RAC REFRIGERATION AND AIR

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.

 

W+Q_L =Q_{H (by first law of therm)}

here Q_L =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 =Q_L/W

(COP)_H.P =Q_H/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.

The devices in red box are an alternate to compressor used in vapour compression cycle

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=Q_E/(Q_G+W_P)

W_P  Is negligible hence COP= Q_E/Q_G

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 =T_E(T_G-T_O)/ T_G(T_O-T_E)                                               ans= .7222