INDOOR CLIMATE IN HEATING CONDITION OF A LARGE GYMNASIUM WITH UNDER-FLOOR SUPPLY/RETURN SYSTEM

Transcription

1 INDOOR CLIMATE IN HEATING CONDITION OF A LARGE GYMNASIUM WITH UNDER-FLOOR SUPPLY/RETURN SYSTEM Mingjie Zheng Research Laboratory, SANKO AIR CONDITIONING CO., LTD. Nagoya, , JAPAN ABSTRACT In large enclosures such as gymnasiums, it is difficult to control the air temperature and create a comfortable climate for people with respect to the heating condition. There are two main reasons that the air temperature in an occupied area is not increased efficiently in large enclosures. 1) Hot supplied air goes up and stays in the upper area due to buoyancy and it takes long time to reach the lower occupied area. 2) The heat capacity of the building structure affects the air temperature and cold air stays in the lower area. In this paper, the results of measurements regarding the pick-up time and the thermal environment of the residential region in a large gymnasium with under-floor air return system were considered. Furthermore, the effects of simultaneous use of the panel heater and the air supply method on the pick-up characteristic in heating condition and the energy consumption of the HVAC system to be investigated is discussed by coupled analysis of AHU outlet air temperature control simulation and CFD including the building structure. KEYWORDS Large Gymnasium, Characteristics in Heating Condition, PI control, CFD INTRODUCTION It is inefficient heating method in large space such as gymnasium when the location of air outlet or inlet is not appropriate even if the capacity of heat source and air conditioner is adequate. For instance, when the warmed air is supplied along the upper wall, the pick-up time in heating condition will become longer since the warm air stays in the upper area of the room and do not come down to the residential region 1) (Figure 1). In this paper, the measurement results regarding the pick-up time in heating condition and the thermal environment of residential region in a large gymnasium with the under-floor air return system were considered as a solution for this problem (Figure 2). The air supply method of HVAC system in the building to be investigated is designed to be alternative between the under-floor air supply method (Figure 2) and the under-floor air return method (Figure 3). The panel heater is also equipped in the seating area on the second floor. As a consequence, the pick-up time in heating condition of the HVAC system to be investigated is affected by the air supply method and whether or not the panel heater is simultaneously used in addition to the initial room temperature, the initial of structures under the floor and the thermal storage property of floor. Therefore, in this study, the effects of the simultaneous use of panel heater and the air supply method is investigated on the pick-up characteristics in heating condition and on the energy consumption of the HVAC system by the coupled analysis of supply air temperature control simulation and CFD including the building structures. Corresponding Author: Tel: , Fax: address: zheng@sanko-air.co.jp

2 Outlets Outlets Inlets Stay of warm air Spaces under Floor Spaces under Floor Exhaust Openin g Stay of cool air Inlets 2F Seats for audiences Outlets Machine Room 2F seats for audiences Machine Room Return air Return air duct Supply air Supply air duct Figure1. Problem of Air Condition in Heating Condition of Large Enclosure Figure2. Outline of Under-Floor Air Return System Figure3. Outline of Under-Floor Air Supply System MEASUREMENT OUTLINE The building to be investigated is Jukai Dome built on June 05 in. The overviews of the building and the HVAC system are presented in Table 1 and 2, respectively. The items, places and devices of measurement are presented in Table 3, and the shape of the building and the measurement points are shown in Figure 4. The set values associated with the control system at the time of measurement are presented in Table 4. Three cases, of which the presence or absence of simultaneous use of panel heater is combined with two types of air supply method, were established in the measurement. However, valid measurement results were obtained only from the cases with simultaneous use of panel heater due to various reasons. Therefore, there was no other way but to examine the effects of the simultaneous use of panel heater and the air supply method on the pick-up time in heating condition by performing coupled analysis of supply air temperature control simulation and CFD including the building structures. Table1 Outline of Building Location Odate City, Akita Prefecture usage Dome Total floor area 8,367m 2 Height 19.1m Main structure RC wooden(parts) S Seats 1,600 Table2 Outline of HVAC System Air(Water) supply [m 3 /h] Outdoor air volume [m 3 /h] Heating ability [kw] Setting position [m] Supply along wall 1FL ,000 42, Supply from floor surface Arena floor surface Panel heater FL+0.3 Table3 Measurement items, places and devices Measurement items Measurement places(refer figure 3) Measurement devices Temperature and humidity in arena A B E 2*3 Wireless small-sized residential region audience seats C D 2*2 thermometer and hygrometer Floor surface D E 1*2 ceiling A B 1*2 Small-sized thermometer Surface temperature Concrete surface of Measurement pointb E 1*2 Small-sized thermometer South wall surface 1 Small-sized thermometer Under floor temperature and humidity Under floor 3 Wireless small-sized thermometer and hygrometer Temperature near ceiling Near point A B 1*2 Central monitor Outlet and inlet temperature Outlet, inlet 4*2 Wireless small-sized thermometer and hygrometer AHU supply and return air temperature Outlet and inlet of AHU 4*2 Central monitor Supply and return water temperature Outlet and inlet of heat source 1*2 Central monitor Outdoor air temperature and humidity Out space of dome 1 Central monitor

3 Table4 Control set value in measurement Warming up time(no outdoor air) 30 minutes Return air set temperature 15 Panel heater operating time 60 minutes Proportional band(p) 3 Outlet set temperature of boiler 75 Integral time(ti) 7 s Control time step 30 s Integral sample period(dt) s WS-2 FL+1,0 FL+0 FL+1,0 FL+0 FL+1,0 FL+1,0 FL+1,0 FL+0 FL+0 FL+0 (a) Cross-sectional view [mm] SA-3 F-1 C SA-1 A E B D F-2 SA-2 WS-2 SA-4 (b) Floor plan [mm] surface temperature air temperature surface temperature Supply air temperature Inlet air temperature Air temperature measurement position Figure4. Building shape and measurement points (A - E) MEASUREMENT RESULTS AND DISCUSSION PICK-UP CHARACTERISTIC IN HEATING CONDITION OF UNDER-FLOOR AIR RETURN SYSTEM The measurement results of the temperature in residential region and the under-floor return air under simultaneous use of panel heater and under-floor air return are shown in Figure 5, and the timely change of air temperature at each measurement point is presented in Figure 6. The pick-up time in heating condition in arena

4 and the seating area was 70 and 60 minutes, respectively. Since the heat is absorbed by the building structures under the floor, the air temperature around the inlet under the floor was lower than that around the floor opening except for the value measured at when the HVAC system started to operate. Hence, the air temperature around the inlet under the floor was lower than that of residential region. temperature [ ] audience seat arena return air set temperature :30 11:30 :30 13:30 temperature[ ] :30 11:30 :30 13:30 inlet return air Figure5. Temperature in residential region Figure6. Under-floor air temperature valve open rate[%] suitable open rate request open rate 0 :00 :30 11:00 11:30 Figure7. AHU valve opening rate(2/) DISCUSSION REGARDING THE SUPPLY AIR TEMPERATURE CONTROL METHOD The AHU supply air temperature is controlled by the return air temperature. However, the return air temperature and the inlet air temperature (similar to the temperature in the residential region of arena) are different as shown above. For instance, as seen in Figure 5, the air temperature measured at 1.1m above the arena floor was higher than the set value of 15 at. This happened because the outlet air temperature was over controlled since the return air temperature was below 15 even though the inlet air temperature around the floor opening was higher than 15. Therefore, it seems that the control temperature (= the return air temperature) or the set value of room air temperature should be set slightly lower if the temperature of structure under floor is lower than the inlet air temperature when heating. COUPLED ANALYSIS OF SUPPLY AIR TEMPERATURE CONTROL SIMULATION AND CFD In order to investigate the effect of simultaneous use of panel heater and the air supply method on pick-up characteristics in heating condition, the coupled analysis was performed for the outlet air temperature control simulation and unsteady state CFD including the building structure. Table5 Calculation condition of building skin, floor and under floor concrete Outer wall Inner wall ceiling window floor slub Surface heat transfer coefficient [W/(m 2 K)] Outer surface temperature [ ] Initial temperature [ ] ANALYSIS METHOD The space to be investigated and the locations for the air outlets are symmetric between East and West sides of the building. Hence, only the West side of the space to be investigated was considered for CFD analysis. It is necessary to replicate the temperature change of return air and outlet air precisely in order to perform the analysis including the HVAC operating state which changes with time. The return air temperature depends on the indoor vertical temperature distribution, the heat transfer characteristic of the surface and the heat capacity of the building structure. Therefore, the material and the construction of each building structure are included in CFD model in order to replicate the heat capacity of structure. The heat transfer coefficient, the surface temperature and the initial temperature, which are all required for calculation, are shown in Table 5. In order to replicate the supply air temperature, it is necessary to model AHU hot water valve opening rate appropriately. In general, the amount of heat exchange of AHU is the exponential function of the hot water flow

5 rate, and it is considered that the amount of heat exchange of AHU is proportional to the valve opening rate since the actual valve has equal percentage characteristic. The temperature difference between the outlet and inlet of the hot water coil is a linear function of the hot water valve opening since the HVAC system is a CAV system. Then, the demand opening rate of AHU hot water valve is obtained by the following formula using the return air temperature calculated with CFD, the set value of room temperature, and the control parameter of hot water valve opening rate as shown in Table Demand Opening Rate = e + e dt (1) p Ti 0 Where, e is the difference between return air temperature and set temperature. However, immediately after the operation start of HVAC system, even if the AHU hot water valve opening rate is demanded to be 0%, the temperature difference between the outlet air and return air could not become the maximum right away since the outlet air temperature is affected by the heat capacity of AHU, operation time of the hot water valve and the heat loss from the supply air duct. In order to calculate the outlet air temperature correctly, the data such as the temperature around the air supply duct are required; however, it is very difficult to obtain these data. Therefore, in this study, suitable valve opening rate is calculated from the following formula, assuming that the maximum temperature difference between the measured outlet air temperature and the return air temperature is equivalent to the temperature difference observed with the maximum AHU hot water valve opening rate (=88%), and it is set to be the hot water valve opening rate for the time period from the HVAC system start until 90 minutes later. suitable open zone rate = MAX outlet air temperature AHU inlet temperature ( AHU outlet air temperature AHU inlet air temperature) Where, the value 88 is the maximum demand valve opening rate calculated by the formula (1) using the return air temperature difference between the measurement data obtained on February and the set value. The demand opening rate and the suitable opening rate of AHU hot water valve calculated by the measurement data taken on February are shown in Figure 7. This shows that the suitable opening rate is becoming larger gradually, unlike the demand opening rate, during the period from the HVAC system start until approximately 90 minutes later due to the reasons associated with the heat capacity of the AHU and duct as well as operation time of the hot water valve, etc. Therefore, in the following simulation, the suitable opening rate which had been calculated by the measurement taken on February is set as the AHU hot water valve opening rate when the demand opening rate is larger than the suitable opening rate during the period from the HVAC system start until 90 minutes later. In addition, the AHU hot water valve opening rate is estimated by the following equation assuming that the heat loss from the air supply duct is approximately 5%. AHU hot water valve opening rate = Demand opening rate 0.95 (3) Based on all above, the AHU hot water valve opening rate can be estimated for all the periods. Then, the outlet air temperature is calculated assuming the inlet/outlet temperature difference of hot water coil to be proportional to the valve opening rate. The calculation method for outlet air temperature hourly in each case is described as the following. step 1. The suitable opening rate of hot water valve is calculated from the temperature difference between outlet air and return air measured hourly on February. step 2. The return air temperature is obtained by performing CFD for 5 minutes with the initial value of the outlet air temperature as the measurement taken on February. step 3. The AHU hot water valve opening rate at the time step now is determined by comparing the calculated return air temperature to the suitable opening rate obtained in the step 1. Then, the outlet air temperature of the next time step is calculated using the following formula. air 88 (2)

6 outlet air + MAX( outlet temperature = AHU inlet air air temperature AHU inlet temperature air temperature ) valve opening rate (4) step 4. Using the calculated return air temperature as an input to CFD, the step 2 is repeatedly performed until the calculation time is over. 3-dimensions computational fluid analysis program based on the standard k-ε model, with which the result is obtained with relatively rough divisions 2), is used for the unsteady state CFD analysis. The SIMPLE method, which is less strict with calculation time interval ( t) and less diverge in calculation 3), is used as the finite difference scheme of time. However, it has been indicated that the number of repeated calculation become unstable and the accuracy of the solution is lost when t is too large with the SIMPLE method though it gives a fairly stable result even if the value of t has been set somewhat large 4). Therefore, the value of t was set as large as possible as a precondition that the stability and accuracy of the solution are to be secured. In terms of the primary analysis condition associated with convergence of unsteady state CFD analysis, the following conditions were set; Mesh Number = 79(x) 56(y) 5(z) (irregular interval), the convergence constant of simultaneous linear equations is to be 0.01, and the convergence judgment condition of pressure modification is to be 1.0E-4. REPERTABILITY OF SIMULATIONS The timely change of the measurement and the calculated value of air temperature at 1.1m above the arena floor, the AHU return air temperature, the surface temperature under the floor and the ceiling temperature on February is shown in Figure Although the calculated value of the temperature at each measurement point did not replicate the measured value perfectly, the change tendency over time of them is almost the same one. Therefore, by using the prepared model, it is possible to consider the effect of each factor on pick-up time in the heating condition. DISCUSSION BY SIMULATION In order to investigate the effects of simultaneous use of the panel heater and the air supply method on the pick-up characteristic in heating condition, coupled analysis of AHU outlet air temperature control simulation and unsteady state CFD including the building structure was performed with the calculation conditions shown in Table 6. The comparisons in changes over time among three cases of the air temperature at 1.1 m above the arena floor, the AHU return air temperature, the surface temperature under the floor, and the ceiling temperature are described in Figures -15. The pick-up time in heating condition and the calculated sum of HVAC energy consumption are shown in Table 7 for each case. Table 6 Calculation of each CASE CASE Panel yes no yes heater HVAC Under-floor Under-floor Under-floor system return air return air supply air Table 7 Calculation of each CASE CASE Measurement Pick-up time in heating condition [min] Energy consumption of HVAC system [kw h] THE EFFECT OF SIMULTANEOUS USE OF PANEL HEATER As shown in Figure, even though the air temperature at 1.1m above the arena floor is lower when only HVAC system (Case-2) is used compared to when the panel heater (Case-1) is simultaneously used in the first 50 minutes period after the start of HVAC system, the difference between the two cases becomes small afterward. This happens since the outlet air temperature is low in the first 50 minutes period after the start of HVAC system and therefore the heating effect of the panel heater becomes obvious while it diminishes as the outlet air

7 temperature rises afterward. Therefore, the pick-up time in heating condition in Case-2 is longer only by minutes than that in Case-1. Also, it is considered as an error of CFD that the calculated value of pick-up time in heating condition in Case-1 is minutes longer than the measured value. THE EFFECT OF AIR SUPPLY METHOD In case of the under-floor air supply system (Case-3), the temperature in the arena residential region is lower than that in Case-1 in the first 45 minutes period after the start of HVAC system. This happens because the temperature of the air outlet from the floor opening is lowered due to the heat absorption by the building structure under the floor. After 45 minutes from the start of HVAC system, since the AHU outlet air temperature rapidly increased due to the integral action of PI controller, the pick-up time in heating condition of Case-3 is shortened by minutes than that of the under-floor air return system (Case-1) to 50 minutes. In addition, according to Figure 13, the floor surface temperature in Case-3 is higher than that in other cases. This indicates that, in case of under-floor air supply system, outstanding floor heating effects are obtained from the under-floor return air. However, the ceiling temperature in Case-3 is lower than those in other cases. This indicates that the indoor vertical air temperature difference of under-floor air supply system is less than that of under-floor air return system. According to Table 7, the sum of the HVAC energy consumption during the calculation period (1.5h) in Case-3 is 155.4[kW h], lower than that in Case-1 by 33.1[kW h]. Namely, the sum of the HVAC energy consumption during the calculation period when the under-floor space is used as air supply route is less by 17% than that when the under-floor space is used as air return route. CONCLUSIONS In this study, the pick-up characteristic in heating condition of a large-scale gymnasium with under-floor air supply/return system is considered by measurement and simulation, and the following findings were obtained. 1. It is considered that the control temperature (= return air temperature) or the set room air temperature should be slightly lowered when the temperature of under-floor structure is lower than the inlet air temperature in heating condition. 2. Compared to the case with the heating system only, the pick-up time in heating condition is shortened approximately by minutes in case of simultaneous use of panel heater. 3. Compared to the under-floor air return system, floor heating effect could be obtained in the under-floor air supply system. Hence, the pick-up time in heating condition is shortened approximately by minutes, and the HVAC energy consumption is reduced by 17.5% approximately too. ACKNOWLEDGMENT I wish to express my most sincere thanks to the members concerned in Odate City, the Urban Regeneration Mechanism and the Odate City JYUKAI park gymnasium for their cooperation to measurement. REFERENCES 1. Nobuyuki Kobayashi et al. (03) Study on Heating System of Gymnasiums, SHASE Transactions, No.89 (Japanese) 2. Shinnsuke Katou ( ) Application to indoor atmosphere of CFD (1~7), SHASE, volume (Japanese) 3. Kouji Sakai and Osamu Ishihara (02) A Measurement of Thermal Performance of Floor Inlet Air-Conditioning System Applied for Gymnasium, AIJ Summaries of Technical Papers of Annual Meeting (Japanese) 4. Koji Sakai and Masaki Manage (03) CFD Simulation of Gymnasium Adopted Floor Outlet Air Conditioning System (Part.1) A Study on Numerical Analysis of Time Dependence Room Air Flow, SHASE Technical Papers of Annual Meeting (Japanese)

8 5. Mingjie Zheng et al (04) Indoor Climate of a Large Enclosure with Under Floor Return System in Heating Condition, SHASE Technical Papers of Annual Meeting (Japanese) 6. Mingjie Zheng (07) Indoor Climate of a Large Enclosure with Under Floor Supply/Return System in Heating Condition, AIJ Tokai Chapter Architectural Research Meeting (Japanese) FL+1.1m temperature [ ] measurement calculation return air temperature [ ] measurement calculation 9: 9: :00 9: 9: :00 Figure 8. Air temperature(fl+1.1m, B) Figure 9. Return air temperature floor surface temperature [ ] 9: measurement calculation 9: :00 ceiling surface temperature [ ] measurement calculation 9: 9: :00 Figure. Floor surface temperature(b) Figure 11. Ceiling temperature(b) CASE-1 CASE-2 CASE-3 CASE-1 CASE-2 CASE-3 FL+1.1m temperature [ ] 9: 9: :00 9: 9: :00 return air temperature [ ] Figure. Air temperature(fl+1.1m, B) Figure 13. Return air temperature floor surface temperature [ ] CASE-1 CASE-2 CASE-3 9: 9: :00 ceiling surface temperature [ ] CASE-1 CASE-2 CASE-3 9: 9: :00 Figure. Floor surface temperature(b) Figure 15. Ceiling temperature(b)