Document wD6Xw2MZ9EJX67jK3G0o3drgQ

422 CHAPTER 29 1960 Guide TEMPERATURE -F fig. 3___ Relation of Saturation Pressure and Enthalpy to Water Temperature perature control. Many are designed for inert gas pres surization with the expansion drum on the suction side of the system circulating pump. In the steam pressurization system, the temperature of the water from the steam drum cannot exceed the temperature of the steam in the drum which cor- Toble 1 ....Properties of Water (212 F to 400 F) Temperatore F Ptia Detail/ Lb/Cit Ft Specific Heal Bfu/tlbHF) Total Heat Above 32 F 8hi/lb* Bhi/Co Ft 212 14.70 59.81 1.007 180.07 10,770 220 17.19 59.63 1.009 188.13 11,216 230 20.78 59.38 1.010 198.23 11,770 240 24.97 59.10 1.012 208.34 12,313 250 29.83 58.82 1.015 218.48 12,851 260 35.43 58.51 1.017 228.64 13,378 270 41.86 58.24 1.020 238.84 13,910 280 49.20 57.94 1.022 249.06 14,430 290 57.56 57.64 1.025 259.31 14,947 300 67.01 57.31 1.032 269.59 15,450 310 77.68 56.98 1.035 279.92 15,950 320 89.66 56.66 1.040 290.28 16,437 330 103.06 56.31 1.042 300.68 16,931 340 118.01 55.96 1.047 311.13 17,409 350 134.63 55.59 1.052 321.63 17,879 360 153.04 55.22 1.057 332.18 18,343 370 173.37 54.85 1.062 342.79 18,802 380 195.77 54.47 1.070 353.45 19,252 390 220.37 54.05 1.077 364.17 19,681 400 247.31 53.65 1.085 374.97 20,117 * Reprinted by persuasion from TJurmodjnamic PiopcrtU* cf Steam. by J. H. Kamnan ftad p. G. Keyes published by John Wiley ftod Sons, Inc., 1934 edition. responds to its pressure at saturation. The point of maximum pressure is at the discharge of the circulating pump. If this pressure is to be maintained below 125 psig, the pressure in the drum, corresponding to the water temperature, cannot exceed 125 psig minus the sum of the pump head and the pressure head due to the difference in elevation between the drum and the circulating pump. Elevation effects and the pressures required to prevent the flashing of water to steam in the supply system can also High-Temperature Water Systems 423 limit the maximum water temperature that may be used and must, therefore, be studied in evaluating the tempera ture-pressure relationships of the system. The properties of water which govern the features of the design are: 1. Temperature vs pressure at saturation (see Fig. 3). 2. Density or specific volume (o.) vs temperature. 3. Enthalpy or sensible heat (At) vs temperature. 4. Viscosity vs temperature. The relation of temperature and pressure, specific vol ume, and enthalpy are all discussed in Chapter 3. Data for the normal temperature-pressure ranges are given in Table 3 of Chapter 3. The properties of water for the range 212 F to 400 F are shown in Table 1. Water temperatures of 350 to 400 F requiring 250 psig working pressure will largely be limited to district and in stitutional installations. In such applications the heat quan tities and distances to be covered usually justify the added cost of the heavier pipe, valves, and equipment. Boilers The boiler plants of the larger high-temperature water systems are comparable to boiler plants of power generating stations operating within the same pressure range. The boil ers should be selected for size and type in keeping with the load and design pressures. Both steam for power or process ing and high-temperature water for heating may be supplied from the same boiler plant. The load is determined by the heat required for: 1. Space heating and winter air conditioning. 2. Generated steam required for power. 3. Water heating or heat processes. The boilers may be water-tube, horizontal return-tube or' Scotch marine type, and may be equipped with any conven tional fuel-firing apparatus. Water-tube boilers are favored for higher pressure ranges--200 psig and upward. Boilers may be of either forced circulation or gravity circulation type. The circulating pump of forced circulation boilers must operate continuously during operation of the boiler. Water- tube boilers frequently require separate vessels for the pressure cushions, depending on the volume of the steam drum relative to system requirements; When systems using fire-tube boilers are cushioned by steam, expansion space may be available within the boiler.1 A separate vessel usually is used when the system is cushioned by air or other gas. Proper distribution of return water and of water flow is essential in all types of boilers to prevent tube failures due to overheating or unequal expansion in boiler tubes. Expansion Pressure Cushion Four methods are used to produce and maintain the neces sary pressures on the system: 1. A steam cushion in the steam space of the boiler or a steam drum. 2. A separate expansion tank for steam presurizing. 3. Compression tank with coxnpremed air or an inert gas. 4. An automatic pressure pump.** ** * A steam cushion or a pneumatic cushion is generally fa vored because .it may also accommodate the expansion. The steam cushion method for maintaining pressure above the water level in the boiler requires space to accommodate the expansion of the water volume within the system when heated to the system operating temperature. Systems which do not use steam for maintaining the required pressure, or in which the boiler steam space is inadequate to accommodate the change in water volume, will require additional tanks or steam drums for expansion. The arrangement of the tanka for pneumatic cushioning is similar to the closed expansion tanka when inert gases are used for cushioning or pressurising. A suitable means of balancing or a connection from the bottom of the tank should be made to the main system return header. A major problem when pressurizing by steam is to main tain a proper boiler water level while providing for the change in water volume due to expansion and contraction. Boiler water level and flow connections should be designed to utilize the steam drum in adjusting to changes in water