muhammad jawhar

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The transfer of heat from a low temperature region to a
high temperature one requires special devices called
refrigerator.
Refrigerator: is a cyclic device&theworking fluid used
in therefrigeration cycles arecalled refrigerants.
As shown in figure bellow the Q1 is the magnitude of the heat
removed from the refrigerated space at temperature T1 and Q2 is
the magnitude of the heat rejected to the warm space at
temperature T2 and Wnet is the net work input to the refrigerator.

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Another device that transfer heat from a low-
temperature medium to a high- temperature one is the
heat pump.
Refrigerators & heat pumps are the same devices they
differ in their objectives only.
The objective of refrigerator is to maintain the refrigerated
space at low temperature by removing heat from it.
The objective of a heat pump is to maintain a heated space at a
high temperature. This is accomplished by absorbing heat from a
low- temperature source such as cold outside air in winter &
supplying this heat to a warmer medium such as a house.

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The performance of refrigerators & heat pump is expressed in terms
of the coefficient of performance (COP)

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Wnet = Q2 - Q1
COPHP = COPR + 1
Where
The cooling capacity of refrigeration system which is the
rate of heat removal from the refrigerated space is often
expressed in terms of Ton of Refrigeration(TR).
TR: is the capacity of a refrigeration system that can
freeze1 ton of liquid water at 0 ºc into iceat 0 ºc in 24 h
1TR = 210 KJ / min = 12000 Btu /hr = 200 Btu/min

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Carnot engine
The heat engine that operates most efficiently between
high-temperature reservoir& a low- temperature
reservoir is the Carnot engine. It is an ideal engine that
uses reversible processes to form its cycle of operation.
Thus it is also called a reversibleengine

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1 -2 Isothermal expansion
2- 3 adiabatic reversibleexpansion
3- 4 Isothermal compression
4- 1 adiabatic reversiblecompression
Applying thefirst lawto thecycle.
Wnet = Q2 - Q1
Thermal efficiency of Carnot cycle.
We can see that the thermal efficiency of a Carnot
engine is dependent only on the high & low absolute
temperature of thereservoirs.
This relation of efficiency is applicable for all working
substances or for all reversible engines regardless of the
particular design characteristics.

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The Carnot engine when operated in reverse become a
heat pump or refrigerator. Depending on the desired
heat transfer. TheCOP for a heat pump becomes:
The a abovemeasures of performanceset limits that
real devices can only approach. The reversiblecycles
assumed areunrealistic but thefact that wehave
limits which weknowwe cannot exceed is often very
helpful in evaluating proposed design &determining
thedirection for further effort.

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The absorption refrigeration system is one of the oldest
method of producing refrigeration effect. The principle of
vapor absorption was first discovery by Michael faraday
in 1824
This system may be used in both the domestic & large
industrial refrigerating plants. The refrigerants
commonly used in a vapour absorption system are
ammonia & water.
The vapor absorption systemuses heat energy instead of
mechanical energy as in vapor compression system.
In the vapor absorption system, the compressor is
replaced by an absorber, a pump, a generator & a
pressurereducing valve.

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The refrigerant vapor leaving the evaporator is absorbed
in the low temperature hot solution in the absorber. This
process is accompanied by the rejection of heat. The
refrigerant in water solution is pumped to the higher
pressure & is heated in the generator. Due to increasing
the temperature of solution in generator as a result of
adding heat, most of the refrigerant is evaporated &
removed from the solution. The vapor then passes to the
condenser & the weak solution is returned to the
absorber.

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The coefficient of performanceof theabsorption
refrigeration systemis
1- In absorption system the only moving parts of the
entire system is a pump which has a small motor. Thus
the operation of this system is essentially quit & need less
power.
2- The absorption system uses heat energy to change the
condition of the refrigerant from evaporator while the
compression systemuses mechanical energy.
3- The absorption system designed to use steam, solar
energy & other heat sources. Thus it can be used where
the electric power is difficult to obtain or is very
expensive.
4- The load variation does not effect the performance of
absorption system. While the performance of
compression systemat partial loads is poor.
5- The absorption systems can be built in capacities well
above 1000 tons of refrigeration each which is largest
sizefor singlecompressor units.

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Is the most widely used cycle for refrigerators, air-
conditioning systems and heat pump. It consists of four
processes:
1-2 isentropic compression in a compressor.
2-3 constant pressureheat rejection in condenser
3-4 throttling in an expansion device
4-1 constant pressure heat absorption in an
evaporator.

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