UNIT-1 1. What is the difference between a refrigerator and heat pump? Derive an expression for the cop for both if they are running on reversed Carnot cycle? The above figure shows the objectives of refrigerators and heat pumps. The purpose of a refrigerator is the removal of heat, called the cooling load, from a low-temperature medium. The purpose of a heat pump is the transfer of heat to a high-temperature medium, called the heating load. When we are interested in the heat energy removed from a low-temperature space, the device is called a refrigerator. When we are interested in the heat energy supplied to the hightemperature space, the device is called a heat pump. In general, the term heat pump is used to describe the cycle as heat energy is removed from the lowtemperature space and rejected to the high-temperature space. The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as COP COP R P Desired output Cooling effect Q = = = Required input Work input W Desired output eating effect Q = = = Required input Work input W net, in net, in
COP COP R, Carnot P, Carnot = T 1 / T 1 = = 1 T / T T 1 = T T T T T Both COP R and COP P can be larger than 1. Under the same operating conditions, the COPs are related by COP P = COP +1 R 2. (a) Discuss the advantages of the dense air refrigeration system over an open air refrigeration system? Advantages of the dense air refrigeration system: 1. Volume of air handled by the compressor and expander are smaller due to higher suction pressure compared to open cycle. 2. igher coefficient of performance due to reduction in operating pressure ratio. 3. As the air does not comes in contact with the cooled chamber or stuff to be preserved, contamination is avoid. 4. As the system is closed, there are no chances of absorption of moisture and it freezing during expansion, 2. (b)explain the term tone of refrigeration? Unit of refrigeration: In SI system, the unit of refrigeration used is kilo watts,where in MKS system the unit of refrigeration is tonne. The practical unit of refrigeration is expressed in terms of Tonne of Refrigeration (TR).
Definition of tonne of refrigeration : A tonne of refrigeration may be defined as the amount of refrigeration effect produced by the uniform melting of one tonne (1000kg) of ice from at 0 0 C, in24hours. Since the latent heat of ice is 335kj/kg, therefore one tonne of refrigeration effect is given as, TR=1000x335kj in 24hr =1000x335 =232.6kj/min 24x60 But in actual practice, 1TR is taken as 210kj/min or 3.5kw 3. A refrigerator working on a bell-coleman cycle operate between the pressure limits of 1.05 bar and 8.5bar.air is drawn from the cold chamber at 10 0 C,compressed and then it is cooled to 30 0 C before entering in to the expansion cylinder. The expansion and follows law pv 1.3 =constant. Determine the theoretical cop of the system? Given data: p 1 =p 4 =1.05bar, p2=p 3 =8.5bar, T1=10 0 C=10+273=283k; T 3 =30 0 C=30+273=303k;n=1.3 et T 2 and T 4 =temperature of air at the end of the compression and expansion respectively.
Pv 1.3 =constant
Unit-IV AMMONIA ABSORPTION REFRIGERATION CYCE.
VAPOUR ABSORPTION SYSTEM The vapour absorption refrigeration is heat operated system. It is quite similar to the vapour compression system. In both the systems, there are evaporator and condenser. The process of evaporation and condensation of the refrigerant takes place at two different pressure levels to achieve refrigeration in both the cases. The method employed to create the two pressure levels in the system for evaporation and condensation of the refrigeration makes the two processes different. Circulation of refrigerant in both the cases is also different. In the absorption system the compressor of the vapour compression system is replaced by the combination of absorber and generator. A solution known as the absorbent, which has an affinity for the refrigerant used, is circulated between the absorber and the generator by a pump (solution pump). The absorbent in the absorber draws (or sucks) the refrigerant vapour formed in the evaporator thus maintaining a low pressure in the evaporator to enable the refrigerant to evaporate at low temperature. In the generator the absorbent is heated. There by releasing the refrigerant vapour (absorbed in the absorber) as high pressure vapour, to be condensed in the condenser. Thus the suction function is performed by absorbent in the absorber and the generator performs the function of the compression and discharge. The absorbent solution carries the refrigerant vapour from the low side (evaporator absorber) to the high side (generator-condenser). The liquefied refrigerant flows from the condenser to the evaporator due to the pressure difference between the two vessels; thus establishing circulation of the refrigerant through the system. 27 Refrigeration Cycle The COP of actual absorption refrigeration systems is usually less than 1.
Air-conditioning systems based on absorption refrigeration, called absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop. DETERMINING TE MAXIMUM COP OF AN ABSORPTION REFRIGERATION SYSTEM.