Document 6od5djzzqN8R5OgRdZj20aZ4

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 so as to admit steam to the heating coil whenever heat is required so as to maintain a fixed temperature in the space. Combination controller 7\ also measures the relative humidity in the conditioned space and opens control valve Vi so as to admit water to the sprays whenever moisture is required in the space. 2. During the Cooling Cycle. Combination controller Ti measures the temperature and humidity in the conditioned space and opens refrigerant control valve V3, thereby admitting refrigerant to the cooling coil when ever cooling is required to maintain the temperature or relative humidity within predetermined maximum limits. The temperature control point of controller T\ must be set at a lower point than that of controller T* in order to provide for the automatic change-over between the cooling and heating cycles. As an example, controller 7\ might be set at 72 F and 35 per cent and T* at 76 F and 60 per cent. As the room conditions rise above the settings of 7\, the I-------------------------- Fig. 7. All Year Air Conditioning Unit with Complete Automatic Control heat and humidification are shut off and when they rise above the settings of Tj the cooling and dehumidification are turned on. CONTROL OF AUTOMATIC FUEL APPLIANCES It is essential that automatic controls be used with oil burners, gas burners, and stokers in order to maintain even temperatures and provide safe and economical operation of the heating plant. There are many types of burners and many types of automatic control, and it' is essential that the proper type of control equipment be selected to fulfill the require ments of the burner equipment and its application. Combustion regulation equipment should be used on the larger com mercial and industrial applications to control the secondary air supply and thereby provide for economical operation. This type of control will usually consist of a pressure regulator which measures and controls the pressure over the fire and which thereby indirectly regulates the carbon dioxide percentage in the flue gas. On all automatically-fired steam boilers it is advisable to provide 666 CHAPTER 34. AUTOMATIC CONTROL control equipment which will stop the supply of fuel in case the boiler water line falls below a predetermined level of safety. For hot water and warm air systems, control devices can be arranged to vary the water and air temperatures from outdoor thermostats. As the weather moderates, lower temperatures are maintained. Inside thermostats are usually installed to correct any improper results from the outside controls. Thermostats used to control automatic fuel appliances may be supplied with clock mechanisms which will automatically shut off the heat or maintain lower temperatures during night hours for economy of fuel. For buildings that are not used every day of the week, clocks may be supplied to provide night conditions from Saturday noon or night to Monday morning. Oil Burner Controls In the normal oil burner installation as encountered in residential and small commercial installations, the burner operation is frequently regu lated by electric controls and primarily governed by a room thermostat. It is essential that a limiting control be incorporated in the control system to prevent the temperature of the heating medium from exceeding any predetermined safe maximum. The type of limit control selected will depend on the type of the heating system. In a warm air furnace instal lation, a limit control would be used, reacting to the temperature of the heated air in the bonnet of the furnace; in a hot water system a control reacting to the temperature of the water in the boiler; and in a steam system a control reacting to the pressure of the steam in the boiler. In addition to the normal control of the burner from the room ther mostat and limit control, it is necessary that a combustion safety device be used to prevent operation of the burner under hazardous conditions! The oil fire is automatically ignited by means of gas, electric spark or incandescent element and the combustion safety control acting through a sequence device permits the burner operation only when the fire is prop erly established as the burner starts up. A further function of the com bustion safety control is to react to any major disturbance in the flame during the running operation, shutting down the burner and preventing the discharge of unburned fuel if for any reason the flame is extinguished. Gas Burner Controls In the case of the domestic burner, full automatic operation is the normal requirement and the burner is started and stopped at the com mand of a room thermostat which, in turn, opens and closes a control valve in the gas supply line. Modulating controls and controls providing a high and low fire are also available for gas burners. For purposes of preventing abnormally high temperatures in the bonnet of gas-fired furnaces or in the temperature of the water in gas-fired hot water heating boilers or.excessive pressures in gas-fired steam boilers, temperature and pressure limit controls are used. Ignition is normally secured through the use of a gas pilot flame and a safety device is provided, utilizing the heat of the pilot flame in such a manner that if the pilot light is extinguished for any reason, the main gas valve cannot be opened. For satisfactory 667