ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 15-2012 Medium-And High-Temperature Water Heating.pdf

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1、15.1CHAPTER 15MEDIUM- AND HIGH-TEMPERATURE WATER HEATINGSystem Characteristics. 15.1Basic System 15.1Design Considerations. 15.2Distribution Piping Design 15.5Heat Exchangers 15.6Air-Heating Coils. 15.6Space-Heating Equipment 15.6Instrumentation and Controls 15.6Water Treatment . 15.7Heat Storage. 1

2、5.7Safety Considerations. 15.7EDIUM-TEMPERATURE water systems have operatingMtemperatures ranging from 120 to 175C and are designed toa pressure rating of 860 to 1030 kPa (gage). High-temperature watersystems are classified as those operating with supply water temper-atures above 175C and designed t

3、o a pressure rating of 2000 kPa(gage). The usual practical temperature limit is about 230C becauseof pressure limitations on pipe fittings, equipment, and accessories.The rapid pressure rise that occurs as the temperature rises above230C increases cost because components rated for higher pressuresar

4、e required (see Figure 1). The design principles for both medium-and high-temperature systems are basically the same.This chapter presents the general principles and practices thatapply to MTW/HTW and distinguishes them from low-temperaturewater systems operating below 120C. See Chapter 13 for basic

5、design considerations applicable to all hot-water systems.SYSTEM CHARACTERISTICSThe following characteristics distinguish HTW systems fromsteam distribution or low-temperature water systems:The system is a completely closed circuit with supply and returnmains maintained under pressure. There are no

6、heat losses fromflashing from steam condensate, and heat that is not used in theterminal heat transfer equipment is returned to the MTW or HTWgenerator. Tight systems have minimal corrosion.Mechanical equipment that does not control performance of indi-vidual terminal units is concentrated at the ce

7、ntral station.Piping can slope up or down or run at a variety of elevations to suitthe terrain and the architectural and structural requirements with-out provision for trapping at each low point. This may reduce theamount of excavation required and eliminate drip points, accessports, and return pump

8、s required with steam. Manual air ventsmust be provided at all high points in the system.Greater temperature drops are used and less water is circulatedthan in low-temperature water systems.The pressure in any part of the system must always be above thepressure corresponding to the temperature at sa

9、turation in the sys-tem to prevent the water flashing into steam. Pressure at theexpansion/compression tank is customarily 200 kPa (gage) abovethe saturation pressure.Terminal units requiring different water temperatures can beserved at their required temperatures by regulating the flow ofwater, mod

10、ulating water supply temperature, placing some unitsin series, and using heat exchangers or other methods.The high heat content of the water in the HTW circuit acts as athermal flywheel, evening out fluctuations in the load. The heatstorage capacity can be further increased by adding heat storagetan

11、ks or by increasing the temperature in the return mains duringperiods of light load.The high heat content of the heat carrier makes MTW/HTWunsuitable for two-pipe dual-temperature (hot and chilled water)applications and for intermittent operation if rapid start-up andshutdown are desired, unless the

12、 system is designed for minimumwater volume and is operated with rapid response controls.Basic engineering skills are required to design a HTW system thatis simple, yet safer and more convenient to operate than a compa-rable steam system.HTW system design requires careful attention to basic laws ofc

13、hemistry, thermodynamics, and physics because these systemsare less forgiving than standard hydronic systems.BASIC SYSTEMMTW/HTW systems are similar to conventional forced hot-water heating systems. They require a heat source (which can be adirect-fired HTW generator, a steam boiler, or an open or c

14、losedheat exchanger) to heat the water. The expansion of the heated wateris usually taken up in an expansion vessel, which simultaneouslyFig. 1 Relation of Saturation Pressure and Enthalpy to Water Temperature15.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)pressurizes the system. Heat transp

15、ort depends on circulatingpumps. The distribution system is closed, comprising supply andreturn pipes under the same basic pressure. Heat emission at the ter-minal unit is indirect by heat transfer through heat transfer surfacesin devices such as converters (heat exchangers) or steam generators.The

16、basic system is shown in Figure 2.The main differences of MTW/HTW systems from low-temper-ature water systems are the higher pressure, heavier equipment,generally smaller pipe sizes, and manner in which water pressure ismaintained.Most systems use insert gas or pump-pressurized system, inwhich the p

17、ressure is imposed externally.HTW generators and all auxiliaries (such as water makeup andfeed equipment, pressure tanks, and circulating pumps) are usuallylocated in a central station. Cascade MTW/HTW converters use anexisting steam distribution system and are installed remote from thecentral plant

18、.DESIGN CONSIDERATIONSSelection of the system pressure, supply temperature, tempera-ture drop, type of HTW generator, and pressurization method are themost important initial design considerations. The following aresome of the determining factors:Type of load (space heating and/or process); load fluc

19、tuationsduring a 24 h period and a 1 year period. Process loads mightrequire water at a given minimum supply temperature continu-ously, whereas space heating can allow temperature modulationas a function of outdoor temperature or other climatic influences.Terminal unit temperature requirements or st

20、eam pressures re-quired in process systems.Distance between heating plant and space or process requiringheat.Quantity and pressure of steam used for power equipment in thecentral plant.Elevation variations within the system and the effect of basic pres-sure distribution.Usually, distribution piping

21、is the major investment in an HTWsystem. A distribution system with the widest temperature spread(t) between supply and return will have the lowest initial and oper-ating costs. Economical designs have a t of 85 K or higher.The requirements of terminal equipment or user systems deter-mine the system

22、 selected. For example, if the users are 70 kPa (gage)steam generators, the return temperatures would be 120C. A2000 kPa (gage) rated system operated at 200C would be selected toserve the load. In another example, where the primary system servespredominantly 60 to 80C hot-water heating systems, an H

23、TW sys-tem that operates at 160C could be selected. The supply temperatureis reduced by blending with 60C return water to the desired 82Chot water supply temperature in a direct-connected hot-watersecondary system. This highly economical design has a 60C returntemperature in the primary water system

24、 and a t of 100 K. How-ever, water-to-water converters are often used to limit pressures inoccupied spaces, and the MTW return temperature will be 80C orthe t 85 K.Because the danger of water hammer is always present when thepressure drops to the point at which pressurized hot water flashes tosteam,

25、 the primary HTW system should be designed with steelvalves and fittings of 1 MPa. The secondary water, which operatesbelow 100C and is not subject to flashing and water hammer, canbe designed for 860 kPa and standard HVAC equipment.Theoretically, water temperatures up to about 175C can be pro-vided

26、 using equipment suitable for 860 kPa. In practice, however,maximum water temperatures are limited by the system design,pump pressures, and elevation characteristics to values between 150and 160C.Most systems are designed for inert gas pressurization. The pres-surizing tank is connected to the syste

27、m by a single balance line onthe suction side of the circulating pump. This establishes the pointin the system at which the pressure does not change during opera-tion and is significant in controlling system pressures and ingestionof air into the system. The circulating pump is located at the inlets

28、ide of the HTW generator. There is no flow through the pressuriz-ing tank, and a reduced temperature will normally establish itselfinside. A special characteristic of a gas-pressurized system is theapparatus that creates and maintains gas pressure inside the tank.In designing and operating an MTW/HT

29、W system, it is impor-tant to maintain a pressure that always exceeds the vapor pressure ofthe water, even if the system is not operating. This may require lim-iting the water temperature and thereby the vapor pressure, orincreasing the imposed pressure.Elevation and the pressures required to preven

30、t water from flash-ing into steam in the supply system can also limit the maximumwater temperature that may be used and must therefore be studied inevaluating the temperature-pressure relationships and method ofpressurizing the system.The properties of water that govern design are as follows:Tempera

31、ture versus pressure at saturation (Figure 1)Density or specific volume versus temperatureEnthalpy or sensible heat versus temperatureViscosity versus temperatureType and amount of pressurizationThe relationships among temperature, pressure, specific volume,and enthalpy are all available in steam ta

32、bles. Some properties ofwater are summarized in Table 1 and Figure 3.Direct-Fired High-Temperature Water GeneratorsIn direct-fired HTW generators, the central stations are compa-rable to steam boiler plants operating within the same pressurerange. The generators should be selected for size and type

33、in keep-ing with the load and design pressures, as well as the circulationrequirements peculiar to high-temperature water. In some systems,both steam for power or processing and high-temperature water aresupplied from the same boiler; in others, steam is produced in theboilers and used for generatin

34、g high-temperature water; in contem-porary systems, the burning fuel directly heats the water.HTW generators are predominantly water-tube, and can beequipped with any conventional fuel-firing apparatus. Water-tubegenerators can have either forced circulation, gravity circulation, ora combination of

35、both. The recirculating pumps of forced-circulation generators must operate continuously while the genera-tor is being fired. Forced-circulation HTW generators are usuallythe once-through type and rely solely on pumps to achieve cir-culation. Depending on the design, internal orifices in the various

36、circuits may be required to regulate the water flow rates in propor-tion to the heat absorption rates. Circulation must be maintained atall times while the generator is being fired, and the flow rate mustFig. 2 Elements of High-Temperature Water SystemMedium- and High-Temperature Water Heating 15.3n

37、ever drop below the minimum indicated by the manufacturer. Fire-tube boilers designed for MTW/HTW service may be used, but ther-mal shock, which damages tubes and tube sheets, caused by suddendrops in the return temperature (sudden load increases) is difficult toavoid. Therefore, fire-tube generator

38、s should be avoided, used insmall systems, or at least be used with caution.A separate vessel is always used when the system is cushioned byan inert gas. Proper internal circulation is essential in all types ofgenerators to prevent tube failures caused by overheating or unequalexpansion.Expansion an

39、d PressurizationIn addition to the information in Chapter 13, the following fac-tors should be considered:The connection point of the expansion tank used for pressuriza-tion greatly affects the pressure distribution throughout thesystem and the avoidance of HTW flashing. For stable pressurecontrol,

40、the expansion tank balance line should be attached to theinlet side of the MTW/HTW generator. It is the only point of nopressure change in the system.Proper safety devices for high and low water levels and excessivepressures should be incorporated in the expansion tank and inter-locked with combusti

41、on safety and water flow rate controls.The following four fundamental methods, in which pressure in agiven hydraulic system can be kept at a desired level, amplify thediscussion in Chapter 13 (Blossom and Ziel 1959; National Acad-emy of Sciences 1959).1. Elevating the storage tank is a simple pressu

42、rization method,but because of the great heights required for the pressure encoun-tered, it is generally impractical.2. Steam pressurization requires the use of an expansion vesselseparate from the HTW generator. Steam pressurization systems areseldom used in contemporary systems. Detailed discussio

43、n of thismethod of pressurization may be found in previous editions of thischapter or noted in references therein.The expansion vessel must be equipped with steam safety valvescapable of relieving the steam generated by all the generators. Thegenerators themselves are usually designed for a substant

44、iallyhigher working pressure than the expansion drum, and their safetyrelief valves are set for the higher pressure to minimize their liftrequirement.The basic HTW pumping arrangements can be either single-pump, in which one pump handles both the generator and systemloads, or two-pump, in which one

45、pump circulates high-temperaturewater through the generator and a second pump circulates high-temperature water through the system. The circulating pump movesthe water from the expansion vessel to the system and back to thegenerator. The vessel must be elevated to increase the net positivesuction pr

46、essure to prevent cavitation or flashing in the pump suc-tion. This arrangement is critical. A bypass from the HTW systemreturn line to the pump suction helps prevent flashing. Cooler returnwater is then mixed with hotter water from the expansion vessel togive a resulting temperature below the corre

47、sponding saturationpoint in the vessel.3. Nitrogen, the most commonly used inert gas, is used for gaspressurization. Air is not recommended because the oxygen in aircontributes to corrosion in the system.The expansion vessel is connected as close as possible to the suc-tion side of the HTW pump by a

48、 balance line. The inert gas used forpressurization is fed into the top of the cylinder, preferably througha manual fill connection using a reducing station connected to aninert gas cylinder. Locating the relief valve below the minimumwater line is advantageous, because it is easier to keep it tight

49、lysealed with water on the pressure side. If the valve is located abovethe water line, it is exposed to the inert gas of the system.Table 1 Properties of Water, 100 to 210CTemperature,CAbsolute Pressure, kPa*Density,kg/m3Specific Heat,kJ/(kgK)Total Heat above 0 CDynamic Viscosity, mPaskJ/kg* MJ/m3100 101.3 958.1 4.22 419.1 401.5 0.284110 143.3 950.7 4.23 461.3 438.6 0.257120 198.5 942.8 4.25 503.7 474.9 0.234130 270.1 934.6 4.27 546.3 510.6 0.215140 361.4 925.9 4.29 589.2 545.5 0.198150 476.0 916.9 4.32 632.2 579.7 0.184160 618.1 907.0 4.35 675.5 612.7 0.171170 792.