Document omayRDpgRY6wOKVzq7RYK36a3

American Society of Heating and Ventilating Engineers Guide, 1936 similar in appearance to the ordinary steam or hot water radiator con nected to a basement boiler. A separate combustion chamber is provided in the base of each radiator and is usually fitted with a one-piece burner. They may be secured in either the vented or unvented types, and with steam pressure, thermostatic or room temperature controls. Warm air radiators are similar in appearance to the steam or hot water radiators. They are usually constructed of pressed steel or sheet metal hollow sections. The hot products of combustion circulate through the sections and are discharged out a flue or into the room, depending upon whether the radiator is of the vented or unvented type. Garage heaters are usually similar in construction to the cabinet circulator space heaters, except that safety screens are provided over all openings into the combustion chamber to prevent any possibility of explosion from gasoline fumes or other gases which, might be ignited by an open flame. They are usually provided with automatic room tem perature controls and are well suited for heating either residence or commercial garages. Conversion Burners Residence heating with gas through the use of conversion burners in stalled in coal-designed boilers and furnaces represents a common type of gas-fired house heating system, especially in natural gas territories. In many conversion burners radiants or refractories are employed to convert some of the energy in the gas to radiant heat. Others are of the blast type with luminous flames, operating without refractories. In each case an attempt is made to transfer the majority of the heat from the gas to the medium to be heated within the fire pot itself because of the low heat transfer that takes place in the flue passages. '' Many conversion units are equipped with sheet metal secondary air ducts which are inserted through the ash-pit door. The duct is equipped with automatic air controls which open when the burners are operating and close when the gas supply is turned off. This prevents a large part of the circulation of cold air through the combustion space of the ap pliance vjjien not in operation. By means of this duct the aiV necessary for proper combustion is supplied directly to the burner, thereby making it possible to reduce the amount of excess air passing through the com bustion chamber.' , Conversion units are made in many sizes both round and rectangular to"' fit different types and makes of boilers and furnaces. They may be secured with manual, push button, or room temperature control. Sizing Gas-Fired Heating Plants While gas-burning equipment can be and usually is so installed as to be completely automatic, maintaining the temperature of rooms at a pre determined and set figure, there are in use installations which are manually controlled. Experience has shown that in order to effectively overcome the starting load and losses in piping, a manually-controlled gas boiler should have an output as much as 100 per cent greater than the equivalent standard cast-iron column radiation which it is expected to serve. Boilers under thermostatic control, however, are not subject to such severe pick-up or starting loads. Consequently, it is possible to use a 504 28Chapter --Automatic Fuel Burning Equipment much lower selection, or safety, factor, : A gas-fired boiler under ther mostatic control is so sensitive to variations in room temperatures that in most cases a factor of 25 per cent is sufficient for pick-up load. . The factor to be allowed for loss of heat from piping, however, must vary somewhat, the proportionate amount of piping installed being con siderably greater for small installations than for large ones. Consequently, a selection factor for thermostatically controlled boilers must be variable. Table'2 gives liberal selection factors to be added to the installed steam radiation under thermostatic control. They have been established by experience and are recommended by the American Gas Association. The same factors may be used in determining the gas demand for which conversion burners installed in steam or hot water boilers should be set. , Multiplying the equivalent direct heating surface (radiation): by 240 and adding the appropriate percentage from Table 2, and then dividing by the heat value of the gas and by the heating efficiency (see discussion of Table 2. Selection Factors for Gas Boilers Cast-Iron Steam Radiation (Equivalent Square Feet) Selection Factor (Per Cent) 500 800 1,200 1,600 2,000 3,000 4,000 and over 56.0 54.0 51.0 48.0 45.0 42.5 40.0 heating efficiencies in Chapter 29), gives the proper hourly rate of gas consumption. However, inadequate boiler heating surface for gas burning, often encountered in coal-designed boilers converted to gas, may necessitate operation at a lesser demand, resulting in much slower pick-up and less margin of safety for piping loss. Appliances used for heating with gas should bear the approval seal of the American Gas Association Testing Laboratory. Installations should be made in accordance with the recommendations shown in the publica tions of that association. : Ratings for Gas Appliances Since a gas appliance has a heat-generating capacity that can be pre dicted accurately to within 1 or 2 per cent, and since this capacity is not affected by such things as condition of fuel bed and soot accumulation, makers of these appliances have an opportunity to rate their product in exact terms. Consequently all makers give their product an hourly Btu output rating. This is the amount of heat that is available at the outlet of a boiler in the form of steam or hot water, or at the bonnet of the furnace in the form of warm air. The output rating is in turn based upon the Btu input rating which has been approved by the American Gas Association Testing Laboratory and upon an average efficiency which has been assigned by that association.