RACA Journal June 2016 - Page 71

Back to basics Andrew Perks Andrew Perks is a subject expert in ammonia refrigeration. Since undertaking his apprenticeship in Glasgow in the 1960s he has held positions of contracts engineer, project engineer, refrigeration design engineer, company director for a refrigeration contracting company and eventually owning his own contracting company and low temperature cold store. He is now involved in adding skills to the ammonia industry, is merSETA accredited and has written a variety of unit standards for SAQA that define the levels to be achieved in training in our industry. The importance of refrigerant Basic pumped ammonia refrigeration system #BackToBasics By Andrew Perks Every component of a refrigeration cycle is important, but without refrigerant, there would be no cooling. L ast month I deviated from the theme of the refrigeration cycle and principles but what use is all this knowledge if no-one is actively out there passing it on? So let’s get into the meat of an ammonia system. The refrigeration cycle is the same for all the refrigerants – cold attracts heat; how we do it is the difference. Before work of any kind is carried out on an ammonia system, the circuiting of the pipe work and the method of plant operation should be understood. The following is a basic description of the operation of a single-stage pumped ammonia refrigeration system: • Sub cooled liquid (by virtue of the elevated pump pressure) is pumped from the suction accumulator to the evaporator by the ammonia pumps. The cold room fans blow the room air over the colder evaporator coils, causing the liquid ammonia inside to evaporate (change from a liquid to a gas) as it extracts the heat from the air cooling the room. The evaporated liquid (saturated suction gas) together with the residual over supplied liquid (as the plant is over feeding liquid at a ratio of 4-to-1 to the cooling duty) returns to the suction accumulator in a saturated liquid/gas mixture. • In the suction accumulator, the excess liquid returning in the suction gas separates, and dry saturated suction gas is drawn from the suction accumulator to the compressor. www.hvacronline.co.za • • • The suction gas is compressed from the suction pressure to the condensing pressure in the compressor. Some of the oil from the compressor sump mixes with the discharge gas during compr ession. The majority of this oil is removed from the delivery gas in the oil separator and returned to the compressor sump via an automatic oil return system. However, some oil is carried over in the gas as a vapour and passes through the condenser and liquid receiver to the suction accumulator. The high pressure superheated hot gas from the compressor is delivered to the condenser where it is condensed to a subcooled liquid. High pressure liquid flows from the condenser to the liquid receiver. While in the liquid receiver the sub-cooled liquid heats up to a saturated liquid. The liquid receiver acts as a buffer to allow storage of liquid which circulates in the system. The level will fluctuate from time to time due to system demand. Liquid The high pressure ammonia liquid is fed to the expansion valve at the suction accumulator on demand where it reduces to the lower pressure/temperature. The oil carried over separates from the ammonia in the suction accumulator and this oil must be RACA Journal I June 2016 69