Document jymMxpQ80awn1v2RjZNek5E72

HEATiNC VENTILATINO AIR CONDITIONING GUIDE 1944 the gas injects the primary air into the burners and the resistance to secondary air flow to burners is slight. Most gas-fired boilers carry the approval of the American Gas Associa tion as evidenced by display of its Laboratories Approval Seal. In order to obtain this approval the boilers must comply with American Standard Approval Requirements for Central Heating Gas Appliances sponsored by the A.G.A. The boiler ratings must be such that they meet the limitations as set forth in these approval requirements. HOT WATER SUPPLY BOILERS Boilers for hot water supply are classified as direct, if the water heated passes through the boiler, and as indirect, if the water heated does not come in contact with the water or steam in the boiler. Direct heaters are built to operate at the pressures found in city supply mains and are tested at pressures from 200 to 300 lb per square inch. The life of direct heaters depends almost entirely on the scale-making properties of the water supplied. If water temperatures are maintained below 140 F the life of the heater will be much longer than if higher temperatures are used, owing to decreased scale formation and minimized corrosion below 140 F. Direct water heaters in some cases are designed to burn refuse and garbage. Indirect heaters generally consist of steam boilers in connection with heat exchangers of the coil or tube types which transmit the heat from the steam to the water. This type of installation has the following advantages: 1. The boiler operates at low pressure. 2. The boiler is protected from scale and corrosion. ------- 3. The scale is formed in the heat exchanger in which the parts to which the scale is attached can be cleaned or replaced. The accumulation of scale does not affect efficiency although it will affect the capacity of the heat exchanger. 4. Discoloration of water may be prevented if the water-supply comes in contact with only non-ferrous metal. Where a steam heating system is installed, the domestic hot water usually-, is obtained from an indirect heater placed below the water line of the; boiler. Indirect heaters may also be used with hot water heating' systems to obtain domestic-hot water and should be located as:high; as' possible with respect to the boiler for most satisfactory performance. FURNACE, DESIGN A Good efficiency and proper boiler performance are dependent on correct furnace design embodying sufficient volume for burning the' particular fuel at hand, which requires thorough mixing of air and gases at a high temperature with a velocity low enough to permit complete combustion of all the volatiles. (See Chapters 8 and 10.) Heating Surface Boiler heating surface is that portion of the surface of the heat transfer apparatus in contact with the fluid being heated on one side and the gas or refractory being cooled on the other side. Heating surface on which the 240 CHAPTER 12. HEATING BOILERS fire shines is known as direct or radiant surface and that in contact with hot gases only, as indirect or convection surface. The amount of heating surface, its distribution and the temperatures on either side thereof influence the capacity of any boiler. Direct heating surface is more valuable than indirect per square foot because it is subjected to a higher temperature and also, in the case of solid fuel, because it is in position to receive the full radiant energy of the fuel bed. The heat transfer capacity of a radiant heating surface may be as high as 6 to 8 times that of an indirect surface. This is one of the reasons why the water legs of some boilers have been extended, especially in the case of stoker firing where the extra amount of combustion chamber secured by an extension of the water legs is important. For the same reason, care should be exercised in building a refractory combustion chamber in an oil-burning boiler so as not to screen any more of this valuable surface with refractories than is necessary for good combustion. The effectiveness of the heating surface depends on its cleanliness, its location in the boiler, and the shape of the gas passages. The area of the gas passages must not be so small as to cause excessive resistance to the flow of gases where natural draft is employed. Inserting baffles so that the heating surface is arranged in series with respect to the gas flow increases boiler efficiency and reduces stack temperature and increases the draft loss through the boiler. Heat Transfer Rates Practical rates of heat transfer in heating boilers will average about 3300 Btu per square foot per hour for hand-fired boilers and 4000 Btu per square foot per hour for mechanically fired boilers when operating at design load. When operating at maximum load as defined in this chapter under heading Selection of Boilers, these values will run between 5000 and 6000 Btu per square foot per hour. ' Boilers operating under favorable conditions at the above heat transfer rates will give exit gas temperatures that" are considered consistent with good practice. TESTING AND RATING CODES The Society has adopted four solid fuel testing codes, a solid fuel rating code and an oil fuel testing code. A.S.H.V.E. Standard and Short Form Heat Balance Codes for Testing Low-Pressure Steam Heating Solid Fuel Boilers--Codes 1 and 2--(Revision of June 1929)1, are intended to provide a method for conducting and reporting tests to determine heat efficiency and performance characteristics. A.S.H.V.E. PerformanceTest Code for Steam Heating Solid Fuel Boilers--Code No. 3--(Edition of 1929)1 is intended for use with A.S.H.V.E. Code for Rating Steam Heating Solid Fuel Hand-Fired Boilers2. The object of this test code is to specify the tests to be conducted and to provide a method for conducting and reporting tests to determine the efficiencies and performance of the boiler. The A.S.H.V.E. Standard Code for Testing Steam Heating Boilers 1See A.S.H;V.E. Transactions, Vol. 35, 1929, pp. 322 and 332. *See A.S.H.V.E. Transactions, Vol. 36,'1930, p. 42.