REFRIGERATION & AIR CONDITIONING
SYSTEM
1. INTRODUCTION (1)
2. TYPES OF REFRIGERATION AND AIR CONDITIONING (3)
3. ASSESSMENT OF REFRIGERATION AND AIR CONDITIONING (9)
4. ENERGY EFFICIENCY OPPORTUNITIES (12)
5. OPTION CHECKLIST (17)
6. WORKSHEETS (19)
7. REFERENCES (21)
1. INTRODUCTION
闻名This ction briefly describes the main features of the refrigeration and air conditioning system.
1.1 What is Refrigeration and Air Conditioning
Refrigeration and air conditioning is ud to cool products or a building environment. The refrigeration or air conditioning system (R) transfers heat from a cooler low-energy rervoir to a warmer high-energy rervoir (e figure 1).
Figure 1. Schematic reprentation of refrigeration system
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There are veral heat transfer loops in a refrigeration system as shown in Figure 2. Thermal energy moves from left to right as it is extracted from the space and expelled into the outdoors through five loops of heat transfer:
§ Indoor air loop . In the left loop, indoor air is driven by the supply air fan through a cooling coil, where it transfers its heat to chilled water. The cool air then cools the building space.
§ Chilled water loop . Driven by the chilled water pump, water returns from the cooling coil to the chiller’s evaporator to be re-cooled.
§ Refrigerant loop . Using a pha-change refrigerant, the chiller’s compressor pumps heat from the chilled water to the condenr water.
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§ Condenr water loop . Water absorbs heat from the chiller’s condenr, and the condenr water pump nds it to the cooling tower. § Cooling tower loop . The cooling tower’s fan drives air across an o pen flow of the hot condenr water, transferring the heat to the outdoors.
1.2 Air-Conditioning Systems
Depending on applications, there are veral options / combinations of air conditioning, which are available for u:
§ Air conditioning (for space or machines) § Split air conditioners
牙垢怎么去除§ Fan coil units in a larger system § Air handling units in a larger system
1.3 Refrigeration Systems (for process)不要讲话
The following refrigeration systems exists for industrial process (e.g. chilling plants) and domestic purpos (modular units, i.e. refrigerators):
§ Small capacity modular units of the direct expansion type similar to domestic refrigerators.
§ Centralized chilled water plants with chilled water as a condary coolant for a temperature range over typically 5 o C. They can also be ud for ice bank formation.
Figure 2. A typical Heat Transfer Loop in Refrigeration System
(Bureau of Energy Efficiency, 2004)
§Brine plants, which u brines as a lower temperature, condary coolant for typically sub-zero temperature applications, which come as modular unit capacities as well as large centralized plant capacities.
§The plant capacities up to 50 TR (tons of refrigeration) are usually considered as small capacity, 50 – 250 TR as medium capacity and over 250 TR as large capacity units.
A large company may have a bank of units, often with common chilled water pumps, condenr water pumps, cooling towers, as an off site utility. The same company may also have two or three levels of refrigeration and air conditioning such as a combination of:
§Comfort air conditioning (20 – 25 o C)
§Chilled water system (80 – 100 C)
§Brine system (sub-zero applications)
2. TYPES OF REFRIGERATION AND AIR CONDITIONING
This ction describes the two principle types of refrigeration plants found in industry: Vapour Compression Refrigeration (VCR) and Vapour Absorption Refrigeration (VAR). VCR us mechanical energy as the driving force for refrigeration, while VAR us thermal energy as the driving force for refrigeration.
2.1 Vapour Compression Refrigeration System
2.1.1 Description
Compression refrigeration cycles take advantage of the fact that highly compresd fluids at a certain temperature tend to get colder when they are allowed to expand. If the pressure change is high enough, then the compresd gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature. In this ca, fluid is ud to cool a low temperature environment and reject the heat to a high temperature environment.
Vapour compression refrigeration cycles have two advantages. First, a large amount of thermal energy is required to change a liquid to a vapor, and therefore a lot of heat can be removed from the air-conditioned space. Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of th e working fluid to the temperature of whatever is being cooled. This means that the heat transfer rate remains high, becau the clor the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer.
The refrigeration cycle is shown in Figure 3 and 4 and can be broken down into the following stages:
§ 1 – 2. Low-pressure liquid refrigerant in the evaporator absorbs heat from its surroundings, usually air, water or some other process liquid. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated. § 2 – 3. The superheated vapour enters the compressor where its pressure is raid. The temperature will also increa, becau a proportion of the energy put into the compression process is transferred to the refrigerant.
§ 3 – 4. The high pressure superheated gas pass from the compressor into the condenr.
The initial part of the cooling process (3-3a) de-superheats the gas before it is then turned
back into liquid (3a-3b). The cooling for this process is usually achieved by using air or water. A further reduction in temperature happens in the pipe work and liquid receiver (3b - 4), so that the refrigerant liquid is sub-cooled as it enters the expansion device.
§ 4 - 1 The high-pressure sub-cooled liquid pass through the expansion device, which both reduces its pressure and controls the flow into the evaporator.
Figure 3. Schematic reprentation of the vapour compression refrigeration cycle
Figure 4. Schematic reprentation of the refrigeration cycle including pressure changes
(Bureau of Energy Efficiency, 2004)
The condenr has to be capable of rejecting the combined heat inputs of the evaporator and the compressor. In other words: (1 - 2) + (2 - 3) has to be the same as (3 - 4). There is no heat loss or gain through the expansion device.
2.1.2 Types of refrigerant ud in vapour compression systems
A variety of refrigerants are ud in vapor compression systems. The required cooling temperature largely determines the choice of fluid. Commonly ud refrigerants are in the family of chlorinated fluorocarbons (CFCs, also called Freons): R -11, R-12, R-21, R-22 and R-502. The properties of the refrigerants are summarized in Table 1 and the performance of the refrigerants is given in Table 2 below.
Table 1. Properties of commonly ud refrigerants (adapted from Arora, C.P., 2000)
Enthalpy * Refrigerant Boiling Point ** (o C) Freezing Point (o C) Vapor Pressure * (kPa) Vapor
Volume * (m 3
/ kg) Liquid (kJ / kg) Vapor (kJ / kg) R – 11 -23.82 -111.0 25.73 0.61170 191.40 385.43 R – 12 -29.79 -158.0 219.28 0.07702 190.72 347.96 R – 22 -40.76 -160.0 354.74 0.06513 188.55 400.83 R – 502 -
45.40 --- 414.30 0.04234 188.87 342.31 R – 7
(Ammonia)
-33.30
-77.7
289.93
0.41949
808.71
487.76
* At -10 o C
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At Standard Atmospheric Pressure (101.325 kPa)
Table 2. Performance of commonly ud refrigerants (adapted from Arora, C.P., 2000)
Refrigerant Evaporating Press (kPa)
Condensing Press (kPa) Pressure Ratio Vapor Enthalpy
(kJ / kg)
COP **carnot R – 11 20.4 125.5 6.15 155.4 5.03 R – 12 182.7 744.6 4.08 116.3 4.70 R – 22 295.8 1192.1 4.03 162.8 4.66 R - 502 349.6 1308.6 3.74 106.2 4.37 R - 717
236.5
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103.4
4.78
*
At -15 o C Evaporator Temperature, and 30 o C Condenr Temperature
**
COP carnot = Coefficient of Performance = Temp.Evap . / (Temp.Cond . -Temp Evap.)
The choice of refrigerant and the required cooling temperature and load determine the choice of compressor, as well as the design of the condenr, evaporator, and other auxiliaries. Additional factors such as ea of maintenance, physical space requirements and availability of utilities for auxiliaries (water, power, etc.) also influence component lection.
