Book - School of Science and Technology

Refrigeration plant components 547All pipeworkor plant carrying refrigerant at low temperatures (low pressure/evaporatorpart of the circuit) or chilled water must be insulated to prevent the formation ofcondensation and to reduce heat gains. To this end an effective vapour barrier must beapplied, or be integral with the insulation, on the outer surface. BS 5422: 1990 gives therecommended thicknesses for given system operating temperatures.EvaporatorsIn its most simple form, the evaporator is tubular, the tubes containing the refrigerant andthe whole array immersed in the liquid to be cooled. In the dry system the evaporator coilsare filled with vapour having little liquid present but when operated on the flooded system,the liquid refrigerant discharges into a cylinder feeding the coils by gravity. As evaporationtakes place, the gas returns to the top of the cylinder and from there returns throughthe suction pipe to the compressor.The most common form of evaporator for application to air-conditioning practice isthe shell and tube type, used with a closed circuit chilled water system serving cooling coils.The water is contained in the tubes and the refrigerant in the shell. A development of theshell and tube type is the direct-expansion shell type evaporator in which the refrigerant isin the tubes and the water in the shell. The tubes may be arranged in two or more circuits,each with its own expansion device and magnetic valve on the liquid inlet to allow stepcontrol. Different arrangements of baffles in the shell control the water velocity over thetubes in order to improve heat transfer.It is generally not necessary to resort to ethylene glycol solutions for air-conditioningpurposes, since chilled water at about 4 C satisfies all normal requirements. Precautionsagainst accidental freezing of the water in the evaporators include low suction pressurecut-outs and low water temperature cut-outs as well as water-flow switches.In instances where the cooling load may be less than the minimum output of therefrigeration machine and where some severe limitation exists as to the number of startsper hour, it is possible to fit a hot gas by-pass valve which will, as required, direct hot gasfrom the condenser into the evaporator to provide an artificial load. This can be arrangedto come into operation automatically at a predetermined point of the capacity controllerrange and will enable the machine to run continuously even when no true cooling load exists.Modern water-chilling plant does not generally require the addition of chilled-waterstorage to act as a `flywheel', because it can be arranged in suitable steps of capacitycontrol to suit the variations in load. Little complication is experienced with pistoncompressors having four to eight steps of control. Centrifugal units which turn down to10% can virtually run on pipeline `losses' as can screw-type machines.Evaporators in the form of direct expansion (DX) cooling coils in the air stream, aredescribed in Chapter 17. The refrigerant flow rate is controlled by a thermostatic expansionvalve, or a capillary tube on smaller plant; the sensor to control the valve beinglocated in the suction line to the compressor to maintain the correct degree of superheat atthe compressor intake and to avoid the damage which would occur if liquid entered. Coilshandling volatile refrigerants present fluid distribution problems and to ensure equal flowof refrigerant through each circuit in the cooling coil assembly, the fluid is passed througha distributor at the coil inlet to divide the flow equally.CondensersThe evaporative condenser, consisting of coils of piping over a tankfrom which water isdrawn and then circulated to drip over the pipes through which the refrigerant is passed,