Document G62XQXBMnpr78JjjqLKEj2ZJV

American Society of Heating and Ventilating Engineers Guide, 1929 appliance designed for gas, when gas is to be the fuel, perhaps apply even more strongly to furnaces than they do to boilers. Codes for the proportion of warm-air heating plants, such as that formulated by the National Warm Air Heating Association (see Chapter VIII), are equally applicable to gas furnaces and coal furnaces. Re circulation should always be practiced with gas-fired warm-air furnaces. It not only aids in heating, but is essential to economy. Where fans are used in connection with warm-air furnaces for residence heating, it is well to have the control of the fan and the gas so coordinated that there will be sufficient delay between the turning on of the gas and the starting of the fan, to prevent blasts of cold air being blown into the heated rooms. SPACE HEATERS In residence heating work, the space heater is most commonly used in the form of a radiant heater. Radiant heaters make admirable auxiliary heating appliances to be used during the occasional cool days at the beginning and end of the heating season; when heat is wanted in some particular room for an hour or two. The radiant heater gives off its heat in the form of radiant energy emitted by an incandescent refractory that is heated by a Bunsen flame. Radiant heaters, and all other types of space heaters as well, are preferably flue connected and should never be operated in unvented rooms, particularly bedrooms. Luminous flame reflector heaters diffuse a large part of their heat by virtue of. the radiating power of the bright yellow flame. They are usually backed by a polished copper sheet which reflects radiant heat into the room. Convection type heaters are typified by the gas-fired steam radiator, the gas-fired tubular heater (built in radiator form) and the floor furnace. The radiator types of gas-fired heaters provide an economical form, of heating apparatus for intermittently heated spaces, such as stores, small churches, and some types of offices and apartments. Since they give off the greater portion of their heat by convection they do not produce a hot spot within the room and are therefore capable of being operated under thermostatic control. The floor furnace is a convection type heater that is hung underneath the floor. It is generally installed with a duplex register having a warm-air outlet and a cold-air return, so that complete recirculation is secured. COMBUSTION OF GAS Most gas appliances consume the gas with a blue or Bunsen flame, although some room heaters use a luminous or yellow flame. Bunsen flames and luminous flames differ in the way in which the air necessary for combustion is supplied. Bunsen type burners are provided with an external mixer, in which a portion of the air is mixed with the gas previous to ignition. This is called primary air. The aspirating effect of the jet of the mixture issuing from the burner ports, and the chimney effect of the gas passages above, draw currents of air past the flame and into it, in sufficient quantity to cause complete combustion. This is known as 236 Chapter XIV--Heating with Gas secondary air. In order for combustion to be complete and for the products to be free of carbon monoxide, it is necessary that heating sur-. faces be sufficiently high above the burner to prevent the pale blue inner cone, which is a characteristic part of a Bunsen flame, from striking any cold heating surfaces. A yellow tip on the flame indicates insufficient primary air and is corrected by opening the adjustable air shutters. Some room heaters which consume gas in very small amounts use the luminous flame. A luminous flame is limited in its application to appliances burning small amounts of gas on account of the increased furnace volume required for combustion. No part of a luminous flame can be allowed to touch any cold surface, as this will result in incomplete combustion with the formation of soot and carbon monoxide. Table 1 shows the heat values of a cubic foot of three common varieties of gas, together with the theoretical air requirements and the air require ments with 50 per cent excess. In practice excess air must be admitted to the fire in order to insure complete combustion. Table 1. Volume of Air Required for Combustion of Different Gases Gas Mixed Coke Oven and Water Gas..... B.t.u. pm Cubic Foot 1131 560 537 Theoretical Air per Foot op Gas 10.70 C. F. 4.78 C. F. 5.03 C. F. Actual Am with 50 Peb Cent Excess 16.05 C. F. 7.17 C. F. 7.55 C. F. In designing and installing gas-burning installations, particularly where large amounts of gas are to be consumed, it is important that provision be made for the easy access of sufficient air to support combustion and to provide boiler room ventilation. Very little motive force exists to draw air into the boiler room. Therefore, the opening through which it enters must be large enough to keep the velocity to a very low value. It is well to see that appliances have the A. G. A. approval seal and installation should be made in accordance with the recommendations of A. G. A. Bulletin on Requirements for House Piping and Appliance Installations. CHIMNEYS See Chapter VI and the material relating to gas appliance chimneys contained therein. HEAT VALUE AND EFFICIENCY When gas is burned a large amount of water vapor is produced as one of the products of combustion. This ordinarily escapes up the chimney, carrying away with it a certain amount of heat. However, when the heat value of gas is determined in an ordinary calorimeter, this water vapor is condensed and the latent heat of vaporization that is given up during the condensation is reported as a portion of the heat value of the gas. The heat value so determined is termed the "Higher Heat Value" and this is what is ordinarily meant when the heat value of gas is specified. 237