ASHRAE HVAC APPLICATIONS SI CH 50-2015 SERVICE WATER HEATING.pdf

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1、50.1CHAPTER 50SERVICE WATER HEATINGSystem Elements. 50.1Water-Heating Terminology. 50.1System Planning. 50.2Design Considerations. 50.2End-Use Fixtures . 50.3Distribution 50.3Water-Heating Equipment 50.8Building Applications. 50.11Hot-Water Load and Equipment Sizing 50.12Water-Heating Energy Use. 50

2、.29Health and Safety . 50.31Water Quality, Scale, and Corrosion 50.32Special Concerns 50.33ATER HEATING energy use is second only to space condi-Wtioning in most residential buildings, and is also significant inmany commercial and industrial settings. In some climates andapplications, water heating

3、is the largest energy use in a building.Moreover, quick availability of adequate amounts of hot water is animportant factor in user satisfaction. Both water and energy wastecan be significant in poorly designed service water-heating systems:from over- or undersizing pipes and equipment, from poor bu

4、ildinglayout, and from poor system design and operating strategies. Goodservice water-heating system design and operating practices willreduce operating costs and can often reduce first costs. The informa-tion in this chapter is thus critical for the sustainable design andoperation of many buildings

5、.Research documenting hot-water use in modern systems is lim-ited to certain segments. Some of the data in this chapter on hot-water demands for some types of buildings, applications, and fix-tures may be outdated. Nevertheless, these data are provided forguidance, because they are often still the b

6、est available; however,these demand values are not intended for use as designers sole ref-erences for hot-water system sizing purposes.1. SYSTEM ELEMENTSA service water-heating system has (1) one or more heat energysources, (2) heat transfer equipment, (3) a distribution system, and(4) end-use fixtu

7、res.Heat energy sources may be (1) fuel combustion; (2) electricalconversion; (3) solar energy; (4) geothermal, air, or other environ-mental energy; and/or (5) recovered waste heat from sources such asflue gases, ventilation and air-conditioning systems, refrigerationcycles, and process waste discha

8、rge.Heat transfer equipment is direct, indirect, or a combination ofthe two. For direct equipment, heat is derived from combustion offuel or direct conversion of electrical energy into heat and is appliedwithin the water-heating equipment. For indirect heat transfer equip-ment, heat energy is develo

9、ped from remote heat sources (e.g., boil-ers; solar energy collection; air, geothermal, or other environmentalsource; cogeneration; refrigeration; waste heat) and is then trans-ferred to the water in a separate piece of equipment. Storage tanksmay be part of or associated with either type of heat tr

10、ansfer equip-ment.Distribution systems transport hot water produced by water-heating equipment to end-use fixtures. For locations where constantsupply temperatures are desired, circulation piping or a means ofheat maintenance must be provided.End-use fixtures are plumbing faucets, accessories, and e

11、quip-ment requiring hot water that may have periods of irregular flow,constant flow, and no flow. These patterns and their related waterusage vary with different buildings, process applications, and per-sonal preference. Examples of end-use accessories are prerinsespray valves, faucet aerators, show

12、erheads, washdown sprayers, andhose bibbs. Examples of end-use equipment are dishwashers,clothes washers, and pressure washers.2. WATER-HEATING TERMINOLOGYDistribution system efficiency. Heat contained in the water atpoints of use divided by heat delivered at the heater outlet duringflow periods.Ene

13、rgy factor. The delivered efficiency of a residential waterheater when operated as specified in U.S. Department of Energy(DOE) test procedures (DOE 2001). See also ASHRAE Standard118.2.First-hour rating. An indicator of the maximum amount of hotwater a residential water heater can supply in 1 h. Thi

14、s rating is usedby the Federal Trade Commission (FTC) for comparative purposes.Because peak draws taken over periods less than 1 h frequently driveresidential equipment sizing, first-hour rating alone should not beused for equipment sizing. As for larger systems, storage tank vol-ume and heating rat

15、e also play important roles.Fixture unit. A number, on an arbitrarily chosen scale, that ex-presses the load-producing effects on the system of different kinds offixtures.Heat trap. A device to counteract the natural convection ofheated water in a vertical pipe. Commercially available heat traps for

16、large equipment are generally 360 loops of tubing; heat traps canalso be constructed of pipes connected to the water heater (inlet oroutlet) that direct flow downward before connecting to the verticalsupply or hot-water distribution system. Tubing or piping heat trapsshould have a loop diameter or l

17、ength of downward piping of at least300 mm. Various prefabricated check-valve-like heat traps are avail-able for residential-sized equipment, using balls, flexible flaps, ormoving disks.Input efficiency. Heat entering water in the heating devicedivided by energy input to the heating unit over a spec

18、ific period ofsteady-state conditions, or while heating from cold to hot, dependingon how stated (steady-state versus average input efficiency); it doesnot include heat losses from the water heater jacket and/or tank.When used with fossil-fuel-fired equipment, this is commonly calledcombustion effic

19、iency.Operating efficiency. Heat delivered at the heater outlet (Qout=mcpThot out Tcold in) divided by heat input to the heating unit(includes heat losses from water heater jacket and/or tank) for anyselected period for systems without recirculation pumps. For dis-tribution systems with recirculatio

20、n pumps, heat losses includerecirculation line losses, because hot water at a reduced tempera-ture is returned back to the heater. Thus, operating efficiencyequals the heat delivered to the middle of the distribution line (Qout=mcp (Thot out+ Thot return)/2 Tcold in) divided by heat input toThe prep

21、aration of this chapter is assigned to TC 6.6, Service Water HeatingSystems.50.2 2015 ASHRAE HandbookHVAC Applications (SI)heating unit. The operating efficiency of water heaters in systemswith continuous recirculation can be further reduced by loss of strat-ification in storage heaters. Elevated re

22、turn temperatures associatedwith continuous recirculation systems further reduce the operatingefficiency of condensing water heaters (see Figures 1 and 2). This isalso referred to the heaters real-world efficiency, which can be eas-ily measured and used to estimate the energy use or operating cost.A

23、 system with higher operating efficiency may not always equate toa higher-performing system, because operating efficiency considerswater temperature leaving the tank, not water temperature reachingthe fixtures. A system with extremely long hot-water distributionpiping and no recirculation may show a

24、 high operating efficiency,but hot water may never reach the farthest fixtures.Overall system efficiency. Heat energy in the water delivered atpoints of use divided by the total energy supplied to the heater forany selected period.Recovery efficiency. Heat absorbed by the water divided by heatinput

25、to the heating unit during the period that water temperature israised from inlet temperature to final temperature (includes heatlosses from water heater jacket and/or tank).Recovery rate. The amount of hot water that a water heater cancontinually produce, usually reported as flow rate in litres per

26、hourthat can be maintained for a specified temperature rise through thewater heater.Standby loss. As applied to a tank water heater (under test con-ditions with no water flow), the average hourly energy consumptiondivided by the average hourly heat energy contained in stored water,expressed as a per

27、cent per hour. This can be converted to the averagewatts energy consumption required to maintain any water/air tem-perature difference by taking the percent times the temperature dif-ference, times 1.15 kWh/(m3K) (a nominal specific heat for water),times the tank capacity, and then dividing by 100.S

28、tandby loss coefficient. The heat input (in W/K) into a storagewater heater when operated as specified in U.S. Department of En-ergy (DOE) test procedures (DOE 2001). This value is essentiallythe standby loss divided by the difference in temperature betweenthe average stored water temperature and th

29、e surrounding air tem-perature. Care should be taken to understand whether a quotedstandby loss coefficient includes the heat input efficiency of theheating device. It is possible to directly measure the heat lost froma storage water heater independently of how that water is heated.Sometimes, the re

30、ported standby loss coefficient represents only theheat lost; at other times, it represents the amount of energy to makeup that heat loss, and considers the heat input efficiency of the heat-ing device.System standby loss. The amount of heat lost from the waterheating system and the auxiliary power

31、consumed during periods ofnonuse of service hot water.Thermal efficiency. Heat in water flowing from the heater outletdivided by the energy input to the heating unit over a specific periodof steady-state conditions (includes heat losses from the waterheater jacket and/or tank).3. SYSTEM PLANNINGThe

32、goals of system planning are to (1) size the system properly;(2) optimize system efficiency; and (3) minimize first, operating,and overall life-cycle costs. It is important to design systems so thatthey perform well from both functional (hot-water delivery) andenergy-use perspectives. Flow rate, tem

33、perature, and total flow overspecific time periods are the primary factors to be determined in thedesign of a water-heating and piping system for delivering adequateamounts of hot water. Operating pressures, time of delivery, andwater quality are also factors to consider. Presently, separate proce-d

34、ures are used to select water-heating equipment and to design thepiping system. However, water-heating equipment sizing and pipingsystem design should be considered together for best system design.Oversized or excessively long piping exacerbates delivery delayand/or energy waste.Water-heating equipm

35、ent, storage facilities, and piping should(1) have enough capacity to provide the required hot water whileminimizing waste of energy or water and (2) allow economical sys-tem installation, maintenance, and operation.Water-heating equipment types and designs are based on the(1) energy source, (2) hea

36、t exchange method, and (3) controlmethod used to deliver the necessary hot water at the required tem-perature under varying water demand conditions. Application ofwater-heating equipment within the overall design of the hot-watersystem is based on (1) location of the equipment within the system,(2)

37、related temperature requirements, (3) volume of water to beused, and (4) flow rate. Consideration of electricity demandcharges on the utility bill is also of growing importance. Additionalplanning is required when the system providing the potable hotwater is also used for space heating or other purp

38、oses. Some specialwater heater designs, made for this purpose, are known as combi-nation space- and water-heating systems.Energy SourcesChoice of energy source(s) is influenced by local availability ofthe various energy sources, equipment type, space considerations,locations of water heaters in stru

39、ctures, initial cost, operating cost,maintenance requirements, and other factors. A life-cycle cost anal-ysis is highly recommended.In making energy conservation choices, consult ASHRAE Stan-dards 90.1 and 90.2, or the sections on Service Water Heating Sys-tems of ASHRAE Standard 100, as well as the

40、 section on DesignConsiderations in this chapter.4. DESIGN CONSIDERATIONSHot-water system design should consider the following:Water heaters of different sizes and insulation may have differentstandby losses, thermal efficiency, or energy factors.A distribution system should be properly laid out, si

41、zed, and insu-lated to deliver adequate water quantities at temperatures satisfac-tory for the uses served. This reduces standby loss and improvesdistribution system efficiency. Locating fixtures or usage devicesclose to each other and to the water-heating equipment is partic-ularly important for mi

42、nimizing piping lengths and diameters,and thus reducing wait times as well as water and energy waste.Heat traps between recirculation mains and infrequently usedbranch lines reduce convection losses to these lines and improvedistribution system efficiency. In small residential systems, heattraps can

43、 be applied directly to the water heater for the same pur-pose.Controlling circulating pumps to operate only as needed to main-tain proper temperature at the end of the main reduces losses onreturn lines.Provision for shutdown of circulators during building vacancyreduces standby losses.Design Path

44、for SavingsReducing hot-water consumption not only results in lower waterand sewer costs, it is the most effective way to reduce water-heatingenergy use. Designing in a reverse direction, starting with the hot-water-using equipment and moving back to the water heater, is aneffective thought process

45、to achieve higher system efficiency andperformance.Step 1: Specify high-performance equipment and accessoriesthat use less hot water, or alternative processes that eliminate theneed for hot water for that particular task.Service Water Heating 50.3Step 2: Locate sinks and equipment in proximity to ea

46、ch otherand to the water heater, and optimize the plumbing layout; these arekey factors to the efficiency and performance of the overall system.Delivering hot water more efficiently yields permanent energy sav-ings and improved hot-water delivery performance.Step 3: Specify high-efficiency water hea

47、ters that are compatiblewith the distribution system and end-use fixtures. This is impera-tive.Step 4: Before the hot-water system design is finalized, considerintegrating preheating technologies such as heat recovery or solarheating.Step 5: Verify proper installation of the system, including simple

48、monitoring equipment, which can play an important role in com-missioning and maintaining the system.5. END-USE FIXTURESAdvanced end-use fixtures play an important role in reducing thesize of the primary water heater(s) and simplifying the distributionsystem design. Use of high-efficiency, low-water-

49、use equipmentand fixtures, such as faucet aerators, reduced-flow (but still ade-quate) showerheads, and advanced clothes washers and dishwash-ers, achieves multiple benefits, including the ability to use lowerwater temperatures and lowered total hot water consumption. Thus,in many instances, it is practical to provide localized heatingdevices, either near or built into the fixture or device, thereby addi-tionally reducing distribution system heat losses and piping firstcosts. Providing localized heating devices also reduces demands onpiping diameter and length for the remaining h