TITLE.PM5

REFRIGERATION CYCLES 715

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Then, C.O.P. =

R

W

n

and Relative C.O.P. =

Actual C. O. P.

Theoretical C.O. P.

where, Actual C.O.P. = Ratio of Rn and W actually measured during a test

and, Theoretical C.O.P. = Ratio of theoretical values of Rn and W obtained by applying laws of

thermodynamics to the refrigeration cycle.

14.1.5. Standard rating of a refrigeration machine

The rating of a refrigeration machine is obtained by refrigerating effect or amount of heat
extracted in a given time from a body. The rating of the refrigeration machine is given by a unit of
refrigeration known as ‘‘standard commercial tonne of refrigeration’’ which is defined as the
refrigerating effect produced by the melting of 1 tonne of ice from and at 0°C in 24 hours. Since
the latent heat of fusion of ice is 336 kJ/kg, the refrigerating effect of 336 × 1000 kJ in 24 hours is
rated as one tonne, i.e.,

1 tonne of refrigeration (TR) = 336 1000

24

× = 14000 kJ/h.

Note : Ton of refrigeration (TR). A ton of refrigeration is basically an American unit of refrigerating
effect (R.E.). It originated from the rate at which heat is required to be removed to freeze one ton of water from and
at 0°C. Using American units this is equal to removal of 200 BTU of heat per minute, and MKS unit it is adopted as
50 kcal/min or 3000 kcal/hour. In S.I. units its conversion is rounded of to 3.5 kJ/s (kW) or 210 kJ/min.
In this book we shall be adopting,
1 tonne of refrigeration = 14000 kJ/h (1 ton = 0.9 tonne).

14.2. Air Refrigeration System

14.2.1. Introduction

Air cycle refrigeration is one of the earliest methods of cooling developed. It became obsolete

for several years because of its low co-efficient of performance (C.O.P.) and high operating costs. It

has, however, been applied to aircraft refrigeration systems, where with low equipment weight, it

can utilise a portion of the cabin air according to the supercharger capacity. The main character-

istic feature of air refrigeration system, is that throughout the cycle the refrigerant remains in

gaseous state.

The air refrigeration system can be divided in two systems :

(i) Closed system (ii) Open system.

In closed (or dense air) system the air refrigerant is contained within the piping or compo-

nents parts of the system at all times and refrigerator with usually pressures above atmospheric

pressure.

In the open system the refrigerator is replaced by the actual space to be cooled with the air

expanded to atmospheric pressure, circulated through the cold room and then compressed to the

cooler pressure. The pressure of operation in this system is inherently limited to operation at

atmospheric pressure in the refrigerator.

A closed system claims the following advantages over open system : (i) In a closed system

the suction to compressor may be at high pressure. The sizes of expander and compressor can be

kept within reasonable limits by using dense air ; (ii) In open air system, the air picks up moisture

from the products kept in the refrigerated chamber ; the moisture may freeze during expansion

and is likely to choke the valves whereas it does not happen in closed system and (iii) In open

system, the expansion of the refrigerant can be carried only upto atmospheric pressure prevailing

in the cold chamber but for a closed system there is no such restriction.