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182 CHAPTER 13 1965 Guide And Data Bool plished automatically with an electric motorized damper which closes when the fan is off, or by an electropneumatic relay in the air line to the pneumatic damper operator. In either case, de-energizing the fan closes the outdoor air damper. Seasonal Changeover Where automatic changeover from summer to winter is to be provided under automatic control, there'should be' some period of inoperation of both heating and cooling. Otherwise; serious cycling from one function to the other will'occur. This period of moperation can be accomplished by a two-position controller with adjustable differential sensing'outdoor tem perature, or by a controller sensing the temperature, at a representative conditioned space. lowered Night Temperature Where lower temperatures are employed during unoccupied periods than ore normally maintained during periods of occupancy, the proper day and night temperature time cycle is often established by an automatic timer. Sufficient time should be allowed in the morning to pick up the conditioning load well before there is any heavy load increase in the con ditioned spaces. PART III: CONTROL APPLICATIONS Applications of controls to systems or apparatus of a general nature will-be found in this section. Applications pertaining to specific systems or apparatus will be found in the chapters covering these subjects. The application- of- controls may conveniently be considered for the three divisions of a system indicated in fig. 13: (1) primary source apparatus, (2) dis tribution system, and (3) terminal equipment. The control of primary source apparatus is described in chapters of this volume treating system components, such as: Chapter 38, Compressors; Chapter 40, Absorption AirConditioning and Refrigeration Equipment; Chapter ,45, Refrigerant. Control. Devices; Chapter 51, Automatic Fuel-Burning Equipment; Chapter 52, Heating Boilers, Furnaces, and Space Heaters; and Chapter 66, Heat Pumps. The second and third divisions are considered here. - O . DISTRIBUTION SYSTEMS The control of distribution systems is discussed separately for central fan systems, hot water systems, zoned steam sys tems, and zoned hot'water systems, due to their differences. CENTRAL FAN SYSTEMS Since there are numerous control combinations' that can be employed with.central fan'systems, the following sections will deal with only a few of the typical combina tions for controlling outdoor- and retum-air dampers, pre heat coils, beating coils, cooling coils, Mid humidity control: The complete control system is obtained by combining the selected method for each item into an integrated set of controls. Proper consideration must be given to the inter relation of the several parts and the sequence of operation Outdoor-Air Damper Control Outdoor air for ventilation is usually controlled by a damper arrangement providing a homogeneous mixture of outdoor air and recirculated air. Outdoor air to meet mini, mum ventilation requirements should be available whenever the fan is running and is usually provided in one of the fop lowing ways: a. By a minimum outdoor-air damper which is fully open when the fan is started. A maximum outdoor-air damper may also be provided if additional outdoor air for natural cooling is desired. 5. By an outdoor-air damper which opens to a Tninm^tni position when the fan is started. Additional outdoor air ia provided by further opening of the damper under automatic control. v Recirculated-air and exhaust-air dampers are synchronized to operate with the outdoor-air dampers. Mechanical inter connections are preferable, but separate operators on of-the dampers can be used when mechanical interconnec tions are impractical. In most arrangements, provisions ate made to prevent admission of more than the- minimum out door air required for ventilation, both during heating opera tion and when the outdoor-air temperature is too high to provide some natural cooling. Provisions should aly> be to close tiie outdoor-air and exhaust-cur dampers and'open the retum-air damper whenever the fan is stopped. .Fig. 14 shows a system having manual adjustment for maximum outdoor air and an optional, but very desirable, mixed-air thermostat. Hus thermostat overrides the manual switch whenever the mixed-air temperature drops below its set point. This provides for a minimum mixed-air temperature for safety of downstream equipment. Whenever the fan motor is stopped, the' outdoor-air and exhaust-air dampers close, while the return-air damper.opens. - Fig. 15 shows an all-season ventilation control system with a fixed minimum quantity of outdoor air and auto matic control of maximum outdoor- and retum-air dampers. This arrangement is sometimes referred-to as an economizer ' control of outdoor air. With outdoor-air temperatures be-; low the set point of the summer thermostat, the mixed-air thermostat positions the outdoor- and returb-air dampers! When: outdoor-air temperature rises to the set point of the outdoor high-Umit thermostat, usually about 70 F, the maximum outdoor-air damper is closed, and the. retum-air damper is opened. The minimum quantity of outdoor air is assured by the separate minimum outdoor-air damper. Both outdoor-air dampers are closed and the retum-air damper opened whenever the fan motor is stopped. By the PRIMARY SOURCE APPARATUS <j~ TERMINAL * EQUIPMENT DISTRIBUTION SYSTEM fig. 13...-. Three- General Divisions of > Control Applications- fig.- 14.... Manual Adjustment of Outdoor-Air Quantity > With Low-limit Control of Mixture Temperature Automatic .Control' 183 Rg 15 . i. All-Season Ventilation Control Wilh Fixed Minimum Outdoor Air Plus Additional Outdoor Air when Practicable 0f a thermostat in the conditioned space or in the re tum-air duct, tiie opening of the maximum outdoor-air damper can be to-only.the desired minimum quantity of out door air, until the space heating requirements are satisfied. This feature ib desirable for quick warm-up during winter heating.. ... Preheater Coil .Control The function of a preheater is to temper the outdoor air to prevent the possibility of freezing beyond the coil. Care must be taken to avoid freezing in the coil itself. The proper installation, and sizing of steam traps are very important to assure rapid elimination of condensate to'pre vent freezing. Other important features are vaccum breakers to insure condensate removal, provisions for rapid air elimi nation from coil, the use of vertical rather than horizontal tubes wherever possible. ' Fig. 16 shows a ample application of an outdoor thermo stat, usually set'for 35 F, controlling the preheat coil valve,.. Careful sizing of the coil is very important with this type-of control. The temperature rise-through the coil should be limited to a maximum of 30 deg when the entering air is 35 F. A higher temperature rise will very likely cause overheating of the space. If more than a 30-deg temperature-rise is needed, multiple coils should be used and their control valves operated in sequence at selected outdoor temperatures. - Fig. 17 offers a method of control-which-reduces the overheating problem, outlined for Fig. -16. .Here, a mixedair thermostat operates m outdoor-air bypass damper in conjunction-with-the exhaust- and retum-air dampers. The use of- bypassed outdoor air makes it possible - to control the mixed-air-temperature without ,endangering -the pre heat coil. A minimum quantity of outdoor air is assured by use of a two-position face damper on the coil. Steam-distributing type coils can be controlled by a proportional-type thermostat set for 55 F or higher, and located downstream? from the coil to operate" a valve in the steam supply line. Where a temperature rise of more t-hfm 30 deg is required, two or more coils in series should be used, each having a separate valve. This arrangement is shown in Fig. 18. The valves should be arranged to operate in sequence, with the valve on the upstream coil opening first. Hot water preheat coils should be controlled by face- and-bypass dampers operated by a thermostat downstream from the coil, with continuous water circulation when the outdoor air is below freezing. A pressure switch to stop-the fan in ^ of pump failure is often employed. Further freeze -protection can be accomplished by a thermostat in the coil outlet, arranged to stop the fan if the water temperature drops to 35 F. In all applications where face and bypass dampers are used, precautionary measures must be teke" to prevent stratification- of- the mixture beyond the coil. One method is to locate the bypass damper, above the face damper, so that the cooler bypassed air will tend to drop while the warmer air through the face damper tends to rise, thereby mixing the air by gravity.. . Heating Coil Control TTpftting coils, when used for tempering air for ventila tion, can be controlled by a.proportional type of- insertion thermostat, preferably located in the fan discharge where the air is usually least stratified. This thermostat, operates the heating coil valve or valves, or face and bypass dampers, to maintain a constant discharge air temperature.- When face and bypass dampers are used, it is good practice to have a valve on the coil which closes as the face damper closes to prevent overheating due to damper leakage or heat picked up by air wiping the exposed coil face. Fig. lO-sbows a typical control arrangement for a heat ing coil when the fan system is used for heating as well as