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Effect of condensing unit layout at building re-entrant on split-type air-conditioner performance
Energy and Buildings 34 (2002) 237±244
Effect of condensing unit layout at building re-entrant on split-type air-conditioner performance T.T. Chowa,*, Z. Lina, J.P. Liub a
Division of Building Science and Technology, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi Province, China Received 2 January 2001; accepted 29 July 2001
Abstract The use of split-type air-conditioners is popular in residential buildings of Hong Kong. In new apartment buildings, the building reentrants are the most frequent choice of the project team for placing the outdoor condensing units. Nevertheless, the effect of heat dissipation from all condensing units at a re-entrant can be detrimental. Improper condenser layout incurs energy wastage, equipment derating and malfunction problems. Because of the continuing changes in outdoor air conditions, individual air-conditioner usage and loading, accurate prediction of the thermal impact at a re-entrant is virtually dif®cult. Based on computational ¯uid dynamics (CFD) analysis incorporating with an energy performance evaluation model, the effectiveness of various condenser layout schemes in building re-entrants under steady-state conditions was explored in this paper. It was found that there are limited hard-and-fast rules to give de®nite solutions. Provided that there is no physical access problem, an alignment of condensing units facing inward to induce a positive in¯ow of air into the re-entrant is a desirable solution. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Air-conditioner performance; Energy conservation; CFD application
1. Introduction In recent years, split-type air-conditioners are commonly standard provisions in new residential projects of Hong Kong. In the form of either single or multisplit units, they are typically serving bedrooms, living and dining rooms, and in some cases, maid rooms and kitchens. Each apartment is therefore likely incorporated with two to four outdoor condensing units. According to the government energy statistical data [1], the consumption of electricity in the domestic sector of Hong Kong has been doubled in the 10year period from 1988 to 1998. One reason for the increase was the wide spread use of household air-conditioners. Building re-entrant Ð a recessed space between two neighbouring apartments Ð is by far most common to accommodate the condensing units of the split-type airconditioners. Fig. 1 shows an example of arrays of condensing units installed at a tall building re-entrant. Thermal buoyancy, as a consequence of heat dissipation from these condensing units, drives an upward air movement within the
* Corresponding author. Tel.: 86-2788-7622; fax: 86-2788-9716. E-mail address: [email protected] (T.T. Chow).
re-entrant space. Outside air enters this congested space via the narrow opening at the front end and leaves at the top; the ¯ow of the warm air plume is assisted by the wind effect if the wind comes in the right direction. Under unfavourable climatic conditions in summer, however, insuf®cient air exchange at this congested space elevates the air temperature, particularly at the upper portion of the re-entrant. Accordingly, this signi®cantly deteriorates the condenser working conditions at the upper ¯oors, owing to inadequate cooling as a result of high refrigerant working pressure. The accumulating effect is an overall degradation in the capacity and ef®ciency of the air-conditioners of the entire building. The condenser heat dissipation problem is expected intensifying at taller buildings, where more condensing units are installed. Since most condensing units cannot function properly at an on-coil temperature (i.e. the temperature of cooling air that enters the condenser coils) above 458C, this is therefore not only an energy wastage problem but also an equipment operation problem. At present, several apartment buildings with 60 ¯oor levels or more are being constructed in Hong Kong; some more are planned for construction. How to arrange condensing units at a re-entrant to achieve desirable equipment performance is a challenging subject to the air-conditioner design engineers.
0378-7788/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 8 8 ( 0 1 ) 0 0 1 1 1 - 6
238
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
Nomenclature B BF Cm C1 C2 C3 CGPI COP G g K n p RF T V
body force vector body force coefficient to calculate the turbulent viscosity, 0.09 coefficient in the turbulent kinetic energy dissipation equation, 1.44 coefficient in the turbulent kinetic energy dissipation equation, 1.92 coefficient in the turbulent kinetic energy dissipation equation, 1.0 condenser group performance indicator coefficient of performance source term to deal with buoyancy factor gravity vector (0, 0, 9.8) turbulent kinetic energy number pressure resistance speed factor air temperature velocity vector
Greek letters b constant coefficient introduced by Boussinesq approximation e turbulent energy dissipation F the source term produced by shear me turbulent viscosity r fluid density se turbulent Prandtl number for e, 1.3 sK turbulent Prandtl number for K, 1.0 sT Prandtl number of air Subscripts m mean o on-coil r room ref referenced 2. Air-conditioner performance evaluation 2.1. Use of energy performance index One of the complications in the thermal analysis lies in the diversity in equipment operation. In a tall apartment building, not all air-conditioners are in service at any moment. For those in operation, the thermal loads vary dynamically. On the other hand, the air¯ow and temperature distribution within a re-entrant are governed not only by the equipment operation conditions but also by the climatic conditions such as outdoor air temperature and velocity. Field measurements in a real building may take care of these uncertainties. However, the execution is costly and time consuming judging from the scale of work required. A huge amount of test
Fig. 1. Array of condensing units at a building re-entrant.
data must be available in order to identify the worst operating conditions, and to generalise the results for evaluating alternative design schemes. Laboratory work on the other hand, allows a good control of the testing conditions; the dif®culties in anemometry measurements involving slow air movements in a scaled-down physical model and the inherent similarity problems in ¯uid ¯ow sometimes impede the experimental accuracy and representation [2]. Measurements with care, however, offer valuable data sets that are crucial for computation work validations and benchmark testing. In order to have a systematic and consistent evaluation, an energy performance index ``condenser group performance indicator'' (CGPI) was introduced and found very helpful in performing comparative studies via computer simulation [3]. To evaluate the energy performance of n number of air-conditioning units working together as a group, say those at a building re-entrant, CGPI can be used. This term quanti®es the average percentage drop in coef®cient of performance (COP) of all n number of air-conditioners. By de®nition, n 100 X COPTr T0 CGPITr Tref 1 (1) n i1 COPTr Tref i for any referenced on-coil temperature Tref and room temperature Tr. Tref can be an outdoor temperature arbitrarily assigned, say 338C which is the summer design condition of Hong Kong [4]. It was shown that the corresponding CGPI expression for T r 25 C and T ref 33 C can be given by CGPI25 33 2:72Tom
89:76
(2)
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
where Tom is the mean on-coil temperature of all condensing units. 2.2. Airflow analysis by computational fluid dynamics The computational ¯uid dynamics (CFD) is a suitable tool to help understanding the complex ¯uid ¯ow phenomena, like air¯ow in and around buildings [5±7]. With the Boussinesq approximation, the K e turbulence models are able to predict buoyancy-driven ¯ow with various physical scales [8,9]. It is therefore applicable to determine the on-coil temperature at each condensing unit installed at a re-entrant. Our previous testing and validation work using the CFX 4.2 software [10] showed that the standard K e model [11,12] could achieve high level of accuracy in predicting buoyancydriven air¯ow in building light wells and re-entrants. The simulation results were in good agreement with analytical solutions and laboratory data [13]. The laboratory data was obtained from an experimental study of the effect of gas water heater discharges in a light well using a physical model of a high-rise apartment building. The experimental rig was a 1:100 reduced model that represented a 41-storey building installed with 480 gas water heaters. CFD analysis was executed based on the same experimental conditions. Both the air¯ow pattern and the air temperature pattern at the light well as predicted by the CFD simulation were found closely matched with the experimental results. For solving a steady-¯ow thermal problem with the standard K e model, the basic transport equations in a 3D vector space are as follows: Continuity: rV 0
(3)
Momentum: V rV
rp 1 r me rV r r
bg T
Tref
Thermal energy: 1 m V rT r e rT r sT Turbulent kinetic energy: 1 me rK F G V rK r r sK Turbulent kinetic energy dissipation: 1 m V re r e re r se e C1 F C3 max G; 0 K
(4)
(5)
K2 e
In Eqs. (6) and (7), the source term F is the shear production defined by m (9) F e rV rV rVT r and the source term G dealing with the buoyancy factor is defined by m G e bg rT (10) rsT The meaning of all other symbols are listed in the `Nomenclature'. In the simulation work below, hypothetical building reentrants with 20 ¯oor levels, corresponding to an overall building height of 60 m, were used in the numerical simulation. Non-uniform 3D rectangular grids with multiblock representation were employed. The enclosing walls were taken as adiabatic, and applied with logarithmic wall functions Ð the same treatment as in the validation test [13]. Only one single type of condensing units with a rated capacity of 4 kW was used throughout the study. The casing of a condensing unit was represented by four solid surfaces, leaving the front and rear ends open for cooling air delivery. The combined condenser fan and coil were modelled by adding appropriate body forces to the enclosed cells, in that B BF
RF jVjV
(11)
where BF is the body force and RF is the resistance speed factor; each of these carries three values in the rectangular coordinate system. Volumetric heat energy was added to the enclosed cells to simulate the condenser heat dissipation. All investigations were based on no wind situation. In the computation domain, a vertical zero-velocity boundary was used at 15 m ahead of the open end of the re-entrant. Two vertical symmetry boundaries were used at 13.5 m away from the side walls of the re-entrant. Other than the solid surfaces, mass flow boundaries were used at 5 m above the re-entrant top (horizontal) and at 5 m downstream the rear wall (vertical). Detailed description of the technique including the underlying assumptions and boundary conditions can be found in Chow et al. [3]. 3. Effect of condenser orientation
e
(6)
C2
e2 K
(7)
Turbulent diffusivity: me Cm
239
(8)
The condenser orientation de®nes the condenser fan positions. These fans are the only mechanical devices in the ¯ow ®eld, which is otherwise dominated by natural- and plume-convection [14]. This part of the study was based on a small re-entrant 3 m 3 m carrying two condensing units per ¯oor level. At each ¯oor level, a 0.6 m H 0:3 m (W) beam is lying horizontally across the front entrance of the reentrant (Fig. 1 referred). The following four cases of condenser layout, as depicted in Fig. 2(a)±(d), were studied: Case 1: The pair of condensing units is mounted face-toface at the centre position of the side walls.
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T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
Fig. 2. Four different condenser arrangement schemes at a 3 m 3 m building re-entrant.
Case 2: The units, labelled positions A and B, are mounted at one of the side walls, discharging the warm air towards the opposite wall. Case 3: The units are mounted on the end wall, discharging the warm air towards the re-entrant front. Case 4: The units are mounted on the beam at the reentrant front, discharging the warm air inward.
In Case 4 above, the merit of hiding the condensing units behind the beams is to maintain a neat external view of the building. In the four simulation cases, all units were assumed operating at 50% of the rated capacity. The results of the on-coil temperature of the condensers are given in Fig. 3(a)±(d). Fig. 4(a)±(d) show in each of the four cases, respectively, the temperature contours on the
Fig. 3. Rise of condenser on-coil temperature with floor level at 3 m 3 m building re-entrant.
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
241
Fig. 4. Temperature contours across the condensing units on the 11/F.
11/F (11th ¯oor) at a horizontal plane cutting across the condensing units (i.e. approximately at the middle level of the 20-storey building). In the three cases: Cases 1, 3 and 4, it can be observed that the temperature curves of the two condensing units A and B are indistinguishable. This is owing to the symmetry in equipment layout. Adverse temperature environment is found at the ®rst ¯oor level of Case 3. This is because re-circulating ¯ow occurs at the dead-end corner where the condensing units lie. The CGPI values of Cases 1±4 as determined by Eq. (2) are, respectively, 12.98, 8.14, 18.12 and 8.77. The layout in Case 3 appears to be the worst and incurs a COP drop of about 18% in average, as re¯ected by the CGPI value. The adverse situation is because the direction of warm air discharge is opposing the in¯ow of outside air towards the condensing units. This lowers the air exchange rate. Comparatively, Case 2 gives the best performance. The position of the outer condensing unit A allows portion of the hot air stream slipping out of the re-entrant at the opposite wall. In Case 4, it was discovered that in this shallow re-entrant the condensers are ``submerged'' in the rising thermal plume. Hence, the temperature of the cooling air entering the condensers are generally higher than the outdoor temperature. The overall energy performance is expected to be better in Case 4 if the re-entrant is a lengthy one. Among the four cases, the condenser performance in Case 1 is only moderate; but the face-to-face layout in Case 1 is a
practical solution from the user point of view, i.e. the condensing units are readily accessible by the owner of either apartment. 4. Effect of condenser positions This part of the study was originated from a belief that introducing staggered arrangement of the condensing units can improve the overall energy performance. The idea was that the staggered arrangement hopefully allowed a more uniform distribution of warm air, and hence eliminating the chances of developing hot spots at the vicinity of the cooling air intakes. The investigation was based on a 6 m 3 m rectangular re-entrant with four condensing units (A±D) per ¯oor level. Six cases of layout arrangements, as shown in Fig. 5(a)±(e), were compared. These include the following cases: Case 5: The four units are arranged symmetrically faceto-face, resulted in an array of four columns of condensing units throughout the building height. Case 6: The two units at one side are stacked one above the other and at equal vertical spacing (i.e. with a centreto-centre separation of 1.5 m) with unit B above units A and D above unit C. This leaves two staggered columns of 40 condensing units throughout the building height.
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T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
Fig. 5. Condenser arrangements at 6 m 3 m building re-entrant.
Case 7: The four units are placed at one side in two columns with one stacked above the other, i.e. unit B above units A and D above unit C as in Case 6. Case 8: The four units form four staggered columns of condensers at two opposite walls throughout the building height. Case 9: The units are in the same staggered layout as in Case 8 for the lower 10 floor levels, but the layout is shifted to the ``mirror'' arrangement (shown in dotted lines) for the upper 10 floor levels. Case 10: The units are in staggered positions, but the layout is shifted to the mirror arrangement at alternate floor levels, i.e. one for odd-number levels and the other for even-number levels. The simulation results of condenser on-coil temperature at all positions are given graphically in Fig. 6(a)±(f). For the six cases from 5 to 10, respectively, the CGPI values were found to be 10.38, 11.95, 11.91, 11.19, 11.03 and 10.76. These values fall within a narrow range of less than 2% difference in COP drop, indicating that the position effect is
not fairly signi®cant. The overall energy performance is the best with the in-line arrangement in Case 5. It can be observed from Fig. 6(a) that in Case 5, the temperature condition at the outer positions A and C are much better than the inner positions B and D. In Case 6, the two-column arrangement, the temperature conditions at all four units are very close, as in Fig. 6(b). This is quite different from the other ®ve cases. Moreover, the energy performance is most undesirable. In Fig. 6(c) for Case 7, there is a marked difference in the on-coil temperature of the outer and inner columns of condensers. The overall performance appears unfavourable. This is not the same as the prediction in Case 2. The observation was that in Case 7 the outermost positions A and B are of considerable distance from the front entrance and therefore the rising thermal plume stays well inside the re-entrant. The ``slipping'' effect does not occur. In Case 8, position A is found in a better condition than position C in energy performance; however, there appears no much difference between the inner positions B and D. The energy performance in Case 9 is slightly better than Case 8, though there are more ¯uctuations in on-coil temperature as a result
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
243
Fig. 6. Variation of condenser on-coil temperature with floor level at 6 m 3 m building re-entrant.
of condenser position shift after the 10/F. Fluctuations can be observed again in the results of Case 10. But the energy performance is relatively good and happens to be the second best among the six cases. 5. Discussions A comparison of the CGPI values in the ®rst four cases indicates that Case 4: `installing the condensing units along the beam' can be a good choice. In reality this layout may incur the technical problem of equipment access, and hence may not be a practical solution. Case 2: `condenser at one side wall' gives the best result, but this is not always true especially when there are more columns of condensers in the array; those condensers located deep in the re-entrant are unable to receive outside air and their performance deteriorate considerably. Case 1 is then a workable arrangement. It can be seen that in all cases, the temperature environment is better at a condenser position closer to the front end of the re-entrant, where the condenser receives more outside air. The staggered effect cannot improve the overall performance of the air-conditioners. Nevertheless, the performance appears better if the condensing units are placed far apart rather than concentrated at several locations. The possibility of direct suction or entrainment of hot discharge air from one condenser into another should be avoided.
The above results and discussions are based on the hypothetical study of a 20-storey apartment building. Experience tells that the ®ndings are extendable to taller buildings as well. In a taller re-entrant, the ventilating ¯ow is expected higher since the thermal buoyancy increases with the overall heat rejection rate. With the face-to-face condenser arrangement as in Fig. 2(a), the CGPI values were obtained for a range of building height up to 60 ¯oor levels. Fig. 7 shows graphically the variation of CGPI with building height. It can be seen that the CGPI increases with building height at a diminishing rate. While it has a value of 10.38 for a 20-storey building, it increases by 50% when the building height is doubled (i.e. 40-storey), and by 87% when the building height is tripled (i.e. 60-storey).
Fig. 7. CGPI values for different overall building height.
244
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
Generally speaking, the thermal environment produced by condensing units working inside a re-entrant can be complicated. In most situations, the shape of the re-entrant is not exactly rectangular and the equipment utilisation factor varies. Whenever, there is uncertainty about the equipment performance, the method of analysis introduced here is able to provide very useful information to support the rationale in making decision. 6. Conclusions The placement of condensing units at a tall building reentrant is crucial since any improper layout can result in substantial energy wastage and deteriorated equipment operation. With the presence of more condensing units at taller buildings, the problem can be intensi®ed because of the accumulative effect of heat energy pick-up, though the trend of increase is at a diminishing rate. CGPI is a useful indicator to evaluate different layout schemes. A number of cases have been compared and discussed in this paper. In the ®rst place, it is most desirable to maximise the in¯ow of outside air to individual condensing units. Therefore, the condensing units should be placed as close as possible to the re-entrant front. Provided that there is no physical access problem, one desirable solution is to have an alignment of condensing units facing inward to induce a positive in¯ow of air into the re-entrant. Placing the units at the end wall is undesirable since the fan pressure head works against the ingress of outside air. Placing the units along the two side walls is comparatively better. A face-to-face layout appears to be appropriate. There is no evidence that the use of staggered layout can improve the condenser on-coil condition, though position interchange at alternative ¯oor can be helpful. In all cases, the condensing units should be placed far apart where possible. The thermal environment developed by condensing unit operation at a re-entrant is virtually complex. There are limited hard-and-fast rules to give concrete recommendations to the condenser arrangement. Whenever there is hesitation about the possible outcome, CFD and the energy performance model are very useful tools to predict and evaluate different equipment operating schemes.
Acknowledgements The research project was ®nancially supported by the Strategic Research Grant of the City University of Hong Kong. References [1] HKSAR, Hong Kong Energy Statistics: Annual Report, 1998 Edition, Census and Statistics Department, Hong Kong, 1999. [2] T.T. Chow, Z. Lin, Prediction of on-coil temperature of condensers installed at tall building re-entrant, Applied Thermal Engineering 19 (1999) 117±132. [3] T.T. Chow, Z. Lin, Q.W. Wang, Effect of building re-entrant shape on performance of air-cooled condensing units, Energy and Buildings 32 (2000) 143±152. [4] ASHRAE, ASHRAE Handbook: Fundamental, Atlanta, USA, 1993 (Chapter 24). [5] T. Kurabuchi, N. Kobayashi, A. Arashiguchi, T. Anai, Application of wind tunnel experiment and CFD simulation on estimation of wind environment inside and outside a large-scale building complex with an atrium space, in: Proceedings of the 6th International Conference (ROOMVENT'98), Stockholm, Sweden, June 1998, pp. 1±8. [6] H. Boyer, A.P. Lauret, L. Adelard, T.A. Mara, Building ventilation: a pressure airflow model computer generation and elements of validation, Energy and Buildings 29 (1999) 283±292. [7] Y. Li, A. Delsante, J. Symons, Prediction of natural ventilation in buildings with large openings, Building and Environment 35 (2000) 191±206. [8] J.E. Lovatt, A.G. Wilson, Stack effect in tall buildings, ASHRAE Transactions 100 (Part 2) (1994) 420±431. [9] M.J. Cook, K.J. Lomas, Buoyancy-driven displacement ventilation flows: evaluation of two eddy viscosity turbulence models for prediction, Building Services Engineering Research and Technology 19 (1) (1998) 15±21. [10] AEA, CFX 4.2 Flow Solver User Guide, Computational Fluid Dynamics Services, Harwell Laboratory, UK, 1998. [11] B.E. Launder, D.B. Spalding, The numerical computation of turbulent flow, Computer Methods in Applied Mechanics and Engineering 3 (1974) 269±289. [12] J.H. Ferziger, M. Peric, Computational Methods for Fluid Dynamics, Springer, Germany, 1996. [13] T.T. Chow, Z. Lin, Q.W. Wang, Applying CFD simulation in analysing split-type air-conditioner performance at buildings, Architectural Science Review 43 (3) (2000) 133±140. [14] A.G. Li, Prediction of natural convection from an array of horizontal line heat sources in a large space, in: Proceedings of the 6th International Conference (ROOMVENT'98), Stockholm, Sweden, June 1998, pp. 609±616.
Effect of condensing unit layout at building re-entrant on split-type air-conditioner performance T.T. Chowa,*, Z. Lina, J.P. Liub a
Division of Building Science and Technology, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi Province, China Received 2 January 2001; accepted 29 July 2001
Abstract The use of split-type air-conditioners is popular in residential buildings of Hong Kong. In new apartment buildings, the building reentrants are the most frequent choice of the project team for placing the outdoor condensing units. Nevertheless, the effect of heat dissipation from all condensing units at a re-entrant can be detrimental. Improper condenser layout incurs energy wastage, equipment derating and malfunction problems. Because of the continuing changes in outdoor air conditions, individual air-conditioner usage and loading, accurate prediction of the thermal impact at a re-entrant is virtually dif®cult. Based on computational ¯uid dynamics (CFD) analysis incorporating with an energy performance evaluation model, the effectiveness of various condenser layout schemes in building re-entrants under steady-state conditions was explored in this paper. It was found that there are limited hard-and-fast rules to give de®nite solutions. Provided that there is no physical access problem, an alignment of condensing units facing inward to induce a positive in¯ow of air into the re-entrant is a desirable solution. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Air-conditioner performance; Energy conservation; CFD application
1. Introduction In recent years, split-type air-conditioners are commonly standard provisions in new residential projects of Hong Kong. In the form of either single or multisplit units, they are typically serving bedrooms, living and dining rooms, and in some cases, maid rooms and kitchens. Each apartment is therefore likely incorporated with two to four outdoor condensing units. According to the government energy statistical data [1], the consumption of electricity in the domestic sector of Hong Kong has been doubled in the 10year period from 1988 to 1998. One reason for the increase was the wide spread use of household air-conditioners. Building re-entrant Ð a recessed space between two neighbouring apartments Ð is by far most common to accommodate the condensing units of the split-type airconditioners. Fig. 1 shows an example of arrays of condensing units installed at a tall building re-entrant. Thermal buoyancy, as a consequence of heat dissipation from these condensing units, drives an upward air movement within the
* Corresponding author. Tel.: 86-2788-7622; fax: 86-2788-9716. E-mail address: [email protected] (T.T. Chow).
re-entrant space. Outside air enters this congested space via the narrow opening at the front end and leaves at the top; the ¯ow of the warm air plume is assisted by the wind effect if the wind comes in the right direction. Under unfavourable climatic conditions in summer, however, insuf®cient air exchange at this congested space elevates the air temperature, particularly at the upper portion of the re-entrant. Accordingly, this signi®cantly deteriorates the condenser working conditions at the upper ¯oors, owing to inadequate cooling as a result of high refrigerant working pressure. The accumulating effect is an overall degradation in the capacity and ef®ciency of the air-conditioners of the entire building. The condenser heat dissipation problem is expected intensifying at taller buildings, where more condensing units are installed. Since most condensing units cannot function properly at an on-coil temperature (i.e. the temperature of cooling air that enters the condenser coils) above 458C, this is therefore not only an energy wastage problem but also an equipment operation problem. At present, several apartment buildings with 60 ¯oor levels or more are being constructed in Hong Kong; some more are planned for construction. How to arrange condensing units at a re-entrant to achieve desirable equipment performance is a challenging subject to the air-conditioner design engineers.
0378-7788/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 8 8 ( 0 1 ) 0 0 1 1 1 - 6
238
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
Nomenclature B BF Cm C1 C2 C3 CGPI COP G g K n p RF T V
body force vector body force coefficient to calculate the turbulent viscosity, 0.09 coefficient in the turbulent kinetic energy dissipation equation, 1.44 coefficient in the turbulent kinetic energy dissipation equation, 1.92 coefficient in the turbulent kinetic energy dissipation equation, 1.0 condenser group performance indicator coefficient of performance source term to deal with buoyancy factor gravity vector (0, 0, 9.8) turbulent kinetic energy number pressure resistance speed factor air temperature velocity vector
Greek letters b constant coefficient introduced by Boussinesq approximation e turbulent energy dissipation F the source term produced by shear me turbulent viscosity r fluid density se turbulent Prandtl number for e, 1.3 sK turbulent Prandtl number for K, 1.0 sT Prandtl number of air Subscripts m mean o on-coil r room ref referenced 2. Air-conditioner performance evaluation 2.1. Use of energy performance index One of the complications in the thermal analysis lies in the diversity in equipment operation. In a tall apartment building, not all air-conditioners are in service at any moment. For those in operation, the thermal loads vary dynamically. On the other hand, the air¯ow and temperature distribution within a re-entrant are governed not only by the equipment operation conditions but also by the climatic conditions such as outdoor air temperature and velocity. Field measurements in a real building may take care of these uncertainties. However, the execution is costly and time consuming judging from the scale of work required. A huge amount of test
Fig. 1. Array of condensing units at a building re-entrant.
data must be available in order to identify the worst operating conditions, and to generalise the results for evaluating alternative design schemes. Laboratory work on the other hand, allows a good control of the testing conditions; the dif®culties in anemometry measurements involving slow air movements in a scaled-down physical model and the inherent similarity problems in ¯uid ¯ow sometimes impede the experimental accuracy and representation [2]. Measurements with care, however, offer valuable data sets that are crucial for computation work validations and benchmark testing. In order to have a systematic and consistent evaluation, an energy performance index ``condenser group performance indicator'' (CGPI) was introduced and found very helpful in performing comparative studies via computer simulation [3]. To evaluate the energy performance of n number of air-conditioning units working together as a group, say those at a building re-entrant, CGPI can be used. This term quanti®es the average percentage drop in coef®cient of performance (COP) of all n number of air-conditioners. By de®nition, n 100 X COPTr T0 CGPITr Tref 1 (1) n i1 COPTr Tref i for any referenced on-coil temperature Tref and room temperature Tr. Tref can be an outdoor temperature arbitrarily assigned, say 338C which is the summer design condition of Hong Kong [4]. It was shown that the corresponding CGPI expression for T r 25 C and T ref 33 C can be given by CGPI25 33 2:72Tom
89:76
(2)
T.T. Chow et al. / Energy and Buildings 34 (2002) 237±244
where Tom is the mean on-coil temperature of all condensing units. 2.2. Airflow analysis by computational fluid dynamics The computational ¯uid dynamics (CFD) is a suitable tool to help understanding the complex ¯uid ¯ow phenomena, like air¯ow in and around buildings [5±7]. With the Boussinesq approximation, the K e turbulence models are able to predict buoyancy-driven ¯ow with various physical scales [8,9]. It is therefore applicable to determine the on-coil temperature at each condensing unit installed at a re-entrant. Our previous testing and validation work using the CFX 4.2 software [10] showed that the standard K e model [11,12] could achieve high level of accuracy in predicting buoyancydriven air¯ow in building light wells and re-entrants. The simulation results were in good agreement with analytical solutions and laboratory data [13]. The laboratory data was obtained from an experimental study of the effect of gas water heater discharges in a light well using a physical model of a high-rise apartment building. The experimental rig was a 1:100 reduced model that represented a 41-storey building installed with 480 gas water heaters. CFD analysis was executed based on the same experimental conditions. Both the air¯ow pattern and the air temperature pattern at the light well as predicted by the CFD simulation were found closely matched with the experimental results. For solving a steady-¯ow thermal problem with the standard K e model, the basic transport equations in a 3D vector space are as follows: Continuity: rV 0
(3)
Momentum: V rV
rp 1 r me rV r r
bg T
Tref
Thermal energy: 1 m V rT r e rT r sT Turbulent kinetic energy: 1 me rK F G V rK r r sK Turbulent kinetic energy dissipation: 1 m V re r e re r se e C1 F C3 max G; 0 K
(4)
(5)
K2 e
In Eqs. (6) and (7), the source term F is the shear production defined by m (9) F e rV rV rVT r and the source term G dealing with the buoyancy factor is defined by m G e bg rT (10) rsT The meaning of all other symbols are listed in the `Nomenclature'. In the simulation work below, hypothetical building reentrants with 20 ¯oor levels, corresponding to an overall building height of 60 m, were used in the numerical simulation. Non-uniform 3D rectangular grids with multiblock representation were employed. The enclosing walls were taken as adiabatic, and applied with logarithmic wall functions Ð the same treatment as in the validation test [13]. Only one single type of condensing units with a rated capacity of 4 kW was used throughout the study. The casing of a condensing unit was represented by four solid surfaces, leaving the front and rear ends open for cooling air delivery. The combined condenser fan and coil were modelled by adding appropriate body forces to the enclosed cells, in that B BF
RF jVjV
(11)
where BF is the body force and RF is the resistance speed factor; each of these carries three values in the rectangular coordinate system. Volumetric heat energy was added to the enclosed cells to simulate the condenser heat dissipation. All investigations were based on no wind situation. In the computation domain, a vertical zero-velocity boundary was used at 15 m ahead of the open end of the re-entrant. Two vertical symmetry boundaries were used at 13.5 m away from the side walls of the re-entrant. Other than the solid surfaces, mass flow boundaries were used at 5 m above the re-entrant top (horizontal) and at 5 m downstream the rear wall (vertical). Detailed description of the technique including the underlying assumptions and boundary conditions can be found in Chow et al. [3]. 3. Effect of condenser orientation
e
(6)
C2
e2 K
(7)
Turbulent diffusivity: me Cm
239
(8)
The condenser orientation de®nes the condenser fan positions. These fans are the only mechanical devices in the ¯ow ®eld, which is otherwise dominated by natural- and plume-convection [14]. This part of the study was based on a small re-entrant 3 m 3 m carrying two condensing units per ¯oor level. At each ¯oor level, a 0.6 m H 0:3 m (W) beam is lying horizontally across the front entrance of the reentrant (Fig. 1 referred). The following four cases of condenser layout, as depicted in Fig. 2(a)±(d), were studied: Case 1: The pair of condensing units is mounted face-toface at the centre position of the side walls.
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Fig. 2. Four different condenser arrangement schemes at a 3 m 3 m building re-entrant.
Case 2: The units, labelled positions A and B, are mounted at one of the side walls, discharging the warm air towards the opposite wall. Case 3: The units are mounted on the end wall, discharging the warm air towards the re-entrant front. Case 4: The units are mounted on the beam at the reentrant front, discharging the warm air inward.
In Case 4 above, the merit of hiding the condensing units behind the beams is to maintain a neat external view of the building. In the four simulation cases, all units were assumed operating at 50% of the rated capacity. The results of the on-coil temperature of the condensers are given in Fig. 3(a)±(d). Fig. 4(a)±(d) show in each of the four cases, respectively, the temperature contours on the
Fig. 3. Rise of condenser on-coil temperature with floor level at 3 m 3 m building re-entrant.
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Fig. 4. Temperature contours across the condensing units on the 11/F.
11/F (11th ¯oor) at a horizontal plane cutting across the condensing units (i.e. approximately at the middle level of the 20-storey building). In the three cases: Cases 1, 3 and 4, it can be observed that the temperature curves of the two condensing units A and B are indistinguishable. This is owing to the symmetry in equipment layout. Adverse temperature environment is found at the ®rst ¯oor level of Case 3. This is because re-circulating ¯ow occurs at the dead-end corner where the condensing units lie. The CGPI values of Cases 1±4 as determined by Eq. (2) are, respectively, 12.98, 8.14, 18.12 and 8.77. The layout in Case 3 appears to be the worst and incurs a COP drop of about 18% in average, as re¯ected by the CGPI value. The adverse situation is because the direction of warm air discharge is opposing the in¯ow of outside air towards the condensing units. This lowers the air exchange rate. Comparatively, Case 2 gives the best performance. The position of the outer condensing unit A allows portion of the hot air stream slipping out of the re-entrant at the opposite wall. In Case 4, it was discovered that in this shallow re-entrant the condensers are ``submerged'' in the rising thermal plume. Hence, the temperature of the cooling air entering the condensers are generally higher than the outdoor temperature. The overall energy performance is expected to be better in Case 4 if the re-entrant is a lengthy one. Among the four cases, the condenser performance in Case 1 is only moderate; but the face-to-face layout in Case 1 is a
practical solution from the user point of view, i.e. the condensing units are readily accessible by the owner of either apartment. 4. Effect of condenser positions This part of the study was originated from a belief that introducing staggered arrangement of the condensing units can improve the overall energy performance. The idea was that the staggered arrangement hopefully allowed a more uniform distribution of warm air, and hence eliminating the chances of developing hot spots at the vicinity of the cooling air intakes. The investigation was based on a 6 m 3 m rectangular re-entrant with four condensing units (A±D) per ¯oor level. Six cases of layout arrangements, as shown in Fig. 5(a)±(e), were compared. These include the following cases: Case 5: The four units are arranged symmetrically faceto-face, resulted in an array of four columns of condensing units throughout the building height. Case 6: The two units at one side are stacked one above the other and at equal vertical spacing (i.e. with a centreto-centre separation of 1.5 m) with unit B above units A and D above unit C. This leaves two staggered columns of 40 condensing units throughout the building height.
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Fig. 5. Condenser arrangements at 6 m 3 m building re-entrant.
Case 7: The four units are placed at one side in two columns with one stacked above the other, i.e. unit B above units A and D above unit C as in Case 6. Case 8: The four units form four staggered columns of condensers at two opposite walls throughout the building height. Case 9: The units are in the same staggered layout as in Case 8 for the lower 10 floor levels, but the layout is shifted to the ``mirror'' arrangement (shown in dotted lines) for the upper 10 floor levels. Case 10: The units are in staggered positions, but the layout is shifted to the mirror arrangement at alternate floor levels, i.e. one for odd-number levels and the other for even-number levels. The simulation results of condenser on-coil temperature at all positions are given graphically in Fig. 6(a)±(f). For the six cases from 5 to 10, respectively, the CGPI values were found to be 10.38, 11.95, 11.91, 11.19, 11.03 and 10.76. These values fall within a narrow range of less than 2% difference in COP drop, indicating that the position effect is
not fairly signi®cant. The overall energy performance is the best with the in-line arrangement in Case 5. It can be observed from Fig. 6(a) that in Case 5, the temperature condition at the outer positions A and C are much better than the inner positions B and D. In Case 6, the two-column arrangement, the temperature conditions at all four units are very close, as in Fig. 6(b). This is quite different from the other ®ve cases. Moreover, the energy performance is most undesirable. In Fig. 6(c) for Case 7, there is a marked difference in the on-coil temperature of the outer and inner columns of condensers. The overall performance appears unfavourable. This is not the same as the prediction in Case 2. The observation was that in Case 7 the outermost positions A and B are of considerable distance from the front entrance and therefore the rising thermal plume stays well inside the re-entrant. The ``slipping'' effect does not occur. In Case 8, position A is found in a better condition than position C in energy performance; however, there appears no much difference between the inner positions B and D. The energy performance in Case 9 is slightly better than Case 8, though there are more ¯uctuations in on-coil temperature as a result
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Fig. 6. Variation of condenser on-coil temperature with floor level at 6 m 3 m building re-entrant.
of condenser position shift after the 10/F. Fluctuations can be observed again in the results of Case 10. But the energy performance is relatively good and happens to be the second best among the six cases. 5. Discussions A comparison of the CGPI values in the ®rst four cases indicates that Case 4: `installing the condensing units along the beam' can be a good choice. In reality this layout may incur the technical problem of equipment access, and hence may not be a practical solution. Case 2: `condenser at one side wall' gives the best result, but this is not always true especially when there are more columns of condensers in the array; those condensers located deep in the re-entrant are unable to receive outside air and their performance deteriorate considerably. Case 1 is then a workable arrangement. It can be seen that in all cases, the temperature environment is better at a condenser position closer to the front end of the re-entrant, where the condenser receives more outside air. The staggered effect cannot improve the overall performance of the air-conditioners. Nevertheless, the performance appears better if the condensing units are placed far apart rather than concentrated at several locations. The possibility of direct suction or entrainment of hot discharge air from one condenser into another should be avoided.
The above results and discussions are based on the hypothetical study of a 20-storey apartment building. Experience tells that the ®ndings are extendable to taller buildings as well. In a taller re-entrant, the ventilating ¯ow is expected higher since the thermal buoyancy increases with the overall heat rejection rate. With the face-to-face condenser arrangement as in Fig. 2(a), the CGPI values were obtained for a range of building height up to 60 ¯oor levels. Fig. 7 shows graphically the variation of CGPI with building height. It can be seen that the CGPI increases with building height at a diminishing rate. While it has a value of 10.38 for a 20-storey building, it increases by 50% when the building height is doubled (i.e. 40-storey), and by 87% when the building height is tripled (i.e. 60-storey).
Fig. 7. CGPI values for different overall building height.
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Generally speaking, the thermal environment produced by condensing units working inside a re-entrant can be complicated. In most situations, the shape of the re-entrant is not exactly rectangular and the equipment utilisation factor varies. Whenever, there is uncertainty about the equipment performance, the method of analysis introduced here is able to provide very useful information to support the rationale in making decision. 6. Conclusions The placement of condensing units at a tall building reentrant is crucial since any improper layout can result in substantial energy wastage and deteriorated equipment operation. With the presence of more condensing units at taller buildings, the problem can be intensi®ed because of the accumulative effect of heat energy pick-up, though the trend of increase is at a diminishing rate. CGPI is a useful indicator to evaluate different layout schemes. A number of cases have been compared and discussed in this paper. In the ®rst place, it is most desirable to maximise the in¯ow of outside air to individual condensing units. Therefore, the condensing units should be placed as close as possible to the re-entrant front. Provided that there is no physical access problem, one desirable solution is to have an alignment of condensing units facing inward to induce a positive in¯ow of air into the re-entrant. Placing the units at the end wall is undesirable since the fan pressure head works against the ingress of outside air. Placing the units along the two side walls is comparatively better. A face-to-face layout appears to be appropriate. There is no evidence that the use of staggered layout can improve the condenser on-coil condition, though position interchange at alternative ¯oor can be helpful. In all cases, the condensing units should be placed far apart where possible. The thermal environment developed by condensing unit operation at a re-entrant is virtually complex. There are limited hard-and-fast rules to give concrete recommendations to the condenser arrangement. Whenever there is hesitation about the possible outcome, CFD and the energy performance model are very useful tools to predict and evaluate different equipment operating schemes.
Acknowledgements The research project was ®nancially supported by the Strategic Research Grant of the City University of Hong Kong. References [1] HKSAR, Hong Kong Energy Statistics: Annual Report, 1998 Edition, Census and Statistics Department, Hong Kong, 1999. [2] T.T. Chow, Z. Lin, Prediction of on-coil temperature of condensers installed at tall building re-entrant, Applied Thermal Engineering 19 (1999) 117±132. [3] T.T. Chow, Z. Lin, Q.W. Wang, Effect of building re-entrant shape on performance of air-cooled condensing units, Energy and Buildings 32 (2000) 143±152. [4] ASHRAE, ASHRAE Handbook: Fundamental, Atlanta, USA, 1993 (Chapter 24). [5] T. Kurabuchi, N. Kobayashi, A. Arashiguchi, T. Anai, Application of wind tunnel experiment and CFD simulation on estimation of wind environment inside and outside a large-scale building complex with an atrium space, in: Proceedings of the 6th International Conference (ROOMVENT'98), Stockholm, Sweden, June 1998, pp. 1±8. [6] H. Boyer, A.P. Lauret, L. Adelard, T.A. Mara, Building ventilation: a pressure airflow model computer generation and elements of validation, Energy and Buildings 29 (1999) 283±292. [7] Y. Li, A. Delsante, J. Symons, Prediction of natural ventilation in buildings with large openings, Building and Environment 35 (2000) 191±206. [8] J.E. Lovatt, A.G. Wilson, Stack effect in tall buildings, ASHRAE Transactions 100 (Part 2) (1994) 420±431. [9] M.J. Cook, K.J. Lomas, Buoyancy-driven displacement ventilation flows: evaluation of two eddy viscosity turbulence models for prediction, Building Services Engineering Research and Technology 19 (1) (1998) 15±21. [10] AEA, CFX 4.2 Flow Solver User Guide, Computational Fluid Dynamics Services, Harwell Laboratory, UK, 1998. [11] B.E. Launder, D.B. Spalding, The numerical computation of turbulent flow, Computer Methods in Applied Mechanics and Engineering 3 (1974) 269±289. [12] J.H. Ferziger, M. Peric, Computational Methods for Fluid Dynamics, Springer, Germany, 1996. [13] T.T. Chow, Z. Lin, Q.W. Wang, Applying CFD simulation in analysing split-type air-conditioner performance at buildings, Architectural Science Review 43 (3) (2000) 133±140. [14] A.G. Li, Prediction of natural convection from an array of horizontal line heat sources in a large space, in: Proceedings of the 6th International Conference (ROOMVENT'98), Stockholm, Sweden, June 1998, pp. 609±616.