ASHRAE HVAC SYSTEMS AND EQUIPMENT IP 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 operatingM temperatures ranging from 250F to 350F and are designedto a pressure rating of 125 to 150 psig. High-temperature water sys-tems are classified as those operating with supply water tempera-tures above 350F and designed to a

3、 pressure rating of 300 psig. Theusual practical temperature limit is about 450F because of pressurelimitations on pipe fittings, equipment, and accessories. The rapidpressure rise that occurs as the temperature rises above 450Fincreases cost because components rated for higher pressures arerequired

4、 (see Figure 1). The design principles for both medium- andhigh-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 250F. See Chapter 13 for basicdesign con

5、siderations 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 heat losse

6、s 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 central stat

7、ion.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 pumps required

8、 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 saturation i

9、n the sys-tem to prevent the water flashing into steam. Pressure at theexpansion/compression tank is customarily 30 psig above the sat-uration pressure.Terminal units requiring different water temperatures can beserved at their required temperatures by regulating the flow ofwater, modulating water s

10、upply 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 storagetanks or by increa

11、sing 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 system is desi

12、gned 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 ofchemistry, therm

13、odynamics, 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 closedheat excha

14、nger) to heat the water. The expansion of the heated wateris usually taken up in an expansion vessel, which simultaneouslyThe preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.Fig. 1 Relation of Saturation Pressure and Enthalpy to Water Temperature15.2 2012 A

15、SHRAE HandbookHVAC Systems and Equipment pressurizes the system. Heat transport 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 surf

16、acesin devices such as converters (heat exchangers) or steam generators.The 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 ismain

17、tained.Most systems use insert gas or pump-pressurized system, inwhich the pressure 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 anexist

18、ing steam distribution system and are installed remote from thecentral plant.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 th

19、e determining factors:Type of load (space heating and/or process); load fluctuationsduring 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 temperatu

20、re or other climatic influences.Terminal unit temperature requirements or steam 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

21、and the effect of basic pres-sure distribution.Usually, distribution piping 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 150F or higher.

22、The requirements of terminal equipment or user systems deter-mine the system selected. For example, if the users are 10 psig steamgenerators, the return temperatures would be 250F. A 300 psigrated system operated at 400F would be selected to serve the load.In another example, where the primary syste

23、m serves predomi-nantly 140 to 180F hot-water heating systems, an HTW system thatoperates at 325F could be selected. The supply temperature isreduced by blending with 140F return water to the desired 180Fhot water supply temperature in a direct-connected hot-watersecondary system. This highly econom

24、ical design has a 140Freturn temperature in the primary water system and a t of 185F.However, water-to-water converters are often used to limit pressuresin occupied spaces, and the MTW return temperature will be 175For the t 150F.Because the danger of water hammer is always present when thepressure

25、drops to the point at which pressurized hot water flashes tosteam, the primary HTW system should be designed with steelvalves and fittings of 150 psi. The secondary water, which operatesbelow 212F and is not subject to flashing and water hammer, can bedesigned for 125 psi and standard HVAC equipment

26、.Theoretically, water temperatures up to about 350F can be pro-vided using equipment suitable for 125 psi. In practice, however,maximum water temperatures are limited by the system design,pump pressures, and elevation characteristics to values between 300and 325F.Most systems are designed for inert

27、gas pressurization. The pres-surizing tank is connected to the system 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 ingestiono

28、f air into the system. The circulating pump is located at the inletside 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 maintai

29、ns gas pressure inside the tank.In designing and operating an MTW/HTW 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

30、 the imposed pressure.Elevation and the pressures required to prevent 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 syst

31、em.The properties of water that govern design are as follows:Temperature 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,

32、pressure, specific volume,and enthalpy are all available in steam tables. 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

33、 pressurerange. The generators should be selected for size and type 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 boile

34、r; in others, steam is produced in theboilers and used for generating 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

35、 either forced circulation, gravity circulation, ora combination of 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

36、-culation. Depending on the design, internal orifices in the variouscircuits 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-Temp

37、erature Water SystemMedium- and High-Temperature Water Heating 15.3never 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

38、load increases) is difficult toavoid. Therefore, fire-tube generators 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

39、tube failures caused by overheating or unequalexpansion.Expansion and 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 the s

40、ys-tem and the avoidance of HTW flashing. For stable pressurecontrol, 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 shoul

41、d be incorporated in the expansion tank and inter-locked with combustion 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-

42、emy of Sciences 1959).1. Elevating the storage tank is a simple pressurization 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 pressurizat

43、ion systems areseldom used in contemporary systems. Detailed discussion 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 ge

44、nerators. Thegenerators themselves are usually designed for a substantiallyhigher 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

45、handles both the generator and systemloads, or two-pump, in which one 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 thegenerat

46、or. The vessel must be elevated to increase the net positivesuction pressure 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 fr

47、om the expansion vessel togive a resulting temperature below the corresponding saturationpoint in the vessel.Table 1 Properties of Water, 212 to 400FTemperature,FAbsolute Pressure, psia*Density,lb/ft3Specific Heat,Btu/lbFTotal Heat above 32FDynamic Viscosity, CentipoiseBtu/lb* Btu/ft3212 14.70 59.81

48、 1.007 180.07 10,770 0.283822023017.1920.7859.6359.381.0091.010188.13198.2311,21611,7700.27120.256724025024.9729.8359.1058.821.0121.015208.34218.4812,31312,8510.24360.231726027035.4341.8658.5158.241.0171.020228.64238.8413,37813,9100.22070.210728029049.2057.5657.9457.641.0221.025249.06259.3114,43014,

49、9470.20150.193030031067.0177.6857.3156.981.0321.035269.59279.9215,45015,9500.18520.177932033089.66103.0656.6656.311.0401.042290.28300.6816,43716,9310.17120.1649340350118.01134.6355.9655.591.0471.052311.13321.6317,40917,8790.15910.1536360370153.04173.3755.2254.851.0571.062332.18342.7918,34318,8020.14840.1436380 195.77 54.47 1.070 353.45 19,252 0.1391390 220.37 54.05 1.077 364.17 19,681 0.1349400 247.31 53.65 1.085 374.97 20,117 0.1308*Source: Thermodynamic Properties of Steam, J.H. Keenan and F.G. Keyes, John Wiley it is usually small.To prevent corrosion-causing