Document g5nJENnKw7G2Bjo9qLQRgjmN

848 CHAPTER 38 1952 Guide - 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 ctulled 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 dehumidified 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 temperature. 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 desired moisture content. Proportioning control of the water and two-position control of the steam are recommended 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 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 used. It is impossible to state in detail the control ap paratus 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. 5. Control of zone temperatures. 6. Year 'round air conditioning with automatic change-over. 7. Constant temperature and humidity. 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. Automatic Control 849 Usually 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 dampers 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 m the entrance to the chamber between the groups of coils) controls the pre..heaters: but it is essential that the preheater coils be of the steam distributing type. .Similarly, a duct.thermostat in the fan discharge, where any effect of stratification has been dissipated, operates the valves and dampers associated with the reheaters. In some cases, where there are more than one bank of preheaters, the practice is to place a freeze protection thermostat in the outdoor air mtake to controf 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 con trolling 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 con trol, in conjunction with a thermostat in the heated space or in the return air. Fig. 1. Control Diagram fob Year .'Rounb Air Conditioning System 2. Humidity control may be obtained by means of a hygrostat, usually located in the conditioned space or in the return air. Such controlling instruments may operate steam supply valves to humidifiers, mixing valves to control the temperature of the water io 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, ac cording to dew-point temperature, is sometimes accomplished by means of a thermo stat in the outlet of the humidifying chamber. However, the setting of the dew-point thermostat 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 positions are regulated by proper types of thermostats, located in the minimum outdoor air intake, the fan discharge, the conditioned space or the return air. In struments are available which, with an adequate arrangement of dampers, will cause a maximum quantity of outdoor air to be handled until it becomes more eco nomical to utilize return air. 4. Cooling and dehumidifying may be controlled by means of thermostats, and hygrostats or dew-point thermostats, which regulate dampers and mixing valves to maintain air of the: proper temperature and humidity in the discharge from the central fan plant. Such controlling instruments normally, are located in the fan