Abstract
Two of the major problems of heat pump systems working in extreme conditions are the loss of efficiency
of the system and the high compressor discharge temperatures. One possibility in order to overcome these
issues is to perform the compression in two stages. In this frame the use of vapor injection two stage cycles
represent an economic and effective solution. This study analyzes the influence of design parameters and
injection conditions for two different configurations of two stage cycles, for four refrigerants (R407C, R290,
R22 and R32). Design parameters, such as the displacement ratio, are optimized in terms of COP in ideal
conditions for both injection systems. A deeper analysis taking into account the efficiencies of the
compressor is done finding that two stage systems could reach improvements of 30% in terms of COP
compared with one stage systems and that a bad design of this type of systems could represent a loss
of improvement between 6% and 10%. Finally a method to control the system at any operating point in
order to make it works in its optimum is done. From all the analysis, guidelines for the optimum design
and control of such systems are obtained in terms of capacity, Coefficient of Performance (COP), seasonal
performance factor and discharge temperature criteria.