Document jBpR715gYMDQx0E1ODod9r015

338 CHAPTER 17 1948 - Guide The burners are-Jocated in the base, at the bottom of an enclosed com bustion chamber. The products of combustion pass around baffles within the heating element, and out the flue at the back neat the top. They are well adapted for residence room heating and also for stores arid offices. Radiant heaters give off a considerable portion of their heat in the forin of radiant energy emitted by an iricaridescent refractory that is heated by a Bunsen flame. They are made in numerous shapes and designs and in sizes ranging from two to fourteen or more radiants. An atmospheric burner is supported near the center of the base. Others have a group of small atmospheric burners supported on a manifold attached to the base. Most radiant heaters are portable; however, there are also types which are encased in a jacket that fits into the wall with a grilled front.. Gas-fired steam and hot water radiators are other types of room heating appliances. They are made in a large variety of shapes and sizes and are similar in appearance to the ordinary steam or hot water radiator. A separate combustion chamber is provided in the base of each radiator and is usually fitted with a one-piecc 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 steam or hot water radiators. They are usually constructed of sheet metal hollow sections. The products of combustion circulate through the sections and are discharged from a flue or into the room, depending upon whether the radiator is of the vented or unvented type. Unit heaters are used extensively for heating large, spaces such as stores, garages, and factories. These heaters consist of a burner, heat exchanger, fan for distributing the air; draft hood, thermostatic pilot, and controls for burners and fan. They are usually mounted in an elevated position from which the heated air is directed downward by louvers. Some unit heaters are suspended from the ceiling, and others are free-standing floor units of the heat tower type. Unit heaters are available in two types, classified according to their use, with, or without ducts. Only those types of unit heaters tested and approved as warm air furnaces can be connected safely to ducts, as they have sufficient blower capacity to deliver an adequate air supply agairist duct resistance and are equipped with limit controls. Combustion Process and Adjustments Most domestic gas burners are of the atmospheric injection (Bunsen) type in which primary air is introduced,, and mixed with the gas in the throat of the mixing tpbe. A ratio of about 3 parts primary air to 1 part gas for manufactured gas, and a 5 to 1 ratio for natural gas, are generally used as theoretical values; The amount of excess air required in practice depends upon several factors, notably: uniformity of air distribution and mixing, direction of gas travel from burner, and the height and tem perature of cotnbustion chamber. : Secondary air is drawn into gas appliances by natural draft. As with other fuels, excess secondary air constitutes a loss, arid should be reduced to a proper minimum, which usually cannot be less than 25 to 35 per cent if the appliance is to meet A.S.A. approval. Yellow flame burners depend upon secondary air, alone, for combustion. The flame produced by atmospheric injection burners is non-luminous. Air.shutter adjustments for manufactured gas should be made by closing Automatic Fuel Burning Equipment 339 the air shutter until yellow flame tips appear arid then by opening the air shutter to a final position at which the yellow tips just disappear. This type of flame obtains ready ignition from port to port and also favors quiet flame extinction. When burning natural gas the air adjustment is generally made to secure as blue a flame as obtainable. Little difficulty should be hadin maintaining efficient combustion when burning gas. The fuel supply is normally held to close limits of variation in pressure and calorific value and the rate of heat supply is noininally constant. Because the force necessary to introduce the fuel into the combustion chamber is an inherent factor of the fuel, no draft lay the chimney is required for this purpose. The use of a draft hood insures the maintenance of constant low draft:condition in the combustion chamber with a resultant stability of air supply. A draft hood is-also helpful in controlling the amount of excess air and preventing back drafts that might extinguish the flame. (See Chapter 16.) Due to the use of draft hoods and gas pressure regulators both the input and combustion conditions of gas appliances are rriaintained quite uniform until deposits of dirt, corrosion, or scale accumulate, in the air inlet openings, burner ports, or on the heating surface. Periodic cleaning is necessary to keep any gas appliance in proper operating condition-. Measurement of the Efficiency of Combustion The efficiency of combustion may be judged from the percentage of carbon dioxide (C02), oxygen (02) and carbon monoxide (CO) in the flue gases. The C02 and 0 may be obtained by means of an Orsat apparatus but the CO must be determined by more accurate equipment. It is customary to use simple indicators to determine whether CO is present and to make adjustments of the appliances to reduce the CO below 4/100 of one per cent before continuing tests in which the C02 and 02 can then be found by use of the Orsat apparatus. Since the ultimate CO% for any gas depends on the total hydrogen content the quality of the combustion should not be judged from the value of the CO% in the flue gas without reference to the ultimate C02 obtainable. Practical values of C02 will usually be from 8 to 14 per cent depending on the gas used. Ratings for Gas Appliances Input rating for a gas appliance is established by demonstrating that the appliance can meet the Approval Requirements of the A.S.A. . The tests are conducted at the A.G.A. Testing Laboratories. Output rating is determined from the approved input and an average efficiency stated in the Approval Requirements and is the heat available at the outlet. Sizing Gas-Fired Heating Plants Although gas-burning equipment usually is completely automatic, maintaining the temperature of-rooms at a predetermined figure, there are some manually controlled installations. In order to overcome effectively 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 radiation which it is expected to serve. Boilers under thermostatic control, however, are not subject to such severe pick-up loads and consequently, it is possible to use a lower selec tion factor. For a gas-fired boiler or furnace under thermostatic control a factor of 20 to 25 per cent is usually sufficient for pick-up allowance.