ASHRAE 4724-2004 Method of Thermal Metering on the Air Side for Fan Coil Units《风机盘管对空方的热计量方法》.pdf

1、4724 Method of Thermal Metering on the Air Side for Fan Coil Units ABSTRACT Energy cost allocation systems apportion cooling and/or heating costs among the individual apartments in centrally metered buildings based on various methods of energy meter- ing. The most commonly used systems measure one o

2、r more parameters related to the thermal output of the terminal element but do not measure enough parameters to provide an accurate energy measurement because of many reasons, such as investment, installation, maintenance cost, etc. They pi- cally assume that cooling or heating output is linearyprop

3、or- tional to a quantity, which may be a single temperature, a difference between two temperatures, or an average of two temperatures. Most of them only perform well over a certain limited range ofapplication conditions (Hewett 1994). To solve this problem, this paper puts forward a new method calle

4、d “cooling/heating metering on the air side, which uses jive parameters, including indoor air temperature and humidity, entering water temperature, airflow rate, and water flow rate, to calculate the thermal output of the terminal elements. These neededparameters can be obtained by a direct or indir

5、ect way, which is discussed at length in this papel: Theoretic analysis shows that the metering errors of the method are lower than 25% in most cases and will be further reduced when some measurements are made in the actual situation. The new method is comparatively economicalandeficient in air-cond

6、i- tioning thermal metering. INTRODUCTION Coolingheating cost allocation in apartment buildings began in the U.S. as a cottage industry in the early 1980s as a result of the 1970s oil embargo. Much literature shows that energy cost allocation is an effective way to influence resi- Zhiwei Lian Yixion

7、g Hu Zhijian Hou dents energy behavior, typically reducing energy use by 15% or more (McClelland 1980, 1983; Hewett et al. 1989; Paler- mini 1991). In China, energy cost allocation is currently a small industry. Before the 1990s, there was little incentive to conserve energy in China. The Chinese pe

8、ople havent the least consciousness of energy conservation because the govemment paid for most of the cooling /heating cost. However, with the development of a market economy in recent years in China, more and more buildings equipped with central air-conditioning systems are rented or offered for sa

9、le. The govemment does not pay for the energy cost any longer. It tends to be allocated among the tenants, and the tenants have begun to care about how much they pay for energy use. Furthermore, the government has given increasing attention to effective incentives to conserve energy. Therefore, a cu

10、rrent trend in the multifamily housing industry of China is to convert coolingheating systems to tenant-metered ones (Yixiong and Ya0 2001). The energy costs associated with the operation of central hydronic cooling/heating systems can be allocated to residents of multifamily housing by using one of

11、 several types of indi- vidual energy metering devices. Over the past years, a number of approaches and relevant devices have been developed. They include the following: 1. Volumetric thermal meters, which measure the volume of water flow through the fan coil unit. These meters do not account for ch

12、anges in the delivery temperature of water, and their accuracy is therefore questionable. Time meters, which measure the amount of time the cool- ingheating system is used in the apartment. These meters are popular in China in spite of their poor accuracy because they are very cheap in price. 2. Ye

13、Ya0 and Zhijian Hou are doctoral students and Zhiwei Lian is a professor at the Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China. Yixiong Hu is an associate professor at the Civil Engineering Institute of Central-South University, Chang- sha, Hunan, China. 02

14、004 ASHRAE. 325 3. Relative-Btu meters, which measure the temperature drop of water as it passes through the fan coil unit, assuming a constant water flow rate. This assumption is viable only if there is not a zone valve in each fan coil unit that stops the flow of water through the coil when the fa

15、n is off (Scott 199 1). Unless flow restrictors are used in each fan coil unit, the error of the meters may exceed 50% at times (Freischlag 1986). Comfort level monitors, which measure and accumulate the difference between ambient room air and outside air temperatures. These meters work based on “eq

16、ual cost for equal comfort amenity,” rather than “equal cost for equal energy use” (Anderson 1993). These types of devices need little investment. However, they may bring about great error as well as waste of energy when the residents open all of the windows of the apartments during the cooling or h

17、eating. Actual-Btu meters, which measure the inlet water temper- ature, outlet water temperature, and water flow volume using a flowmeter. They are quite accurate (typically 98% Scott 1991), but their use is limited in China because of the relatively high cost to install and maintain them. The meter

18、s mentioned above are very imprecise, except for the actual-Btu meters, because they do not measure enough parameters. Most of them only perform well over a certain limited range of application conditions. In order to solve this problem, this paper puts forward a new method called “cool- ingheating

19、metering on the air side” that uses five parameters, which include indoor air temperature and humidity, entering water temperature, airflow rate, and water flow rate, to calcu- late the thermal output of the fan coil units using the theory of coil heat exchange. These parameters can be obtained by a

20、 direct or indirect way, which is discussed in detail in this paper. Then, the metering precision is analyzed and several measures are presented to reduce its error and make it perform well over a wide range of application conditions. 4. 5. THERMAL OUTPUT CALCULATION FOR FAN COIL During operation, t

21、he instantaneous cooling/heating capacity of a fan coil unit changes from time to time. In a period of AT, the thermal output QAT of a fan coil unit can be divided into two parts, the fluctuant one and the stable one (see Figure I), and is expressed by where k = the dividend number in the period AT;

22、 Aq(T + n . AT / k) = the fluctuant thermal output rate at the time (T + n . AT I k), W and CL(4 = the stable thermal output rate at the time T, w. The fluctuant thermal output at the time T, Aq(T), is the result of small perturbations of the five input parameters of the fan coil and can be calculat

23、ed by where A = a small variation of the input parameters. The coeficients A, A, A, A, and A, reflect the degree of effect of the respective input parameters on the fluctuant ther- mal output rate Aq(2). These coefficients are difficult to obtain because they change at every turn. In fact, Aq(7) mer

24、ely accounts for a small portion of the total thermal output rate of coil. So long as AT is adequately small, the fluctuant part in Equation 1 can be neglected. CL(T) is related to the input parameters of coil that include inlet air temperature, inlet air humidity, airflow rate, entering water tempe

25、rature, and water flow rate (Zhiwei et al. 2001). It can be computed by Equations 3 to 9. The calculating flow chart is shown in Figure 2. ta,-u,i - 1 - exp -NTU( 1 - C,) E,=- tw,l-tu,l l-C,exp-NT(l-C,) -1 = - g=i.O r Using Equation 9a to obtain CL Using Equation 9b to obtain CL I Using Equation 8 t

26、o calculate E;! r 7 -1 I I Calculating t, by Equation 8 , then obtain 3 II Calculating fi, by Equation 3b I 1 Calculating K, andcby Equation 6 and 7, respectively Figure 2 Flow chart of computing the fan coil 3 stable thermal output. CL = Ga(ta,2 - t,l) = GwCw(,1 - tw,2) (9b) where A, B, m, n, and z

27、 are coefficients obtained by experi- ment. The subscript“1“denotes inlet; the subscript“2“denotes outlet. The thermal output, - N Equation 10 shows that the water flow rate of every indi- vidual fan coil unit can be obtained from that ofthe main water supply instead of measuring by a flowmeter on e

28、ach coil. The correction coefficient, k, which is obtained through prelimi- nary debugging in the field, reflects the difference in the resis- tance of watercourse among all fan coils in the system. Equation 10 may be fairly accurate in estimating the water flow rate of an individual fan coil if k,

29、is fixed. However, it is certain to cause errors in practice. As a result, the precision of thermal metering will be influenced negatively by the errors of estimation. To see the degree of influence, it is necessary to analyze how the water flow rate influences the thermal output of fan coil. In ord

30、er to make a comprehensive analysis, six typical conditions of inlet air are given in Table 1. Citing the I Cooling conditions 42,l (“Cl 24.0 RH (%) 50 thermal performance data of a fan coil unit from a certain manufacture (York 1999) graphs showing the relationship between a fan coils coolingheatin

31、g capacity and the water flow rate are obtained (see Figure 3). It is obvious from Figure 3 that the water flow rate has less influence on the coils thermal output as it increases. When it reaches a certain value, the influence will become small. For example, in the case of 600 kgh in the water flow

32、 rate and the type-I condition of inlet air, the fluctuation of thermal output is within the range of5 170 W (7.1 YO) under cooling conditions and +270 W (6.4%) under heating conditions, even ifthe water flow rate changes at a considerable value of h120 kglh. This indicates that the accuracy of ther

33、mal metering will not be influenced greatly as the water flow rate of individual fan coil is estimated by Equation 1 O. Besides, if all of the fan coils in the system are in parallel connection, the water flow rate of every fan coil fluctuates in the same geometric proportion while the total water f

34、low rate of the main water supply changes. Therefore, it does not influence the equity of charg- ing for air conditioning as the cooling/heating cost is allocated among the tenants according to the thermal meters. II III IV V VI 26.0 27.0 27.0 27.0 28.0 55 50 55 60 55 Heating conditions ta,1 (“C) 18

35、.0 19.0 20.0 21.0 22.0 23.0 RH(%) 50 50 55 60 65 70 L i 300 360 420 480 540 600 660 720 78 Chilled water flow rate (kg/h) Figure 3 Injluence of water flow rate on the thermal output of fan coil. 3000 I 300 360 420 480 540 600 660 720 780 Hot water flow rate (kgh) 328 ASHRAE Transactions: Research -

36、35 40 45 50 55 60 65 Inlet chilled water temperature (r) Inlet hot water temperature (r) Figure 4 Influence of inlet water temperature on the thermal output of fan coil. Inlet Water Temperature Inlet water temperature may be one of the important parameters that affect the thermal output of fan coils

37、. As shown in Figure 4, the thermal output of coils fluctuates within the range of about *200 W under the cooling condition and about *lo0 W, respectively, as the inlet water temperature changes only 1C. It indicates that the temperature of inlet water should be measured for the thermal metering of

38、fan coils. However, a thermometer on each fan coil is not needed, for the fan coils are the systems terminal equipment and their inlet water temperatures are closely related to the temperature of supply water from the chillers or boilers. The authors have investigated several fan coil air-conditioni

39、ng systems in hotels and find that the difference between the inlet water tempera- ture of fan coils and the supply water temperature from central hydronic cooling/heating systems is very small (about 0.3“C to 1 .O“C under cooling conditions and 1 .O“C to 2.0“C under heating conditions). Thus, the i

40、nlet water temperature of fan coils-one of the important parameters for thermal meter- ing-can be obtained from the central coolingheating station instead of measuring. It is reasonable because the changes in the delivery temperature of water indicate the thermal loss from the supply water pipes and

41、 the tenants should be obli- gated to pay for losing thermal energy when they enjoy the central air-conditioning systems (Ye and Hu 2001). The inlet water temperature, tw of every fan coil may be calculated by Airflow Rate Average fan coil units have three grades of nominal airflow rate-enoted as hi

42、gh, mid, and low-that are controlled by the fans speed regulator. So the airflow rate of fan coils may be obtained from the state of the fans speed regulator instead of measuring. It is stipulated in Chinese Industry Standard (The Fan Coil UnitJB/T4283-91) that the nominal airflow rate should be mea

43、sured under the special condition that the coil does not have a water supply and the static pressure between the air inlet and the air outlet equals zero. Obviously, the running conditions of fan coils differ greatly from that described in the standard. The actual airflow rate of the fan coil will b

44、e 10% to 20% lower than the nominal one because the air resistance increases in actual situations (Zhongyuan 1996). It is easy to see fiom Figure 5 that airflow rate has a great effect on the thermal output of fan coils, which indicates that considerable errors will occur in the thermal metering if

45、the nominal airflow rate simply substitutes for the actual one without proper correction. The airflow rate, Ga, of the fan coil during running may be calculated by The correction coefficient, kfan, is ascertained by the performance of the fan in the fan coil unit and the real position ofinstallation

46、. Inmost case, kfann=0.8-0.9(Zhongyuan 1996). Inlet Air Temperature and Humidity The inlet air temperature and humidity of fan coils are closely related to the conditions of indoor air that, to a certain degree, reflect the air-conditioning thermal load of the occu- pants room. In the case of coolin

47、g conditions, inlet air temper- ASH RAE Transactions: Research 329 3400 3200 3000 3 2600 . .- o o 2200 o . . 1800 . w,ate.flw.r.te.lGw4.801h. i:nIet water temperature tw=7C 250 300 350 400 450 500 550 Air flow rate (rn3/h) 4000 3800 z 3600 3400 m Q m o c m al O) 32oa 3ooa .- - 280C 260: 1 Figure 5 I

48、nfluence of airflow rate on the thermal output of fan coil. ature and humidity produce a great effect on the thermal output of the fan coils, while in the case of heating conditions, the effect of the two parameters on the fan coils thermal output seems small. As shown in Figures 3 to 5, when the in

49、let air condition changes from one to another, the cooling capacity of the fan coil fluctuates a lot while the heating capacity fluctu- ates a little. It indicates that the inlet air temperature and humidity need to be known in the cooling metering, while in the heating metering, inlet air temperature may be enough on the air side. To make the thermal meters be more efficient under different conditions, the inlet air temperature and humidity of fan coils should be measured by relevant super- visory instrume