AL-2499 -- A Parametric Study of an Open Air Cycle Air Conditioner Using Moist Air Thermodynamics PDF

The open air cycle or reversed Brayton cycle has been known for·many years. An inefficient form of the open air cycle has been used for cooling in places where a source of high pressure air has been available, such as bleed air from aircraft engines. In the 1930's, Bosnjakovic, in his book Technical Thermodynamics (1) , suggested spraying moisture into the inlet of an air compressor to reduce the discharge temperature. However, no mention of the effect of this moisture injection on efficiency was made. The present.study was prompted by the renewed interest shown in the reverse Brayton cycle and the possibility that moisture in the air could have a significant impact on the overall cycle efficiency.

The parametric study described herein uses the psychrometric properties of moist air as found in the ASHRAE Handbook of Fundamentals) except for the development of the basic entropy equation by the authors. An example calculation will be discussed using the psychrometric chart as well as a specially developed entropy vs humidity ratio chart. The effect of isentropic compressor and expander efficiencies on the C.O.P. as well as the effect of moisture addition to the cycle will be discussed. Several typical kinds of cycle inefficiencies will be investigated and, finally, air cycle performance relative to a Refrigerant 22 Rankine cycle operating between the same ambient temperature levels will be discussed.