Document MGoMa1aL2aegrJbEEe09dzny

416 CHAPTER 29 1959 Guide 7fcnrcn;eters end Pr@Mwrs Cgqss Pressure gages should be installed in the pump discharge and suction and at locations where pressure readings will assist operation and maintenance. Thermometers (prefer ably dial type) or thermometer wells should be installed in the flow and return pipes, the pump suction and discharge, and at any other points of major temperature change or where temperatures are of importance in operating the sys tem. Thermometers and gages are desirable in the piping at the entrance to each building, hot water converter, or steam converter. (See Chapter 44.) SPACE HEATING EQUIPMENT In industrial areas, space heating equipment may often be operated with high-temperature water due to its avail ability. Convectors and radiators may require water tem peratures in the low and medium temperature range (13) to 180 F or 200 to 250 F) depending upon how close they are to the occupants and upon the pressure for which they are designed.'The water velocity through the heating equipment affects its capacity. This must be taken into account in se lecting the equipment because, if a large water-temperature drop is used, the rate of circulation is reduced and conse quently the velocity of flow may be reduced sufficiently to lower, appreciably the rate of heat transfer. (See Chapter 5.) TEMPERATURE CONTROL . The principles'discussed under Control of Flow in Chapter 43 apply to high-temperature water systems, but added con sideration should be given to the selection and location of control components. Valve Selection Valve selection and sizing are very important because of the relatively high temperature drops and smaller flows en countered in high-temperature water systems. To obtain Table 2....Head Loss In Fittings* (For ferbafeaf far) Fe*f of Soiao Srt Pip* Having Eqvnftoa* fBfhgt 10 8 6 s 4 3 2X 2 1% 1% HM Globe Valve.................. ........................................... Gate Valve................................................................ 90* Welding Elbow.............................................. 45* Welding Elbow............................................... 340 270 200 170 135 105 83 7 5.75 4.5 3.5 3 2.2 1.7 11 8.5 6.5 5.5 4.5 3.5 2.6 76 4.5 3.6 3 2.4 1.7 70 1.5 2.2 1.5 55 1.2 1.8 1.2 45 1 1.5 1 36 0.8 1.2 0.8 28 0.6 0.9 0.6 21 0.5 0.7 0.5 22%* Welding Elbow.......................................... Lateral6...................................................................... Lateral*...................................................................... Welding Tee........................................................... 54 26 20 17 14 13 10 3 2.6 2 16 13 11 1.6 1.3 1.1 0.9 0.7 0.6 0.5 0.3 8 6.5 5.5 4 3.5 2.8 2.2 1.6 10 8.5 7 5 4 3.5 2.8 2.5 1.8 1.5 1.1 8 7 6.6 4 3.2 2.6 2 1.7 1.4 1.2 0.8 Concentric Front. N. m10 . .6 6 S A 3 2 X H 1 H M 10 8 6 5 4 3 2% 2* .IX 1% 1 X % 1 2.5 3.5 1.2 10 3.5 12 7.5 1.5 12 9.6 4.5 10 7 7.5 3 2 0.5 1.5 5 6 6 a,.E fL A1IGIH 2.8 2.2 1.3 0.5 1.8 3.8 5 5 2.2 1.8 1.2 0.6 0.7 2.5 3.8 4 1.7 1.4 1.2 0 0.8 2.8 3 3 C ONT RAC TIN 1.5 1.4 1.5 0.8 1 1 0 0.7 0.8 0.75 0 0.6 0.6 0.6 9 0 0.5 0.5 2 0.3 \ 0 0 2.5 1.25 0.5 \ 0 2.5 2 1.3 0.5 2 1.8 1.5 0.8 0.5 0.3 0.3 0 * Compiled from dote published by Tub* JStnw, Ioe* * Division at tbe National Cylinder G&a Co. Used' by b Water deflected throotb branch. Water flowing straight throogh fit ting. 4 Per eccentric reducers, the vetoes read for eaneentrie reducer* most be increased by 33%%. * Eadivalent length of pipe having a Fanning (rictus coefficient of OJXM. High-Temperature Water Systems 417 rood control, the valve must be sized so that it is effective over its full range of stem travel. It is recommended that the valve be selected on the basis of a pressure drop at full flow equal to approximately 50 percent of the pressure dif ference between supply and return mains at the location of the equipment served. The pressure drop across the valve must not be sufficient to-result in a downstream pressure below the saturation pressure at the temperature existing at any point or flashing into steam will result. A valve with equal percentage flow characteristics is desirable. Sometimes two all valves are used rather than one large valve to improve the controllability of the system. Stainless steel trim is recommended, and all valve body materials and packing should be suitable for the high temperatures and pressures encountered. -The valve should have a close-off rating at least equal to the maximum head produced by the circu lating pump. Generally, two-way valves are more desirable tfhnn the three-way type because of the equal percentage flow characteristics and the smaller capacities available in two-way valves. Single-seated valves are recommended in Fig. 14....Methods of Venting High-Temperature Water . Mains preference to double-seated valves since the latter do not provide tight dosing. Valve Location Control valves preferably should be located in the return ling* from heat transfer units in order to reduce the valve operating temperature and to prevent the high-temperature water from flashing to steam at the lower discharge pressure. Sometimes valves are located 8S low as practicable to in crease the static head at the valve and further minimize the possibility of flashing. Typical Applications A typical application is the control of the temperature of water leaving a heat exchanger in which high-temperature water is used as the heating medium. The temperature measuring element should be located where it can best detect changes arid thus prevent overheating of the outlet water anH possible flashing. When the measuring dement is lo cated in the leaving water, there must be a continuous flow through the exchanger and past the dement. If the water leaving the exchanger is used for space heating, the set point of the thermostat in this water (which controls the temperature of the leaving water by controlling the inlet temperature to the exchanger) can be readjusted in accord ance with outdoor temperature. Another typical application is the control of a low- or medium-pressure steam generator, usually less than 50 psdg, using high-temperature water as the source of heat. In this application a proportional pressure controller in the steam generator supply positions a high-temperature water valve located in the return line to maintain the desired steam pres sure. High-temperature water is' sometimes used to supply coils in tur handling units where hpAting surfaces are so placed that possibility of personal contact is avoided. Care ful sizing of coil and valve is required to obtain stable con trol. In some cases face and bypass damper control is used,