A new proposal of a control method for the stack effect in super tall buildings: E/L Shaft cooling system

Transcription

1 A new proposal of a control method for the stack effect in super tall buildings: E/L Shaft cooling system Hyun Woo Lim 1, a, June ho Lee 1, b, Doo Sam Song 2, c, Joong hoon Lee 3, d, Holger Wallbaum 4, e 1 Dept. of Civil and Environmental system Eng., Sungkyunkwan University, Suwon, Korea, Dept. of Architectural Eng., Sungkyunkwan University, Suwon, Korea, Samsung C&T co., Seoul, Korea, Dept. of Civil, Environmental and Geomatic Eng., ETH Zürich, 8093 Zürich, Switzerland a imeru@skku.edu, b ifeelings@skku.edu, c dssong@skku.edu, d jh6925.lee@samsung.com, e holger.wallbaum@ibb.baug.ethz.ch ABSTRACT The stack-effect in super-tall buildings in winter causes many problems such as difficulties in opening or closing doors, infiltration, energy loss, noise and fire protection. Stack effect is influenced by temperature difference between the interior and exterior of building and the height of building. As an attenuation method for stack effect, the architectural measures such as strengthening air-tightness are generally used. However, as though architectural measures were fully adopted, the problems are reported as ever in tall building. In this study, a new method to reduce stack effect will be suggested. As an active control method against the stack effect, E/L shaft natural cooling method is suggested. In this paper, the concept of E/L shaft natural cooling system and its reduction performance of stack effect by simulation and field measurement will be reported. KEYWORDS: Stack Effect, E/L shaft cooling, simulation, field measurements 1. INTRODUCTION Super tall buildings are being constructed continuously with the concentration of population in downtown areas and the development of architectural technology. In recent times, as skyscrapers symbolize the status of a country, many countries are competitively planning to build skyscrapers. However, tall buildings pose new problems that are not experienced in low-rise buildings, and one representative problem is stack effect. Stack effect is caused by the pressure difference between the inside and outside of a building due to the difference of air temperature. Under the condition of same temperature difference between indoor and outdoor, the higher the building and the lower the general air-tightness, the higher the stack effect. Problems caused by stack effect include unpleasant noise and strong draft from elevator doors, malfunctioning of elevator and hall door due to the excessive pressure difference, increased cooling and heating loads, and slow down the performance of fire safety system. As an attenuation method for stack effect, the architectural measures are generally used. The architectural measures such as the installation of windproof rooms, revolving doors, additional separations, and greater air-tightness of air flowing paths have been suggested as reduction strategies of stack effect. Architectural measures focus on limiting air flow in a building because air flow is recognized as the main cause of stack effect. Although architectural methods are effective to the applied parts, but have limitations in reducing stack effect of the whole building because the main cause of stack effect is the air flow in and outside of a building. Moreover, the architectural measures may cause secondary problems such as the transfer of the problem to other parts of the building ([1] Park et al. 2007). 719

2 As a measure to improve the limitations of such architectural measures to counter stack effect, this study proposes an E/L shaft cooling system. This paper describes the concept of the E/L shaft cooling, and examines its applicability and stack effect reduction performance through simulation. Furthermore, the proposed system was applied to actual buildings and the stack effect reduction characteristics of the proposed system were analyzed through actual measurements. 2. OUTLINE OF THE E/L SHAFT COOLING SYSTEM 2.1 Concept of the E/L shaft cooling The E/L shaft cooling system naturally cools down the E/L shaft space by introducing cool outside air into the E/L shaft which is the main vertical air flow path of a building. This is an attempt to reduce the driving force of stack effect by lowering the difference in density of the air due to the temperature difference between the interior of the building (shaft space) and outside air. That is, as shown in Fig. 1, this system can reduce stack effect by lowering the pressure difference between outside air and the shaft by changing the pressure inside the shaft through the E/L shaft cooling. What is important here is to introduce an appropriate quantity of outside air into the shaft and facilitate the mixing of shaft air to minimize the temperature difference between upper and lower parts, and to reinforce the heat insulation performance of the shaft to minimize the heat loss by conduction ([2] Park et al. 2008). Figure 1. Principle of stack effect reduction using the E/L shaft cooling system 2.2 Performance analysis of the E/L shaft cooling by simulation The performance of E/L shaft cooling system was analyzed through simulation. A network simulation technique (CONTAMW) was used in this analysis which could easily predict the air characteristics (e.g., pressure, temperature) at various nodes and the air flow rates between nodes in a building. The simulation was accomplished to analyze the stack effect reduction performance for the target building (see chapter 3.1) prior to E/L shaft cooling system installation. 720

3 The simulation modeling conditions are summarized in Table 1. The air leakage data, which is the core of the modeling, was used in CONTAMW Library, ASHRAE Data, and the measured data for the target building as mentioned in chapter 3.1. Table 2 shows the simulation cases and conditions. Case 1 is corresponding to base case and there is no shaft cooling in this case. In Case 2, the E/L shaft for high-rise is cooled with outdoor air (-11.3 o C), the shaft temperature is cooled down as o C in case of the induced outdoor air flow. The shaft cooling temperature of case 2 is the temperature in the shaft with the outside air inflow rate (design air flow rate) set to 7,200 CMH. Fig. 2 shows the simulation result indicating the pressure distribution in the main air flow path from outdoor to indoors, that is outdoor, exterior wall, E/L hall door, E/L shaft, E/L doors, hall door, building envelope in order. Compared to the basic Case 1 (see Fig. 2-(a)), the pressure differences of E/L doors, which is the main cause of stack effect, is reduced as about 3/4 in Case 2 (see Fig. 2-(b)). The decrease rate of pressure difference at E/L door of entire building in Case 2 is almost same. This is the main feature of the E/L shaft cooling, that is, the E/L shaft cooling can be possible to reduce the stack effect accompanied problem as same rate for the entire building. Table 1. Simulation modeling Classification Conditions Stairwell -Modeling as one node on each floor -All nodes are connected vertically E/L shaft -Modeling as one node on each floor -All nodes are connected vertically E/L hall -Modeling as one zone Interior zone Atmospheric pressure Temp. Air leakage data -Simplified considering air flow path -Modeling on the first floor as the standard pressure -Considering the changes in outside air pressure by floor -Indoor(air-conditioning zone) : 24, -Ambient : (winter), -Non-air-conditioning zone : 15 -Shaft space : 24 (without cooling), (cooling) CONTAMW Library, ASHRAE Data([3]), Calculated data, Measured data([4]) Table 2. Simulation cases and conditions Cases Case 1 Case 2 condition Base -E/L shaft space Temp. : 24 Base + E/L shaft cooling (Highrise) -E/L shaft space cooling Temp.: air flow rate of outdoor air : 7,200CMH The pressure differences at the E/L doors on the lobby floor and the highest floor (40F) where the pressure differences are particularly great are summarized in Table 3. As shown in Table 3, after the E/L shaft cooling, the pressure on the E/L doors decreased by 25.7% and 23% on 1 st floor and 40 th 721

4 floor, respectively. Fig. 3 shows outdoor air flow rate induced by stack effect. In case of the E/L shaft cooling, the air flow rate is decreased about 17% compare to non-shaft cooling condition of Case 1. (a) case 1 (b) case 2 Figure 2. Simulation results: Pressure difference at E/L door Table 3. Simulation results Cases Case 1 Case 2 [m 3 /h] air flow rate F E/L door 96.7 Pa 71.9 Pa (25.7% reduction) case1 case2 40F E/L door 61.1 Pa 47.1 Pa (23% reduction) Figure 3. Simulation result: outdoor air flow rate induced by stack effect 3. APPLICATION OF THE E/L SHAFT COOLING SYSTEM AND PERFORMANCE ANALYSIS 3.1 Target building outline Fig. 4 shows a section of the target building. This is a large office building named 'S building located at Seoul, Korea with 41 floors above ground and 8 floors underground. The total area of building is 197,428 m 2. The E/L for passengers, emergency, cargoes and shuttles were installed. The E/L for passengers were divided into E/L for low floors (1 st ~17 th floors), medium floors (17 th ~28 th floors) and high floors (28 th ~40 th floors) and all of them may be ridden in 1 st floors. The E/L for emergency and for cargoes was operated to all the floors and the E/L for shuttles was operated between the underground parking lots and the 1 st floor. For indoor air conditioning, air supply was set to be about 5% higher than air exhaustion, but this amount considered the amount of air exhausted from the toilets, and so the building was basically planned to have the indoor pressure not change by air conditioning. The VAV air conditioning method was applied to whole floors. In standard floors except for the VIP zones, E/L halls and indoor spaces were divided by swing doors. Because high stack effect was reported in winter, the architectural measures such as a revolving door on the lobby floor, additional separations, and reinforced air-tightness for elevator doors were adopted. 722

5 However the stack effect was as high as ever. So E/L shaft cooling system was installed and the performance of its system was examined by field measurement. 3.2 Application of the E/L shaft cooling system Figure 4. Sectional Plan of Target Building The E/L shaft cooling system proposed in this study was applied to an actual building and the characteristics of the E/L shaft cooling system to reduce stack effect were reviewed by measurement. Since the E/L shaft cooling system had to be installed after completion of the S building, the cooling system had to be applied under limited conditions. As shown in Fig. 5 and Fig. 6, the air supply and exhaust ducts for E/L shaft cooling was decided on the rooftop. Air supply from the rooftop was branched to two sides of the E/L shaft and the outside air was supplied at eight points through the six internal vertical ducts, and air was exhausted through the upper part. Table 4 shows the air flow rate of the two shafts measured through T.A.B. 6 5 Shaft A Shaft B Table 4. Air-flow rate of E/L shaft cooling system Shaft Supply (m 3 /h) Exhaust (m 3 /h) Shaft A 6,733 6, Shaft B 10,036 9,337 Figure 5. Horizontal duct distribution in E/L shaft Figure 6. Vertical duct distribution in E/L shaft 3.3 Performance analysis of the E/L shaft cooling system To analyze the performance of the installed E/L shaft cooling system, the temperature changes at various positions in the shaft by the changes of outside air temperature were examined through monitoring of the temperature sensors in the shaft. Moreover, for the lobby floor and top floor (40F) where particularly large drafts occur due to stack effect, the passing wind velocity at the opening of the elevator doors was periodically measured. Fig. 7 shows the average temperature distribution in the E/L shaft (shaft A) with and without shaft cooling (Fig. 7-a) and the temperature distributions of three points in the shaft when the E/L shaft cooling was operated or not (Fig. 7-b). Temperature sensors are installed at the heights of 10 th, 20 th, and 30 th floors inside 723

6 the E/L shaft, and the measurements were recorded once every hour. Rate of air velocity [%] Shaft Temperature( ) (a) Average Temperature Figure 7. Changes in temperature inside E/L shaft (b) Point Temperature As shown in Fig. 7-a, the average temperature in shaft space were lower about 2 ~ 3.5 when the E/L shaft space were cooled compared to when the E/L shaft cooling system was not activated. On the other hand, when the outside air temperature was -11.3, which was the design condition of the cooling system, the average temperature measured inside the E/L shaft was approximately 21, there were some difference between measured temperature and predicted temperature of This result implicated the E/L shaft cooling system installed in S building was partially performed compared to the initial design intend. This is the reason that the actual cooling system installed S building was a little different from the initial design of E/L shaft cooling system. Furthermore, as shown Fig.7-b, the temperatures of the upper part (30F) of the shaft were lower than that of lower part of the shaft (10F) during the activation of the shaft cooling system. Because the outside air supplied from the rooftop acquires heat while being supplied to the lower part of the shaft by the conduction between duct and shaft. From the above results, to properly express the cooling effect of the E/L shaft cooling system, the methods to minimize heat acquisition in the air supply path by shortening the air supply path and to minimize the pressure drop in the duct are required. As an evaluation item for the stack effect reduction by the E/L shaft cooling system, the draft at E/L door when the door is opening was chosen, because excessive draft was one of the serious problem caused by the stack effect in the S building. Also, wind velocity is easily done in actual building. As shown in equation (1), the pressure difference by stack effect is proportionate to the air velocity when the air leakage area is fixed. 2ΔP Q = αa Av 120 ρ OFF (1) F 87 Floors 40F Figure 8. Rate of E/L door passing wind velocity ON Fig. 8 shows the result of the inflow air velocity (1F) into the shaft and the outflow air velocity (40F) from the shaft to the interior by stack effect when the E/L door was opened. Because the size of stack effect is changed by the outside air temperature, the measurement results were corrected and compared on the basis of the outside air conditions. Following the activation of the E/L shaft

7 cooling system, the inflow air velocity on the first floor decreased by approximately 13%, and the outflow air velocity on the 40 th floor decreased by approximately 37%. The reason why the reduction effect on the 40th floor was better than that of 1 st floor was the indoor pressurization effect, because 40 th floor was not air conditioned during the measurement with no E/L shaft cooling, however the 40 th floor was air conditioned during the measurement with E/L shaft cooling. 4. IMPROVEMENT STRATEGIES OF PERFORMANCE OF THE E/L SHAFT COOLING SYSTEM 4.1 Improvement of performance of the E/L shaft cooling system The E/L shaft cooling system installed in the target building did not achieve proper cooling effect. As a result, the stack effect reduction by the E/L shaft cooling system was lower than expected. Based on the above measurements, to improve the effects of the installed E/L shaft cooling system in target building, the air supply inlets were relocated to the lower part in the shaft. As shown in Fig. 7, the current air supply rate is insufficient to cool the entire shaft. Therefore, concentrating the air supply in the lower part of the shaft, the air density of the lower part of the shaft will be increased and as result the air flow into the shaft from the lobby floor will be decreased. For this reason, the outdoor air supply was adjusted from the eight supply inlets to two supply inlets in lower part of the shaft. However the total supply air volume was not changed. 4.2 Performance measurement of the E/L shaft cooling system after the adjustment Fig. 9 shows the average temperatures (Fig. 9-a) in the E/L shaft (shaft A) and temperatures at various points (Fig. 9-b) after the adjustment. Compared to the air supply through eight inlets in the shaft, the air supply through two inlets in the lower part decreased the average temperature inside the shaft by approximately 1~1.5. In particular, the cooling effects in the 10th and lower floors were improved. Fig. 10 shows the rate of wind velocity at elevator door opening. The lower part concentrated cooling decreased the E/L door passing wind velocity on the 1F and 40F by 41% and 48%, respectively compared to no operation of the cooling system. Moreover, with the concentrated cooling at the lower part of shaft, the reduction effect on 1F and 40F were more improved by 28% and 21% as compared to the 8ea air supply. The rate of pressure difference at E/L doors was decreased by 13% on the 1st Floor and 22% on the 40th floor when the shaft cooling system was operated. As a result, the lower part concentrated cooling improved the stack effect reduction performance as compared to the 8ea air supply in target building OFF Avg. ON(8ea) Avg. ON(2ea) Avg. trend line (OFF Avg.) trend line(on(8ea) Avg.) trend line(on(2ea) Avg.) Ambient Temp.( ) Shaft Temperature( ) (a) Average Temperature (b) Point Temperature 725

8 Rate of air velocity [%] Figure 9. Changes in temperature inside E/L shaft (a) Wind velocity (b) pressure difference Figure 10. Wind velocity and pressure difference rate of E/L door with/without adjustment 5. CONCLUSIONS As a measure to reduce stack effect in high-rise office buildings, this study introduced an E/L shaft cooling system, examined its effects through simulation and measurements. Moreover, the improvement strategies to maximize the stack effect reduction performance by E/L shaft cooling system were proposed. The results from this study are summaries as follows: (1) As a result of examining the stack effect reduction by the E/L shaft cooling system as a design draft through simulations found that the pressure differences at E/L doors which are one of the problems caused by stack effect in super tall buildings could be reduced evenly on all floors. (2) The application of the design draft to an actual building found that the installed E/L shaft cooling system did not fully express the cooling effect due to heat acquisition in the air supply path and pressure loss in the air supply duct from the T.A.B. and measurements. (3) As a measure to improve the performance of the installed cooling system, the concentrated air supply to the lower part of the E/L shaft was suggested in this study, which greatly improved the performance of the E/L shaft cooling system to attenuate the stack effect. (4) To maximize the efficiency of the E/L shaft cooling system, the air supply and exhaust locations and air supply rates must be examined from the design stage. ACKNOWLEDGEMENTS This work was supported by Sustainable Building Research Center which was supported by the SRC/ERC program of MOST (grant R ). And it was researched as an international joint research with Prof. Dr. Holger Wallbaum at ETH Zurich University. REFERENCES [1] Park, D. R., Lee, J. H., Song, D. S.: (2007) A study on the Reduction Strategies of Stack Effect in High-rise Residential Buildings. SB07 Seoul Conference, pp [2] Park, D. R., Lee, J. H., Song, D. S.: (2008) A Control Method of E/V Shaft Cooling System for Reduction of Stack effect. Proceedings of the KIAEBS, pp [3] ASHRAE: (2001) ASHRAE guide and data book-fundamentals and equipment. American Society of Heating, Refrigerating, and Air- Conditioning Engineers, Inc., Chapter 26. [4] Jo, J. H.: (2005) Prediction of pressure distribution due to stack effect in high-rise residential buildings and evaluation of its impact. Ph. D. thesis, Seoul University, Seoul, Korea. 726