UNITS OF REFRIGERATION

Domestic  and  commercial  refrigerators  may  be  rated  in  kj/s,  or  Btu/h  of cooling. Commercial refrigerators in the US are in tons of refrigeration, but elsewhere in kw. One ton of refrigeration capacity can freeze one short ton of water at 0 ºC (32 ºF) in 24 hours.

Latent heat of ice (i.e. heat of fusion) = 333.55 kj/kg

≈ 144 Btu/lb

One short ton = 2000lb

Heat extracted = (2000)*(144)/24 hr = 288000 Btu/24 hr

= 1200 Btu/24 hr

= 200 Btu/min

1 tonne of refrigeration = 200 Btu/min

= 3.517 kj/s

= 3.517 kw

The  practical   unit  of  refrigeration   is  expressed   in  terms  of  ‘tonne  of refrigeration’ (briefly written as TR). A tonne of refrigeration is defined as the amount of refrigeration effect produced by the uniform melting of one tonne (1000 kg) of ice from and 0  ºC in 24 hours. Since the latent heat of ice is 335 kj/kg, therefore one tonne of refrigeration,

1 TR = 1000 * 335 kj in 24 hours

= (1000) * (335) / (24) * (60)

= 232.6 kj/min

In actual practice, one tonne of refrigeration is taken as equivalent to 210 kj/min or 3.5 kw (i.e. 3.5 kj/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,

Theoretically C.O.P.  = Q/W

Where Q = Amount of heat  extracted  in  the  refrigerator  ( or  the amount   of   refrigeration  effect  produced,  or the capacity of a refrigerator), and

W = Amount of work done.