ABSTRACT
In Chapter 12, we covered the Carnot cycle based on the second law of thermodynamics as applied to power, refrigeration, and heat pump cycles. This chapter extends this treatment to the description and analysis of practical vapor compression refrigeration and heat pump cycles, and then to actual cooling systems with heat exchangers both at the evaporator and the condenser. In 2013, vapor compression systems accounted for 99% of the cooling needs and 11% of heating needs in U.S. residential and commercial buildings. We also discuss absorption cycles whereby cooling can be achieved, not by work input, but by heat input to the system. The attraction of heat pumps is that they can deliver more thermal power than they consume electrically during an appreciable portion of the heating season. In moderate climates requiring both heating and cooling, the heat pump can also be operated as an air conditioner during summer, thereby avoiding the additional cost of a separate air conditioning system. The design methodology to size heat pumps and the associated auxiliary heater is presented. We then discuss the differences between air, water and ground source heat pumps, and their advantages and disadvantages. How cooling equipment performance is affected by off-rating conditions and degraded by part-load operation is described, and several energy analysis methods widely used to model this phenomenon are presented and illustrated with several solved examples. Though not attainable in practice, the concept of theoretical efficiency limits of cooling equipment specified by the second law of thermodynamics can provide useful insights. We derive such expressions for ideal and actual heat pumps and illustrate their use via simple examples. The last section includes an overview of refrigerants, their method of designation, and desirable thermodynamic and physical properties, along with international efforts to minimize their adverse effect on climate change and human health.
