Document yM70N7wejqwxnn5pn8J0eNyX

American Society of Heating and Ventilating Engineers Guide, 1937 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 control equipment which will stop the burner operation in case the boiler water line falls below a predetermined level of safety. Thermostats used to control automatic fuel appliances may be provided with clock mechanisms which will operate to maintain lower temperatures during night hours for economy of fuel. 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 dowi\ 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. For purposes of preventing abnormally high temperatures in the bonnet of gas fired furnaces or in the temperature of 290 Chapter 14--Automatic Control jjje water in gas fired hot water heating boilers or excessive pressures in eas 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 and economical operation, all automatically fired gas burners should be equipped with pressure regu lators on the gas supply line. Stoker Controls Domestic stokers are normally placed under command of a room thermostat for primary operation subject also to the command of a limit control to prevent their operation when conditions in the boiler or furnace exceed predetermined safe maximums. Utilizing coal as fuel, automatic ignition is not provided and the stokers, once ignited, maintain their fire, merely changing the rate of combustion by changing the draft and the rate at which the coal is fed. Thus, at the command of the room ther mostat the stoker motor is started, driving a forced draft fan and fuel feeding mechanism. The rate of combustion is thus increased and this operation continues until the thermostat has been satisfied when the motor is stopped and the fuel in the combustion chamber continues to burn at a slow rate with reduced draft. At certain seasons of the year, the operation of the stoker under the requirements of the thermostat may be so infrequent that there is a possibility of the fuel in the combustion chamber burning out or the fire going out between operations. To prevent this occurrence, automatic controls may be utilized to operate the stoker independently of ther mostat requirements, sufficiently to sustain the fire either through a timing device functioning for short periods at predetermined intervals or through a temperature control device reacting to minimum stack or boiler temperatures. Control may also be utilized to prevent stoker operation and the delivery of coal into the combustion chamber in the event that the fire has gone completely out. This control is- governed normally by the stack temperature and shuts down the stoker after a predetermined minimum stack temperature is reached. RESIDENTIAL CONTROL SYSTEMS The control installation in a residence may vary from the simple regulation of a coal-fired heating plant to the completely automatic all year air conditioning system. Residential installations with automatic fuel burning appliances, such as oil burners, gas burners or stokers, are normally equipped with single room thermostat, limit and safety controls as outlined above under Control of Automatic Fuel Appliances. Coal-Fired Heating Plant Control in the normal coal-fired domestic heating plant consists of regulating the combustion rate in accordance with requirements. This function is accomplished by a spring or electric-driven damper motor which under the command of a room thermostat and through chain linkage, operates the draft and check dampers of a boiler or warm air 291 I i /,