Keywords: Chilled beam, Thermal plume, Air diffusion, Draught risk

Transcription

1 Topic 6. Ventilation, infiltration and air distribution Impact of Heat Load Location and Strength on Air Flow Pattern with a Passie Chilled Beam System Risto Kosonen 1,*,Pekka Saarinen 2, Alex Hole 3 and Hannu Koskela 2 1 Halton Oy, Vantaa, Finland 2 Finnish Institute of Occupational Health, Turku, Finland 3 Arup, Sydney, Australia * Corresponding risto.kosonen@halton.com Keywords: Chilled beam, Thermal plume, Air diffusion, Draught risk SUMMARY A chilled beam is a source of natural conection, creating a flow of cold air directly into the occupied zone. Experiments hae been conducted in a mock up of an office room to study the air elocities in the occupied spaces. In addition, elocity profiles of a passie beam are registered with underneath heat loads when the cool and warm air flows interact. Experimental laboratory study shows that the underneath heat gains, een when no upward plume is generated and the dummy only affects as a flow obstacle, hae a significant effect on the elocity profile. Furthermore in an actual occupied office enironment, the thermal plumes and the supply air diffuser mix effectiely the whole air olume. The maximum air elocity measured was still below 0.25 m/s with the high heat gain of 16 W/m². The results demonstrate that a simplified analysis of an undisturbed conection flow can not accurately describe air moement and draught risk. INTRODUCTION The maximum elocities inside the occupied zone increase with a rising thermal load (Muller et al. 200) and the location and strength of the heat gain has a significant effect on air flow pattern (Kosonen et al. 2007). Thus, elocity profiles together with thermal comfort should be analyzed using heat load higher than 60 W/m 2. There are seeral international and local standards and guidelines that gie recommendations and propose design criteria for achieing good indoor enironment (ISO , EN , ASHRAE , CEN CR , etc.). Recommended alues for the mean air elocity, air temperature, relatie humidity, ertical temperature gradient and mean radiant temperature both for winter and summer conditions are listed in these documents. A model for local discomfort that predicts the percentage of dissatisfied due to draught was introduced by Fanger et al. (1988). An index called draught rating () index was deried as a function of mean air elocity, air temperature and turbulence intensity. The chilled beam system is an attractie solution to achiee good indoor air conditions in an energy efficient manner. A common design encompasses a combination of entilation air, distributed by mechanical means to a ceiling or floor diffuser, and mechanical cooling ia the passie chilled beam element integrated into a suspended ceiling. The passie battery type element itself is essentially a series of water tubes with fins. Cooled water circulation through the coil generates airflow across the beam by the natural buoyancy. Under undisturbed conditions, a flow of cold air is directly falling down in the occupied zone.

2 Experimental study of the elocity field generated by passie beams has been conducted in undisturbed conditions (Fredrikson el al. 2001). The study indicated that the air elocity can create draught with the specific cooling capacity higher than 200 W per linear metre of beam. As a rule of thumb, the REHVA guide (REHVA 2006) recommends a specific cooling capacity to be lower than 150 W/m when the beam is in an occupied zone, and up to 250 W/m when installed in the non-occupied zone. During the design phase, howeer, the location of the passie beam should be carefully considered. Basic principle is to aoid installation oer the working places. Neertheless, in many cases when the layout changes, it is possible that someone is sitting just below a passie beam. Therefore, more information is required about the actual air flow pattern in the occupied offices. In this paper, the results of the laboratory study on the effect of the underneath heat gains on the elocity profile of a passie beam are presented. Furthermore in a mock-up office, the air elocities are studied when thermal plumes and a supply air diffuser are introduced in the room space. METHODS The heat load used in the measurements comprised a PC and a human dummy, simulating an office situation. This load was supplemented by underfloor heating, if necessary, to attain the cooling power required. The heated floor then coered a much larger area than the load below the beam. The basic arrangement is shown in Fig. 1. (Saarinen et al. 2006). Two additional arrangements were also used. In one of them the heat load was shifted 0.5 m aside from the beam centre line, and in the other it was remoed altogether. The human dummy was constructed according to the standard DIN (DIN 1995), containing a description of a cooling load simulator. The dummy was heated electrically with the power of 90 W, whereupon, depending on the cooling power, of the heat loss was through conection. Moreoer, the dummy used in the experiments originally had three large holes on its flank, near the top surface. They had to be blocked, since the warm air blew horizontally out of them, when the dummy was under the chilled beam. Figure 1. Basic arrangement obeyed in most experimental setups. All measures are in millimeters. Vertical middle line (red) marks the position of the beam centre point.

3 A list of all the experimental setups measured is gien in Table 1. In the setups with a B affixed to the setup number, the load has been shifted 0.5 m aside (to the left in the scheme of Fig. 1), and an A tells that the load was in its normal position, but unheated. In setups 3, 9, and the load was remoed from under the beam. The dimensions of the beam were mm and of the heat exchanger mm. The difference between the cooling and heating powers is due to the leakage of heat through the surfaces of the test room. Table 1. Test setups measured in the research. A = load unheated, B = load shifted by 0.5 m. setup cooling cooling additional balance power PC + dummy power load (underfloor of passie display (cylinder) per linear m heating) beam [W] [W] [W] [W] [W] B A B A The flow field was measured on four horizontal planes at different heights between the beam and the load, and on a ertical plane below the beam centre line. The density of the measuring grid on each plane was cm cm. The elocity measurements were carried out by two direction sensitie ultrasonic anemometers, and the temperature field was always measured simultaneously, enabling calculation of the distribution of the draught rating. The measurement planes were scanned using a computerized measurement robot, coneying the sensors through the points of measurement. In each point there was a s waiting period to allow the swinging to damp out, followed by a measurement period of 60 s. Another test room (L x W x H = 3.6m x 3.6m x 3.3/2.8m) was used to simulate a real office (Hole and Kosonen 2006). In this test room, two passie beams (CPT /200) were installed aboe the perforated false ceiling (3195 mm from the floor leel). Three light fittings (each: 2x28 W/T5) and a swirl diffuser (TSR-160) were installed in the false ceiling at the leel of 2800 mm from the floor leel. The gross free area of the ceiling panels was 37 and the diameters of the perforation holes were 5mm. Close to the window wall, there was arranged an additional opening (300 mm) to improe the buoyancy effect, Fig. 2. In the basic case of 122 W/m 2, the heat loads were: two computers (260 W), two dummies (150 W), light fittings (168 W), warm window surface (35.0 o C and ~ 600 W) and an electrical foil (00 W) (size of 3.6 m x 1.5 m close to the window) on the floor. The used airflow rate and supply air temperature of the swirl diffuser were 35 L/s (2.7 L/s per m 2 ) at 13 o C. The inlet water temperature of the chilled beams was 15 o C. The requested cooling

4 capacity was arranged by modulating water flow rate of the chilled beams. The room air temperature was in the test cases between o C. Figure 2. Simulated office (workplace layout 1) and the cross-section of the room space. Two higher cooling capacity 153 W/m 2 and 16 W/m 2 were also studied. In the case of 153 W/m 2, two extra computers and dummies were installed. The highest cooling capacity (16 W/m 2 ) was arranged from the case of 153 W/m 2 by increasing the heat gain of the dummies. In this study, the effect of the location of the workplaces on the performance of the passie chilled beam system was also analyzed. One additional case without false ceiling was also measured to analyze the effect of the perforated ceiling panels on the air distribution. The cooling capacities and office layouts in the test cases are shown in Table 2. Table 2. Studied heat load and workplace layout cases. Case Room Water flow Heat loads Workplace (WP) layout: air rate (l/s) (W/m 2 ) :WP in the middle :WP close to the side wall :WP close to the window :WP in the middle 23. (no ceiling panels) :WP in the middle :WP in the middle Mean air elocity, air temperature and turbulence intensity were measured within the occupied zone at the locations of 0.1m, 0.2m, 0.5m, 1.1m, 1.5m and 1.8m aboe the floor. The measurement grid consisted altogether of 16 locations at each height (96 points in total), Fig. 2. Air flow elocities were measured with elocity sensor type Sensor HT 12 haing an accuracy of ± 1. The room air temperature, inlet water temperature, supply and exhaust air temperatures were measured with temperature sensors of type PT0 class A. The temperature sensors were shielded against radiation. Instantaneous alues of elocity and temperature were measured simultaneously. Measurements of elocity and temperature were time aeraged oer a period of 180 s.

5 RESULTS Contrary to the expectations, the plumes from the heat loads were not powerful enough to fight the downward flow from the beam. Instead the heated air was captured by the downward flow from the beam and no significant upward buoyancy force was generated. This was true een with low cooling powers, as can be seen from Fig. 3, showing the horizontal cross sections of z for the cooling power of 223 W. Only when the load was shifted (setups 1B and B), or when an extra load was added (setup 2), did an upward plume build up. 2.1 m 1.8 m 1.5 m 1.2 m ertical flow speed z setup No cooling power: W dummy: 90 W PC+dspl: 95 W Figure 3. Vertical component of the flow field at different heights in setup No... The positions of the dummy, PC, display, and the table hae been marked on the lowermost measurement plane (height 1.2 m). On the uppermost plane (height 2.1 m), the beam is outlined. The centre point of the beam has been marked by a red cross on each plane. Measurement grid ( cm cm) is also displayed

6 Figure shows that in the absence of the dummy the downward air elocity below the beam, 0.1 m aboe the default position of dummy top surface, was m/s with cooling powers W ( W/m). In the presence of the dummy the obtained z and alues were considerably smaller. When the dummy is present, it reduces the downward flow elocity cm aboe its top surface by m/s regardless of whether it is heated or not. Howeer, since the effect of the dummy on the flow seems to be based on its geometry rather than on its plume, its shape becomes important. Therefore, replacing the dummy by a real human, or a manikin, might result in considerably different results Air elocity z at cylinder axis, height 1.2 m warm load cold load shifted load load absent cooling power [W] Figure. Vertical air elocity [m/s] cm aboe the dummy top surface as a function of the cooling power. In case of shifted load (yellow triangles), the point of measurement is not shifted. In Fig 5, there are listings of the measured air elocity, temperature, turbulence intensity and draft rate profiles in the mock-up test case 1. The elocity profile reeals that the air is well mixed oer the whole occupied zone. Een with a heat gain of 122 W/m 2, the maximum air elocity was only 0.20 m/s. Also, the draft rate alues were at an acceptable leel. Changes in office layout had no effect on the performance of the air distribution. This is seen from the results of cases 2 and 3. The maximum air elocity was 0.20 m/s when the workplaces were located in the middle of the room, close to the side wall or close to the window. Also when the false ceiling was remoed (case ), the air elocities did not increase. In fact, the maximum elocity slightly reduced. When the cooling capacity was increased from 122 W/m 2 to W/m 2, the maximum air elocity was increased up to the alue of 0.23 m/s. Still, the draft rate alues were at acceptable leel: only in a couple of points - index was higher than 15. It should be noted that the typical limit of air elocity is set to 0.25 m/s during cooling season. Thus, the chilled beam concept studied does not exceed this international standard threshold for the air elocity een with the cooling capacities as high as W/m 2.

7 Height (m) () () () () () () Height (m) () () () () Height (m) () Figure 5. Measured air elocity, temperature, turbulence intensity and draft rate in case 1. The numbers on the top rows refer to the horizontal locations listed in Fig. 2. Table3. Measured air elocity and draft rate () in the test cases. Case Heat gain (W/m 2 ) Workplace location Middle Side Window Middle Middle Middle max ag max () ag () Number of point > Number of lines > DISCUSSION () () The draught rating is designed to be measured without the presence of human beings. Howeer, the occupants constitute a significant heat load, affecting the cooling power of the beam and the flow field in the room. The question then arises whether this heat load should be present during the measurement, and how far from the load surface the elocity measurements should be made. The results of this study suggest that when a cooled downward air flow dominates, it is distorted if measured less than cm away from the surface. Seeral tests were performed in order to establish the draught and heat gain performance of a mock up passie chilled beam installation under a number of scenarios. The results identify that within the range of these tests the draught rate () could be maintained below 20 een with relatiely high heat gains ( W/m 2 ). The measurement indicates that the influence of the mechanically introduced air supply ia swirl outlet diffusers together with thermal plumes has a profound effect on the air flow within the space. Similarly, adjustments to the internal layout of equipment produced little effect on the air moement. In all cases, the whole olume is fully mixed as demonstrated by the elocitiy measurements and smoke tests. There was no location in the occupied zone that exhibited excessiely high () () ()

8 elocities. The maximum elocity was approximately two times higher than aerage elocity in all cases. CONCLUSIONS It is apparent that the total performance is a combination of the influence of the cooling elements, air diffusion and thermal plumes of heat gains. Focusing on the performance of a chilled beam element in empty space will not gie the total iew. When the dummy is present, it reduces the downward flow elocity cm aboe its top surface by m/s. These tests found that under certain conditions a combination of cooled air supply through swirl outlet diffusers, and battery type passie chilled beams can proide room sensible cooling capacities of 120 W/m² and potentially up to 16 W/m² without exceeding the requirements of ISO7730. It is belieed that a mechanically introduced air supply together with thermal plumes plays a significant role in the deelopment of the elocity field within the space. ACKNOWLEDGEMENT The study is supported by Technology Agency of Finland (TEKES). REFERENCES ASHRAE Thermal Enironmental Conditions for Human Occupancy, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. CEN CR 1752, Ventilation for Buildings: Design Criteria for the Indoor Enironment. Brussels. European Committee for Standardization. DIN Chilled surfaces for rooms Part 1: Measuring of the performance with free flow Test rules (English ersion). Deutsche Institut für Normung. EN Indoor enironmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal enironment, lighting and acoustics. Fanger P. O., Meliko A. K., Hanzawa H., Ring J Air turbulence and sensation of draught, Energy and buildings, 12, pp Hole A and Kosonen, R. An experimental inestigation of a passie chilled beam system in sub-tropical conditions. WellBeing Indoors Clima June Helsinki Finland. Proceedings of Clima 2007 ISO Ergonomics of the thermal enironment- Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. Fredriksen, J, Sandberg M and Moshfegh Experimental inestigation of the elocity field and airflow pattern generated by cooling ceiling beams. Building and Enironment 36 (2001) Kosonen R, Meliko A, Yordonoa B and Bozkho L. Impact of heat load distribution and strength on airflow pattern in rooms with exposed chilled beams. RoomVent 2007 th International Conference on Air Distribution in Rooms. Proceedings of Rooment 2007 Helsinki June REHVA.2006 Guidebook no.5. Chilled Beam Cooling. Federation of European Heating and Air-conditioning Associations. Saarinen P, Kosonen, R and Koskela H. Interaction of a passie chilled beam and underneath heat load. RoomVent 2007 th International Conference on Air Distribution in Rooms. Proceedings of Rooment 2007 Helsinki June 2007.