Document JJQ8dVR62KddnX8kD9D1xzdV6

138 CHAPT 9 1962 Guide And Data Book --J/ 200 250 300 350 400 450 500 rcMPCRAnnc -r Rg. 2 .... Relation of Saturation Pressure and Enthalpy to Water Temperature at s given minimum supply temperature continuously, while space heating may permit modulating the temperature as a function of the outdoor temperature or other climatic influences. 2. Temperature which must be obtained in the terminal unit. 3. Distance from heating plant to space or process requiring beat. 4. Terrain of land on which buildings are located. 5. Zoning requirements based on occupancy and load distri bution. 6. Quantity and pressure of steam used for power equipment in central plant, if desired. 7. Variations in elevation within the system. These may make it desirable to locate the central station on the highest ground in order to maintain a maximum static pressure head on the system. Theoretically, water temperatures up to about 350 F may be provided using equipment suitable for 125 psig, but prac tically, maximum water temperatures will be limited by the system design, pump heads, and elevation characteristics to values between 300 to 325 F. Water temperatures of 350 F to 420 F, requiring 250 to 300 psig working pressure, permit a greater temperature drop to be realized, and are employed where the savings resulting from the reduced pipe sizes offset the added cost of heavier fittings and equipment, or where the terminal units require the higher temperatures. Most systems designed for steam pressurization have a steam drum through which the entire flow is taken, and which also serves as expansion vessel A circulating pump is in the supply line taking water from the tank. The temperature of the water from the steam drum cannot exceed the tempera ture of the steam in the drum which corresponds to its pres sure at saturation. The point of maximum pressure is at the discharge of the circulating pump. If, for example, this pres sure 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 dram and the circulating pump. The steam drum (expansion tank) be placed above the high-temperatore water generators. Many systems are designed for inert gas pressurization. In most of these systems, the pressurizing tank is connected to the system by a angle balance line on the suction ride of the circulating pump. The circulating pump is located at the inUj side of the high-temperature water generator. There is qq flow through the pressurizing tank, and a reduced temperature will normally establish itself inside. This tank may be placed wherever is most suitable in the central station. A special char acteristic of the gas-pressurized systems is the apparatus for creating and maintaining gas pressure inside the tank. Of primary importance in the design and operation of the high-temperature water system is the requirement of main taining at all points of the system a pressure which at aD times, whether or not the system is in operation, exceeds the vapor pressure of the water. This may require limiting the water temperature, and thereby the vapor pressure. Elevation effects, and the pressures required to prevent the flashing of water into steam in the supply system, can also limit the maximum water temperature that may be used, and must therefore be studied in evaluating the temperaturepressure relationships of the system.1 The properties of water which govern the features of the design are: 1. Temperature versus pressure at saturation. (See Fig. 2.) 2. Density or specific volume versus temperature. 3. Ehithafpy or n*nihle heat versus temperature. 4. Viscosity versus temperature. The relation of temperature and pressure, specific voluas and enthalpy, are all available in the steam tables. The proper ties of water for the range 212 to 400 F are summarized in Table 1. Direct-fired High-Temperature Water Generators Where the high-temperature water generators are directfired using conventional fuels, the central stations are coxn- Tabfe 1 ....Properties of Water (212 F to 400 F) TeapMvfvT* PreostfB* F Poo Dsmcfy Uj/Cb H Specific fW 6fv/(Lb)(f) ratal Hwif Abo** 32 F Btw/Lb* Bta/Ceft 212 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 14.70 17.19 20.78 24.97 29.83 35.43 41.86 49.20 57.56 67.01 77.68 89.66 103.06 118.01 134.63 153.04 173.37: 195.77 220.37 247.31 59.81 59.63 59.38 59.10 58.82 58.51 58.24 57.94 57.64 57.31 56.98 56.66 56.31 55.96 55.59 55.22 54.85 54.47 54.05 53.65 1.007 1.009 1.010 1.012 1.015 1.017 1.020 1.022 1.025 1.032 1.035 1.040 1.042 1.047 1.052 1.057 1.062 1.070 1.077 1.085 180.07 188.13 198.23 208.34 218.48 10,770 11,216 11,770 12,313 12,851 228.64 238.84 249.06 259.31 269.59 13,378 13,910 14,430 14,947 15,450 279.92 290.28 300.68 311.13 321.63 15,950 16,437 16,931 17,409 17,879 332.ik 342.79 353.45 364.17 374.97 18,343 18,802 19,253 19,681 20,117 * Beprinted by penma a frotn rScrMdpiwat*e Frvpotacf of Slcewi, byi. BKwwiin and F. C. Keye* p bliabed by Jobs WUey and Saba, Inc., 1938 edition. High-Temperature Water Systems 139 to steam boiler plants operating within the same Trr range. The generators should be selected for size and Sjein keeping with the load and design pressures, as well as he circulation requirements peculiar to high-temperature tar There are many systems in existence in which both fOT power or processing and high-temperature water ^resupplied from the same boiler; others in which steam is Produced in.boilers and used for generating high-temperature Ureter; and again others where the burning fuel directly heats waterthe high-temperature water generators may be of the water-tube or fire-tube type, and may be equipped with any conventional fuel firing apparatus. The water-tube type may have either forced circulation or gravity circulation, or a com bination of both. The recirculating pumps of forced-circulation generators must operate continuously while the generator is frpjng fifed. Steam boilers relying on natural circulation may require internal baffling when used for high-temperature water geoeration. In Scotch Marine type boilers the problem of thfrnml shock caused by a sudden drop in the temperature of the return water impinging on the fire tubes must be con sidered. Forced circulation high-temperature water generators are generally of the once-through type and rely solely on pnmpq to achieve the circulation. Depending on the design they may require the use of internal orifices in the various circuits to apportion the water flow rates in proportion to thehttt absorption rates. Circulation must be maintained at all tirnpg while the generator is being fired, and the flow rate pmst never be allowed to drop below a minimum indicated by the manufacturer. Where gravity circulation type steam boilers are used for high-temperature water generation, the steam dram usually serves as expansion vessel.1 In forced circulation type hightemperature water generators, a separate vessel into which all generators discharge is commonly used for maintaining the steam pressure cushion and for expansion. A separate vessel is usually used when the system is cushioned by an inert gas. Proper internal circulation is essential in all types of boilers to prevent tube failures due to over-heating or unequal ex pansion. Pressurization The method whereby the pressure is maintained above satu ration should take into consideration the following factors: 1. The pressurization system should be relatively simple and incorporate a high degree of reliability. T^na of pressurization results in flashing withm the system, which can lead to problems within pumps, generators, heat exchange devices, and the piping system. 2. The pressurization system should maintain the generally Rg. 4 .... inert Gas Pressurization for Two-Pump Type System corrosion-free characteristics of hot water heating. Oxygen should be excluded from the system, both in the means of pressurization and in the operating cycle. This precludes the use of air as a pres surizing "Winn Cycles which periodically withdraw and reintro duce water should not be used. 3. The control of the pressurization system should maintain the inherently stable conditions of the high-temperature water cycle. Fluctuations in system pressure or temperature should be minimized by taking advantage of the thermal storage of the cycle. This will improve combustion efficiency through relatively steadyfiring rates. 4. The location of the compression tank used for pressuriza tion has a great effect on the net positive suction bead (NPSH) at the pumps and may limit the allowable friction losses of the piping system. Flexibility in the location of this tank is an advan tage in wdapiing a HTW system to a given problem. 5. Proper safety devices for high and low water levels and ex cessive pressures should be incorporated in the compression system and interlocked with combustion safety and water flow rate controls. There are a number of fundamental methods whereby pres sure in a given hydraulic system may be kept at a desired level1,4 1. An elevated storage tank is a ample means of pressuriza tion, but due to the great heights required for the pressures en countered, this system is generally impractical 2. A hydraulic pump may be used. However, difficulties, in maintaining constant preraure-temperature relationship within the system, plus the withdrawal and reintroduction of system water make Aia system generally impractical. When pressurising with a feed pump automatically operated by pressure control, a relief valve lor the maximum system pressure is provided (see' Figs. 3 and 4).a The pressure control is set to operate the feed pump and circulate water from the make-up tank to the system whenever the pressure falls to the regulator cut-in point, and to stop the pump when the cutrout point is reached. If the pressure increases to the relief valve setting, excess water will flow through the relief valve to the make-up water tank. Since this system is not entirely closed, the miww for preventing the entry of cor rosion-causing elements, principally oxygen, or for their elimina tion, should be considered. 3. Steam pressurization through the U9e of an expansion vessel which is separate from the HTW generator. The vessel must be above the HTW generators and connected in the supply water line from the generator. Water in the expansion vessel flashes to steam to rrmintain system pressure above the saturation point of the cooler water in the balance of the system. The circulating pump moves the water from the expansion vessel to the system andback to the generator. Since the system pump draws water from the bottom of the vessel, the vessel must be elevated to increase the net positive suction head to prevent cavitation or fluhing in the pump suction. This arrangement is critical. To in the prevention of flashing, a bypass is provided from the HTW system return line to the pump suction. Cooler return water is then mixed with hotter water from the expansion vessel to give a resulting temperature below the corresponding satura tion point in the vessel.