Document zQqXBwrje4Yq3Z3EvnRyd9mr6

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 natural circulation of the water. The design temperature difference is usually assumed from between 20 to 35 F. After having determined the temperature difference and the temperature of the flow water, data given in Fig. 4 can be used to obtain the pressure head. With information' obtained concerning the required pressure head the same procedure is CHAPTER 16. HOT WATER HEATINC SYSTEMS AND PIPINC and reading horizontally a pressure head of 150 milinches per foot or 600 milinches for 4 ft. Assuming first floor radiators are located 3 ft,above the mains and second floor radiators 12 ft above the mains, third floor,21`ft and fourth floor 30 ft, the circulating pressure heads are 450, 1800, 3150 and 4500 milinches respectively. FRICTION HEAD IN MlONCHES PEB FOOT OF PIPE Fig. 3. Friction Heads in Black Iron Pipes for a 20 F Temperature Difference of the Water in the Flow and Return Lines For other temperature drops the pipe capacities may be changed correspondingly. For example, with ' a temperature drop of 30 F, the capacities shown in this figure are to be multiplied by 1.5., .. followed for conlputing the necessary data for a gravity circulation system as was previously outlined for a forced hot water system. Radiator heat emission rates from 150 to 200 Btu per square foot are commonly used so that flow temperatures generally range from 180 to 200 F or higher. Assuming a flow temperature of 200 F and a 35 F drop, and with the mains located'4 ft above .the top of the.boiler, a circulating pressure head of 600 milinches results. This is obtained by following the 200 F i floor riser line in Fig. 4 to where it. intersects the 165 F return riser line , ' .314 Fig. 4. Gravity Pressure Heads for Various Temperature Differences EXPANSION TANKS Expansion tanks may be either of the open or closed type. In the open type, (see Fig. 5) the water is subject to atmospheric pressure only, but in the closed tank (see Fig. 6) the system is under pressure and, therefore, a relief valve should be placed on the tank. Water expands about 4 per cent when, being heated from 40 F to 200 F, and the expansion tank should have a volume about twice the actual expansion or about 8 per cent of the total volume of water in the entire system including boiler, radiators, pipes, etc. . Open expansion tanks should be at least 3 ft above the 315