PERFORMANCE ANALYSIS OF VAPOUR COMPRESSION CHILLERS

Abstract

Vapour compression chillers are commonly used in a wide range of commercial and industrial cooling applications. This thesis will study the performance characteristics of both reciprocating and centrifugal chillers from the framework of irreversible thermodynamics. For reciprocating chillers, the author has shown first that the performance of this class of chiller is dominated by the effect of heat transfer losses and fluid friction losses. Using basic thermodynamic principles, viz., the First and Second Laws of Thermodynamics and the concept of process averaged temperatures, energetically optimal operating conditions can be established for each set of chiller operating conditions. When there is no significant difference in the value of the respective space occupied by the condenser and evaporator, the design conditions of the chiller approach that of the energetic optimum. However, when the space occupied by the evaporator becomes a major constraint, such as in the case of split, air-to-air chillers, the need to reduce the evaporator size dictates a reduction in the design cooling capacity so that there is only a minor compromise in its efficiency. This is due to the existence of a broad energetic optimum for the operating conditions of the chiller. For commercial, installed constant speed centrifugal chillers, the same thermodynamic model can be applied to describe full- and part-load performance characteristics. It is observed that, at part-load operation, 1/COP (ratio of net power input to cooling capacity) of the machines increases linearly with the reciprocal of cooling capacity, implying that their behaviour is dominated by internal losses and its 1/COP is minimum at full-load.