SPONSOR CPD PROGRAMME Continuing professional development (CPD) is the regular maintenance, improvement and broadening of your knowledge and skills, to maintain professional competence. It is a requirement of CIBSE and other professional bodies. This Journal CPD programme can be used to meet your CPD requirements. Study the module and answer the questions on the final page. Each successfully completed module is equivalent to 1.5 hours of CPD. Modules are also available at www.cibsejournal.com/cpd Ensuring safe, environmentally sensitive refrigerants for building services applications This module explores the demanding practical requirements for efficient and safe refrigerants and the application of lower-flammability refrigerants Reducing the environmental impact of the built environment is driving technological changes in refrigerants as well as influencing professional and public realities of life-cycle risk assessment and moderation. This has generated renewed activity in the development and rediscovery of refrigerants that, in a world previously ignorant of the challenges of ozone depletion and global warming, would have not have been considered for HVAC system applications because of concerns about flammability. This CPD article will consider the increasingly demanding practical requirements for efficient and safe refrigerants that are ushering in pragmatic acceptance of the lower-flammability refrigerants, and methodology to ensure their appropriateapplication. The vapour compression refrigeration system is employed in the majority of building services systems cooling applications, and is based on the basic simplified system as shown in Figure 1. (Although this article will refer to cooling systems, the same cycle and basic principles hold good for heat pump heating systems.) In a system, the cycle, as shown on the p-h diagram of Figure 2, will diverge from that of the simple cycle as indicated by the dotted line. This will depend on the specific systems components and control, as well as levels of wasteful evaporator superheating, frictional resistances, pressure losses and heat losses. However, the fundamental process remains the same. The refrigerant transfers thermal energy between the colder evaporator and the warmer condenser by changing state both at the upper and lower pressures, generated by the compressor and enabled by the throttling device. (The basic system is discussed more fully in CIBSE Journal CPD module 2, available at cibsejournal.com.) The efficiency of the cycle can be described by the coefficient of performance (COP). The maximum theoretical efficiency is provided by the Carnot COP = Te/(Tc Te) where Tc is the condensing temperature and Te is the evaporating temperature, both in absolute temperature, Kelvin. The value of the evaporating temperature will have the most significant impact on the COP, since it affects both the numerator and 3 2 Condenser 1 Throttling device 4 Evaporator Compressor Figure 1: Schematic of a notional vapour compression system denominator of the equation, but generally for high COP, the (colder) evaporating temperature should be as high as possible and the (warmer) condensing temperature as low as possible. The difference between the two temperatures is known as temperature lift. In a real system, the COP is determined from the power ratio, (useful cooling, kW)/(compressor power input, kW) and is less than the Carnot COP due to the irreversibility of the pressure and thermal processes. The compressor power, described by the increasing concave down curve 2-3, increases disproportionately with www.cibsejournal.com June 2019 55 CIBSE Jun19 pp55-58 CPD 146 v4.indd 55 24/05/2019 16:25