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.
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