ASHRAE 4712-2004 Performance Analysis of Low-pressure Household Water Heaters《低气压家用热水器性能分析》.pdf

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1、4712 Performance Analysis of Low-Pressure Household Water Heaters P. K. Bansal, Ph.D. Member ASHRAE ABSTRACT Thispaper analyzes theperformance of a heat exchanger type, low-pressure household water heater that is commonly used in the northern part OfAustralia. The water in the tank is stored at atmo

2、spheric pressure and is heated with an electric resistance heater to about 75C. The cold water at lowpressure jlows through a heat exchangec which is immersed inside the wafer tank. Australian test standard AS1361 (1995) is used to compare the performance of six diflerent designs that use diserent h

3、eat exchangers for improving the energy eficiency of the water heater. It explores the heat transfer characteristics of one of the heat exchanger designs (HX2) and presents the relative performance of the others. It presents actual designs tested under standard conditions. The test conditions were s

4、et to draw of hot water at a constant jlow rate of 9 liters per minute until the outlet water temperature from the heat exchanger) was at least 30C warmer than the entering cold wateror continuous “rawoj-”ofwarrn waterup to 7minutes, whichever is less. The duration for this temperature drop illus- t

5、rates the delivery capaciq of a heat exchanger: the longer the time taken for the temperature drop, the more usable the hot water is delivered. The results could help the manufacturer develop a base for design of a cost-eflective water heater with higher eficiency. INTRODUCTION A company that manufa

6、ctures various types of water heaters for both local and overseas markets, such as Australia, America, and China, provides huge selections of water heaters in different forms and sizes to suit industrial or household purposes. Currently a new water heater product dealing with “low pressure electric

7、heat exchanger type domestic storage water heater” has been introduced solely for the Australian market. This product is the main focus in this investigation and will be referred as “LPHX” herein. LPHX water heaters were design-targeted to people with lower incomes, who live in a warmer climatic reg

8、ion (in north- em Australia), have lower demand of hot water, and may not be connected with the “mains” water supply system. The LPHX water heater delivers less quantity of usable hot water than the normal storage type heaters with the same storage unit size but stores water at atmospheric pressure.

9、 This feature may be considered as a disadvantage to the LPHX water heater, compared with the other water heater designs, but it is suitable for the unique market of Australia. Although there are several investigations in the literature on normal water heaters (e.g., Probert et al. 1991, 1993; OBrie

10、n et al. 1998), there is not a single study being done on low-pressure water heaters and hence this investigation. The main objective of the study was to investigate the best possible design of the heat exchanger to optimize the rate of heat extraction from the heater and thereby improve the energy

11、effi- ciency of LPHX water heaters. In this process, Australian Test Standard AS1361 (1995) was used to compare the perfor- mance of six different designs that use different heat exchanger configurations (see Table I) for the same operating conditions. This paper presents a comprehensive comparative

12、 analysis of the energy-efficiency performance of these six configurations. WATER HEATERS There are three types of water heaters that are generaliy used in households, namely, the instantaneous, the storage, and the heat exchanger coil (also called indirect storage) types. P.K. Bansal is a professor

13、 in the Department of Mechanical Engineering, The University of Auckland, Auckland, New Zeaiand. 196 02004 ASHRAE. 1 c- . ASHRAE Transactions: Research 197 ORaCt Inlet mer Cornpater 0 +bermocuuples (;rs trlow meter II wiring Figure I Diagram of the test rig with heat exchanger type water heater: The

14、 instantaneous unit is designed to heat water only when needed. The unit operates on the principle that when a hot water tap is opened, the flow of water activates a switch caus- ing gas or electriciy to flow to heat the water and this stops when the hot water tap is closed. A storage type water hea

15、ter is designed to hold a limited quantity of hot water in a ther- mally insulated tank ready for immediate use and operates on the displacement principle. The hot water is drawn from the storage tank near the top while cold water enters near the bottom. Hot water, being less dense than the cold wat

16、er, floats on top of the incoming cold water. On the other hand, a heat exchanger (HX) type water heater (Figure 1) employs a very different concept from the storage type water heater. It consists of a thermally insulated tank in which is immersed a heat exchanger, usually of copper. The stored wate

17、r in the tank is not directly used but serves as a heat reservoir to provide thermal energy to raise the temper- ature of water passing through the HX. Stored water in the tank is first heated with an electric element, and its temperature is controlled through a thermostat. As cold water passes thro

18、ugh the copper HX, it picks up thermal energy from the stored water. The water in the coil is heated and is then delivered to the household. When the water draw-off is stopped, the stored water is reheated back to its original temperature (Australian Standard 1361, 1995). In general, storage typeuni

19、ts are much more flexible in performance than the heat exchanger units. An HX type water heater is not as efficient as the storage type, and for comparable performance they are approximately twice the size of a storage unit. However, it is comparatively cheaper to run and is ideal for families with

20、less hot water demand. Also, an HX type water heater avoids water contamination by trapping the delivered water inside the HX pipes (AS/NZS 157 1 1995). The advantages and disadvantages of the three designs are summarized by Yeh (2001). This design may not be popular in the United Kingdom or New Zea

21、land where hot water demand is high in winter, but it is popular in the northern part of Australia where the weather is always warm, and a low delivery capacity water heater is considered to be sufficient without any need for the “mains“ water supply connection. DESIGN OF HEAT EXCHANGER TYPE WATER H

22、EATER The water heater cylinder shell is made of steel, lined with appropriate glass enamel for protection against corrosion, and is insulated with polyurethane. It is classified as the mains pressure water heater to withstand 1000 kPa with nominal capacity of I40 liters of water in the storage tank

23、 with heat exchanger coil being totally submerged inside the tank. It has an electric element, usually rated at 3.6 kilowatts (with 220 - 240 V at 50 Hz), and the thermostat is adjustable between 63C and 93C. The standard heat exchanger is made of copper (to a coil) and is designed to the Australian

24、 Standard ASNZS 1571 (1995). The nominal diameter, thickness, and length of the copper tube are, respectively, 12.7 mm, 0.71 mm, and 19 rn. The coil is 3 15 mm high and has 16 turns. Outside diameter and inside diameter of the copper coil are 362 mm and 337 111111, respectively. Six different types

25、ofheat exchang- ers (see Table 1) were studied during the project, namely, LPHX (as HXl), tall coil (as HX2, where the coil of HXI was stretched from 31.5 cm to 72 cm), tall coil-stretched (as HX3, where the coil of HX2 was frther stretched to 92 cm), finned coil (as HX4, where the coil had 75 exter

26、nal fins), short double coil (as HX5, where the outer coil had 11 turns while the inner coil 10 turns), and the radiator (as HX6). These designs were tested under identical operating conditions following AS 1361 (1 995) for assessing the overall perform- ance improvement of the low-pressure heat exc

27、hanger type water heater. TEST RIG AND EXPERIMENTAL PROCEDURE The manufacturer supplied a standard LPHX cylindrical tank, where the thermostat of the electric heater was set at 75T. Australian Test Standard and its Requirements. Australian test standard AS1361 (1995) was followed for assessing the c

28、omparative performance of different heat exchangers in this water heater. The water was drawn off from the heat exchanger at 9 literdmin throughout the tests. The draw-off was continued until the outlet water temperature from the heat exchanger was at least 30C warmer than the entering cold water in

29、 the tank (Le., exit water temperature being around 46.5“C, which is considered to be the lowest temperature at which hot water must be supplied) or continu- ous draw-off of hot water for up to 7 minutes (420 seconds), whichever is shorter. The tests were performed inside the labo- ratory where the

30、ambient temperature was around 17OC. Instrumentation. The heat exchanger (Figure 1) was suitably placed in the tank, and T-type thermocouples (having an accuracy of =kO.l“C) were installed at different evenly spaced heights inside the tank to measure temperatures of stored water in the tank. An elec

31、tronic flow meter (RS 257- 133 type having an accuracy of the differences were usually quite small, in the range of hl0 seconds. 17.3 17.6 16.5 16.4 16.4 17.8 17.7 16.9 16.6 16.6 75.4 75.4 74.7 75.4 75.3 75.5 75.7 75.1 75.4 75.3 326 289 414 389 375 341 338 300 362 N/A* 43.9 42.5 46.5 45.4 45.3 44.0

32、44.3 42.5 44.9 38.0 7 9 1 2 3 5 6 8 4 10 EXPERIMENTAL RESULTS The relative merits of each HX depend on several factors including the cost, space requirement, and the required rate of heat transfer. The experimental results on the relative perfor- mance of these HXs under identical operating conditio

33、ns are presented in Table 2 and Figure 2. It may be noted from the results in Table 2 (column 3) and Figure 2 that in accord with Figure 2 Variation ofelapsed timefur all six diflerent HX designs under ?normal? and ?reverse? flow modes. Note that HX3 was operated only under the normal mode, while HX

34、6 did not meet the standard operation criteria. AS1361 performance criteria, HX2 supplied the hot water (at least 30C warmer than the entering cold water; inlet = 16.5?C, outlet = 46.5?C) at the HX outlet for the longest period (ie., for 414 sec). As with the standard criteria, the tests were discon

35、tinued at 414 sec (this time being shorter than the maxi- mum 420 sec) as soon as the outlet water temperature dropped below 465C. For other heat exchangers, the corresponding elapsed time (as shown in Row 5 of Table 3) was measured for their consistent performance comparison with HX2. No heat excha

36、nger could deliver hot water for the maximum period of 420 sec. Interestingly HX2 outperformed the other heat exchangers in the reverse flow mode also. ASHRAE Transactions: Research 199 -I Elements from Tank Bottom 10 9 Table 3. Inlet and Outlet Water Conditions for Each Tank Element for HX2 Inlet O

37、utlet Temperature (“C) Temperature (OC) 54 58 50 54 - 8 7 6 46 50 42 46 37 42 5 4 3 33 29 33 25 29 37 2 1 21 25 17 21 I E 60- 3 -Inlet water tempenhire OC +OuUet water temperature OC 6- Ti, water temperatura OC, (O.Sm fmrn bottom) q- TZ, water temperature OC. (O.Pm from bottom) - Tt, water temperatu

38、re OC, (0.45m horn bottom) - Ii - TI. water temperature OC, (O.am from bottom) 10 ci 1 i - i -4 - , - I -. o sa 1w)iw2oo2oosw3w46o Elapsed tlme in seconds Figure 3 Variation of experimental water temperatures vs. elapsed time under the “normal”f2ow mode for XH2 for the inlet water; outlet water; and

39、 at different heights of the tank. “- , 06 Figure 4 Variation of efectiveness of various heat exchangers with “withdrawal times. almost similar and hence the added cost ofputting fins on HX4 is unjustified. HX6 was made of different material and the tube length was only half of the coil. This HX cou

40、ld not even raise the water temperature over 46.5”C and proved to be unsatis- factory. EFFECTIVENESS ANALYSIS follows: The effectiveness of a heat exchanger can be defined as ESfectiveness E = a = Actual heat transfer rate Maximum possible heat transfer rate emox (1) where (3) 200 ASHRAE Transaction

41、s: Research Elcmcntai layer of stored water Figure 5 Elemental discritisation of tank height for HX2. where Th,mar is the maximum temperature of stored water in the tank. The variation of effectiveness of various HXs is shown in Figure 4. It is obvious from the figure that the effec- tiveness (E) co

42、ncept is inadequate in analyzing the perfor- mance comparison of HXs because the effectiveness values for most of the heat exchangers are so close to each other (except the radiator type HX 6) that it is difficult to differen- tiate among them. The experiments had already demonstrated poor performan

43、ce of this HX6. The only useful information shown in the figure is that the effectiveness of HXs dqgrades with time. Therefore, an elemental approach was used for the HX analysis, as discussed in the following. FINITE ELEMENT ANALYSIS OF HX 2 Since HX 2 was found to be the best configuration from ex

44、perimental results, this HX was chosen for the finite element analysis. HX 2 is a single coiled, 19 m long HX occupying the entire tank height. For the analysis, the HX was assumed to be a long stretched coil divided into 1 O elements of equal length so that each element was 1.9 m long as shown in F

45、igure 5. Similarly the tank height was also dscritized into ten small elements of equal height. Each element was treated as a small HX with more definable geometry. Experimental results show that the temperature of stored water in the tank varies with time as well as with tank height. This implies t

46、hat the transient heat conduction method is to be used to analyze the problem. Temperatures were recorded at 30-second intervals. The over- all heat transfer coefficient (U) was used as the performance parameter for the analysis. Determining Entering (Tin) and Exiting (Tout) Temperatures Inlet water

47、 for the HX tube in element 1 comes directly from the supply system. When it leaves element 1 and enters element 2 it becomes the inlet water for element 2 and so on, as shown in Table 3. The inlet and outlet water temperatures for each heat exchanger element, T, and Tout, were evaluated ASHRAE Tran

48、sactions: Research Figure 6 Water temperature trend-line prediction at diflerent time steps for Hx2. by dividing the temperature difference between the system incoming cold water and the exiting hot water by 1 1 intervals. Water temperature rises sharply as soon as water enters the HX. It is difficu

49、lt to insert thermocouples through the tube wall and measure the temperature directly, and hence linear temperature rise was assumed for the analysis. Determining the Temperature Profile of Stored Water (i,) The measured temperature profiles of the hot water stored in the tank are shown in Figure 6. The top layers are hotter than the layers underneath. As water withdrawal continued, temperature differences in the layers increased. These profiles were used to develop a polynomial equation and to determine its coefficients as discussed in the following. The st