Document DMXB6Dyq1GOxgYVBdQ5x5Eajd

602 CHAPTER 34 1946 Guide the temperature of the steam or return pipe or the pressure of the steam supply. - Where several unit heaters serve a limited area, they may be grouped . for purposes of automatic control, and several heaters placed in operation at the command of one thermostat. By properly grouping the units which will operate together, the benefit of zone control can often be obtained with a minimum of control equipment. Where such group operation is utilized* the thermostat and limit control usually function through a relay, as the combined load of the several motors may exceed the current capacity of the thermostatic control device. In some cases where cold drafts will not result, it is desirable to operate the unit heaters continuously for circulation of: air. In such instances the room, thermostat regulates the supply of steam to the unit through a control valve in the steam supply line and the unit heater motor operation is manually controlled. Unit heaters equipped with dampers arranged for by-passing air around the heating coils are controlled by room thermostats operating modu lating damper motors attached to these dampers so that as the tempera-, tures rise, a decreasing amount of air is heated. When the by-pass is wide open the heating effect is so much reduced that control of the steam supplied to the coil is not generally important. If valve control is added, the throttling of the steam may be concurrent with, or subsequent to, the opening of the by-pass. Cooling Units The recommended form of temperature control for a cooling unit con templates the continuous operation of the fan, with automatic regulation . of the compressor or cooling coil, or both, as determined by a thermostat in the room, or in.the return air to the cooling unit.- Such operation insures continuous circulation of air in the room, and in addition to providing the cooling effect of moving air, overcomes the tendency of the air to stratify. As the temperature begins to rise, the controller opens the valve to a cold water cooling coil, .or for direct expansion coils, opens a valve in the refrigerant line, closes a by-pass around the coil or starts a compressor. . Cooling units may also be controlled by arranging the room thermostats to start and stop the1 fan motors or by a combination of motor and. refrigerant control. Unit Ventilators There are various types of unit ventilators available but in general all types are. designed to. draw .air from the outside or to mix outside and recirculated air, heat it and introduce it into the room under control of a thermostat. . .. . .. ' The design of unit ventilators has to an extent been based on the' requirements for automatic temperature control and the cycles of control have-been developed to include other heating devices in the rooms with unit ventilators. Unit ventilators are frequently used in schools and other types of buildings where states have laws or regulations governing the minimum amount of ventilation to be provided. The control of the amount of oiitdopr air is designed to conform to the various laws. Usually the device circulates a constant amount of air and the amount auto-' matically taken in from outdoors is controlled in one' of these ways: 1 1. Full recirculation lintil.'the room temperature reaches a: certain point,'generally two degrees, below the desired room temperature; then a minimum amount of outdoor Automatic Control 603 air for ventilation while the temperature is maintained by throttling steam; and if the room temperature rises with all steam shut off, a gradual increase in amount of outr door air up to 100 per cent. ' . 2. Full recirculation until the room reaches a set point below room temperature, after which all air is taken from outside. ' ... 3. Gravity recirculation while the fan motor is not running, with full outside air as . soon as the fan starts, obtained by a relay in the motor circuit. 4. Full recirculation or all outdoor air as determined by a manual switch which can be operated at any time whether or not the fan is running.* All the unit, ventilators in a single building may be operated by one or many switches. With arrangements 1 or 2, it is desirable to include a relay to prevent the intake dampers from opening while the fan is not running, regardless of room temperatures. With a dual system of control this is essential to prevent the thermostat keeping the outside damper open until the tem perature falls to the reduced setting. The intake and recirculated air quantities are determined by a single damper or by a pair of dampers working together, and operated by a damper motor. Although this affects the temperature of the air delivered, the main heat control comes from the throttling of the steam supplied to the heating coil, with or without by-pass damper control. To prevent air being delivered at too low a temperature, a low limit thermostat is commonly installed in the air stream and set at some point between 55 and 70 F. The lower settings may cause discomfort, the higher ones overheating, depending on circumstances. The air stream thermostat can be used to turn on steam, reduce the amount of outside air, or both. Rooms with unit ventilators frequently have auxiliary heating, devices, such as direct radiators, convectors or unit heaters, all under control of a single room thermostat. A common control cycle for such rooms is com posed of the following functions, assuming that 72 F is desired: 1. Below 70 F the unit ventilator intake damper is in full recirculating position and . all heat is turned on. _. t 2. At 70 F the intake damper moves to a position that will admit a predetermined minimum amount of air from outdoors. 3. At 71 F the auxiliary heating devices are shut off. 4. From 71 to 72.5 F, the heating effect of the unit ventilator is throttled. 5. From 72.5 to 74 F, the intake damper is gradually moved to increase the amount of- outside air from the set minimum to 100 per cent. 6. If the room thermostat calls for too much cooling, the air stream thermostat holds the delivery temperature at a proper minimum. Other similar cycles may be used. One additional feature is the use of an air stream thermostat that has its control point reset, by the room thermostat. Then as the room temperature rises, the delivery tempera ture is gradually reduced from a maximum to a minimum.. CONTROL OF REFRIGERATION EQUIPMENT The most common means of providing cooling for air conditioning may be divided into four general classifications as follows: ": Refrigeration compressors may furnish refrigerant to direct expansion cooling coils through which air is being passed, or to coils in cooling tanks , through which water is passed which is then pumped to air washers or cooling coils through which the air is passed. In either case the compressor motor may be started and stopped in order to meet the demand for refrigeration or a pressure controller may be used to regulate the low side or suction pressure of the compressor. When the latter method is used, the flow, of refrigerant to cooling coils may be regulated by the opening and closing of a solenoid refrigerant, valve at the command of a temperature controller or thermostat.