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454 CHAPTER 23 , 1948 'Guide FCAST9iRnNYR^^CHr^IOf T toTn! LESS Than 12 Sections Long FIS- Typical Connections, to Gast Iron Blast Heaters More Than 12 Sections Long F.HvtnC?NECnoNS TO Finned Tuse Blast Heaters with Low Pressure Steam Fig- 42. Typical Return Connection , From Finned Tube Blast Heaters with High Pressure Steam steam connection. For stacks of large capacity it is sometimes desirable to run a separate steam main direct from the boiler to the stacks. CONTROL VALVES Gate valves are recommended in all cases where service demands that the valve be either entirely open or entirely closed, but they should never be used for throttling. Angle globe valves and straight globe valves should be used for throttling, as done on by-passes around pressure reducing valves or on by-passes around traps. REFERENCES $?*Bhicl^haw^CA.^IIACE^^rRANSACTl^s.^or/sO.^Dl^p0 S~ of a^nd " ValUe 0f preS3ure drops- foTM,y are noTerp^ in fract (A.I.HAvaE. Ti^o^. Ra,i"8 Re,Ur" Une Low Vacuam Hea,in* Chapter 24 HOT WATER HEATING SYSTEMS AND PIPING Available Head; Friction Loss; Classification; System Design; Examples of Piping Design; One-Pipe Gravity, One-Pipe Forced Circulation, TtcorPipe Gravity, and Two-Pipe Forced Circulation Systems, - Expansion Tanks, Installation Details, Zoning Systems AHEATING system is called a hot water system if water is used to convey heat by flowing through pipes connecting a boiler or water heater with radiators, convectors or other suitable heat dispensing means. There are two types: the gravity system in which the water flows by virtue of thermo-syphon action, and the forced system in which a pump, usually driven by an electric motor, sometimes by a steam turbine or other, means, maintains the necessary flow. Most panel heating systems (see Chapter 31) fall into the category .of forced hot water systems, and the design procedures pertaining to pipe sizing and.friction contained in this chapter are largely applicable- to such systems. ' Historically; the gravity system is much the older, and many such systems have been in satisfactory operation for several decades. Oper ation depends on the difference in density of the water due to. difference in temperature in the flow and return pipes. The available head is therefore limited and the pipes must be ample in size to permit adequate flow of water. In the forced system, the pipes; valves and fittings can be much smaller, with a resultant saving in the cost of installation, since the available head is limited only by consideration of economy in pumping the water. With the forced system, higher boiler temperatures and automatic control of the pump or circulator make possible the use of indirect water heaters with hot water systems when that is desirable. (See Chapter 50). AVAILABLE CIRCULATION HEAD The available head in a gravity circulation system may be found from the equation: Aa = 144 X 2 31 X 12,000 t 0) where Aa = available head per foot of height, milinches (1 milinch = 1/1000 of 1 in. of water). pi = average density of flow water, pounds per cubic foot, pi = average density of return, pounds per cubic foot. 144 = square inches per square foot. 2.31 = height of water column equivalent to 1 psi, feet. 12,000 - milinches equivalent of 1 ft of water column. The available head may also be found from Fig. 1. For example, at a flow temperature of 200 F and a 35 deg drop, and with the mains located 4 ft above the top of the boiler, a head of 600 milinches is obtained. This is found by following the 200 F flow riser line in Fig. 1 to where it inter sects the 165 F return riser line and reading horizontally a head of 150 milinches per foot or 600 milinches for 4 ft. If the first floor radiators are located 3 ft above the mains, second' floor radiators 12 ft above the mains, third floor 21 ft, and fourth floor 30 ft, the heads are 450, 1800, 3150, and 4500 milinches respectively. 455