ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 11-2012 STEAM SYSTEMS.pdf

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1、11.1CHAPTER 11 STEAM SYSTEMSAdvantages. 11.1Fundamentals. 11.1Effects of Water, Air, and Gases 11.2Heat Transfer . 11.2Basic Steam System Design . 11.2Steam Source 11.2Boiler Connections 11.3Design Steam Pressure 11.4Piping. 11.5Condensate Removal from Temperature-Regulated Equipment. 11.6Steam Trap

2、s 11.7Pressure-Reducing Valves 11.9Terminal Equipment . 11.11Convection Steam Heating. 11.11Steam Distribution . 11.12Temperature Control 11.13Heat Recovery 11.14Combined Steam and Water Systems . 11.15Commissioning. 11.16TEAM systems use the vapor phase of water to supply heat orS kinetic energy th

3、rough a piping system. As a source of heat,steam can heat a conditioned space with suitable terminal heat trans-fer equipment such as fan-coil units, unit heaters, radiators, and con-vectors (finned tube or cast iron), or through a heat exchanger thatsupplies hot water or some other heat transfer me

4、dium to the termi-nal units. In addition, steam is commonly used in heat exchangers(shell-and-tube, plate, or coil types) to heat domestic hot water andsupply heat for industrial and commercial processes such as in laun-dries and kitchens. Steam is also used as a heat source for certaincooling proce

5、sses such as single-stage and two-stage absorptionrefrigeration machines.ADVANTAGESSteam offers the following advantages: Steam flows through the system unaided by external energysources such as pumps.Because of its low density, steam can be used in tall buildingswhere water systems create excessive

6、 pressure.Terminal units can be added or removed without making basicchanges to the design.Steam components can be repaired or replaced by closing thesteam supply, without the difficulties associated with drainingand refilling a water system.Steam is pressure/temperature dependent; therefore, the sy

7、stemtemperature can be controlled by varying either steam pressure ortemperature.Steam can be distributed throughout a heating system with littlechange in temperature.In view of these advantages, steam is applicable to the followingfacilities:Where heat is required for process and comfort heating, s

8、uch as inindustrial plants, hospitals, restaurants, dry-cleaning plants, laun-dries, and commercial buildingsWhere the heating medium must travel great distances, such as infacilities with scattered building locations, or where the buildingheight would result in excessive pressure in a water systemW

9、here intermittent changes in heat load occurFUNDAMENTALSSteam is the vapor phase of water and is generated by adding moreheat than required to maintain its liquid phase at a given pressure,causing the liquid to change to vapor without any further increase intemperature. Table 1 illustrates the press

10、ure/temperature relationshipand various other properties of steam.Temperature is the thermal state of both liquid and vapor at anygiven pressure. The values shown in Table 1 are for dry saturatedsteam. The vapor temperature can be raised by adding more heat,resulting in superheated steam, which is u

11、sed (1) where higher tem-peratures are required, (2) in large distribution systems to compen-sate for heat losses and to ensure that steam is delivered at thedesired saturated pressure and temperature, and (3) to ensure thatthe steam is dry and contains no entrained liquid that could damagesome turb

12、ine-driven equipment.Enthalpy of the liquid hf(sensible heat) is the amount of heat inkilojoules required to raise the temperature of a kilogram of waterfrom 0C to the boiling point at the pressure indicated.Enthalpy of evaporation hfg(latent heat of vaporization) is theamount of heat required to ch

13、ange a kilogram of boiling water at agiven pressure to a kilogram of steam at the same pressure. Thissame amount of heat is released when the vapor is condensed backto a liquid.Enthalpy of the steam hg(total heat) is the combined enthalpy ofliquid and vapor and represents the total heat above 0C in

14、thesteam.Specific volume, the reciprocal of density, is the volume of unitmass and indicates the volumetric space that 1 kg of steam or wateroccupies.An understanding of the above helps explain some of the follow-ing unique properties and advantages of steam:Most of the heat content of steam is stor

15、ed as latent heat, whichpermits large quantities of heat to be transmitted efficiently withlittle change in temperature. Because the temperature of saturatedsteam is pressure dependent, a negligible temperature reductionoccurs from the reduction in pressure caused by pipe frictionlosses as steam flo

16、ws through the system. This occurs regardlessof the insulation efficiency, as long as the boiler maintains theinitial pressure and the steam traps remove the condensate. Con-versely, in a hydronic system, inadequate insulation can signifi-cantly reduce fluid temperature.Steam, as all fluids, flows f

17、rom areas of high pressure to areas oflow pressure and is able to move throughout a system without anexternal energy source. Heat dissipation causes the vapor tocondense, which creates a reduction in pressure caused by theThe preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquip

18、ment and Systems.11.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)dramatic change in specific volume (1600:1 at atmosphericpressure).As steam gives up its latent heat at the terminal equipment, thecondensate that forms is initially at the same pressure and temper-ature as the steam. When this

19、 condensate is discharged to a lowerpressure (as when a steam trap passes condensate to the returnsystem), the condensate contains more heat than necessary tomaintain the liquid phase at the lower pressure; this excess heatcauses some of the liquid to vaporize or “flash” to steam at thelower pressur

20、e. The amount of liquid that flashes to steam can becalculated as follows:% Flash Steam = (1)wherehf1= enthalpy of liquid at pressure p1hf 2= enthalpy of liquid at pressure p2hfg2= latent heat of vaporization at pressure p2Flash steam contains significant and useful heat energy that canbe recovered

21、and used (see the section on Heat Recovery). Thisreevaporation of condensate can be controlled (minimized) by sub-cooling the condensate within the terminal equipment before it dis-charges into the return piping. The volume of condensate that issubcooling should not be so large as to cause a signifi

22、cant loss ofheat transfer (condensing) surface.EFFECTS OF WATER, AIR, AND GASESEnthalpies in Table 1 are for dry saturated steam. Most systemsoperate near these theoretically available values, but the presence ofwater and gases can affect enthalpy, as well as have other adverseoperating effects.Dry

23、saturated steam is pure vapor without entrained water drop-lets. However, some amount of water usually carries over as con-densate forms because of heat losses in the distribution system.Steam quality describes the amount of water present and can bedetermined by calorimeter tests. The quality of sat

24、urated steam canbe improved by installing a separator in-line before the equipment.Although steam quality might not have a significant effect on theheat transfer capabilities of the terminal equipment, the backing upor presence of condensate can be significant because the enthalpy ofcondensate hfis

25、negligible compared with the enthalpy of evapora-tion hfg. If condensate does not drain properly from pipes and coils,the rapidly flowing steam can push a slug of condensate through thesystem. This can cause water hammer and result in objectionablenoise and damage to piping and system components.The

26、 presence of air also reduces steam temperature. Air reducesheat transfer because it migrates to and insulates heat transfersurfaces. Further, oxygen in the system causes pitting of iron andsteel surfaces. Carbon dioxide (CO2) traveling with steam dissolvesin condensate, forming carbonic acid, which

27、 is extremely corrosiveto steam heating pipes and heat transfer equipment.The combined adverse effects of water, air, and CO2necessitatetheir prompt and efficient removal.HEAT TRANSFERThe quantity of steam that must be supplied to a heat exchangerto transfer a specific amount of heat is a function o

28、f (1) the steamtemperature and quality, (2) the character and entering and leavingtemperatures of the medium to be heated, and (3) the heat exchangerdesign. For a more detailed discussion of heat transfer, see Chapter4 of the 2009 ASHRAE HandbookFundamentals.BASIC STEAM SYSTEM DESIGNBecause of the v

29、arious codes and regulations governing the designand operation of boilers, pressure vessels, and systems, steam sys-tems are classified according to operating pressure. Low-pressuresystems operate up to 100 kPa (gage), and high-pressure systemsoperate over 100 kPa (gage). There are many subclassific

30、ationswithin these broad classifications, especially for heating systemssuch as one- and two-pipe, gravity, vacuum, or variable vacuumreturn systems. However, these subclassifications relate to the distri-bution system or temperature-control method. Regardless of classi-fication, all steam systems i

31、nclude a source of steam, a distributionsystem, and terminal equipment, where steam is used as the source ofpower or heat.STEAM SOURCESteam can be generated directly by boilers using oil, gas, coal,wood, or waste as a fuel source, or by solar, nuclear, or electricalenergy as a heat source. Steam can

32、 be generated indirectly byrecovering heat from processes or equipment such as gas turbinesand diesel or gas engines. Cogeneration of electricity and steamshould always be considered for facilities that have year-roundsteam requirements. Where steam is used as a power source (e.g.,in turbine-driven

33、equipment), exhaust steam may be used in heattransfer equipment for process and space heating.Steam can be provided by a facilitys own boiler or cogenerationplant or can be purchased from a central utility serving a city or spe-cific geographic area. This distinction can be very important. A facil-i

34、ty with its own boiler plant usually has a closed-loop system andrequires the condensate to be as hot as possible when it returns to theboiler. Conversely, condensate return pumps require a few degrees ofsubcooling to prevent cavitation or flashing of condensate to vapor atthe suction eye of pump im

35、pellers. The degree of subcooling varies,depending on the hydraulic design or characteristics of the pump inTable 1 Properties of Saturated SteamAbsolutePressure, kPaSaturationTemperature, CSpecific Volume, m3/kg Enthalpy, kJ/kgLiquid vfSteam vgLiquid hfEvaporation hfgSteam hg19.9 60 0.00102 7.669 2

36、51 2358 260947.4 80 0.00103 3.405 335 2308 2643101 100 0.00104 1.672 419 2256 2675199 120 0.00106 0.891 504 2202 2706362 140 0.00108 0.508 589 2144 2733618 160 0.00110 0.307 676 2082 27581003 180 0.00113 0.194 763 2015 27781555 200 0.00116 0.127 852 1941 2793Note: Values are rounded off or approxima

37、ted to illustrate the various properties discussed in the text. For calculation and design, use values of thermodynamic properties of watershown in Chapter 1 of the 2009 ASHRAE HandbookFundamentals or a similar table.100 hf1hf2hf g2-Steam Systems 11.3use. See the section on Pump Suction Characterist

38、ics (NPSH) inChapter 44.Central utilities often do not take back condensate, so it is dis-charged by the using facility and results in an open-loop system. Ifa utility does take back condensate, it rarely gives credit for its heatcontent. If condensate is returned at 80C, and a heat recovery sys-tem

39、 reduces this temperature to 25C, the heat remaining in thecondensate represents 10 to 15% of the heat purchased from the util-ity. Using this heat effectively can reduce steam and heating costs by10% or more (see the section on Heat Recovery).BoilersFired and waste heat boilers are usually construc

40、ted and labeledaccording to the ASME Boiler and Pressure Vessel Code becausepressures normally exceed 100 kPa (gage). Details on design, con-struction, and application of boilers can be found in Chapter 32.Boiler selection is based on the combined loads, including heatingprocesses and equipment that

41、 use steam, hot water generation, pip-ing losses, and pickup allowance.The Hydronics Institute standards (HYDI 1989) are used to testand rate most low-pressure heating boilers that have net and grossratings. In smaller systems, selection is based on a net rating. Largersystem selection is made on a

42、gross load basis. The occurrence andnature of the load components, with respect to the total load, deter-mine the number of boilers used in an installation.Heat Recovery and Waste Heat BoilersSteam can be generated by waste heat, such as exhaust from fuel-fired engines and turbines. Figure 1 schemat

43、ically shows a typicalexhaust boiler and heat recovery system used for diesel engines. Aportion of the water used to cool the engine block is diverted as pre-heated makeup water to the exhaust heat boiler to obtain maximumheat and energy efficiency. Where the quantity of steam generatedby the waste

44、heat boiler is not steady or ample enough to satisfy thefacilitys steam requirements, a conventional boiler must generatesupplemental steam.Heat ExchangersHeat exchangers are used in most steam systems. Steam-to-waterheat exchangers (sometimes called converters or storage tanks withsteam heating ele

45、ments) are used to heat domestic hot water and tosupply the terminal equipment. These heat exchangers are the platetype or the shell-and-tube type, where the steam is admitted to theshell and the water is heated as it circulates through the tubes.Condensate coolers (water-to-water) are sometimes use

46、d to subcoolthe condensate while reclaiming the heat energy.Water-to-steam heat exchangers (steam generators) are used inhigh-temperature water (HTW) systems to provide process steam.Such heat exchangers generally consist of a U-tube bundle, throughwhich the HTW circulates, installed in a tank or pr

47、essure vessel.All heat exchangers should be constructed and labeled accordingto the applicable ASME Boiler and Pressure Vessel Code. Manyjurisdictions require double-wall construction in shell-and-tube heatexchangers between the steam and potable water. Chapter 48 dis-cusses heat exchangers in detai

48、l.BOILER CONNECTIONSFigure 2 shows recommended boiler connections for pumpedand gravity return systems; local codes should be checked for spe-cific legal requirements.Supply PipingSmall boilers usually have one steam outlet connection sized toreduce steam velocity to minimize carryover of water into

49、 supplylines. Large boilers can have several outlets that minimize boilerwater entrainment. The boiler manufacturers recommendationsconcerning near-boiler piping should be followed because this pip-ing may act as a steam/liquid separator for the boiler.Figure 2 shows piping connections to the steam header. Al-though some engineers prefer to use an enlarged steam header foradditional storage space, if there is no sudden demand for steamexcept during the warm-up period, an oversized header may be adisadvantage. The boiler header can be the