Refrigeration and air conditioning
Refrigeration:-it is the process of maintaining a space at
lower temperature than surroundings, hence heat must be made to flow from lower
temp to higher temp.
Refrigerant:- it is the substance that extracts heat from lower
temp space and reject it to higher temp. surroundings.eg:-
NH3( industrial use)
Co2(transportation as dry ice)
R12( domestic ref. i.e freezer)
Note: it not in use now due to
presence of chlorine atom , 1 atom has potential of destroying billion ozone
atoms.( banned in Montreal protocol)
R-22 (window A/C)
R-134a(domestic ref.)
Note: Ecosafe , used in car A/C also
Air (aircraft a/c)
R-11 (central a/c)(
water cooled)
Refrigeration effect:- (unit—KJ/Kg):- it is the haet
absorbed from the refrigerated space in order to maintain it at lower temp. than the surroundings.
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W+QL =QH (by first law of therm)
here QL
=Desired effect(D.E)= refrigeration effect(RE)
COP/EPR/ER:-COEFFICIENT OF PERFORMANCE /
ENEGRY PERFORMANCE RATIO/ENERGY RATIO:-
COP= D.E/ENERGY INPUT
(COP)R =QL/W
(COP)H.P =QH/W
NOTE:-it is analogous to
efficiency of an heat engine, but cop can be greater than 1, can be less than 1
also but can’t be infinity.
Greater is the cop
, smaller is the work input required for a desired effect.
Q/A
Q. The disadvantages of
natural refrigeration methods are:
a) They are expensive
b) They are uncertain
c) They are not environment
friendly
d) They are dependent on
local conditions
Ans: b) and d)
Q. Evaporative cooling
systems are ideal for:
a) Hot and dry conditions
b) Hot and humid conditions
c) Cold and humid conditions
d) Moderately hot but humid
conditions
Ans-a
Q. Compared to natural
refrigeration methods, artificial refrigeration methods are:
a) Continuous
b) Reliable
c) Environment friendly
d) Can work under almost all
conditions
Ans. a), b) and d)
Q. In the evaporator of a
vapour compression refrigeration system:
a) A low temperature is
maintained so that heat can flow from the external fluid
b) Refrigeration effect is
produced as the refrigerant liquid vaporizes
c) A low pressure is
maintained so that the compressor can run
d) All of the above
Ans. a) and b)
Q. The function of a
compressor in a vapour compression refrigeration system is to:
a) To maintain the required
low-side pressure in the evaporator
b) To maintain the required
high-side pressure in the condenser
c) To circulate required
amount of refrigerant through the system
d) To safeguard the
refrigeration system
Ans. a), b) and c)
Q. In a vapour compression
refrigeration system, a condenser is primarily required so that:
a) A high pressure can be
maintained in the system
b) The refrigerant
evaporated in the evaporator can be recycled
c) Performance of the system
can be improved
d) Low temperatures can be
produced
Ans. b)
Q. The function of an
expansion valve is to:
a) Reduce the refrigerant
pressure
b) Maintain high and low
side pressures
c) Protect evaporator
d) All of the above
Ans. b)
Q. In a domestic icebox type
refrigerator, the ice block is kept at the top because:
a) It is convenient to the
user
b) Disposal of water is
easier
c) Cold air can flow down
due to buoyancy effect
d) None of the above
Ans. c)
Q. An air conditioning
system employs a refrigeration system to:
a) Cool and dehumidify air
supplied to the conditioned space
b) To heat and humidify the
air supplied to the conditioned space
c) To circulate the air
through the system
d) To purify the supply air
Ans. a)
VAC:VAPOUR ABSORPTION CYCLE
This cycle works on the principle of
selective absorptivity .eg. Water is a strong absorbent of NH3. If NH3 is kept
in a vessel that is exposed to another vessel containing water, the strong
absorption potential of water will cause evaporation of NH3 requiring no compressor to drive the vapours. A liquid pump is
used to increase the pressure of strong solution. The strong solution is then
heated in a generator and passed through a rectification column to separate the
water from ammonia. The ammonia vapour is then condensed and recycled. The pump
power is negligible hence; the system runs virtually on low- grade energy (i.e
heat other cycles use work as input )used for heating the strong solution to
separate the water from ammonia. These systems were initially run on steam.
Later on oil and natural gas based systems were introduced. Figure shows the
essential components of a vapour absorption refrigeration system.
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Theory:--
Ammonia vapour leaves the evaporator and enters the absorber
where it dissolves and reacts with water to form NH3.H20.
This is an exothermic rxn and the amount of NH3 that can be
dissolved in H2O is inversely proportional to temperature.
The liquid (NH3+H20) solution is then
pumped to generator . In generator as temperature is high solution vaporizes
and hence high pressure is created .H2O is removed and send back to
the generator whereas high pressure pure NH3 vapour continues its journey
Another variation of vapour absorption system is the one
based on Lithium Bromide (LiBr)-water. This system is used for chilled water
air-conditioning system. In this system LiBr is the absorbent and water is the
refrigerant.
VAC is used where large waste heat is available or cost of
electricity is very high. Solar and geothermal refrigeration system are based
on VA cycle.
In this cycle, heat is rejected in condenser and absorber
whereas heat is absorbed in evaporator and generator.
Electrolux system:- is a type of VA system using 3 fluids
and no pump(NH3, H2, H2O)
total pressure is const throughout the system .h2 is used to
create less partial pressure of ammonia
fluid flows due to gravity
COP
OF A VAC:-
COP= D.E/ENERGY INPUT
COP=QE/(QG+WP)
WP Is negligible hence
COP= QE/QG
Max cop of
VA system:-
Figure 1 to be attached
Funda for learning –(EGO GOE)
Q- A VA
system work with generator ,ambient & evaporator temp of 360k , 310k and
260 k resp. find the max cop of the sys. If the evaporator temp falls to 250 k
what should be the generator temp in order to keep same cop?
Sol: hint
COP max =TE(TG-TO)/ TG(TO-TE) ans=
.7222
