Document kDK96Vzjaowp3DVgXEvjY6nNE

638 CHAPTER 43 1960 Guide (PROPORTIONAL) . U-------- ! 7 Damper Control From Mixed Air overheating problem outlined for Fig. 13. Here a mixed* air thermostat operates an outdoor-air bypass damper in conjunction with the exhaust- and return-air dampers. The use of bypassed outdoor air makes it possible to control tiie 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 supply line. Where a temperature rise of more than 30 deg is required, two or more coils in series should be having a separate valve. This arrangement is shown in Fig. 15.' 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 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 tftirftn to prevent stratification of the mixture beyond the coil. One method is to locate the bypas 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 Heating coils when used for tempering air for ventila tion can be controlled by a proportional type of insertion Fig. 16___ Control of Heating Coil From Space and Discharge Air Using Valve 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 T""nt-ain a constant discharge air temperature. When face and bypass dampers are used it is good practice to have on toe coil a valve 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. 16 shows a typical control arrangement for a heat ing coil when the fan system is used for beating as well as for ventilation. Here a room or a return-sir thermostat controls the heating coil valve until toe space temperature is satisfied. A discharge duct thermostat then assumes con trol of the valve to iYift.inta.in the desired minimum air dis charge temperature. With this system no portion of toe air entering toe coil should be less than 32 F. Fig. 17 shows an arrangement using face and bypass dampers on the hnating coil and two-position control of the valve. It should be noted that without a low-limit controller toe air entering temperature must be the desired minimum discharge air temperature. Fig. 18 offers a typical water coil control application. The room or the return-air duct thermostat operates the three-way miring valve until the space temperature is satisfied. The discharge duct thermostat then takes over the valve control to maintain the desired minimum air discharge temperature. Cooling and Dehumidificah'on Coil Controls Dehumidification by condensation is so closely related to sensible cooling that it is convenient to discuss them together. Three typical applications of controls are discussed. Direct-Expansion Systems. A thermostat measuring the space temperature may be used to open and close a solenoid refrigerant valve on the coil inlet; control the starting Automatic Control 639 Discharge Air Using Mixing Valve and stopping of one or more refrigerating compressors, or otherwise control the compressor capacity; position face and bypass dampers, or a return-air bypass damper, in com bination with the opening and closing of the refrigerant coil valve or the starting and stopping of toe compressor; or control a back pressure regulating valve with proportional action. The positive opening and closing of the refrigerant valve or the starting and stopping of toe refrigeration compressor will result in wide variations in discharge tem perature and dehumidification. When such variations are objectionable, it is desirable to employ proportional damper controls or proportional control of refrigerant flow in con junction with refrigeration equipment capacity controls as discussed in Chapter 38. Chilled-Water and Brine Cods. A thermostat measuring the space temperature may be used to control a valve on the inlet or outlet of the cooling coil; control a three-way mix ing valve; position face and bypass dampers at toe coil; or position a return-air bypass damper. An apparatus-dew point thermostat may also be used to control the cooling coil valve. When cooling capacity is controlled only by a dry-bulb thermostat the amount of dehumidification will vary with the sensible cooling demand. When independent dehumidification is required it is necessary to employ some form of reheat. fig. 19 shows a ample arrangement where chilled water is supplied to the coil at a constant temperature and toe water volume through the coil is varied. The space or return-air thermostat modulates the three-way valve to increase or decrease the volume of water through the coil. Although this arrangement provides constant flow through the chiller, it should be noted that throttling of chilled water to the coil causes higher coil temperatures with light 2-POSITION UM.VCI----- SUPPU- - RETURN Fig. 20.... Control of Chilled Water Coil From Space Temperature Using Return Bypass Damper and Valve sensible cooling loads which may result in unsatisfactory humidity conditions. Fig. 20 shows a control system tiring two-position control of a cold water coil valve and modulating control of a return-air bypass damper. Here the cooled air through the coil is mixed with bypassed return air to satisfy the space thermostat demands. When the return-air bypass-damper is fully opened the water valve can be closed. It is often desirable to omit the valve control and instead maintain a constant flow of chilled water through the coil. This provides some dehumidification with light sensible cooling loads. Air Washers. Since the air leaving an efficient washer is practically saturated, the temperature of the spray water will determine the dew-point temperature of the air leav ing toe washer. With this condition it is passible to maintain very accurate dew-point temperatures and consequently accurate relative humidities under a given ratio of sensibfe-to-latent heat gain in the conditioned space, provided the space dry-bulb temperature is accurately controlled. In general, the water pump operates continuously and a thermo stat measuring the dew-point temperature is used to position a three-way valve to mix refrigerated water with bypassed washer water. To compensate for variations in latent load, a humidistat measuring toe space relative humidity may be employed as a master controller to readjust the set point of a dew-point submaster controller. Humidity Controls Fig. 21 shows a simple method of humidity control em ploying a room or. a return-air duct humidistat to control a valve supplying steam to an evaporator pan type hu midifier. Additional control provides for closing the valve Fig. 15.... Proportional Control of Two Preheat Coils in Fig. 17.... Control of Heating Coil From Space Temperature Sequence Using Valve and Face and Bypass Dampers