Proceedings of Clima 2007 WellBeing Indoors

Transcription

1 Experimental Study of Thermal Environment and Comfort in an Office Room with a Variable Air Volume (VAV) System under Low Supply Air Temperature Conditions Mari-Liis Maripuu Chalmers University of Technology, Building Services Engineering, Gothenburg, Sweden Corresponding mari-liis.maripuu@chalmers.se SUMMARY The air distribution components in a variable air volume (VAV) flow system influences the overall function of the system. A low supply air temperature and a wide working range in terms of airflow rate, facilitate high energy efficiency. However, in order to achieve these properties, high demands must be set on the function of the supply-air diffusers. This paper analyzes the thermal environment and comfort in an office room where the supply air diffusers have been chosen in accordance with such high demands. The study involves determining thermal comfort properties under different flow conditions and with a low supply air temperature, about +15 o C, in a full scale test room. It also includes measured velocity profiles and calculated draught ratings. The results indicate that the risk of draught was quite low despite high airflow rates and a low supply air temperature with the studied type of a supply-air device. INTRODUCTION The selection of air distribution components in the design of a variable air volume (VAV) flow rate system has a crucial importance for the overall function of the system. Varying airflow conditions need to be managed in a way that the requirements for the indoor climate parameters are always fulfilled. Here the airflow control devices and supply air outlets are critical components that need to be selected carefully, since improper selection of these devices is a common cause of excessive noise and draught in occupied spaces [1,2]. The main demand for the supply air diffuser in a VAV system application is to have high induction properties with varying airflow rates. It is essential that the diffuser supplies cold air to the occupied space evenly under any airflow condition without causing uncomfortable drafts by air dumping or by excessive room air motion. This requirement means that the air should be introduced to the room at a sufficient velocity to ensure good mixing with the room air. If the discharge area of the diffuser remains constant, the velocity of the supply air stream falls in direct proportion to the reduced airflow rate. Due to the naturally denser cold air a dumping, defined as a dropping of a horizontal supply air jet into the occupied zone, can occur and result in a sensation of draught. Thus, using conventional fixed supply air outlets in a variable air volume flow application needs to be carefully considered. The fixed devices are commonly used when VAV boxes are installed for the room airflow control. As a result the supply air temperatures and minimum airflow rates must be kept relatively high in order to avoid problems with thermal comfort. Moreover, over-cooling the premises can occur at low

2 internal heat loads and high minimum airflow rates if the supply air temperature is kept constant. Relatively constant supply air velocity irrespective of the airflow rate is maintained with a diffuser with a variable discharge area, commonly referred to as a VAV diffuser. This is a device, which changes its outlet configuration automatically when controlling the supplied airflow rate to the room. In commonly installed devices the airflow rate is determined by the diffuser s opening (between to 1%) and the constant static pressure at the inlet side. Therefore, a stable pressure should be maintained upstream of the diffuser. This demand is achieved by keeping constant pressure in the branch ducts by active control dampers, which can make the system more complicated and costly. This can especially be the case when adopting VAV systems in existing buildings, where extensive refurbishment is not always possible and the active control dampers are not easy to install in the duct system. This study aims to look for uncomplicated VAV system configurations that can assure a good indoor climate, while at the same time minimize the energy use of the system. A possibility of building up a VAV system with VAV diffusers for airflow control and without active control dampers in the duct system have been considered. For applying this kind of a technical solution it is essential for the VAV diffusers to be able to absorb the pressure unbalance in the system during varying airflow rates, while maintaining good airflow control properties without causing problems for the indoor climate, such as excessive noise. Moreover, from an energy use point of view it is beneficial for the VAV diffusers to be able to control the airflow within a wide range and manage low supply air temperatures without any risk for the thermal comfort in the room. With lower supply air temperatures the cooling capacity of the supply air will be improved and better control of the room temperature can be achieved. Low supply air temperatures are also advantage in systems that work with 1% of outside air and where a possibility for free-cooling can be applied. This paper analyzes the thermal environment and comfort in an office room where the supply air diffusers have been chosen in accordance with the demands discussed above. The paper accounts for laboratory tests, which aimed to study how the requirements from the thermal comfort point of view are met under different supply airflow conditions and with low supply air temperature. The results indicate that is possible to fulfill the demands that must be set on the VAV diffusers in order to apply them in a wide airflow range and at low temperature conditions. Although the tests have been carried out with a specific diffuser, the results are general in the sense that they show that the high demands on supply air diffusers can result in products which fulfil them. METHODS The function properties of the selected VAV supply air diffuser were tested in a simulated indoor environment: in a full size cellular office cube built inside the laboratory hall. The test room was constructed with plaster boards on a wooden framework. The internal dimensions of the test room were 3,9(L) x 2,8(W) x 2,7(H) m. To imitate a common office environment the room was filled with usual office equipment: a table, a chair, a computer and lighting. The internal heat loads were simulated with a PC-model (15 W), a dummy (8 W) and lighting (total 22 W). The artificial lighting consisted of two luminaries with two fluorescent tubes each (85 W per luminaire), plus a desk lamp (5 W).

3 The test set-up included a supply air fan with frequency inverter and with a pressure control (controlled pressure level was approx. 5 Pa.), a sound attenuator, a supply air heater with an air temperature control, an airflow meter, a VAV diffuser and temperature sensors for monitoring temperatures in the duct as well as inside and outside the test room. All sensors were connected via a logger to a personal computer for monitoring purpose. The technical properties of the tested VAV diffuser enable to measure the incoming supply airflow rate and adjust the discharge area respectively. Therefore strict pressure control at the inlet of the device is not needed. The discharge area of the diffuser varies according to the needed airflow rate and is internally controlled by a traversing motor, which gets impulses from the controlling sensor. The control and regulating equipment as well as the sensors are built into the supply air device and the simultaneous values can be read with the computer. Two different VAV diffuser mounting arrangements were tested: one with the diffuser free from the ceiling and one with the diffuser in the suspended ceiling. Without suspended ceiling the height from the ceiling to the discharge area of the device was 3 cm and from the device to the floor 2,4 m. With suspended ceiling the latter height was increased to 2,7 m. For the exhaust air a transfer air device was installed on the wall close to the ceiling. Thermal comfort measurements were carried out with constant supply air temperature +15 C, but with different airflow rates under steady-state conditions. For all the tests the operative temperature in the occupied zone and temperature outside the office cube was kept 22 ± 1 C, which corresponds to the winter conditions and A level comfort class [3]. The heat loads in the test room were adapted to the airflow in order to obtain the correct room temperature. Table 1 presents the conducted test cases with different combined heat loads and cooling capacities. There is a small difference between the heat load and cooling power values in the table. This gap is due to the heat transmission through the envelope of the test room. Table 1. The test cases completed in thermal comfort measurements Case Mounting condition Airflow rate, l/s Supply air temperature, C Cooling capacity, W Balancing heat load, W 1 no suspended ceiling no suspended ceiling no suspended ceiling suspended ceiling suspended ceiling suspended ceiling Air temperature and air velocities were measured in a number of room points as shown in Figure 1. At each position the measurements were taken at 3 heights -,1 m,,6 m, 1,1 m, which is based on the position of a sitting person. All together the results were obtained from 27 room points. The sampling period for each measurement was 3 minutes. Every measurement case described in table 1 was done in three replicates and the results are presented as an average over these three measurements. The risk of draught in the test room was evaluated by using draught rating (DR) model and the calculations for every measured point were done according to ISO 773 [4]. Draught rating expresses the percentage of people predicted to be dissatisfied by draught.

4 Computer desk lamp Ceiling luminaire VAV dummy diffuser Ceiling luminaire 8 9,9m,5m,9m,5m 1,45m 3,9 m Figure 1. The layout of the test room and measurement points in the room. The photo presents the measurement set up. Additionally, the measurement results were statistically analyzed with an analysis of variance (ANOVA) test, in order to see if the measured values of air velocities in the occupied space depend on different parameters that were varied during the experiment. The statistical significance of an effect of different parameters such as the room point, the level of a measured point, the ceiling and the airflow rate was studied. The main and combined effects of the described variables were first found by analyzing all the airflow rates together and then by each airflow case separately. The chosen confidence level in the analysis accounted here is 95% (p =,5). RESULTS The mean air speed and draught rating distributions at different supply airflow rates in two mounting cases, with and without suspended ceiling, are presented in Figures 2 and 3. The figures show the percentage of the measured points being in the specified range of air velocity and draught rating values. For example, it can be seen from the figures that the average air velocity in majority of measured points was less than m/s with maximum airflow conditions 5 l/s and less than m/s at lower airflow rates. According to thermal comfort guidelines, the air velocity in the occupied space should not exceed m/s [3] and the draught rating should be below 15% [4]. These limits were exceeded only in few measured points in the room and that occurred mainly at maximum airflow rates (see Fig. 2 and 3). Fraction of total measured points, % Without suspended ceiling <,5 - - > Mean air velocity, m/s Fraction of total measured points, % With suspended ceiling <,5 - - > Mean air velocity, m/s Figure 2. Air speed distributions in the test room with different supply airflow rates and with different mounting cases. The supply air temperature was +15 C.

5 Fraction of total measured points, % < >15 < >15 Draught Rating, DR% Draught Rating, DR% Figure 3. Draught rating distributions in the test room with different supply airflow rates and mounting cases. The supply air temperature was +15 C. In addition, there seem to be no substantial differences between the results with and without suspended ceiling mounting cases. However, some diversity can be seen at average airflow conditions (25 l/s), where the mean air velocities and draught ratings were somewhat lower with the suspended ceiling. The results from statistical analysis of variance test, summarized in Table 2, also indicate that the ceiling has an effect at average and minimum airflow conditions, but no effect at maximum airflow condition in the specified confidence level (p=,5). The table shows if an effect of each parameter that varied during the experiment is a statistically significant or not. The probability values of calculated F value compared to the value given for the F-distribution in the F-table are also presented for the variables which have an effect. The table accounts for the main effects only, meaning that if the parameter alone and not in interaction with other parameters influences the room air speed. The interaction effects of different parameters were also tested, but no higher order effects were revealed from the results. Nevertheless, even though different parameters such as the room point, the room level and the ceiling revealed to affect mean air velocities in the occupied space, there seem to be no regularities between the main effects. With maximum airflow rate 5 l/s the room point and the room level showed a significant influence, yet in the minimum airflow rate 1 l/s the effect revealed to be only from the room level and the ceiling. However, it was preliminary assumed that the airflow has an effect and as it can be seen from the Table 2, the probability that the variability of the mean air velocity values with different airflow rates can be attributed to experimental error is very low. Table 2. Statistically significant effects of different variables on the mean air velocity in the occupied space. The chosen confidence level is 95% (p =,5) Main Combined all 1 l/s 25 l/s 5 l/s effect airflows Room point NO NO P(F>5,5) =,25 P(F>4,79) =,3 Room level P(F>15,29)=,1 P(F>6,47)=9, NO Pr(F>6,45)=,12 Ceiling P(F>15,29)=,26 P(F>63,5)=3, P(F>17,65)=4, NO P(F>3,8)=,53 Airflow rate Without suspended ceiling 1 l/s 25 l/s 5 l/s Fraction of total measured points, % P(F>577,85) =2, With suspended ceiling

6 In general, it should be noted that the results of an ANOVA test do not show the size of a single effect, e.g. if the room level has a higher effect compared to the ceiling. Moreover, the direction of the variation is not known, e.g. in which case the highest results appear. The test shows statistically if different parameters affect the results or if the variation is mainly due to experimental error. Figures 4 and 5 illustrate the velocity profiles evaluated from the results from the measurement cases 3 and 6 (see Table 1). These were the cases where the required draught rating and air velocity values were exceeded in some room points. The critical points given in the figure, where the air velocities were exceeding m/s are also the points where the draught rating was exceeding 15 %. 5 l/s,1m level,2,18,18 5 l/s,6m level 5 l/s 1,1m level,5 3.9m Figure 4. Iso-velocity profiles in measurement case 3. Crossed dots mark the measuring points., 12,7 5 l/s,1m level,18,23,18,22,2,18,7,5 5 l/s,6m level 5 l/s 1,1m level Figure 5. Iso-velocity profiles in measurement case 6. Crossed dots mark the measuring points. As shown in Fig.4 and Fig.5, all of the critical room points were locating on one side of the room (measuring points 3, 6, 9). This was the empty side of the test room. Moreover, the most,18,17,17,17 3.9m

7 critical point, room point nr 6, was locating on the level of,1 m above the floor. The upper levels of the same measuring point did not have any higher velocities. Since the turbulence of airflow has an important influence to the perception of draught in the occupied space [5], the fluctuation of the air velocities in the test room has been further analysed. Figure 6 presents the standard deviation as a function of the mean air velocity with all different airflow conditions at three measurement levels for no suspended ceiling installing case. The results from the suspended ceiling mounting case were similar. It can be seen that the fluctuation of the air velocity was increasing when the average air velocity in measured points increased. In addition, a small decrease in the gradient of the regression lines, as the measuring level decreased from 1,1 m to,1 m, shows that the velocity fluctuations were more significant at the ankle level. Similar data has also been found in previous studies about airflow characteristics [6].,1m y =,3824x,7 R 2,6m =,8265 1,1m,5 Linear (,1m) y =,3285x,4 Linear (,6m) R 2 =,7915,3 Linear (1,1m) y =,3335x,2 R 2 =,819,1,5,1,,25 Mean air velocity, m/s Figure 6. Standard deviation of air velocity with three different airflow rates and at different levels in the test room. The diagram corresponds to no suspended ceiling mounting case. Standard deviation, m/s DISCUSSION Thermal comfort guidelines address minimal draft levels by placing limits on the allowable mean air velocity as a function of air temperature and turbulence of airflow. The measurements in the test room revealed that the air movements and the draught levels at medium airflow (25 l/s) and minimum airflow (1 l/s) conditions did not exceed the required levels stated by comfort standards and regulations [3,4]. Even at the lowest airflow rate, no risk of air-dumping was indicated. Marginal draught risk was registered in a few measuring points at maximum airflow conditions 5 l/s. In addition, the results showed no substantial differences between the two diffuser mounting cases depending on the ceiling. However, the ceiling had an effect at lower airflow conditions (see Fig.1 and Fig. 2). The statistical analysis test revealed that the measurement point and the measurement level have a statistically significant effect to the results at maximum airflow conditions. As was illustrated in Fig.4 and Fig.5, all critical points were situated on the empty side of the test room, opposite the workplace. No draught risk was observed in the normal working zone. One possible explanation for this could be the specific distribution of heat sources in the room. In general, all of the heat sources in the room have an influence to the air motion in the room by giving rise to buoyancy induced velocity scale which can match the velocities generated by a jet in the occupied zone [7]. The only influencing heat source on the empty side of the test room was the ceiling luminaire, while the other heat sources where distributed to the other side of the room. Nevertheless, since the air motion in the room is complex, it is

8 hard to make any definite conclusions for the causes of the draught risk in these specified room points and further studies should be conducted in order to identify the reason. A direct relation between air velocity and turbulence intensity was indicated from the measurement results. Moreover, the points with higher turbulence intensities were situated close to the floor, measured at ankle level. Coincidentally, this was also the level where the most critical points were located (see Fig. 4 and Fig.5). However in the present case, higher air turbulence on the ankle level may partly have been caused by the floor temperature, which was some degrees lower than the room temperature. This laboratory experiment was part of a study which aimed to look for uncomplicated VAV system configurations that can assure good indoor climate, while at the same time minimize the energy use of the system. A possibility of building up a VAV system with VAV diffusers, which can manage pressure variations (at least 1 Pa) in the system and work with a wide airflow range and at low supply air temperatures, has been considered. However, in order to achieve these properties, high demands must be set on the function of the supply-air diffuser. One type of a device that seemed to have the technical properties to fulfil the demands was tested in a full scale test room with the aim to study the thermal environment and comfort under different supply airflow conditions and with low supply air temperature. The results indicate that it is possible to fulfil the demands that must set on the VAV diffuser in order to apply it in the specified conditions. The VAV system configuration with this type of a VAV diffuser has also been tested in buildings in operation, focusing on indoor climate and the need of energy. The results yielded that this technical configuration provides an adequately functioning system that ensures good indoor climate and work energy efficiently [8]. ACKNOWLEDGEMENT This work described has been carried out as a part of a pilot project CAVA (From Constant Air Volume to Variable Air Volume), which has been conducted under the EUFORI program funded by Swedish Energy Agency (Statens Energimyndighet). REFERENCES 1. Cappellin TE. 1997, VAV Systems- What makes them succeed? What makes them fail?, ASHRAE Transactions, 13(2): Linder R and Dorgan CB. 1997, VAV Systems Work Despite Some Design and Application Problems, ASHARE Transactions, 13(2): CEN Report CR1752, 1998, Ventilation for buildings- Design criteria for the indoor environment, European Committee for Standardization. 4. ISO , Ergonomics of the thermal environment -- Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. ISO International Organization for Standardization. 5. P.O. Fanger, A.K. Melikov, H. Hanzawa, J. Ring, 1988, Air turbulence and sensation of draught, Energy and Buildings, 12: Chow, W.K., Wong, L.T., Chan, K.T. and Yiu, J.M.K., Experimental studies on the airflow characteristics of air-conditioned spaces. ASHRAE Transactions, 1(1): Etheridge, D, Sandberg, M., Building ventilation.theory and Measurement. Wiley 8. Maripuu M-L. 26, Adapting Variable Air Volume (VAV) systems for office buildings without active control dampers - Function and demands for air distribution components, Licentiate thesis, Building Services Engineering, Chalmers University of Technology, Gothenburg, Sweden.