Document 7OjQqrBvRwK8eRRmEExJgvYrR

,814 CHAPTER 38 1950 Guide desired moisture content. Proportioning control of thei.water andtwo-position control of the steam are recoriuriended to maintain constant temperature. . 2. Control by varying the solution temperature so that the moisture content of ' the discharge air remains constant regardless of load variation. Basically, this control is similar to the constant solution temperature control with the addition of a hygrostat or wet-bulb' controller controlling the water valve from space conditions. In order to secure a constant discharge dry-bulb temperature, a coil is provided with proportioning valve controlled by a proportioning thermo stat in the unit discharge, CONTROL FOR CENTRAL FAN SYSTEMS Automatic temperature control for central fan heating, cooling, ventilat ing and air conditioning systems involves the proper application of various types of controlling instruments and associated regulators such as valves, dampers and damper operators, .relays and other auxiliary equipment Automatic'Control 815 The apparatus which enters into the automatic maintenance of tempera tures and humidities for central.fan,systems which are designed to produce each of the effects listed in items 1 to 7, is indicated in the following para graphs : . 1. In heating control, there are three considerations to be borne in mind: (1) to control .space temperature, (2) to prevent drafts, (3) to guard against freezing. IJsually in central fan systems, suitable thermostats operate valves in the steam or hot water supply to heating coils or face dampers across such coils and by-pass damp ers around them. If the heating coils are sub-divided into two or more groups, such as preheaters and reheaters, a duct thermostat following the preheaters, and located in the entrance to the chamber between the groups of coils, controls the preheaters;' but it is essential that the preheater coils be of the steam distributing type. Simi larly, a duct thermostat in the fan discharge, where any effect of stratification has - been dissipated, operates the valves arid dampers associated with the reheaters. In soine cases, where there are more than one bank of preheaters, the practice is to place TWBMOSTAT HYGROSTAT .... TO . otsTRIBUTION Fio; I. Control Diagram fob Year 'Round Air Conditioning System .which are' described in earlier sections of this chapter. . In central fan systems the conditions-required dictate the type of built-up control system to be used.. Otherwise, some, arrangement of available package equipment probably would-be-used1. For that reason, it is impossible-to state in detail the control apparatus which will be required in even the most representative applications.' ,. In general, in so far as automatic temperature and humidity control equipment is concerned, central fan systems may be divided into certain broad classifications, as follows: . ; .1. Heating .. 2. Humidifying 3.- Ventilating and atmospheric cooling 4. Cooling and dehumidifying 5i`Control of zone temperatures 6. Year 'round air conditioning with automatic changerOver ", ; 7. Constant temperature and humidity. ; , ... r a freeze protection thermostat in the outdoor air intake to control the valve on the first bank of heaters, which is designed so that the heat-rise through it will not cause overheating. --A room thermostat in the heated space may serve as-the controlling instrument, with a thermostat in the fan discharge serving to-prevent the delivery of air at a temperature: which might cause drafts. If desired,-similar action may be obtained from,a thermostat in the return air connection. When there is only one heating coil, a limit thermostat in the fan discharge accomplishes the control, in conjunction with a thermostat in the heated space or in the return air. . 2. Humidity control may be obtained by means of a hygrostat, usually located in the conditioned space or in the return air. Such controlling instruriients may operate steam supply valves to humidifiers, mixing valves to control the temperature of the water to sprays, or a system of dampers to regulate the quantity of air passed through the humidifying chamber or by-passed around it. The control of humidity according to dew-point temperature is sometimes accomplished by means of a thermostat in the outlet of the humidifying chairiber. However; the setting of the dew-point thermo stat may have to be changed as the humidity in the conditioned space varies. 3. The control of ventilating and cooling'by the use of outdoor:air consists of an arrangement of dampers, usually determining the relative quantities of outdoor air and return air which are to be delivered to the conditioned space. The damper posi tions are regulated by proper types of thermostats, located in the miriimum outdoor