Theory of Refrigeration

THEORY OF REFRIGERATION The most general method of securing mechanical refrigeration is to use a reversed heat engine whereby heat is absorbed at the lower temperature and is rejected at the upper temperature with a direct acting engine or an electric motor, providing the external work required to make the cycle possible. If the Carnot heat cycle (see INTERNAL COMBUSTION ENGINES) is used for the pro duction of power the reversed Carnot cycle is the one employed in the refrigerating cycle. Such a reversed cycle cannot have a greater efficiency than the direct cycle.

According to the second law of thermodynamics (q.v.) the efficiency of the cycle is independent of the kind of working medium used, and is dependent only on the temperature limits. If it were possible to operate according to the Camot cycle one refrigerant would be as efficient as any other but as all practical refrigerating cycles deviate from the Carnot it will be seen later that such is not the case exactly. This deviation is mainly on account of the use of a throttle valve in order to reduce the pressure of the liquid before it enters the evaporating coils instead of an expansion motor as would be required in the reversed Carnot cycle to provide the necessary adiabatic expansion.

Heat Cycles.

Heat engine cycles can be shown graphically in a number of ways. The p-h (Pressure-specific enthalpy) dia gram, shown in fig. I, is the simplest and most useful diagram for representing the refrigerating cycle. The value of h is given by the expression h = U+Apv, where U is the energy, p the pressure, v the volume, A= I +778; therefore dh=du+Apdvd-Avdp, but dq=du+Apdv; therefore dh=dq+AvdP, so that for a constant pressure process dh = dq or h1—h2=1Q2. From the last equation it is clear that in a constant pressure process the heat absorbed is equal to the difference in the value of h at the beginning and the end of the process.

The Refrigerating Cycle.

In refrigeration the volatile liquid is permitted to pass into evaporating coils under reduced pressure and there it boils, absorbing heat from the commodities to be cooled. The liquid at the condenser pressure is indicated by the point a in fig. i and the line ab is the drop of pressure in the

pressure-reducing (the so-called expansion) valve. Such a throt tling process is one of constant h and is shown by the equation for steady flow. If an expansion cylinder had been used, the light line ak would have indicated the process and this being re versible in the thermodynamic sense, the cycle would be more efficient than the one having the expansion valve. The point b represents the condition of the working medium with the reduced pressure, and with the temperature corresponding to the pressure obtainable from suitable thermodynamic tables. Fig. i shows that the point b corresponds to a point haviqg 16% vapour, or (in other words) 16% of the liquid is evaporated in cooling the liquid refrigerant from the temperature at a to that at b.

Useful refrigeration is shown by the evaporation line bc, during which time the heat absorbed by the refrigerant per pound is The point c represents the gas leaving the evapo rator coils and entering the compressor. The compressor process is shown by the line cd and the work of compression per one pound of the refrigerant is given by the value of Finally the constant pressure line da represents the heat lost by the com pressed gas first in the removal of the superheat df and then the latent heat of liquefaction fa. The heat absorbed by the con densing water per one pound of the refrigerant is As stated, the useful refrigeration per pound of refrigerant is given by the expression In the United States the ton of refrigeration is defined as the absorption of 200 B.T.U. per minute. The ton of refrigeration in Great Britain is given as the absorption of one kilogram calorie per second which is larger than the United States unit by 18.8%. Using the United States definition the piston displacement per ton of refrigerant per min ute theoretically is 200 where is the volume of 'lb. of dry, saturated • reincrerant at the pressure 1),.

Allowance, however, must be made for the volumetric efficiency which may be as small as go% or less under extreme conditions.