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A reciprocating compression chiller plant has a design capacity of 360 kW when chilling water from 13°C to 7°C with an evaporating temperature of 3.5°C. The compressor has six cylinders that can be loaded in four steps of 0 - 2 - 4 - 6 cylinders, or reverse, as the cooling load changes. This is controlled through a step-controller from a thermostat immersed in the chilled water return main. The refrigeration capacities when operating at a constant condensing pressure with six cylinders, four cylinders and two cylinders, respectively, are shown in the diagram below. The plant operates within the following constraints: maximum chilled water supply temperature 7°C; minimum allowable evaporating temperature 2°C; set-point of low water temperature cut-out at 3.5°C. Using the given diagram, a) establish the settings of chilled water return temperature for the step controller to unload the cylinders at reducing load, and the settings to load the cylinders at increasing load; and b) determine the refrigeration duty and chilled water supply temperature before and after unloading/loading at these settings. 450 6 cylinders 300 4 cylinders Refrigeration capacity, kW 150 2 cylinders 0 02 12 14 4 6 8 10 Evaporating temperature, °C

Solutions 450 Unloading 6 cylinders 300 4 cylinders Refrigeration capacity, kW 150 2 cylinders 0 02 4 6 8 10 1214 11 Temperature, °C