REFRIGERATORS
The transfer of heat from a low- temperature region to a high-temperature one requires special devices called refrigerators.
Refrigerators are cyclic device, and the working fluids used in the refrigeration cycles are called refrigerants.
HEAT PUMPS
Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump
Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.
The objective of a refrigerator is to remove heat (QL) from the cold medium.
The objective of a heat pump is to supply heat (QH) to a warm medium.
The cooling capacity of a refrigeration system is the rate of heat removal from the refrigeration space is often expressed in terms of ton of refrigeration.
The capacity of a refrigeration system that can freeze 1 ton of liquid water at 0 °C into ice at 0 °C in 24 h is said to be 1 ton.
VAPOR-COMPRESSION REFRIGERATION CYCLE
Two modes of operations:
1. Ideal vapor-compression refrigeration cycle
2. Actual vapor-compression refrigeration cycle
THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE
The vapor-compression refrigeration cycle is the ideal model for refrigeration systems, air conditions and heat pumps.
It consists of four processes:
1-2 Isentropic compression in compressor.
2-3 Constant-pressure heat rejection in a condenser.
3-4 Throttling in an expansion devise.
4-1 Constant-pressure heat absorption in an evaporator.
The process in ideal vapor compression refrigeration
cycle:
The refrigerant enters the compressor at state 1 as
saturated vapor and is compressed isentropically to the
condenser pressure.
The temperature of the refrigerant increases during this isentropic compression process to well above the temperature of the surrounding medium.
The refrigerant then enters the condenser as superheat
vapor at state 2 and leaves as saturated liquid at state 3
as a result to the heat rejection to the surrounding.
The saturated liquid at state 3 enters an expansion valve or
capillary tube and leaves at evaporator pressure. The
temperature of refrigerant drop below the temperature
of refrigerated space during this stage.
The refrigerant enters the evaporator at stage 4 as saturated mixture and it completely evaporate by absorbing the heat from the refrigerated space. The refrigerant leaves the evaporator as saturated vapor and reenters the compressor, completing the cycle.
Notice that the ideal vapor compression refrigeration cycle is not an internally reversible cycle since it involves an irreversible (throttling) process. This process is maintained in the cycle to make it a more realistic model for the actual vapor compression refrigeration cycle. If the thronling device were replaced by an isentropic turbine, the refrigerant would enter the evaporator at state 4´ instead of state 4. As a result, the refrigeration capacity would increase and the net work input would decrease (by the amount of work output of the turbine). Replacing the expansion valve by the turbine is not practical since the added benefits cannot justify the added cost and complexity
SELECTING THE RIGHT REFRIGERANT
• Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point).
• R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market.
• The industrial and heavy-commercial sectors use ammonia (it is toxic).
• R-11 is used in large-capacity water chillers serving A-C systems in buildings.
• R-134a (replaced R-12, which damages ozone layer) is used in domestic refrigerators and
freezers, as well as automotive air conditioners.
• R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.
• R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.
• CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the ozone layer. Refrigerants that are friendly to the ozone layer have been developed.
• Two important parameters that need to be considered in the selection of a refrigerant are the temperatures of the two media (the refrigerated space and the environment) with which the refrigerant exchanges heat.
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