Document nkaqbdvqOL7wJm6O1v0Oe0ed8

894 CHAPTER SO 1946 Guide gravity circulation from the boiler, as, when there is a. sudden demand followed by an inflow.of cold water, the bottom of a steam heating trans fer element always should be at least 30. in. above the boiler water line, and the steam and condensate return pipes should be of liberal size. Otherwise water hammer and reduced capacity may result due to im perfect drainage of condensate. When connecting a transfer-type hot water heater below.the water line of a cast-iron steam boiler having vertical sections, there should be a separate tapping for water circulation into every section of the boiler, as shown in Fig. 3, unless the boiler has large top nipple* ports providing inter-sectional circulation. If the top nipples - are entirely within- the boiler steam space, no internal circulation occurs between sections and steaming may occur in sections not connected to the heater and further the unconnected sections will not deliver any heat to the heater. COMPUTING GRATE AREA FOR COAL-FIRED HEATER The grate area required for a small coal-fired water heater may be calculated by Equation 2. where W(h- <i) X 100 HXE X C (2) G = grate area, square feet. W -- weight of water, pounds per hour. ' . ty--ti = temperature difference between entering and leaving water, degrees Fahrenheit. H -- heating value of coal, Btu per pound. C = weight of coal burned, pounds per (hour) (square foot of grate). E = efficiency, per cent. In a small heater 4.5 lb is a conservative value for C, and an efficiency of 60 per cent would represent excellent performance. ^Example 6. What grate area is required for a coal-burnirig water heater warming 100 gal per hour of water from 50 to 180 F*. when the combustion rate is 4.5 lb per hour per square foot of grate; if the heating value of the fuel is 12,500 Btu per pound, and the efficiency is 60 per cent? '. _ Substituting: G = TM X 80j* (18 - ^ X 100 = 3 2 sq ft 12,500 X 60 X 4.5 sq it. The quantity of gas, oil, or. other fuel required per hour for water heating may be calculated by Equation 3. /*. ; '.- where .., W fa - h) X 100 F= , H X. E (3) -F = Units of fuel (lb, cu ft, gal, etc). H = heating value of fuel, Btu per unit. W = weight of water, pounds per hour. G--h - temperature difference between entering and leaving water, degrees Fahrenheit * ... ,E = efficiency, percent.. ......... ; Efficiencies for oil and gas may be taken as 75 and 80 per cent respec-. tively. The heating value of the fuel and the temperature rise should be determined, to suit local conditions. CONTROL OF SERVICE WATER TEMPERATURE s Coal-fired boilers are usually controlled b`y: an immersion thermostat located .in. the heated water, which opens or closes draft, dampers at-the Hot Water-Supply- _____ ~:395 boiler to adjust the rate of fuel, combustion. With oil- or gas-fired boilers the immersion thermostat controls the oil burner motor or the automatic gas valve. The gas pilot flame usually bums continuously. With electric heaters the immersion thermostat operates a switch on the source of energy. When steam or hot water is the medium for heating the water in the tank, immersion thermostat controls a valve in the steam or hot water- supply line. In small residence installations, using water as the carrier, a combined immersion thermostat and butterfly valve all in one simple fitting may be installed in the transmitting circuit to prevent overheating of the service water. In residences heated by pump circulated hot water, the house tempera ture is controlled by operating the circulating pump intermittently, while domestic hot water is warmed by transfer from the house boiler, independent of the pump operation. The domestic water is heated from the heating boiler the year 'round. . Under such an arrangement, to prevent overheating the house by thermal circulation when the pump is not running, it is usual to insert a weighted check-valve in the house heating main, so that no circulation to the house heating system can occur unless the pump operates. In summer the fire may be controlled to maintain a-boiler water temperature lower than when heating and generally about 20 F warmer than that desired in the domestic hot water system. * In buildings which have restaurants it is generally, desirable to install two separate service hot water systems so that water at about 180 F minimum may be available for dish washing, while water at 140 F maxi mum may be' used for lavatory and bath purposes. The immersion thermostat in a hot water storage tank should be located no higher than the center of the tank, and possibly should be even closer to the bottom since water in a tank stratifies proportionally to the tem perature. When hot water is removed, the cold water entering to replace it quickly reduces the temperature in the lower parts of the tank. SOLAR WATER HEATERS Solar heaters utilize the" energy of the sun* for heating hot water. The successful operation of such heaters requires the availability of sunshine practically every day. in the year, which has limited their use to Florida and-the southern'portions of California. When supplemented-with some other means of gas, coal or- oil water heating, solar heaters may be used in climates where sunshine may be more or less intermittent. They have been used in summer homes as far north as Chicago. When properly installed and proportioned, solar water heaters render satisfactory service especially in climates where the outside temperatures are high and. extremely hot water is not. necessarily desirable. Such installations consist essentially of a storage tank, heating coil and hot box. The coil is installed in the hot box and is arranged to circulate water to and from the storage tank. The advantage in the use of this type of. heater is the fact that it requires no fuel. The same materials should be used for the coil, circulation -lines and tank. A copper coil is more efficient in absorbing ' heat in the box but galvanized iron or steel may be substituted depending, on the local water conditions, cost and other considerations. The storage tank must be able to store sufficient heated water for the night period of about 16 hours when the coil is not functioning or is operating under such poor sun conditions as to make its heating effect -'