ASHRAE LV-11-007-2011 Hot-Water Distribution System Piping Heat Loss Factors-Phase III Test Results.pdf

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1、2011 ASHRAE 727ABSTRACTThis paper describes results of recent laboratory tests onadditional piping types and sizes in a variety of environmentsincluding in-air, in-attic, and in-floor, both insulated and unin-sulated, and compares them to previous test results by theauthor on other pipe configuratio

2、ns and environments. Newpipes tested included chlorinated polyvinyl chloride (CPVC)and high-density cross-linked polyethylene (PEX), in both in-air and in-attic environments, and rigid copper, in both in-atticand in-floor environments. These tests allowed calculation ofmeasured piping heat loss (UA)

3、 factors under a variety ofdifferent temperature and flow conditions, with various insu-lation levels. This piping UA information can be used to esti-mate piping heat loss and steady-state temperature drop underany desired temperature and flow conditions. The UA data canadditionally be used to deter

4、mine piping cool-down rates afterflow has ceased. Piping heat loss data are one critical part ofthe information necessary to accurately estimate total energylosses associated with piping systems. These energy loss effectsextend beyond just energy loss from the pipe itself, and include,for example, i

5、ncreases in tank heat loss caused by the need toset temperatures higher to overcome temperature drop of waterflowing through the pipe. A separate related paper discussestime, water, and energy waste while waiting for hot water toarrive at fixtures.INTRODUCTIONResults of in-field investigations of ho

6、t-water distribution(HWD) system behavior by the author and others (Hiller andMiller 2002; Hiller et al. 2002; Henderson 2003; Lutz andKlein 1998; Klein 2004) revealed that time, water, and energywaste characteristics of HWD systems were deteriorating innewer building designs compared to earlier pra

7、ctice. Furtherinvestigation revealed that there was a lack of information andrigorous data on how HWD systems really function, especiallywith regard to factors that affect time, water, and energy waste.For that reason, laboratory tests were initiated, aimed at quan-tifying factors that affect HWD sy

8、stem performance (Hiller2005a, 2007, 2010). These laboratory tests quantified numer-ous HWD system piping behaviors under a variety of temper-ature, flow rate, environment, and insulation conditions. Thispaper reports on only the piping heat loss behaviors observed,and includes examples of how that

9、information can be used toanalyze HWD system performance. A related paper discussesthe time, water, and energy waste that occurs while waiting forhot water to arrive at fixtures (Hiller 2011).METHODOLOGYA test laboratory was established in Davis, CA, wherecomplete full-size piping systems could be c

10、onstructed,instrumented, and tested. The tests discussed in this paperwere on horizontal 0.75 in. (19 mm) rigid 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

11、mm)high-density cross-linked polyethylene (PEX) piping both instill air and in-attic, 0.5 inch (13 mm) PEX in-air, and 0.375 in.(10 mm) PEX in-air, all both bare, 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

12、 be consistentwith common practice.) The results are compared to those ofother pipe sizes, types, and environments previously tested(Hiller 2005b, 2006a, 2006b, 2008a, 2008b).Hot-Water Distribution System Piping Heat Loss FactorsPhase III: Test ResultsCarl C. Hiller, PhD, PEFellow ASHRAECarl C. Hill

13、er is president of Applied Energy Technology Co., Davis, CA.LV-11-0072011. 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 reproduction, distribution, or transmis

14、sion in either print or digital form is not permitted without ASHRAES prior written permission.728 ASHRAE TransactionsTest 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 containing2

15、5.5 tons (25 metric tons) of damp sand where horizontalpiping could be tested in a simulated under-slab environ-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) t

16、hick gypsum wallboard on the bottom side, and 8 in.(200 mm) wooden joists spaced on 24 in. (610 mm) centers.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

17、 new piping types were first performed separately“in-air,” both bare, and insulated with 0.75 in. (19 mm) 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-be

18、nds. Fast response immersion thermocouples wereinserted directly through the pipe side wall using a specialcompression 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

19、 intervals for all tests.Figure 1 shows the bare 0.75 in. (19 mm) nominal diam-eter CPVC test section (four-pass, approximately 95 ft long)as tested in air. Figure 2 shows the bare 0.75 in. (19 mm) PEXtest section as tested in air (four-pass, approximately 92 ftlong). Figure 3 shows the bare 0.5 in.

20、 (13 mm) PEX test sectionas tested in air (six-pass approximately, 125 ft long). Figure 4shows the bare 0.375 in. (10 mm) PEX test section as tested inair (six-pass, approximately 160 ft long). Figures 5 and 6 showtwo of the many in-attic 0.75 in. (19 mm) rigid CU test config-urations (four-pass, ap

21、proximately 95 ft long). All the in-airconfigurations were tested both bare, and with 0.75 in.(19 mm) thick foam pipe insulation.Many hundreds of tests were performed, independentlyvarying water flow rate, initial pipe temperature, entering hot-water temperature, room air temperature, and insulation

22、 level.The foam pipe insulation thermal conductivity was approxi-Figure 1 0.75 in. (19 mm) CPVC piping4-pass, in-air.Figure 2 0.75 in. (19 mm) PEX piping4-pass, in-air.Figure 3 0.5 in. (13 mm) PEX piping6-pass, in-air.Figure 4 0.375 in. (10 mm) PEX piping6-pass, in-air.2011 ASHRAE 729mately 0.02 Btu

23、/hftF (0.036 W/mK). Since pipe insulationR-values are 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.The pipe heat loss (UA) factors were determined differ-ently for th

24、e flowing vs. zero-flow (cool-down) tests. To deter-mine UAflowing, the steady-state (or near steady-state) drop intemperature was measured from inlet to outlet in the pipe. Themeasured temperature drop and flow rate were then used tocalculate UAflowingfrom the following formula:Q = (mCp)w(Thot in T

25、hot out) = UAflowing (Thot average Tair) = UAflowing(LMTDflowing) (1)whereQ = heat loss rate(mCp)w= mass flow rate of water times specific heat of waterThot in = water temperature entering pipeThot out = water temperature leaving pipeThot average = log-mean average pipe water temperatureTair = surro

26、unding air temperatureUAflowing= pipe heat loss characteristic under flowing conditions (usually on a per unit length basis)LMTDflowing= log mean temperature difference under flowing conditionsLMTDflowing=(Thot in Tair) (Thot out Tair)/ln(Thot in Tair)/(Thot out Tair)Duration of the flowing pipe hea

27、t loss tests consisted oftwo phases: a transient phase while water temperatures at eachmeasuring location were increasing as the pipe and surround-ing materials (if any) were heating up to their final steady-state(or slowly changing quasi-steady-state) values and the airnatural convection flows beca

28、me established, and a steady-state phase during which all temperatures remained relativelyconstant. The transient phase typically lasted 5 to 30 minutes(the longer times were needed when there was much surround-ing thermal mass), and the steady-state phase typically was runfor 1530 minutes once stea

29、dy conditions were established.To determine UAzero-flow, the drop of pipe temperature wasobserved over time after flow was stopped. The known dimen-sions and properties of the pipe were used to calculate theeffective mass times specific heat per foot of the pipes tested,including pipe, water, and in

30、sulation. The UAzero-flowwas thencalculated at each minute during the pipe cool-down process,and the average value over an extended period was computed.Cool-down periods varied with pipe diameter. Best consis-tency for comparisons was achieved by using UAzero-flowvalues computed during the time the

31、pipe was cooling from itsinitial temperature down to the 105F (40C) minimum usableFigure 5 0.75 in. (19 mm) rigid cu off gypsum wallboard inattic.Figure 6 0.75 in. (19 mm) rigid cu on gypsum wallboard inattic, with attic insulation.Table 1. Pipe Insulation R-ValuesNominal Piping Diameter, in. (mm)0.

32、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)730 ASHRAE Transactionstemperature level. The formula used to calculate UAzero-flowwas:Q = (MCp)pwi(Tpipe time 1 Tpipe time 2)/(time interval)

33、= UAzero-flow(Tpipe average Tair) = UAzero-flow(LMTDzero flow)(2)whereQ = heat loss rate(MCp)pwi=(mass of pipe)(Cppipe) + (mass of water)(Cpw) + (mass of insul.)(Cpi)Cp = specific heat of the materialTpipe time 1 = water temperature at beginning of time intervalTpipe time 2 = water temperature at en

34、d of time intervalTpipe average = log-mean average pipe temperature over the time intervalTair = air temperatureUAzero-flow= pipe heat loss characteristic under zero-flow conditions (usually on a per unit length basis)LMTDzero flow= log mean temperature difference under zero-flow conditionsLMTDzero

35、flow=(Tpipe initial Tair) (Tpipe final Tair)/ln(Tpipe initial Tair)/(Tpipe final Tair)PIPING HEAT LOSS TEST RESULTS IN AIRTwo previous papers by the author (Hiller 2006a, 2008a)report UA test results for 0.5 in. (13 mm) and 0.75 in. (19 mm)rigid copper piping, 0.75 in. (19 mm) rolled pex-aluminum-pe

36、x (PAX) piping, 0.75 in. (19 mm) rolled copper piping (allin-air) and 0.75 in. (19 mm) rolled copper piping buried indamp sand. Summary results for those pipes are repeated herefor comparison to the newly tested pipe results. Space limita-tions prevent us from presenting the detailed UA vs. flow rat

37、ecurves for each configuration. The reader should consult theproject final report (Hiller 2010) for the detailed results. Herewe only present summary curve fits of the UA vs. flow ratecurves for the various pipes and configurations tested. Itshould be mentioned here that the summary curve fits shown

38、are chosen partly to produce plots that are “simple” in appear-ance. That is to say, the curve fits are not necessarily ones thatgive the best statistical fit under all conditions, rather they givevalues that are within acceptable accuracy bounds, whilehaving a simple shape (e.g., a constant value a

39、s opposed tovalues that might vary up and down a little between tests withincreasing flow rate). Accuracy of the UA curve fits is typicallywithin better than about 10%. Accuracy limits are discussedmore fully in the project final reports (Hiller 2005a, Hiller2010). Figure 7 shows plots of the summar

40、y curve fit measuredUA values for all the bare and insulated 0.75 in. (19 mm) diam-eter pipes tested in air, as a function of flow rate. The UAvalues of bare pipe are a slight function of temperatures andtemperature difference between the pipe and the surroundings,whereas the UA values of insulated

41、pipe are almost indepen-dent of temperatures and temperature difference. Note that thebare pipe heat loss UA characteristics for all the plastic pipes,including CPVC, which has very low thermal conductivity, arehigher than for the copper pipes tested. It has been suggestedthat this is because the co

42、pper pipe, which is initially shiny, haslower surface emissivity for radiant heat loss than aged copperpipe, which has a dull-brown appearance due to the oxidationlayer that forms with time. We have not yet tested aged copperpipe, but plan to do so.As an important aside, it should be noted that much

43、 of theheat loss off of piping (typically between 30%70%) is byradiation heat transfer to the surroundings, even though thetemperature difference between the pipe and the surroundingsis small. This is because the convective heat loss per unitlength is also small, such that convective and radiative h

44、eatlosses are of the same magnitude. The overall heat losses,however, are not small because of the significant lengths ofpiping normally employed in piping systems.Figure 8 shows plots comparing UA vs. flow rate for0.375 in. (10 mm) and 0.5 in. (13 mm) PEX piping, comparedto 0.75 in. (19 mm) diamete

45、r PEX piping. Partly because of thelower surface areas, the smaller diameter pipes have lowerheat loss UA values. Previous results for 0.5 in. (13 mm) rigidcopper pipe are also shown in Figure 8. Again, the 0.5 in. (13mm) rigid copper pipe tested was new and shiny, as comparedto aged brown copper pi

46、pe. We plan to test aged pipe in thefuture.PIPING HEAT LOSS TEST RESULTSCEILING/ATTIC/FLOORWhen pipe is installed in a partial or full enclosure, suchas an attic, floor, or wall, we expect that it will have differentheat loss characteristics than when in open air far from othermaterials. The purpose

47、 of the ceiling/attic and floor pipingconfiguration tests was to determine how sensitive heat lossFigure 7 Summary curve-fit UA vs. flow rate0.75 in.(19 mm) diameter pipe in-air.2011 ASHRAE 731was to proximity to other common construction materials. Itwas anticipated, for example, that heat loss mig

48、ht be signifi-cantly higher when bare pipe was touching gypsum wallboardversus when it was spaced away from the gypsum wallboard.(This assumption proved only partially correct as notedbelow.) Table 2 shows the different configurations that weretested in the ceiling/attic and floor test fixtures. The

49、 insulatedcases were achieved using unfaced fiberglass batt attic insu-lation, a single layer for R-19 hft2F/Btu (3.36 m2K/W) anda double layer for R-38 hft2F/Btu (6.72 m2K/W).Figure 9 shows UA vs. flow rate results for bare 0.75 in.(19 mm) rigid CU on and off (3.5 in. or 89 mm above) thegypsum wallboard compared to the same pipe tested in air. Itwas interesting to note that having the uninsulated pipe liedirectly on the gypsum wallboard did not have significantlyhigher heat loss UA values than suspending it above thegy