Document 3xZz1wqGroRQG0gwMoaxNX16

612 CHAPTER 34 1948 Guide presence of two or more: wings, having the same general exposure, may suggest the desirability of more restrictive zoning. , In smaller buildings, and in those of larger extent where cost and other conditions limit the number of zones, a common compromise is to combine the North and West exposures in one zone, and .the East and South ex posures in a second zone. Frequently, when the street floor level is given over to public spaces or to activities which are markedly different from those carried on in the re mainder of the building, it is advisable to provide a separate steam main, with convent ional room thermostats in each individual area. For steam heating systems, the radiator output may be.varied pro portionately to the changes in outdoor temperature, by any of a number of general methpds. Those in most common use are: 1. Turning the steam on and off at appropriate intervals as dictated by temperature or time considerations, proportioned to the need for heating. 2. Varying the pressure of steam in the system in accordance with the demand for heat. 3. Throttling the steam pressure, at the demand of the controller, to allow flow through orifices in proportion to the heating requirements. In hot water heating, the accepted practices are (1) to vary the tem perature of the hot water supplied to the system or, (2) to vary the flow of hot water, both proportionately to the heating requirements. . The regulator for each zone usually is of a type which in some manner responds to the outdoor temperature and effect of sun and wind for the zone. Many of the available zone controllers are arranged in such a way as to be affected also by the temperature of the heating system and the indoor temperature in the zone. Whatever the mechanical features of the regulator, its function is to dictate the flow impulse, or rate of flow, in such a way as to maintain the desired indoor temperature in the zone, regardless of the fluctuations in the. outdoor temperature. Provisions also may be made to maintain a predetermined low economy temperature in each zone during periods of non-occupancy; to facilitate quick warmingup following such periods; and to follow those portions of the,daily cycle of control with normal heating effect during the occupancy period. A control panel, at a central location, may be arranged so that manual switches for each zone may raise or lower the operating temperature, and time switches, if desired, may be provided for obtaining, automatically, any day-night or other predetermined control program which the oper ators of the building may desire. The characteristics of the regulators which are operated by the zone controllers depend upon which of the basic systems of zone control is employed in a given installation. Shut off, mixing or throttling valves and various forms of devices to reset or pilot the action of reducing valves and to control firing means, are some of the more common regulators which are used to control the flow of steam or hot water, under the command of zone controllers. The char acteristics of these regulators usually are determined by the manufacturer of the type of controller 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 controls 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 Automatic Control 61S 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, particularly where units are installed in school rooms. It is desirable to coordinate the selection of control cycle with the unit ventilator manu facturer since, in many cases, modifications of the unit are required for the installation 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 minimum 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 discharge tempera ture from dropping below a predetermined minimum. Control Cycle No. 2. 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 minimum 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 full-open position, control of the recirculated air and outdoor air dampers is transferredto a thermostat installed ahead of the heating coil to maintain a minimum air tem perature. On continued call for cooling, the valve gradually closes. If direct radiation is used, it is desirable that it be controlled.in sequence with.what ever control cycle is adapted for the unit ventilators. Unit Coolers Although most unit coolers can be. adapted to any control cycle, continuous fan operation is recommended to avoid stratification and wide fluctuations in space tem perature. Should the unit be completely self-contained, control of the direct expansion refrigeration unit-may be obtained from the temperature of the recirculated air and from suction pressure. 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 following 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 provide 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 dis-