. In this example, we are rejecting the heat to the ambient air. The temperature of the refrigerant must be higher than that of the ambient air (figure 21). As the heat content of the refrigerant vapor is reduced, it condenses into liquid (D). Figure 21. Condenser WN Mechanical Systems Refrigeration Cycle Page 14 of 21

15 Finally, the expansion device completes the cycle. Several types of expansion devices are available. The high-pressure liquid refrigerant (D) flows through the expansion device, causing a pressure drop (figure 22). This pressure drop reduces the refrigerant pressure and its temperature to that of the evaporator. At the lower pressure, the temperature of the refrigerant is higher than its boiling point. This causes a small portion of the liquid to boil, or flash. Because heat is required to boil this small portion of refrigerant, the boiling refrigerant absorbs heat from the remaining liquid refrigerant, cooling it to the required evaporator temperature. Figure 22. Expansion Device Placing each component in its proper sequence within the system, we complete the cycle (figure 23). The compressor and expansion device maintain a pressure difference between the high-pressure side of the system (condenser) and the low-pressure side of the system (evaporator). Figure 23. Refrigeration Cycle Components WN Mechanical Systems Refrigeration Cycle Page 15 of 21

16 Pressure Enthalpy Chart The pressure enthalpy chart graphically depicts the refrigeration cycle on a chart (figure 24). The horizontal axis represents enthalpy, or energy state of the media. The vertical axis represents pressure. The left hand side of the chart represents the condition at which the refrigerant is in the liquid phase. The right hand side of the chart represents the condition at which the refrigerant exists as a pure gas or is in a vapor phase. The center of the chart represents a mixture of liquid plus vapor. This area is depicted by a curve shaped like a dome, called an envelope. Any state point that exists within this envelope is a mixture of liquid plus vapor. The left portion of the curve represents a saturated liquid condition. The right hand portion of the curve represents a saturated vapor condition. If the state condition lies to the left of the curve, the refrigerant is subcooled. If the state condition lies to the right of the curve, the refrigerant vapor is superheated. Lines of constant temperature are as shown on figure 24. Figure 24. Pressure Enthalpy Chart WN Mechanical Systems Refrigeration Cycle Page 16 of 21

17 We begin our analysis of a PE chart by examining the evaporator. In this example, we will assume refrigerant R-410a. At the inlet to the evaporator, the refrigerant is at a pressure of 145 psia and a temperature of 45.0 F and is a mixture of liquid and vapor. This cool, low-pressure refrigerant enters the evaporator (A) where it absorbs heat from the relatively warm air that is being cooled. This transfer of heat boils the liquid refrigerant inside the evaporator and superheated refrigerant vapor is drawn to the compressor (B). As discussed earlier in this clinic, a greater amount of heat energy can absorbed when a refrigerant changes state as compared to remaining in the same state (heating a superheated vapor in this case). However, we must ensure that no liquid enters the compressor. This requires a margin of safety. This safety margin is called superheat and represents the energy added from points B to B. Superheat is generally ranges between 8 o F and 12 o F. The total amount of energy absorbed from points A to B is called the refrigeration effect (figure 25). Figure 25. Evaporator WN Mechanical Systems Refrigeration Cycle Page 17 of 21

18 The compressor draws in the superheated refrigerant vapor (B) and compresses it to a pressure and temperature (C) high enough that it can reject heat to another media. Compression may occur via either positive displacement or dynamic compressors. The mechanical energy used by the compressor to compress the refrigerant is converted to heat energy. This causes the temperature of the refrigerant to rise as its pressure is increased (figure 26). When the refrigerant vapor is discharged from the compressor, its temperature is considerably higher than the temperature at which it would condense. The increase in enthalpy from C to D is due to heat added by the compressor, known as the heat of compression. In this example, the refrigerant leaves the compressor at 490 psia and 190 F. At this higher pressure, the corresponding saturation temperature (the temperature at which the refrigerant condenses) is 130 F. The refrigerant vapor leaving the compressor is, therefore, 60 F above its saturation temperature. Figure 26. Compressor WN Mechanical Systems Refrigeration Cycle Page 18 of 21

19 The condenser transfers energy from hot, high pressure refrigerant (C) to the cooler ambient air. The energy released from the condensing gas causes it to desuperheat. The refrigerant becomes a saturated vapor and eventually condenses into a saturated liquid. The liquid then further cools (called subcooling) before it finally leaves the condenser (D) to return to be delivered to the expansion device. The refrigerant vapor is cooled (the line from C to C ) to its saturation temperature of 130 F. As additional heat is removed by the condenser, the refrigerant vapor condenses to its saturated liquid condition (line from C to D ). This saturated liquid refrigerant now passes through the area of the condenser called the subcooler. Here, the liquid refrigerant is further cooled (the line from D to D), In this example, the refrigerant is subcooled to 115 o F. Because the saturation temperature at the condensing pressure is 130 F, the refrigerant has been subcooled 15 F. Subcooling is required to ensure that only liquid refrigerant returns to the expansion device. Much like superheat, subcooling provides a margin of safety. Because of the shape of the envelope, if the refrigerant was not subcooled, some of the refrigerant would flash to vapor before entering the expansion device. Expansion devices do not meter reliably when subjected to vapor. Thus, some degree of subcooling will be required (figure 27). Figure 27. Condenser WN Mechanical Systems Refrigeration Cycle Page 19 of 21

20 The function of the expansion device is the drop the pressure of the liquid refrigerant to that of the evaporator pressure. At this pressure, the refrigerant is now lies within the saturation envelope and is a mixture of liquid and vapor (primarily liquid). The high-pressure liquid refrigerant (D) flows through the expansion device, causing a large pressure drop. This pressure drop reduces the pressure and temperature of the refrigerant to that of the evaporator (A). Even at this lower pressure, the temperature of the refrigerant is higher than its boiling point causing some liquid to boil, or flash. The drop in pressure occurs with no change in heat or enthalpy. The temperature of the refrigerant entering the expansion device (D) is 115 F and its pressure is 490 psia. The refrigerant leaves the expansion device (A) at evaporator conditions, 145 psia and 45.0 F. The refrigeration cycle is complete (figure 28). Figure 28. Expansion Device WN Mechanical Systems Refrigeration Cycle Page 20 of 21

21 The refrigeration cycle is complete (figure 29). The example show was assuming refrigerant R-410a. The pressures and associated enthalpy will depend on the actual refrigerant used. Each of the devices involved (evaporator, compressor, condenser and expansion device) will be discussed in greater detail in future clinics. Figure 29. Completed Refrigeration Cycle WN Mechanical Systems Refrigeration Cycle Page 21 of 21