Units of Refrigeration or 1 Ton refrigeration
The practical unit of refrigeration is expressed in terms of 'tonne of refrigeration' briefly written asTR).a tonne of refrigeration is defined as the ammount of refrigeration effect produced by the uniform melting of one tonne(1000kg) of ice from and at ooc in 24 hours.
Since the latent heat of ice is 335 kJ/kg, therefore one tonne of refrigeration,
1 TR = 1000 x 335 kJ in 24 hours
= l000 x 335 / 24x60
= 232.6 kJ/min
In a actual practice, one tonne of refrigeration is taken as equivalent to 210 kJ/min or 3.5 kW or 3.5 tJ/s).
Coefficient of Performance of a Refrigerator
The coefficient of performance (briefly written as C.O.P.) is the ratio of heat extracted in the
refrigerator to the work done on the refrigerant. It is also known as theoretical coefficient of
performance. Mathematically,
refrigerator to the work done on the refrigerant. It is also known as theoretical coefficient of
performance. Mathematically,
Theoretical C.O.P. = Q / W
Q = Amount of heat extracted in the refrigerator (or the amount of refrigeration produced, or the capacity of a refrigerator), and
W = Amount of work done.
1be ratio of the actual C.O.P to the theoretical C.O.P. is known as relative cofficient of performance.Mathematically
Relative C.O.P = Actual C.O.P. / Theoretical C.O.P.
Open Air Refrigeration Cycle
In an open air refrigeration cycle, the air is directly led to the space to be cooled refrigerator allowed to circulate through the cooler and then retwlled to the compressor to start another cycle. Since the air is supplied to the refrigerator at atmospheric pressure, therefore, volume of air handled by the compressor and expander is large. Thus the size of compressor and expander should be large. Another disadvantage of the open cycle system is that the moisture is regularly carried away by the air circulated through the cooled space. This leads to the formation of frost at the end of expansion process and clog the line. Thus in an open cycle system, a drier should be used.
Closed or Dense Air Refrigeration Cycle
In a closed or dense air refrigeration cycle, the air is passed through the pipes and component parts of the system at all times. The air, in this system, is used for absorbing heat from the other fluid (say brine) and this cooled brine is circulated into the space to be cooled. The air in closed system does not come in contact directly with the space to be cooled.
- Since it can work at a suction pressure higher than that of atmospheric pressure, therefore the volume of air handled by the compressor and expander are smaller as compared to an open air refrigeration cycle system.
- The operating pressure ratio can be reduced, which results in higher coefficient of performance.
Merits and Demerits of Air Refrigeration System
Merits
- The air is easily available and there is no cost of the refrigerant.
- The air is non-toxic and non-inflammable.
- The leakage of air in small amounts is tolerable.
- Since the main compressor is employed for the compressed air source, therefore there: is no problem of space for extra compressor.
- The air is light in weight per tonne of refrigeration.
- The chilled air is directly used for cooling, there by eliminating the cost of separ evaporator.
- Since the pressure in the whole system is quite low, therefore the piping, ducting etc. are quite simple to design, fabricate and maintain.
Demerits
- It has low coefficient of performance.
- The rate of air circulation is relatively large.