PowerPoint Presentation by: Associated Technical Authors. Publisher The Goodheart-Willcox Company, Inc. Tinley Park, Illinois

Transcription

1 Althouse Turnquist Bracciano PowerPoint Presentation by: Associated Technical Authors Publisher The Goodheart-Willcox Company, Inc. Tinley Park, Illinois

2 Chapter 3

3 Explain the operation of a simple ice refrigerator. Explain how evaporation provides a cooling effect. Name the basic mechanical refrigeration systems. Explain various applications for mechanical refrigeration systems. Describe the operation of various mechanical refrigeration systems.

4 Compare compression- and absorptiontype systems. Discuss refrigeration systems using icemakers and water coolers. Explain how a system using an expendabletype of refrigerant works. Discuss and compare domestic and commercial refrigeration systems.

5 Explain the operation of thermoelectric refrigeration. Compare the differences between hot-gas and electric defrost systems.

6 3.1 Ice Refrigeration Consists of an insulated cabinet with tray at top to hold ice. Food located below ice compartment. Cold air flows downward, cooling the food. Maintains a fairly high humidity level. Temperatures range between 40 F and 50 F (4.4 C and 10 C).

7 3.1 Ice Refrigeration

8 3.2 Evaporative Refrigeration (Desert Bag) Moisture on surface of the bag evaporates rapidly. Heat that causes the evaporation comes from the desert bag and its water. This heat removal cools the drinking water.

9 3.3 Evaporative Refrigeration (Snow Making) Water flows from nozzle. Air under high pressure breaks water into tiny droplets. Surrounding temperature near or below freezing. Water droplets evaporate and rapidly cool. Ice is formed.

10 3.4 Low-Side Float Refrigerant Control Pressure on low side of flooded system will vary with temperature.

11 3.4 Low-Side Float Refrigerant Control Note the float needle, low-side float, and pressure motor control.

12 3.5 External-Drive (Open) Refrigeration System Compressor usually belt-driven from an electric motor. Compressor must have crankshaft seal to avoid refrigerant flowing out or air flowing in.

13 3.5 External-Drive (Open) Refrigeration System

14 3.6 High-Side Float (Flooded) System Capillary tube refrigerant line frequently used in conjunction with the high-side float system. Frequently specified in commercial applications where high operating efficiency is desired. Easy to service. Charge must be accurately measured. This is a critically charged system.

15 3.6 High-Side Float (Flooded) System

16 Questions What is the temperature range for ice refrigeration? 40 to 50 F. Would the humidity level of ice refrigeration be high or low? High. What causes the cooling effect on a desert bag? Evaporation. What are two important features used for snow making? High-pressure air and freezing temperatures.

17 Questions continued What type of evaporator uses a low-side float? A flooded evaporator. What is used to prevent refrigerant from escaping to the atmosphere on an open-type compressor? A crankshaft seal. Which of the following is a critically charged system, highside float or low-side float? High-side float.

18 3.7 Compression System Using Automatic Expansion Valve (AEV) The refrigerant can flow through the AEV only when the compressor runs and reduces evaporator pressure. Pressures do not balance on the off cycle. Motor compressor must start under load. Used widely in small commercial applications. Must be used with a temperature control.

19 3.7 Compression System Using Automatic Expansion Valve (AEV)

20 3.8 Using Thermostatically Controlled Expansion Valve (TEV) Operation of thermostatically controlled expansion valve (TEV) controlled by: Temperature of TEV control bulb. Pressure in the evaporator. Spring pressure. Pressures do not balance on-off cycle, so compressor often starts under load. Used on large commercial refrigerators as well as in many air-conditioning applications. Can be controlled by pressure or temperature control.

21 3.8 Using Thermostatically Controlled Expansion Valve (TEV)

22 3.9 Compression System Using Capillary Tube One of the most popular compression-type systems. Capillary tube maintains a pressure difference while compressor is operating. Must be operated by temperature control. Capillary tube allows high- and low-side pressures to balance during the off cycle. Used in refrigerators, freezers, air conditioners, and dehumidifiers.

23 3.9 Compression System Using Capillary Tube

24 3.10 Multiple Evaporator Systems One condenser connected to two or more evaporators. Two-temperature valve located in suction line of warmer evaporator. Check valve located in suction line of colder evaporator. Sight glass often included in liquid line. Pressure motor control operates as a safety device. Commonly used in commercial refrigeration applications.

25 3.10 Multiple Evaporator Systems

26 Questions Which type of motor control is used with an AEV? Temperature control. Which pressures control a TEV? Bulb, evaporator, and spring. Does the compressor on a TEV system start under a load? Yes. Which type of motor control can be used on a TEV system? Pressure or temperature control.

27 Questions continued What type of motor control can be used on a capillary tube system? Temperature control. In a multiple evaporator system, where would the twotemperature valve be located? At the outlet of the warmer evaporator. In a multiple evaporator system, where would a check valve be located? At the outlet of the colder evaporator.

28 3.11 Compound Refrigerating System Consists of two or more compressors connected in series. Usually requires an oil separator for each compressor to increase system capacity during low temperatures. A single-temperature motor control operates all motors. A TEV controls liquid refrigerant flow into evaporator. Pressures do not balance on the off cycle. Motors must start under load. Requires heavy maintenance.

29 3.11 Compound Refrigerating System

30 3.12 Cascade Refrigerating Systems Two refrigeration systems connected in series. Both systems operate at the same time. Each system has a TEV. One motor control is used. Motors must be capable of starting under load, since pressures do not balance on-off cycle.

31 3.12 Cascade Refrigerating Systems continued Condenser-evaporator is usually of shell-and-tube flooded evaporator type. Refrigerant must be very dry. Often used in industrial process where objects must be cooled below 50 F ( 46 C). One condenser gives up heat to the other s evaporator.

32 3.12 Cascade Refrigerating Systems

33 3.13 Modulating Refrigeration Cycle Matches machine capacity to needed heat load. May use two or more compressors connected in parallel. Each compressor is operated by a pressure-type motor control. Commonly uses TEV. Same evaporator is connected to all the compressors.

34 3.13 Modulating Refrigeration Cycle

35 3.14 Ice Maker Motor compressor and condenser usually located in bottom of cabinet. Note the accumulator, ice cube mold, and electric defrost. Most surfaces in contact with water and ice are stainless steel.

36 3.15 Drinking Water Cooler Uses a hermetic (airtight) compression refrigerating system. Uses a capillary tube system. Water pressure regulator adjusts the water flow. Condensing unit is air-cooled. Condenser fan increases capacity. Thermostat maintains desired water temperature; approximately 50 F (10 C).

37 3.15 Drinking Water Cooler Note water heat exchanger.

38 Questions In a compound refrigeration system, are the compressors connected in series or in parallel? In series. In a compound refrigeration system, do the pressures balance during the off cycle? balance during the off cycle? No. On a cascade refrigeration system, how are the two systems connected, in series or in parallel? In series. What are the operating temperatures of a cascade refrigeration system? Below 50 F.

39 Questions continued Are the compressors of a modulating refrigeration system connected in series or in parallel? Parallel. From which material is the evaporator of an ice machine most commonly made? Stainless steel. What is the desired water temperature of a water cooler? Approximately 50 F.

40 3.16 Expendable Refrigerant Refrigeration System Often called a chemical refrigeration or open-cycle refrigeration system. Used on trucks or other vehicles to transport or store refrigerated or frozen foods. Consists of a heavily insulated space cooled by liquid nitrogen. The system discards the refrigerant after it has evaporated. Excellent for shipping frozen foods, since it maintains 20 F ( 29 C) temperature.

41 3.16 Expendable Refrigerant Refrigeration System Note temperature-sensing element, control box, and liquid control valve.

42 3.17 Thermoelectric Refrigeration Transfers energy from one place to another using electrons, rather than refrigerants. A thermoelectric couple moves heat from the inside of an insulated space to a heat exchanger. Electrons carry the heat. Fins on the evaporator increase heat flow. Fins on the heat exchanger give off heat to the surrounding air. Contains no moving parts. Thermal efficiency is low.

43 3.17 Thermoelectric Refrigeration This system uses a simple thermoelectric couple.

44 3.17 Thermoelectric Refrigeration continued Semiconductors are of two types. N-type semiconductors conduct electricity by flow of negatively charged particles (usually electrons). P-type semiconductors conduct electricity by the flow of positively charged particles (often called holes or electron holes). Several N-P paired junctions are connected in series to produce significant cooling. Often used in electronic equipment (computers, aerospace devices, etc.).

45 3.17 Thermoelectric Refrigeration continued

46 3.18 Dry Ice Refrigeration Dry ice is solid carbon dioxide. Dry ice changes from solid directly to vapor through sublimation. Dry ice is packed beside or on top of food. Carbon dioxide, as it changes to a vapor, keeps the food frozen. Safety Note: Never handle dry ice with bare hands. It will cause instant freeze burns. Always wear heavy gloves.

47 3.18 Dry Ice Refrigeration

48 3.19 Intermittent Absorption System Uses a generator charged with water and ammonia. Heat source (usually a kerosene flame) heats solution in generator. Piping is welded steel, due to high pressures on the generating cycle. Intermittent absorption systems are popular in areas where electric power is not available.

49 3.19 Intermittent Absorption System

50 3.20 Continuous-Cycle Absorption System Operated by limited amount of heat furnished by gas, electricity, or kerosene. No moving parts are employed. Operation based on Dalton s Law. Consists of four main parts: boiler, condenser, evaporator, and absorber. Unit charge consists of quantity of ammonia, water, and hydrogen. Widely used in domestic refrigerators and recreational vehicles.

51 3.20 Continuous-Cycle Absorption System

52 3.21 Solid Absorbent Refrigeration Based on the principle of an experiment done by Michael Faraday.

53 3.21 Solid Absorbent Refrigeration

54 3.22 Sophisticated Commercial Systems Various conditions and refrigeration requirements require accessory (add-on) devices. Accessory devices can make systems safer and more efficient.

55 3.22 Sophisticated Commercial Systems Note the pressure regulator, vibration damper, crankcase heater, filter-drier, suction service valve, crankcase heater, oil separator, liquid desuperheater valve, and defrost control device.

56 3.23 Hot-Gas Defrost During Refrigerating Cycle Timing mechanism directs hot high-pressure vapor through the evaporator to remove frost and ice.

57 3.23 Hot-Gas Defrost During Refrigerating Cycle Note: Solenoid valve No. 1 is open during refrigerating cycle.

58 3.23 Hot-Gas Defrost During Defrost Cycle Solenoid valve No. 1 is closed no liquid refrigerant is flowing through the TEV into the evaporator. Solenoid valve No. 2 is open hot compressed refrigerant vapor flows through it directly into the evaporator. As the hot vapor passes through the evaporator, it melts ice from the evaporator surface.

59 3.24 Electric Defrost During the Refrigerating Cycle Note: Refrigerant flow and electric heating elements alongside the evaporator surface are green.

60 3.24 Electric Defrost During the Defrost Cycle Note: Refrigerating unit is not running. There is no flow. Note: Resistance heating elements are shown in red alongside the evaporator surface.

61 Questions What is another name for an open cycle refrigeration system? Expendable refrigeration system. How is heat transferred in a thermoelectric refrigeration system? By electrons. Name two types of semiconductors that are commonly used in thermoelectric refrigeration systems. N type and P type. What is sublimation? A process in which a substance changes from a solid directly to a vapor, skipping the liquid state.

62 Questions continued From which substance is dry ice made? Carbon dioxide. In an intermittent absorption system, what is the generator commonly charged with? Ammonia and water. On which law is the operation of a continuous cycle absorption system based? Dalton s law. What controls the hot gas that flows to evaporator during a hot gas defrost cycle? A solenoid valve.

63 Safety Wear goggles and gloves at all times, especially when charging or discharging refrigerant. R-717 (ammonia) is very irritating to the eyes and lungs. Avoid exposure to it. Dry ice will cause instant freeze burns. Never handle it with bare hands.

64 Glossary capillary tube Control system in which pressure difference is maintained through the use of a thin tube. continuous-cycle absorption system System that has a continuous flow of energy input. dry ice refrigeration Use of a refrigerating substance made of solid carbon dioxide, which changes directly from a solid to a gas (sublimates). Its subliming temperature is 109 F ( 78 C). Faraday, Michael Scientist who found that silver chloride absorbs ammonia when cool and releases it when heated. This is the basis on which some absorption refrigerators operate.

65 Glossary intermittent absorption system Refrigerant system using a kerosene burner and ammonia, used in situations where gas and electricity are not available. modulating refrigeration cycle Refrigerating system of variable capacity. pressure motor control Device that opens and closes an electrical circuit as pressures change. semiconductors A class of solids whose ability to conduct electricity lies between that of a conductor and an insulator.

66 Glossary sublimation Condition in which a substance changes from a solid to a gas without becoming a liquid. thermoelectric refrigeration Direct current flowing through an electrical junction between unlike metals produces heating or cooling effect, depending on direction of current flow. thermostatically controlled expansion valve (TEV) Control valve operated by temperature and pressure within evaporator. It controls flow of refrigerant. Control bulb is attached to outlet of evaporator.