1、2009 ASHRAE 345ABSTRACTIndoor air distribution is a major concern in designing indoor thermal environment of a large space, which is different from normal buildings. Today, stratified air conditioning is acommon choice for large-space buildings. Indoor air distri-bution with the stratified air condi
2、tioning is affected by many design parameters. These parameters have direct effects on thermal comfort, environment noise and energy consumption. Selection of these parameters thus becomes the key problem indesign of indoor thermal environment in a large space. In this paper, a large-space building
3、is taken as the study object. It was found that the cooling load varies linearly with the stratified height. The calculations of the air distributions were performed under the conditions of different stratified height, diameter and quantity of the blast nozzles. Based on calculatedresults, the align
4、ment charts were plotted in an effort to determine the design parameters such as the height, the diam-eter and the number of blast nozzles and their selection range for a large-space building. According to the charts, the rela-tionship of design parameters and their optimized selection are analyzed.
5、 It is clear that the lower the height, the less the energy consumption demand is, however, the noise level would be higher. Based on above results, an optimized and practical methodology is formulated for the design of the air distribution in a large space. INTRODUCTIONStratified air-conditioning i
6、s used to air-condition the lower zone of an indoor space (occupied zone), as shown in Figure 1, wherein the thermal parameters of the air-condi-tioned occupied zone can fulfill thermal environment design standard. With an energy-saving strategy, stratified air-condi-tioning system removes the heat/
7、thermal load in the occupied zone only; as a result, the refrigeration capacity will be reduced efficiently. The level of the height, where supply air nozzles are installed, is taken as the stratified surface of the air-conditioned zone. The stratified height is defined as the height from the strati
8、fied surface to the ground. Generally speaking, as long as it meets the requirement of thermal environment design standard, a lower stratified height will be beneficial so as to provide a lower cooling load and improve energy-saving. Figure 1 Stratified air-conditioning in a large space.Discussion o
9、f Design Method and Optimization on Airflow Distribution in a Large-Space Building with Stratified Air-Conditioning SystemC. Huang, PhD X. Wang, PhDMember ASHRAEC. Huang is a professor and X. Wang is a faculty member in the College of Urban Construction and Environment Engineering, University of Sha
10、nghai for Science and Technology, Shanghai, China. LO-09-0312009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 115, Part 2. For personal use only. Additional reproduction, distribution, or transmission in e
11、ither print or digital form is not permitted without ASHRAEs prior written permission.346 ASHRAE TransactionsHowever, a lower stratified height might cause an unreason-able air distribution in the air-conditioned zone, which would bring about such side effects as non-uniform temperature distribution
12、, air draft and thermal discomfort in the partial air-conditioned zone. Furthermore, since the indoor air distribu-tion is affected by the temperature and velocity of supply air, the quantity and position of the blast nozzles, as well as the size and type of the nozzles should also be considered. To
13、 achieve a design with optimal air distribution, these factors should be taken into account. Based on the analysis of the large-space building space studied in this paper, an optimized design methodology is proposed.OUTLINE OF A LARGE-SPACE BUILDING AND THE LOAD CALCULATIONInvestigation and study we
14、re performed in the Shanghai International Gymnastics Center (see Figure 2). It contains a gymnasium, dozens of function rooms and shops, and it can hold 4330 spectators. A round-vaulted dome made of alumi-num-titanium alloy sits on the cylindroid main body with a diameter of 68m(223.09ft). The top
15、height of the gymnasium is 26m(85.3ft). The 180kW(51tons) light fixtures are installed at the height of 18m(59.05ft) above the main gym field.The gymnasium is air-conditioned by a central refrigera-tion plant with a cooling capacity of 4750 kW (1350tons). Two air conditioning systems are located sym
16、metrically in the gymnasium. Each provides approximately 42% (about 2000kW or 570tons) of the installed cooling capacity of the center. The maximum designed air rate is 240000m3/h(141667 CFM). The cooling air is delivered from 38 nozzles installed annularly on the side wall at a height of 14.5m (47.
17、57ft). The return air is exhausted mainly through the return intakes fixed evenly under the seats, with the rest through return air openings on the wall. The main problem of the calculation for the cooling load of the stratified air-conditioning is the heat effect of the upper zone exerting on the o
18、ccupied zone. It has been determined that the heat radiation of the upper wall roof and the electric lighting can reach the occupied zone in a certain proportion1. The literature2introduces a method for estimating the cool-ing load of an occupied zone, including the cooling loads from the partial ra
19、diant loads of the upper skin load, and from the convective cooling load produced by the supply air. This method is used in the present investigation.The calculations are carried out for the height of nozzles of 16.5m(54.13ft), 14.5m(47.57ft), 12.5m (41.01ft), 10.5m (34.45ft) and 8.5m (27.89ft) resp
20、ectively. This height is taken as the stratified height in the calculation. The outdoor dry and wet bulb temperatures are assumed to be 34C (93.2F) and 28.2C(82.76F) respectively. Conditions of the lower zone is that the dry temperature is 26C (78.8F) and the relative humidity is 60%, and the upper
21、zone is 31.5C (88.7F). The compositions of the calculated cooling loads for the stratified surface of 14.5m (47.57ft) are shown in Figure 3. Those for other heights of the stratified surface are listed in Table 1, excluding the occupant load. The occupant load is considered as a constant for all hei
22、ghts, and it is approximately 492kW (140tons), 51% of the total load, which is the largest part among the other cooling loads. The second largest one is the convection heat load, 30% of the total load, which is trans-mitted from non-air-conditioned zone to the air-conditioned zone through the surfac
23、e of stratified height. The convection heat can be reduced, when the stratified height is lowered. For example, the proportion of the convection heat to the total load is 21.8% when the height is 8.5m (27.89ft). The radiation load is caused by the radiation from the ceiling to the floor, there-fore
24、it is not affected by the stratified surface and almost remains as a constant at all heights. The analysis on the cooling load shows that the energy-consumption can be reduced by lowering the stratified height, which yields a decrease in the convection heat load. The air volume of air-conditioning v
25、aries with the cooling load, as shown in Figure 4. Therefore, it is a good way to decrease the height of the stratified surface so as to lower energy consumption vet maintaining desired thermal comfort. For instance, when the stratified surface is elevated by 1m, the Figure 2 Shanghai International
26、Gymnastics Center.Figure 3 Compositions of the cooling loads for the stratified surface of 14.5 m (47.6 ft) in height.ASHRAE Transactions 347cooling load will increase 21kW (6tons) and the air volume would increase 8900m3/h (314 103ft3/h), (3.28ft).THE DESIGN METHOD OF AIR DISTRIBUTION The air distr
27、ibution calculations have been carried out for different height of stratified surface according to the thermal comfort demands. The results of the calculations can be used to optimize the stratified height.A stratified air-conditioning system is suitable for the target. The air outlets are located a
28、round the perimeter of the gymnasium. After the temperature and volume of the supply air have been determined by the cooling load, the characteristics of air distribution of the stratified air-conditioning are mainly dependent on the disposal form and structure of outlet and inlet. The key factors a
29、re the height, diameter and quantity of the nozzles. The characteristics of air distribution with different arrangements and height vary withthe following two models. One is based on the assumption thatthe quantity of the nozzles is calculated under the condition of the definite nozzle diameter. For
30、 the other one, the diameters of nozzles are calculated under the condition of the definite nozzle quantity.All air outlets are arranged circularly and symmetricallyat the sidewall. The typical track of air jet in a large-spaceroom is shown in Figure 5. The reference point for calculation is the poi
31、nt A illustrated in Figure 4. The track of air jet is calculated by Equation 1, in the calculation, the designed drop of air jet is determined by the height of the stratified surface and the radial distance of the air jet at the point A, and the throw of the air jet is calculated by Equation 2, wher
32、e 0.93 is an insurance coeffient. The calculation2 of air distribution is based on the following prerequisites. 1) The average air velocity of the occupied zone should remain between 0.15 m/s (0.5ft/s) and 0.25 m/s (0.82ft/s), i.e. the centerline velocity of the air jet should be 0.30 m/s (1ft/s) to
33、 0.50 m/s (1.6ft/s). 2). The velocity at outlet of the nozzles should be controlled at no more than 12 m/s (40ft/s) due to the requirement of maximum noise level.The drops of the air jet arranged at different heights are calculated by Equation 3. The rule of calculation is that the calculated drop a
34、nd the calculated throw should be close totheir design values respectively. Two design models can be used, in which either the quantity or the diameter of the air jet is given, then, v0, R0(or N), vxand vAcan be calculated by Equations 47. A set of curves are obtained for the optimal design of nozzl
35、es. Figure 4 The calculated results of the cooling load and the air volume for different heights of stratified surfaces.Figure 5 The typical track of air jet in a large-space room.Table 1. Calculation Results of Cooling LoadStratified Height,m (ft)Conditioned Space Ratio, %Skin Load, kW (ton)Radiati
36、on Heat Load, kW (ton)Convection Heat Load, kW (ton)Cooling Load, kW (ton)Decreased Load Ratio,%Volume of Supply Air,m3/h (106ft3/h)16.5 (54.13) 84.4 110 (31.3)78 (22.2)324 (92.1) 1003 (285) 3.4 224048 (7.91)14.5 (47.57) 74.2 103 (29.3) 293 (83.3) 966 (275) 6.9 209066 (7.38)12.5 (41.01) 63.9 96 (27.
37、3) 254 (72.2) 920 (262) 11.4 189417 (6.69)10.5 (34.45) 53.7 86 (24.5)79 (22.5)221 (62.8) 879 (250) 15.3 172836 (6.69)8.5 (27.89) 43.5 81 (23.0) 182 (57.7) 834 (237) 19.7 152835 (5.40)348 ASHRAE Transactions(1)(2)(3)(4)(5)(6)(7)ANALYSES ON OPTIMIZED DESIGN FROM THE RESULTS OF CALCULATIONSFigure 6 sho
38、ws that the quantity and diameter of air jets varies linearly with the stratified height, and their change ratio decrease slightly with the increase of the stratified height. It is important to design the layout of air jets for the air distribution in large space. Upon confirmation of the installati
39、on height of the air jet based on the building structure, different combina-tions of the diameter and quantity of the air jets can be applied through trial and error with the help of Figure 6. The upper parts of Figure 7 show how the relationship changes with the quantity and diameter of the nozzles
40、, and the initial velocity and the height of the nozzle respectively. All of the calculated drops and throws of the point A should meet the design requirements. The initial velocity of the air jet and the velocity of the point A can be deduced by the quantity and diameter of the nozzle from the lowe
41、r parts of Figure 6. The velocity data in the frames are taken as the logical values for the optimized design. It was found that both the initial velocity of the air jet and the energy-consumption of the fan would be higher, if the height of the nozzle is lower. The thermal discomfort, including noi
42、se and air draft, becomes greater when initial velocity is increased. The discussion on the opti-mized selection of the stratified surface is determined by systematic consideration of such factors as energy-saving, environment discomfort and building structure, etc.The Shanghai International Gymnast
43、ics Center is taken as the study object in the calculations. The stratified surface is 14.5m (47.57ft) and 38 nozzles are used. By using Figure 6, we can conclude that the nozzle diameter of 0.53m (1.74ft), the initial velocity of 6.9m/s (23ft/s) and the velocity of the point A of 0.42 m/s (1.4ft/s)
44、 would be favorable. Actually, the diam-eter of the nozzle in the gymnastics center is 0.55m (1.80ft). From the field measurements of the gymnastics in summer, winter and the transitional seasons from 1998 to 2002, it was found that the indoor thermal environment was comfortable. CONCLUSIONBased on
45、the present investigation, it is found that both the cooling load and air volume of a stratified air-conditioning system will increase with the stratified height. Taking Shang-hai International Gymnastics Center as an example, when the stratified surface is elevated by 1 meter (3.3ft), the cooling l
46、oad will be increased by 21kW (6tons)and the supply air rate will be increased by 8900 m3/h (314 103 ft3/h). Thus the reduction of energy consumption can be realized by decreas-ing the stratified height. Additionally, the cooling load would also decrease, if the convective heat transfer from the non
47、-air conditioned zone to the air-conditioned zone is reduced, espe-cially when the proportion of the occupant load to the whole cooling load is small. Based on the design calculations outlined in this paper, the quantity or the diameter of nozzles increases with the height of the stratified surface,
48、 which is an important reference to the design process of air distribution in a large space. The alignment charts for the design of air distri-bution are presented in this paper, and the properties of the nozzle can be obtained from the charts. The initial velocity of the air jets and the velocity a
49、t the reference position can also be obtained from the charts. It is concluded that by lowering the stratified height, the energy-consumption could be reduced. However, the thermal comfort may be compromised due to higher noise and draft. Consequently, energy consump-tion, environment discomfort, and the building structure should be equally and synthetically considered in the opti-mized design of the air distribution.ACKNOWLEDGMENTSThe authors would like to thank the World EXPRO Item of Shanghai Science and Technology Commission (No. yHh haRA2=x 0.93 BE()=yR0-x
