Document mgBE90pYm8ZqZwGE84KgVzEQ

614 Chapter 33 .\ 1945 Guide A high pressure cutout as an individual unit or in combination with .either a temperature or pressure controller provides a safety feature against excessive pressures on the high side of the compressor. . Many compressors may be unloaded by instruments sensing room or duct conditions, or by refrigerant pressures, thus reducing the frequency of'Starting and stopping. If two or more compressors are used 'for a single cooling system, step controllers are used to. start them in sequence at intervals of a few seconds to avoid the large momentary electric input that simultaneous starting would demand. When condensers are water cooled, thermostatic control to vary the quantity of water is needed for economical operation. Mechanical, air condensers may be started and stopped with temperature demands! Chilled water may be stored in tanks at temperatures slightly lower Automatic Control _________ ' . - , ' . ; &15 discharge and its function is to recondense the water .Vapor drawn fromthe evaporator, plus.the steam which supplies the energy for compression. The temperature of the cold water leaving the flash chamber should be measured by a temperature controller which will in turn operate a twoposition or positive-control valve installed in the steam line to the jet so as to permit steam to flow only when cooling is required. If city water is Fig. 1. Location of Control Devices for Year 'Round Air Conditioning System than required for air cooling coils. The control of temperature for the water distribution system is as described for Ice Cooling. When ice is used for the cooling, or dehumidification of air, it is usually placed in bunkers and water is sprayed over it. This water, after being cooled, may be used in air washers or surface cooling coils and is usually returned to the bunker for additional cooling, after being used. Control of the water temperature leaving the cold water tank may be maintained by a temperature controller, which measures the temperature of the water in the tank and modulates a control valve in a by-pass which permits a portion of the return water to return directly to the tank instead of passing through the sprays. A vacuum refrigerating system consists of an evaporator, compressor, condenser and auxiliaries. The refrigerant used is water, and water vapor (steam) is the power medium. Water which has been passed through an air washer or cooling coil is sprayed directly into the evaporatpr or water cooler where it is cooled by its own evaporation. A condenser is attached directly to the compressor Fig. 2. Control Diagram for Year 'Round Air Conditioning System used in the condenser, the amount of water should be modulated according to the demand as measured at the condenser outlet by means of a tem perature controller and control valve. ' . Well water, if available in sufficient quantities at low temperatures during the cooling season, may be pumped directly. ,to air washers or cooling coils. Control is usually effected through control valves on the water supply to the cooling unit, actuated by temperature or humidity con trollers, or both, located either at the outlet of the conditioner or in the conditioned space.