Document m7mqRZaa7O53kwgaqwQB0Ddg

812 CHAPTER 38 1950 Guide under the command of zone controllers. The characteristics of these regulators usually are determined by the manufacturer of the type'of con troller which is selected. AUTOMATIC CONTROL APPLICATION Some of the considerations affecting the selection of automatic controls for applications are given in the following paragraphs which describe con trols and operation for various types of units. Unit Heater Control Two-position (on-off) control by means of a room thermostat is the standard method of control for unit heaters. A limit control should be incorporated to prevent operation of the unit heater fan motor when steam or hot water is not available. The' limit control can be a surface thermostat or pressure control. Where there is no possibility of drafts, and continuous air circulation is required, the unit heater fan motor may operate continuously. In this case, a room thermostat controls a valve (two-position or proportioning) in the steam or in" the hot water supply line. Unit Ventilator Control Various makes of unit ventilators are designed for different control cycles. Selection of automatic temperature control for unit ventilators is largely determined by the design of the particular unit. The choice of control cycle may also be determined by local and state ventilating codes, par-, ticularly where, units are installed in school rooms. It is desirable to co ordinate the selection of control cycle with the unit ventilator manufacturer since, in many cases, modifications of the unit are required for the in stallation of the control equipment; and, in some cases, it is desirable to have the equipment factory-mounted. Two typical control cycles are: (1) Variable Outdoor Air with Fixed Minimum; and (2) Variable Outdoor Air without Fixed Minimum. Control Cycle No. 1. In full heating position, the outdoor air damper is closed, the recirculating air damper is open, and the supply valve is open. In full cooling position the outdoor air damper is open, the recirculating air damper is closed, and the supply valve is closed or at a minimum position to maintain a mini mum discharge air temperature. The sequence of control operations is: On call for cooling, under control of a room thermostat, outdoor air damper opens to minimum setting. The supply valve then closes gradually, after which the outdoor air damper gradually opens to the maximum position and, simultaneously, the recirculating air damper closes. A low limit thermostat mounted above the coil prevents the dis charge temperature from dropping below a predetermined minimum. Control Cycle Not S. In full heating position the outdoor air damper is closed, the recirculating air damper is open and the supply valve is open. In full cooling position the supply valve is closed, the outdoor air damper is open, and recirculating air damper is closed or at any position required to maintain a mini mum discharge air temperature. The sequence of control operations is: On call for cooling, under control of a room thermostat, the valve gradually closes. As the valve leaves lull-open position, control of the recirculated air and outdoor air dampers is transferred to a thermostat installed ahead of the heating coil to maintain a minimumair temperature. On continued call for cooling, the valve gradually closes. . If direct radiation is used, it is desirable that it be controlled in sequence with whatever control cycle is adapted for the unit ventilators. Unit Coolers Although most unit coolers can be adapted to any control cycle, con tinuous fan operation is recommended to avoid stratification and wide Automatic Control 813 fluctuations in space temperature. Should the unit be completely selfcontained, control of the direct expansion refrigeration unit may be ob tained from the temperature of the recirculated air and from suction pres sure. In the case of multiple unit systems supplied with refrigerant or chilled water from a central source, a valve in the supply to each cooling coil may be controlled thermostatically from space temperature. Refrigeration and Dehumidification Equipment Typical control equipment and its functions are described in the follow ing paragraphs. Well Water. Where well water is used directly in air washers or cooling coils, control of temperature or humidity is usually obtained by thermostats or humidistats operating valves (two-position or proportioning). The two-position valve will pro vide better dehumidification since a lower coil temperature will be maintained, but the temperature of the discharge air will fluctuate. With proportioning control, better control of discharge air temperatures will be maintained. In both cases the sensible-latent heat ratio is basically a matter of coil-design rather than automatic control. Proportioning three-way valves may be used as mixing or diverting valves for better pump performance, and may also be applied to an air washer used with a recirculating pump to control water temperature rather than volume. Ice Bunkers. Where water is sprayed over the ice in bunkers and circulated to air washers or cooling coils, control is obtained by a thermostat in the water line from the bunker. The thermostat proportions a three-way valve to by-pass enough return water around the ice bunker to maintain a constant discharge temperature. Compressors. Compressors may supply refrigerant to direct expansion cooling coils in air conditioning units, or to direct expansion coils in water-chilling unitB. In either case, the compressor motor may be started and Btopped directly by a room or. duct thermostat, or a pressure controller may be used to regulate the suction pressure of the compressor, fn the latter case, a room or duct thermostat may be used to control a solenoid valve in the refrigerant supply line to the cooling coil. A high and low pressure cut-out is standard safety equipment on most compressor installations. Reduced capacity of the refrigerating unit may be obtained by means of temperature or pressure controlled unloading devices which vary the capacity of the compressor in some proportion to variations of cooling load. Program or step controllers, act tuated by temperature or pressure, are commonly used in multiple compressor in stallations. It is desirable in such installations to return the program or step con troller to the off position when the system is shut down to prevent the full electrical load of multiple compressors from being thrown across the line at the same time. Thermostatic control of water supply to water-cooled condensers may be achieved by means of self-contained controllers or valve and thermostat application. Steam Jet.. A steam jet refrigeration system is commonly controlled by means of a thermostat in the chilled (secondary) water. The thermostat operates a twoposition valve in the steam line to the jet. In the case of multiple jet units, program or step control can be achieved by controlling the jets in sequence. As in the case of direct expansion refrigeration units, a system of control is advisable for the watercooled-condensing unit. Centrifugal Units. The control of centrifugal refrigeration units or other types of vacuum systems is customarily achieved by means of a thermostat in the chilled water to control the operating cycle of the equipment at full or reduced capacity. Adsorption Units. Control of adsorption units consists of a damper control which, in'response to humidity, controls the air flow through or around the activated bed of adsorption material. Standard controls for cooling are used to reduce the dehumidi fied air to a desired dry-bulb. Absorption Units. Since at constant density, the absorption solution will extract water from the treated air in an amount proportional to the solution temperature, the moisture content of the air leaving an absorption unit is regulated by solution tem perature. Solution density is held constant by a combination of float control and steam valve controlling the solution regenerator. Two basic methods of control are standard: 1. Constant solution temperature where the solution temperature is set so that, at the full load for which the unit is designed, the discharge air will have the