Air refrigerating machines (ARMs) refer to compressor ones because the compressor for refrigerant (air) compression is applied in them. Such machines were used before appearance of vapour-compression refrigerating machines, in which low-boiling substances served as refrigerants – ammonia and carbonic acid and, moreover, freons.
Schematic diagram of simplest ARM is shown in Fig. 1, and its theoretical cycle is shown in Fig. 2 (air state in appropriate places of machine is designated with figures 1, 2, 3 and 4 on charts and installation diagrams).
Fig. 1. Schematic diagram of simplest air refrigerating machine: П – compartment; К – compressor; Т – turbine (expander); ПО – intermediate cooler; М – motor; ЗВ – overboard water.
Air from compartment П, where temperature T1 is maintained, is drawn into compressor К and compressed from pressure p0 to pressure p (process 1-2). At that its temperature increases to T2, and thereby the air can be cooled in intermediate cooler ПО by overboard water ЗВ (process 2-3). Compressed cooled air with temperature T3 enters the expander – turbine Т, where the air expanding to pressure p0 (process 3-4) is cooled and passed to the compartment with temperature T4 < T1. The air, heated in the compartment at a constant pressure of р0 from T4 to T1 (process 4-1), cools down this compartment.
Fig. 2. Theoretical cycle in v-p diagram (a) and s-T diagram (b) of simplest air refrigerating machine: process 1-2 – air compression in compressor; process 2-3 – cooling of compressed air in intermediate cooler; process 3-4 – expansion of compressed cooled air in turbine; process 4-1 – air heating in compartment.
As it is seen in Fig. 2, adiabatic processes of air compression and expansion, as well as isobaric processes of air cooling (by ambient medium – overboard water) and heating are implemented in the theoretical cycle.
Air specific refrigeration capacity q0 = i1 – i4, kJ/kg, where i1 and i4 – enthalpy in states characterized by points 1 and 4 in the diagram. It is proportional to area c-4-1-d (Fig. 2, b).
The specific work, expended for completing the cycle, is proportional to area 1-2-3-4 and is determined by the formula:
l = lк.а – lр.а ,
where lк.а – compressor work (negative), kJ/kg, lк.а = i2 – i1 = area 1-2-b-a (Fig. 2, а); lр.а – expander work (positive), kJ/kg, lр.а = i3 – i4 = area 3-4-a-b.
Theoretical performance coefficient of reversible cycle of air refrigerating machine:
At p/p0 equal to 3, 4, 6, εт is equal to 4.56; 2.05; 1.50.
Reverse Carnot cycle 1-2′-3-4′ for temperature interval T1-T3 in the compartment to be cooled (T1 = T0 = const) and of ambient medium – cooling water (T3 = T = const) is shown in s-T diagram (see Fig. 2, b). As it can be seen, refrigeration capacity for this cycle is more and expended work is less than in the cycle of air refrigerating machine.
Performance coefficient of Carnot cycle for p/p0 = 4; t1 = -5 °C; t2 = 120 °C; t3 = 20 °C; t4 = -75 °C is equal to εк = 10.7, and the degree of thermodynamic perfection of ARM cycle:
i.e. it is very low.
ARM practical cycle is shown in Fig. 3. It differs from theoretical one by pressure losses in ПО (from pд to p) and inner losses in compressor and expander – turbine, which are estimated by adiabatic (inner) efficiency of compressor ηк.а = 0.7…0.9 and turbine ηр.а = 0.7…0.85.
Fig. 3. Practical cycle of air refrigerating machine in s-T diagram.
Actual specific refrigeration capacity, kJ/kg:
q0д = q0 – lр.а·(1 – ηр.а).
It is less than theoretical one q0 by the value of turbine losses (shaded area а-4-4d-b).
Actual specific work, kJ/kg, is more than the theoretical one by the value of compressor and turbine losses:
Then the actual coefficient of performance is as follows:
It is much less than theoretical coefficient of performance; usually εд < 1.
ARMs sufficiently rank below the most economical vapour-compression refrigerating machines in mode of conditioning and medium-temperature cooling. Power consumed by them in conditioning mode is 2…3 times as much as the power for VCRM.
However the ARM actual coefficient of performance at cooling temperatures of -70 °C and lower is εд = 0.46…0.58 and exceeds εд for VCRM. Economical operation of low-temperature ARMs, which can be applied for fish freezing aboard, increases by means of regeneration introduction.
Such ARMs are implemented in industrial production and operation in stationary practice.
Absence of special refrigerant in ARMs is their doubtless merit. In this case, free harmless air fulfills the function of refrigerant, and although ARMs have not found particular widespread application yet, they are used, for example, for air conditioning in airplanes, cars, sometimes on ships, at cold treatment of hardware items (t0 < -70 °C), in thermal vacuum chambers for aircraft engine testing, as well as in deep cooling units for gases separation, air liquefying and oxygen obtaining.
Waste heat of power plants can be used for ARM drive (on ships included).