Q6. (a) Sketch a diagrammatic arrangement of a fully automatic direct expansion domestic refrigeration system which supplies a number of cold rooms. (5)
(b) State, with reasons, FIVE desirable thermodynamic properties of a refrigerant. (5)
2019/DEC>> Some candidates still giving very basic diagrams and stating thermodynamic properties but not the reasons.
2018/MAR >>This was by far the worst answered question and yet there is no reason why it should be at this level.
Part (a) Four boxes labelled compressor, condenser, expansion valve and evaporator is not worth any marks at the management level and scored zero. At this level it should be a given that basic OOW knowledge is not being tested.
>> To get the 5 marks a candidate needs to show LP and HP cut outs, oil separator, a relief valve on the condenser (yes this is a vital safety device) a dryer, sight glass, solenoid valve with thermostat and an expansion valve adjacent to the cold room not on the other side of the page. Room temperatures should be another detail that is included.
Part (b) – very few candidates seem to understand what thermodynamic properties mean. The favoured one was high Co-efficient of Performance; this is not a thermodynamic property – it is a ratio. Other properties such as environmentally friendly, non-toxic, cheap etc are indeed desirable but are not thermodynamic.
>> The question also asked for the reason for each property and a lot of candidates never gave reasons.
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