ASHRAE LV-11-008-2011 Hot-Water Distribution System Piping Time Water and Energy Waste-Phase III Test Results.pdf

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1、742 ASHRAE TransactionsABSTRACTThis paper describes results of recent laboratory tests onadditional common hot-water distribution system piping sizesand types under a variety of flow rate, temperature, and envi-ronmental conditions. The new tests include chlorinated poly-vinyl chloride (CPVC) and hi

2、gh-density cross-linkedpolyethylene (PEX) piping. They also include attic and floorenvironments. The tests measured the time spent waiting for hotwater to arrive at fixtures and the amount of water wasted todrain while waiting. From these, the energy waste associatedwith the water waste can be compu

3、ted. Results are presentedas plots of the ratio of actual flow volume divided by pipevolume (AF/PV) vs. flow rate, temperatures, and other param-eters of importance. Several unanticipated flow regimes wereobserved that dramatically alter behavior from what might beexpected based on either plug-flow

4、or fully developed flowtheory. A separate related paper discusses piping heat loss UAfactors that were measured.INTRODUCTIONResults of in-field investigations of hot-water distribution(HWD) system behavior by the author and others (Hiller andMiller 2002; Hiller et al. 2002; Henderson 2003; Lutz andK

5、lein 1998; Klein 2004), revealed that time, water, and energywaste characteristics of HWD systems were deteriorating innewer building designs compared to earlier practice. This isoccurring for a variety of reasons, including: (1) having morehot water using fixtures located further distances apart;(2

6、) more prevalent use of under-slab piping, which is bothgenerally longer than above-slab piping (Hiller 2005) andwhich represents a high-heat-transfer environment due tomoisture presence under the slab; (3) use of lower-flowfixtures, which exacerbate water waste during the deliveryphase of hot-water

7、 flow; and (4) a lower level of care in design-ing HWD piping systems for energy-efficient operation thanin the past. Further investigation revealed that, while someinformation is available (Schultz and Goldschmidt 1978,1983), there is a lack of information and rigorous data on howHWD systems really

8、 function, especially with regard tofactors that affect time, water, and energy waste. For thatreason, laboratory tests were initiated, aimed at quantifyingfactors that affect HWD system performance for an initial setof pipe types and sizes (Hiller 2005). Additional piping typesand environments were

9、 studied under phase II of the research(Hiller 2007, 2006a, 2006b, 2008a, 2008b). An even largervariety of pipe materials, sizes, and environments were studiedunder the Phase III research reported here (Hiller 2010). Theselaboratory tests quantified numerous HWD system pipingbehaviors under a variet

10、y of temperature, flow rate, environ-ment, and insulation conditions. This paper reports on only thetime, water, and energy waste behaviors observed during thedelivery phase of flow, and includes examples of how thatinformation can be used to analyze HWD system perfor-mance. Related papers discuss p

11、ipe heat loss characteristicsthat were measured (Hiller 2006a, 2008a; Hiller 2011).METHODOLOGYA test laboratory was established in Davis, CA, wherecomplete full-size piping systems could be constructed,instrumented, and tested. The tests discussed in this paperwere on horizontal 0.75 in. (19 mm) rig

12、id copper pipe inseveral in-attic and in-floor configurations. The tests alsocovered 0.75 in. (19 mm) chlorinated polyvinyl chloride(CPVC) piping both in still air and in-attic, 0.75 in. (19 mm)Hot-Water Distribution System Piping Time, Water, and Energy WastePhase III: Test ResultsCarl C. Hiller, P

13、hD, PEFellow ASHRAECarl C. Hiller is president of Applied Energy Technology Co., Davis, CA.LV-11-0082011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduc

14、tion, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.2011 ASHRAE 743high-density cross-linked polyethylene (PEX) piping both instill air and in-attic, 0.5 in. (13 mm) PEX in-air, and 0.375 inch(10 mm) PEX in-air, all both bare,

15、 and insulated with 0.75 in.(19 mm) thick foam pipe insulation. (All the piping tested istechnically tubing, but we call it piping here to be consistentwith common practice.) The results are compared to those ofother pipe sizes, types, and environments previously tested(Hiller 2005, 2006a, 2006b, 20

16、08a, 2008b).Test fixtures in the laboratory consisted of the “in-air”test fixture, where horizontal piping could be tested far fromsurrounding structures, and a “buried” test fixture containing25.5 tons (25 metric tons) of damp sand where horizontalpiping could be tested in a simulated under-slab en

17、viron-ment. For the most recent tests, an additional new piping testfixture was constructed. This ceiling/attic/floor test fixturewas constructed overhead in the laboratory with 0.5 in. (13mm) thick gypsum wallboard on the bottom side, and 8 in.(200 mm) wooden joists spaced on 24 in. (610 mm) center

18、s.In the ceiling/attic configuration, the top side of the test rigwas open to air. In the floor configuration the top side of thetest rig was covered with 0.75 in. (19 mm) thick plywood.Tests on new piping types were first performed separately“in-air,” both bare, and insulated with 0.75 in. (19 mm)

19、thickfoam pipe insulation, and then were also tested in the ceiling/attic/floor test fixture. All piping setups were serpentinearrangements with four to six parallel passes and three to fiveU-bends. Fast response immersion thermocouples wereinserted directly through the pipe side wall using a specia

20、lcompression fitting fashioned by the principle investigator.Thermocouples were located at the entrance to each pipe testsection, at each U-bend, and at the outlet. Data were storedat one-second intervals for all tests.Space does not permit showing pictures of the test setupsin this paper. The pictu

21、res are shown, however, in a companionpaper (Hiller 2011). The 0.75 in. (19 mm) nominal diameterCPVC test section was 4-pass, approximately 95 ft long. The0.75 in. (19 mm) PEX test section was 4-pass, approximately92 ft long. The 0.5 in. (13 mm) PEX test section was 6-pass,approximately 125 ft long.

22、 The 0.375 in. (10 mm) PEX testsection was 6-pass, approximately 160 ft long. The 0.75 in.(19 mm) rigid copper piping tested in the ceiling/attic/floorfixture was approximately 95 ft long. All the in-air configura-tions were tested both bare, and with 0.75 in. (19 mm) thickfoam pipe insulation.Many

23、hundreds of tests were performed, independentlyvarying water flow rate, initial pipe temperature, entering hot-water temperature, room air temperature, and insulation level.The foam pipe insulation thermal conductivity was approxi-mately 0.02 Btu/hftF (0.036 W/mK). Since pipe insulationR-values are

24、based on the pipe outer diameter, this yielded theR-values shown in Table 1 for the pipe insulation tested. Theseare the values that were printed on the outside of the insulation.Each test consisted of three distinct phases. These werethe “delivery phase,” where hot water was traversing the pipe,thr

25、ough to the time where “hot-enough-to-use” water arrivedat the outlet end of the test section (defined as 105F (40C) fortest and analysis purposes); the “use phase,” where aprolonged draw of water was taken for the purpose of measur-ing pipe heat loss under flowing conditions; and the “cool-down pha

26、se,” where data were taken for a prolonged periodafter flow stopped in order to obtain pipe heat loss informationduring zero-flow conditions. The results of this paper focusonly on behavior during the delivery phase. Companionpapers present pipe heat loss information taken from the othertwo flow pha

27、ses (Hiller 2006a, 2008a, 2011).After much investigation, it was decided that presentingthe water waste results as the ratio of actual flow volumerequired to obtain 105F (40C) water divided by pipe volume(AF/PV) gave the best resolution of the data. Given the AF/PVratio and a flow rate, the time spe

28、nt waiting for 105F (40C)water to arrive could then be computed. Moreover, givenknowledge of the water heater heat input efficiency and enter-ing cold- and leaving hot-water temperatures, energy impactof the water waste could be computed.A short discussion about results accuracy is informative.The a

29、ccuracy of the time measurement at which the hot/coldinterface passes each measuring station changes with flowvelocity and position along the pipe. The flow velocityincreases with increasing volumetric flow rate, and withdecreasing pipe diameter. Given the fixed one-second datacollection and storage

30、 interval, the worst case “interface pass-ing” time error is one second. At short pipe lengths and highflow rates in small diameter pipes, this can result in unaccept-able measurement error. Since we can anticipate the level oferror at each position along the pipe for each pipe size and flowrate, we

31、 can compensate for the error by making the pipe testsection long enough so that end sections have low measure-ment error. In the plots below, instances where measurementerror was greater than about 15% were deleted. This was typi-cally the first temperature measurement position along thepipe length

32、 of smaller diameter pipes, at high flow rates.TEST RESULTSWhen waiting for “hot-enough” water to arrive at fixtures,at a minimum an amount of water equal to the volume of thepipe must be wasted to drain if that water is below the mini-mum useful temperature. In reality, the amount wasted to drainis

33、 greater than pipe volume because a number of mechanismsTable 1. Pipe Insulation R-ValuesNominal Piping Diameter, in. (mm)0.5 in. (13 mm) Foam,hft2F/Btu (m2K/W)0.75 in. (19 mm) Foam,hft2F/Btu (m2K/W)0.375 (10) 5.5 (0.97)0.5 (13) 3.1 (0.55) 5.2 (0.92)0.75 (19) 2.9 (0.51) 4.7 (0.83)744 ASHRAE Transact

34、ionsdegrade the temperature of some of the hot water to belowusable levels as it traverses the pipe. This paper describes andquantifies those mechanisms.The tests conducted show that delivery-phase hot-waterflow in pipes is a highly transient, non-steady-state process,and that neither plug-flow (sha

35、rp hot/cold interface with notemperature degradation or mixing), nor fully developed flowassumptions are valid as a general rule. In fact, fully developedflow may never happen before a draw is terminated. Severaldistinct unanticipated flow regimes were observed that char-acterize delivery-phase flow

36、 in pipes where there is a hot/coldinterface.Flow Regimes and Residence Time Effects in Delivery-Phase FlowOne of the reasons flow during the delivery phase isunusual is because both hot and cold water are flowing in thesame pipe at the same time, and the amount of water waste isgoverned by what hap

37、pens at the interface between the two.The behavior of the hot/cold interface is transient, as opposedto steady-state, and varies significantly under different condi-tions, and with pipe material, length, flow rate, temperatures,and time. The flow regimes observed can be classified in threegeneral ca

38、tegories: normal fully developed flow, stratifiedflow, and shear-flow (called plug-flow in an earlier paper).These flow regimes were described more fully in an earlierpaper (Hiller 2006b); hence, they will only be summarizedhere (see Figures 1 and 2).Stratified Flow. At low flow rates, where flow of

39、 the coldwater is laminar, the hot water sometimes stratifies and travelsa further distance down the top side of a horizontal pipe thanon the bottom side. This stratification is detrimental to time,water, and energy waste because it increases the length of thehot-/cold-water interface in the pipe, a

40、llowing more heat trans-fer to occur between the hot and cold water in the pipe. Thisdegrades the temperature of more of the hot water to below auseful level. In the case of the copper pipes, the stratificationalso creates greater surface area for heat transfer from hot tocold water through the high

41、ly conductive pipe wall. Whenhorizontal stratification occurs, it causes the plots of AF/PVvs. flow rate and pipe length to have certain characteristicshapes, as described in Figures 1 and 2.Flow stratification in horizontal pipes causes a significantincrease in AF/PV ratio for short lengths of pipe

42、 over whatwould be expected based on normal fully developed flow. Atgreater pipe lengths, normal hot/cold mixing in the pipe causethe stratified region to disappear, such that at long pipe lengths,the effect of the entrance-region stratification is negated.It is important to note that stratified flo

43、w is driven by grav-ity, not by pressure difference as in normal flows. This meansthat stratified flow can occur even if no bulk flow is occurring,and also that it can occur superimposed on the bulk flow. Whenbulk flow is laminar, we can see the stratification, but when itis turbulent, the turbulenc

44、e obscures the stratification flow.When shear flow occurs (see below), there is also little or noflow turbulence, again allowing stratification flows to occursuperimposed on bulk flow. Additionally, it should be recog-nized that while stratification easily forms when there is asignificant temperatur

45、e difference between the hot and coldwater, it also occurs at fairly low temperature (and, hence,density) differences between the hotter and colder water.During testing, stratification was observed to occur at temper-ature differences of as little as 1F2F (1C) between thehotter and colder water.The

46、effect of stratification is different in vertical pipeswith downward flow compared to horizontal flow. The buriedpipe test systems previously tested had vertical downward-flowing entrance legs ranging from 2 to 3 ft (0.6 to 1 m) long.At low flow rates, flow was observed to stratify as before, butsin

47、ce flow was vertically downward, this resulted in a sharphot/cold interface approximating plug-flow rather than theextended length hot/cold interface that develops in horizontalpipes. In retrospect, this behavior was to be expected. It high-lights the fact, however, that pipe orientation and flow di

48、rec-tion can impact water and energy waste, especially in shortpiping. Vertical downward flow produces less water wasteFigure 1 Example behaviors: AF/PV vs. flow rate. Figure 2 Example behaviors: AF/PV vs. pipe length.2011 ASHRAE 745compared to horizontal or upward vertical flow, under lowflow rate

49、conditions.Shear-Flow. Another extremely important flow phenom-enon observed was that above certain flow velocities, whichvaried with pipe size, type, and temperature conditions, AF/PV could actually drop to 1.0. It is speculated that this occursbecause high shear develops at the water/pipe interface undercertain conditions such that the flow shears off the boundarylayer, resulting in reduced turbulence and mixing compared tofully developed flow. The most recent tests, taken with one-second data collection and storage intervals, revealed thatwhen shear flow occurs,