Document MG4rQRoY9R666LMEOoEm9yVDx

Heating Ventilating Air Conditioning Guide 1938 flues, as (1) horizontal return tubular (HRT) boilers, (2) portable (selfcontained) firebox boilers with either water or fire tubes, and (3) water tube boilers of the power type. Fire tube boilers are constructed so that the water available to produce steam is contained in comparatively large bodies distributed outside of the boiler tubes, the hot gases passing within the tubes. In water lube boilers, the water is circulated within the boiler tubes, heat being applied ex ternally to them. The-HRT boiler is the oldest type and consists of a horizontal cylin drical shell with fire tubes, enclosed in brickwork to form the furnace and Table 1. Practical Combustion Rates for Small Coal-Fired Heating Boilers Operating on Natural Draft of from 14 in. to in. Water3 Kind op Coal No. 1 Buckwheat Anthracite Anthracite Pea Anthracite Nut and Larger Bituminous Sq Ft Grate Up to 4 5 to 9 10 to 14 15 to 19 20 to 25 Up to 9 10` to 19 20 to 25 Up to 4 5 to 9 10 to 14 15 to 19 20 to 25 Up to 4 5 to 14 15 and above Lb op Coal per Sq Ft Grate per Hour 3 3'A 4 m 5 5 5H 6 8 9 10 11 13 9.5 12 15.5 aSteel boilers usually have higher combustion rates for grate areas exceeding 15 sq ft than those indicated in this table. combustion chamber. All heating surfaces and the interior of the boiler are accessible for both cleaning and inspection. Horizontal return tubular boilers, especially the larger sizes, should be^uspended from structural columns and beams independent of the brick setting. Small HRT boilers sometimes are supported by brackets resting on the brick setting. Portablefirebox boilers are the more generally used type of steel heating boilers, their outstanding characteristic being the water-jacketed firebox which eliminates virtually all brickwork. They are shipped in one piece from the factory and'come to the job ready for immediate hook-up to jpiping. They may be of welded or riveted construction and have either water or fire tubes. Manufacturers' catalogs usually list heating surface as well as grate area. The elimination of brickwork also makes this type the most compact of steel boilers as well as the lowest in first cost. Water tube boilers. For large heating loads water tube boilers are quite frequently used. They usually require more head room than other types of. boilers but require considerably less floor space and make possible a 244 Chapter 13. .Heating Boilers much higher, rate of evaporation per square foot of heating surface, with proper setting, baffling and draft. Water`tube boilers, used for heating purposes are either completely supported, insplattd and, encased in steel, or else brick set, supported on structural steel columns and have the brick setting encased in an insulated steel housingjto prevent air infiltration and to minimize heat losses. For large heating loads at a,high rate of evapora tion,"such boilers should be operated at pressures above 15 lb per square inch with a pressure-reducing valve on the.connection to the heating main. SPECIAL HEATING BOILERS A special, type, of boiler, known as the magazine feed boiler, has been developed for the burning of small sizes of anthracite and coke. These are built of both cast-iron and steel, and have a large fuel carrying capacity which results in longer firing petiods than would be the case with the standard types using buckwheat sizes ofcoal. Special attention must be given to insure adequate draft and proper chimney sizes and connections. Oil-burner boiler units, in which a special boiler has been designed with a furnace shaped to meet the general requirements qf oil burners or are specially adapted to one particular burner have been developed by a number of manufacturers. These usually are. compact units with the burner and all controls enclosed within an insulated steel jacket.' Ample furnace, volume is provided for efficient combustion, and the heating surfaces are proportioned for effective heat transfer. Consequently, higher efficiencies are obtainable than with the ordinary coal fired boiler converted to oil firing. GAS-FIRED BOILERS Gas boilers have assumed a well-defined individuality. The usual boiler is sectional in construction with a number of independent burners placed beneath the sections. In most boilers each section has its own burner. In all cases the sections are placed quite closely together, much closer than would be possible when burning a soot-forming fuel. The effort of the designer is always to break the hot gas up into thin streams, so that all particles, of the heat-carrying gases can come as close as possible to the heat-absorbing surfaces. Because there is no fuel bed resistance and because the gas company supplies the motive power to draw in the air necessary for combustion (in the form of the initial gas pressure), draft losses through gas boilers are low. See Chapter 11. 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 iF. Direct water heaters in some cases-are designed to burn refuse.and garbage. * 245