Document ekzM90YQw672KMgydoQqNL84

HEATING VENTILATING AIR CONDITIONING CUIDE 1940 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. 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, operating without refractories. Many conversion units are equipped with sheet metal secondary air ducts which are inserted through the ashpit 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 when not in operation. With this duct the air necessary for proper combustion is supplied directly to the burner, thereby making it possible to reduce the excess air passing through the combustion 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. Combustion Process and Adjustments Because of the varying composition of gases used for domestic heating it is difficult to generalize on the subject of gas burner combustion. Little difficulty should be experienced in maintaining efficient com bustion conditions when burning gas. The fuel supply is normally held to close limits of variation in pressure and calorific value and, therefore, the rate of heat supply is nominally constant. Since the force necessary to introduce the fuel into the combustion chamber is an inherent factor of the fuel, no draft by the chimney is required for this purpose. The use of a draft diverter insures the maintenance of constant low draft condition in the combustion chamber with a resultant stability of air supply. A draft diverter is also helpful in controlling the amount of excess air and pre: venting back drafts which might extinguish the flame. (See Chapter 9). Measurement of the Efficiency of Combustion It is possible to determine the results of combustion by analyzing the gases of combustion with an Orsat apparatus. It is desirable to determine the percentage of carbon dioxide (C02), oxygen (02) and carbon monoxide (CO) in the flue gases. While ultimate C02 values of 10 to 12 per cent may be obtained from the combustion of gases commonly used for domestic heating, a combustion adjustment which will show from 8 to 10 per cent COt represents a practical value. Under normal conditions no CO will be produced by a gas-fired boiler or furnace. Limitations as to output rating 'by the A.G.A. are based upon operation with not more than 0.04 per cent CO in the products of combustion. This is too small an amount to be determined by the ordinary flue gas analyzer. Controls Gas burner controls may be divided into two parts: (a) devices to regulate burner operation so that the desired house heating results may be obtained, and (6) devices for the safety of the boiler and burner. Control devices are treated in detail in Chapter 38. A room thermostat may be used as a control of house heating effect. These may be obtained in a 220 CHAPTER 11. AUTOMATIC FUEL BURNING EQUIPMENT number of types. Some central heating plants are equipped with pushbutton.or manual control. Class (b) controls include a device to shut off the burner if the gas fails to ignite, a device actuated by boiler pressure, water temperature, or furnace bonnet temperature to shut off the burner when the pressure or temperature becomes excessive, a device on the boiler to shut off the.burner if the water level falls below safe limits or one which automatically feeds additional water to the boiler, and a device for controlling the gas pressure, within desired limits. The main gas valve may be either of the snap action or throttling type. Sizing Cas-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 equiva lent 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 much lower selection, or safety factor. A gas-fired boiler under thermo static control is sensitive to variations in room temperatures so that in most cases a factor of 20 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. Liberal selection factors to be added to the installed steam radiation under thermostatic control are given in Fig. 1 of Chapter 13. . 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 iii the publi cations of that associatiorf. 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 Asso ciation Testing .Laboratory and upon an average efficiency which has been assigned by that association. In the case of boilers, the rating can be put in terms of square feet of equivalent direct radiation by dividing it by 240 for steam, and 150 for water. This gives what is called the American Gas Association rating, and is the manner in which all appliances approved, by the American Gas Association Laboratory are rated. To use these ratings it is only necessary 221