Document jBzmKVL3NDaVgdw5jEKKG5wvO

HEATINC VENTILATiNC AIR CONDITIONING CUIDE 1944 For instantaneous submerged heaters the surface required will depend upon (1) the velocity of water in the tubes (2) the boiler water tem perature (3) the inlet water temperature (4) the outlet water temperature (5). the cleanliness of the coil surface and (6) the condition of the boiler water surrounding the coil. If the heater is located in the water of an actively steaming part of a boiler the heat transfer may be twice as great as would be obtained if the water surrounding the coil were circulating slowly. Ratings of instantaneous water heating coils will therefore vary greatly depending upon the assumptions made regarding the conditions of operation. .The values of the coefficient of heat transmission for instantaneous heaters shown in Table 3 are conservative. Table 3. Coefficient of Heat Transfer of Instantaneous Water Heaters k = Btu per hr per sqft per degree Fahrenheit logarithmic mean temperature difference Boiler Water Temperature k 210 225 200 175 180 150 For a coil in which heat is transferred from steam to water the value of k = 300 Vv may be safely used, (u = velocity of water in feet per second). The rate of heat transfer between steam or water as the carrier and the domestic water is influenced by the rate of movement of both the carrier and the water which receives the heat. For this reason, where the transfer occurs from heating system water to domestic water, it is good practice to install a circulating pump to insure rapid movement of the boiler water. In view of the high condensation rates obtained when steam is used with 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 any section not connected to the indirect heater and further the unconnected sections will not deliver any heat to the water 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. - W (tt -- tj) x 100 HXEXC (2) 808 CHAPTER 46. HOT WATER SUPPLY where C = grate area, square feet. W = weight of water, pounds per hour. /2--tt = 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 per 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 5. What grate area is required for a coal-burning 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: 100 X 8.3 X (180 - 50) X 100 3.2 sq ft. 12,500 X 60 X 4.5 The quantity of gas, oil, or other fuel required per hour for water heating may be calculated by Equation 3. F=wiu-t,)xim (3) where ti X * F -- units of fuel (lb, cu ft, gal, etc). H = heating value of fuel, Btu per unit. W -- weight of .water, pounds per hour. la --1\ = temperature difference between entering and leaving water, degrees Fahrenheit, E -- efficiency, per cent. 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 Coal-fired boilers are usually controlled by. an.immersion thermostat located in the heated water, which opens or closes draft dampers at the 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 burns 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 vqlve 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 ah arrangement, to 809