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Air Conditioning Clinic Refrigeration Cycle One of the Fundamental Series TRG-TRC003-EN.
NO POST AGE NECESSARY IF MAILED IN THE UNITED ST A TES BUSINES S REPL Y MAIL FIRST -CLAS S MAIL PERMIT NO. 11 LA CROS SE, WI POST AGE WILL BE P AID BY ADDRESSEE THE TRANE COMP ANY Attn: Applications E.
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Refrigeration Cycle One of the Fundamental Series A publication of The T rane Company— W orldwide Applied Systems Group.
Preface © 1999 American Standard Inc. All rights reserved TRG-TRC003-EN ii The T rane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience.
TRG-TRC003-EN iii Contents period one Heat and Refrigeration ....................................... 1 What is Heat? ......................................................... 2 Principles of Heat Transfer ...................................... 4 period two Refrigerants .
iv TRG-TRC003-EN.
TRG-TRC003-EN 1 notes period one Heat and Refrigeration Before discussing the refrigeration system, we need to understand the terms heat and refrigeration. The term refrigeration is commonly associated with something cold. A household refrigerator , for example, keeps food cold.
2 TRG-TRC003-EN notes period one Heat and Refrigeration What is Heat? Heat is a form of energy . Every object on earth contains heat energy in both quantity and intensity . Heat intensity is measured by its temperature, commonly in either degrees Fahrenheit (°F) or degrees Celsius (°C).
TRG-TRC003-EN 3 period one Heat and Refrigeration notes These two different masses of water contain the same quantity of heat, yet the temperature of the water on the left is higher . Why? The water on the left contains more heat per unit of mass than the water on the right.
4 TRG-TRC003-EN notes period one Heat and Refrigeration Principles of Heat T ransfer Air -conditioning and refrigeration systems use the principles of heat transfer to produce cooling and heating.
TRG-TRC003-EN 5 period one Heat and Refrigeration notes substance. This is commonly referred to as the principle of “conservation of energy .” Ice cubes are typically placed in a beverage to cool it before it is served. As heat is transferred from the beverage to the ice, the temperature of the beverage is lowered.
6 TRG-TRC003-EN notes period one Heat and Refrigeration The third principle is that heat is transferred from one substance to another by one of three basic processes: conduction, convection, and radiation. The device shown is a baseboard convector that is commonly used for heating a space.
TRG-TRC003-EN 7 period one Heat and Refrigeration notes In refrigeration, as in heating, emphasis is placed on the rate of heat transfer , that is, the quantity of heat that flows from one substance to another within a given period of time.
8 TRG-TRC003-EN notes period one Heat and Refrigeration In the English system of units, there is a larger and more convenient measure of the rate of heat flow . It is called a ton of refrigeration . One ton of refrigeration produces the same cooling effect as the melting of 2000 lb of ice over a 24-hour period.
TRG-TRC003-EN 9 notes In this period we will discuss refrigerants, the substances used to absorb and transfer heat for the purpose of cooling. Ice can be used to preserve food.
10 TRG-TRC003-EN notes period two Refrigerants Pure ice, however , does have an important disadvantage. It absorbs heat and melts at 32 °F [0 °C]. Ice cream, for example, melts at a temperature lower than 32 °F [0 °C]. In the same frozen display case, ice cannot keep the ice cream frozen because ice melts at a higher temperature than ice cream.
TRG-TRC003-EN 11 period two Refrigerants notes Finally , Refrigerant-22 (R-22) is a chemical used in many refrigeration systems. If, hypothetically , an open container of liquid R-22 were placed in the frozen display case, when exposed to atmospheric pressure, it would absorb heat and boil violently at -41.
12 TRG-TRC003-EN notes period two Refrigerants Change of Phase This question is best answered by examining the effects of heat transfer on water . Consider 1 lb of 60 °F water . By adding or subtracting 1 Btu of heat energy , the water temperature is raised or lowered by 1 °F .
TRG-TRC003-EN 13 period two Refrigerants notes In fact, 970.3 Btu must be added to 1 lb of 212 °F water to completely transform it to 1 lb of steam at the same temperature. Similarly , 244.5.3 kcal (1023 kJ) must be added to 1 kg of 100 °C water to completely transform it to 1 kg of steam at the same temperature.
14 TRG-TRC003-EN notes period two Refrigerants The quantity of heat that must be added to the water in order for it to evaporate cannot be sensed by an ordinary thermometer . This is because both the water and steam remain at the same temperature during this phase change.
TRG-TRC003-EN 15 period two Refrigerants notes have different capacities for absorbing heat. This capacity is a property of the substance called specific heat. Suppose equal quantities of two different liquids, $ and % , both at room temperature, are heated.
16 TRG-TRC003-EN notes period two Refrigerants Modern Refrigerants Refrigerants are substances that are used to absorb and transport heat for the purpose of cooling.
TRG-TRC003-EN 17 notes period three Refrigeration Cycle The frozen display case example used in the last period demonstrates that, at a given pressure, refrigerants absorb heat and change phase at a fixed temperature.
18 TRG-TRC003-EN notes period three Refrigeration Cycle liquid refrigerant to the coil so that all the refrigerant evaporates before it reaches the end of the coil. One disadvantage of this system is that after the liquid refrigerant passes through the coil and collects in the drum as a vapor , it cannot be reused.
TRG-TRC003-EN 19 period three Refrigeration Cycle notes The liquid refrigerant absorbed heat from the air while it was inside the evaporator , and was transformed into a vapor in the process of doing useful cooling.
20 TRG-TRC003-EN notes period three Refrigeration Cycle At higher pressures, refrigerant boils and condenses at higher temperatures. This can be explained by examining the properties of water . At atmospheric pressure (14.7 psia [0.10 MPa]), water boils and evaporates at 212 °F [100 °C].
TRG-TRC003-EN 21 period three Refrigeration Cycle notes increase the pressure of the resulting refrigerant vapor to 280 psia [1.93 MPa]. This increase in pressure raises the temperature at which the vapor would condense back into liquid to 121.5 °F [49.
22 TRG-TRC003-EN notes period three Refrigeration Cycle Basic Refrigeration System This diagram illustrates a basic vapor -compression refrigeration system that contains the described components. First, notice that this is a closed system. The individual components are connected by refrigerant piping.
TRG-TRC003-EN 23 period three Refrigeration Cycle notes At the inlet to the evaporator , the refrigerant exists as a cool, low-pressure mixture of liquid and vapor . In this example, the evaporator is a finned-tube coil used to cool air . Other types of evaporators are used to cool water .
24 TRG-TRC003-EN notes period three Refrigeration Cycle The condenser is a heat exchanger used to reject the heat of the refrigerant to another medium. The example shown is an air -cooled condenser that rejects heat to the ambient air . Other types of condensers are used to reject heat to water .
TRG-TRC003-EN 25 period three Refrigeration Cycle notes The high-pressure liquid refrigerant ( ' ) flows through the expansion device, causing a large pressure drop. This pressure drop reduces the refrigerant pressure, and, therefore, its temperature, to that of the evaporator .
26 TRG-TRC003-EN notes period four Pressure–Enthalpy Chart During this period we will again analyze the basic vapor -compression refrigeration cycle.
TRG-TRC003-EN 27 period four Pressure–Enthalpy Chart notes envelope, the refrigerant exists as a mixture of liquid and vapor . If the enthalpy of the refrigerant lies to the right of the envelope, the vapor is superheated . Similarly , if the enthalpy of the refrigerant lies to the left of the envelope, the liquid is subcooled .
28 TRG-TRC003-EN notes period four Pressure–Enthalpy Chart The distance between the edges of the envelope indicates the quantity of heat required to transform saturated liquid into saturated vapor at a given pressure. This is called the heat of vaporization .
TRG-TRC003-EN 29 period four Pressure–Enthalpy Chart notes At the inlet to the evaporator , the refrigerant is at a pressure of 85 psia [0.59 MPa] and a temperature of 41.
30 TRG-TRC003-EN notes period four Pressure–Enthalpy Chart The compressor draws in the superheated refrigerant vapor ( & ) and compresses it to a pressure and temperature ( ' ) high enough that it can reject heat to another fluid. As the volume of the refrigerant is reduced by the compressor , its pressure is increased.
TRG-TRC003-EN 31 period four Pressure–Enthalpy Chart notes temperature is 121.5 °F [49.7 °C]. The refrigerant vapor leaving the compressor is therefore 70 °F [38.9 °C] above its saturation temperature. This hot, high-pressure refrigerant vapor then travels to the condenser .
32 TRG-TRC003-EN notes period four Pressure–Enthalpy Chart The primary purpose of the expansion device is to drop the pressure of the liquid refrigerant to equal the evaporator pressure. At this lower pressure, the refrigerant is now inside the saturation envelope where it exists as a mixture of liquid and vapor .
TRG-TRC003-EN 33 period four Pressure–Enthalpy Chart notes The temperature of the refrigerant entering the expansion device ( * ) is 110 °F [43.3 °C] and its pressure is 280 psia [1.93 MPa]. (The refrigerant condensed at 121.5 °F [49.7 °C] and was subcooled to 110 °F [43.
34 TRG-TRC003-EN notes period four Pressure–Enthalpy Chart This cool mixture of liquid and vapor refrigerant leaving the expansion device then enters the evaporator ( $ ) to repeat the cycle.
TRG-TRC003-EN 35 notes W e will now review the main concepts that were covered in this clinic reagrding the vapor -compression refrigeration cycle. Period One introduced the concept of heat and how it is transferred from one substance to another . Recall that heat is a form of energy and can vary in both quantity and intensity (temperature).
36 TRG-TRC003-EN notes period five Review Period T wo discussed refrigerants and how they are used in the process of removing and transporting heat. Remember that refrigerants absorb significant amounts of heat when they change phase (e.g., from a liquid to a vapor).
TRG-TRC003-EN 37 period five Review notes Refrigerant enters the evaporator as a cool, low-pressure mixture of liquid and vapor . It absorbs heat—from the relatively warm air or water to be cooled—and boils. The cool, low-pressure vapor is then pumped from the evaporator by the compressor .
38 TRG-TRC003-EN notes period five Review For more information, refer to the following references: ■ T rane Air Conditioning Manual ■ T rane Reciprocating Refrigeration Manual ■ ASHRAE Handbook – Fundamentals ■ ASHRAE Handbook – Refrigeration ■ ASHRAE Handbook – Systems and Equipment Visit the ASHRAE Bookstore at www .
TRG-TRC003-EN 39 Questions for Period 1 1 Heat intensity is measured in terms of its __________? 2 Heat quantity is measured with units of __________? 3 Heat always flows from a substance of ________ (higher , lower) temperature to a substance of ________ (higher , lower) temperature.
40 TRG-TRC003-EN Quiz 10 What is the state of the refrigerant when it enters the compressor? 11 What is the state of the refrigerant when it enters the expansion device? Questions for Period 4 12 What.
TRG-TRC003-EN 41 Quiz 14 Referring to Figure 60 and given the following conditions, A - 44.5 °F , 90 psia, 41.6 Btu/lb [6.9 °C, 0.62 MPa, 96.8 kJ/kg] B - 44.5 °F , 90 psia, 108.5 Btu/lb [6.9 °C, 0.62 MPa, 252.4 kJ/kg] C - 54.5 °F , 90 psia, 110.3 Btu/lb [12.
42 TRG-TRC003-EN 1 T emperature or degrees Fahrenheit [degrees Celsius] 2 British Thermal Unit (Btu) [kilocalorie (kcal) or kiloJoule (kJ)] 3 Higher to lower 4 Conduction, convection, and radiation 5 Boiling the water requires more energy–970.3 Btu/lb [244.
TRG-TRC003-EN 43 Answers 14 a 10 °F [5.6 °C] (temperature rise from % to & ) b 14 °F [7.8 °C] (temperature drop from ) to * ) c 68.7 Btu/lb [159.
44 TRG-TRC003-EN ASHRAE American Society of Heating, Refrigerating and Air -Conditioning Engineers British Thermal Unit (Btu) A measure of heat quantity , defined as the quantity of heat energy required to change the temperature of 1 lb of water by 1 °F .
TRG-TRC003-EN 45 Glossary radiation The process transferring heat by means of electromagnetic waves emitted due to the temperature difference between two objects.
The T rane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse, WI 54601-7599 www .trane.com An American Standard Company Literature Order Number TRG-TRC003-EN File Number E/A V -.
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