CHILLED BEAMS IN HEATING: DESIGN CRITERIA AND CASE STUDY. Halton Oy, Haltonintie 1-3, Kausala, Finland

Transcription

1 CHILLED BEAMS IN HEATING: DESIGN CRITERIA AND CASE STUDY Maija Virta 1 and Risto Kosonen 1 Halton Oy, Haltonintie 1-3, Kausala, Finland maija.irta@halton.com Keywords: chilled beams, entilation, heating, thermal comfort ABSTRACT The chilled beam system is suitable for spaces with a high cooling requirement, low humidity load and relatiely small entilation requirement. Although the chilled beam system is quite popular and well known, there is still limited experience in using actie chilled beams for primary heating. The main objectie of this paper is to gie design criteria for actie chilled beam system and present results of a case study, where actie chilled beams were used for heating. The study is carried out in laboratory conditions. The results show that the occupied zone conditions are good and the temperature gradient is low. BACKGROUND The design of the heating system begins by defining the required heating capacity. In traditional heating systems the design is often based on high safety margins when heat losses are calculated. When a chilled beam system is used for heating, proper system operation cannot be achieed by oersizing the heating system. In a new office building W/m 2,floor of heating capacity is typically enough. The required heating capacity is strongly dependent on the thermal properties of the building and the outdoor climate conditions. Figure 1 shows the required heating capacity as a function of the aerage thermal conductance and the outdoor temperature. In this case, the room dimensions are 2.8 m (room width), 4.5 m (room length) and 3.0 m (room height), giing a the floor area of 12.6 m2 and a total external wall area of 8.4 m2. In this case, the glazing ratio of the total outdoor wall area is fixed to 50. In the European case, there is a double-glazed window (U-alue 3 W/m 2 K) and the U-alue of the wall is 0.45 W/m 2 K. In the Scandinaian case, there is a triple-glazed window (U-alue 2 W/m 2 K) and the U- alue of the wall is 0.28 W/m 2 K. Figure 1 also shows an example, which incurs high heat losses: this case is like a European one, except for haing a single-glazed window.

2 In addition, a case representing low heating demand is shown. Here, the heat losses are simply fixed 50 lower than the Scandinaian case. Based on this calculation, the required heating capacity of 500 W per room-module is enough for the normal case in arious European climate and design conditions. Heating demand (W) High losses G=27.1 W/m2K European G=14.5 W/m2K Scandinaia G=9.6 W/m2K Low energy G=4.8 W/m2K Outdoor temperature oc Figure 1. The required heating capacity as a function of thermal conductance and external temperature for a case room-module (room area 12.6 m 2 and external wall area 8.4 m 2 ). The basic guideline for thermal comfort is laid down in the ISO 7730 Standard (1990). The major part of this report concerns thermal comfort in terms of the temperature gradient in the room space and the room air elocities. ISO 7330 recommends the following: Air elocity (to aoid draughts): < 0.15 m/s (winter) < 0.25 m/s (summer) Vertical air temperature difference: < 3 C from foot to head when sitting, 0.1 m to 1.1 m, or standing, 0.1 m to 1.7 m. With the traditional heating system, the maximum gradient is typically 3 5 C. Of course, the existing conditions considerably affects a lot for the gradient, e.g. heat losses, positioning of the heating deice, surface temperatures, and the design temperature leels of system. All in all, the installation has a consequential effect on the gradient: it is more difficult to keep the gradient low when the installation height of the heat emitter is high. HEATING DESIGN CRITERIA WITH ACTIVE CHILLED BEAMS The actie chilled beam is one of the most popular air-conditioning systems in Europe. With chilled beams, it is possible to create high-quality indoor climate conditions, including thermal comfort and a low noise leel within reasonable life-cycle costs. Chilled beam systems are dedicated outdoor air systems to be applied primarily in spaces where internal humidity loads are moderate. They can also be used for heating. Actie chilled beams are connected to both the entilation supply air ductwork, and the chilled water system. When desired, hot water can be used in this system for heating. The main air-handling unit supplies primary air into the arious rooms through the chilled beam. Primary air supply induces room air to be recirculated through the heat exchanger of the chilled beam. In order to cool or heat the room either cold (14-18 C) or warm (30-45 C) water is cycled through the heat exchanger. Recirculated room air and the primary air are mixed prior to diffusion in the space. Room temperature is controlled by the water flow rate through the heat exchanger.

3 Figure 2. Typical exposed installation of actie chilled beams in office enironment. If the heating inlet water temperature of a chilled beam is higher than 45 C (linear output of an actie beam is higher than a. 150 W/m) in a typical installation, secondary air is often too warm to mix properly with the room air. The relatiely low temperature gradient in the space raises the air temperature near the floor thus maintaining comfortable thermal conditions, as well as ensuring the energy efficiency of the system (by decreasing the short circuit and thus the exhaust air temperature). Figure 3. Typical example of an actie chilled beam system operation principle in heating mode. This has been confirmed in preious papers based on laboratory measurements, where both the heating capacity and the temperature gradient of the room were analyzed. The studied inlet water temperature leels were 40 C, 60 C and 70 C.

4 3 Room heigh 2,5 2 1, deg.c 60 deg.c 74 deg.c 0, Temperature difference dt (C) Figure 4. Temperature gradient in the space with different inlet water temperatures. The data in Fig. 4 shows the effect of the inlet water temperature leel on the temperature gradient. With relatiely high temperatures, the gradient in the room rises: the temperature leel of 74 C leads to far too high gradient 8 10 C in the room. 40 C inlet water temperature gies much lower gradient in the room of under 3 C. Increasing the supply airflow rate decreases the temperature gradient but in this measured case, using 22 l/s instead of 13 l/s reduced the gradient only about 1 C. Therefore, the key factor is to control the inlet water temperature at a reasonably low leel. With the temperature leel of 40 C, it is possible to reach 500 W, which is in most cases enough. The mixing is also dependent on the window size and surface temperature. The higher and colder the window, the colder the air falling down to the floor, and the temperature gradient between secondary air and room air becomes higher. For this reason using beams for heating is recommended when the heat transmission of the windows is moderate (e.g. surface temperature is higher than 14 C and height is not more than 1.5 m). The heating capacity of actie beams is dependent on the primary airflow rate. This is why entilation must be operating when heating is required. When an office room is occupied the internal heat sources normally reduce the required heating output and the temperature gradient stays at an acceptable leel. The control principle in heating case is ery important. It can be on-off, time proportional on-off or modulating. The selection of the control system is dependent on the system design. In most cases all the aboe mentioned control principles proide reliable operation of the system. In cases where the difference between the design and normal operating conditions is large (e.g. the design is based on a much higher heating capacity leel than the real leel), time proportional on off or modulating controls are recommended. Otherwise the mixed air temperature of chilled beam becomes too quickly too warm, and there is no sufficient mixing in the space and therefore the occupied zone becomes too cold. In some cases the operation of heating system can be improed by installing the room air temperature measurement at higher leel in the space. In that case the heating mode is controlled based on higher room air temperature in heating mode and in cooling mode with fully mixed situation, the positioning of temperature sensor is irreleant.

5 CASE STUDY OF EXPOSED CHILLED BEAMS IN HEATING The main idea of the conducted case study measurement was to study a chilled beam system in heating mode with correctly calculated heat losses (35 W/m 2 ) without oerdesign and relatiely low inlet water temperature (35 O C). The office room is also relatiely high (3.6m) and chilled beams are installed at 800 mm under the soffit. This measurement was so-called early morning case without any internal loads. The measurements were carried out in a laboratory measurement room with room length of 6.5 m, width of 4.4 m and height of 3.6 m. There was cold window (surface temperature of 15 O C and height 2m) in the room. This project was designed at London weather conditions with double-glazed window. Two open types of chilled beams with actie length of 4.6 m and width of 0.3 m were installed exposed at height of 2.8 m. The beam installation can be seen in Fig. 5. Figure 5. The measurement room sizes, measurement grid and chilled beam location during the measurement.

6 The comfort conditions were measured in 25 different points throughout the space at 6 different heights (0.1m, 0.2m, 0.6m, 1.1m, 1.5m and 1.8m). The air elocity, air temperature and turbulence intensity was measured in each measurement point and from these results the draught rate was calculated using following formula: 0,62 = (34 Ta )( 0,05) (0,37* * Turb + 3,14) (1) Where: = draught rate = air temperature ( O C) = air elocity Turb = turbulence intensity Table 1. gies all the input data alues. The design inlet water temperature was 35 O C and with 0.04 kg/s water flow rate, the chilled beam heating output was 1425 W. The primary air was slightly colder than room air in order to generate free cooling in the space when all internal heat loads are effectie and therefore cooling is needed instead of heating. At room air temperature of 21.3 O C the 18 O C primary air has a cooling effect of 487 W. Therefore the net heating capacity to compensate heat loads was 937 W, corresponding heating capacity per floor area of 33 W/m 2. Table 1. Input alues of a chilled beam case study. T R Room Temperature 21,3 q S air Supply Air Flow Rate 116,0 l/s q S air/m Supply Air Flow Rate /m 25,8 l/s/m q E air Exhaust Air Flow Rate 116,0 l/s T S Supply Air Temperature 17,8 T E Exhaust Air Temperature 21,7 Window Surface Temperature 14,6 T W T W1 T W2 Inlet Water Temperature Outlet Water Temperature 35,0 26,5 q mw Total water Flow Rate 0,040 kg/s P W Total capacity of water 1425 W P A Capacity of air -487 W P T Total Capacity 937 W dt T R -aerage (T W1 andt W2 ) -9,45 The chilled beam heating operates fine also in this kind of installation. The achieed temperature gradient is low enough and elocities are ery low. Results of comfort measurement are shown in Table 2. In all measured points the air elocity is under 0.18 m/s and majority of them is under 0.15 m/s. Calculated draught rates in all points are under 15 (except 3 points at 1.8m height). Temperature gradient in the occupied zone is less than 2 O C and is below the comfort criteria of ISO 7730 Standard (<3 O C between 0.1m and 1.7m). This means that the airflow from chilled beam is well mixed with room air in the whole occupied zone and therefore the entilation efficiency is good. In addition it also shows relatiely good energy efficiency of system with exhaust air temperature of 21.7 O C een the space was 800 mm higher than the installation height of chilled beams. With internal loads (lights, persons and computers all together 55 W/m 2 ), the room air temperature increases a. 2 O C if water flow rate is not reduced. This means that een the maximum heating output is defined with room air temperature of 21 O C, in practice the personal control of +2 O C can be achieed.

7 Table 2. Comfort measurement results and calculated draught rates in arious measurement points ,80 0,05 21,4 55-0,05 21,4 51-0,03 21,6 56-0,04 21,6 45-0,03 21,6 42-1,50 0,07 21, ,05 21,3 47-0,04 21,4 47-0,04 21,4 35-0,03 21,4 44-1,10 0,06 21, ,04 21,1 39-0,04 21,1 38-0,03 21,1 39-0,02 21,2 38-0,60 0,03 20,6 42-0,03 20,6 37-0,03 20,6 31-0,04 20,6 48-0,03 20,7 35-0,20 0,02 20,3 35-0,02 20,4 40-0,03 20,5 34-0,03 20,4 53-0,02 20,5 44-0,10 0,02 20,1 25-0,03 20,3 27-0,05 20,3 36-0,04 20,3 36-0,03 20, ,80 0,09 21, ,13 21, ,18 22, ,09 21, ,07 21, ,50 0,10 21, ,09 21, ,14 21, ,10 21, ,04 21,6 47-1,10 0,10 21, ,06 21, ,06 21, ,06 21, ,02 21,2 40-0,60 0,06 20, ,04 20,4 35-0,04 20,5 36-0,03 20,5 34-0,05 20,6 34-0,20 0,03 20,4 59-0,02 20,3 44-0,02 20,4 32-0,02 20,4 43-0,03 20,4 33-0,10 0,03 20,2 36-0,02 20,2 27-0,03 20,3 34-0,03 20,3 32-0,03 20, ,80 0,07 21, ,12 21, ,15 22, ,10 21, ,05 21,8 51-1,50 0,07 21, ,08 21, ,11 21, ,07 21, ,05 21,6 40-1,10 0,08 21, ,05 21,3 54-0,06 21, ,03 21,1 38-0,03 21,3 54-0,60 0,04 20,7 57-0,03 20,6 42-0,03 20,5 34-0,03 20,5 36-0,04 20,6 50-0,20 0,02 20,4 47-0,02 20,5 41-0,02 20,4 34-0,02 20,4 31-0,03 20,4 52-0,10 0,03 20,2 34-0,03 20,3 28-0,02 20,3 24-0,02 20,3 23-0,03 20, ,80 0,09 21, ,10 21, ,13 22, ,08 21, ,05 21,6 35-1,50 0,10 21, ,08 21, ,08 21, ,04 21,3 54-0,05 21,4 36-1,10 0,10 21, ,04 21,1 46-0,04 21,1 47-0,03 20,9 30-0,05 21,0 22-0,60 0,04 20,6 56-0,02 20,5 41-0,03 20,4 40-0,03 20,3 41-0,04 20,3 37-0,20 0,02 20,3 34-0,02 20,3 28-0,03 20,3 36-0,03 20,3 48-0,06 20, ,10 0,02 20,1 24-0,03 20,2 29-0,03 20,1 30-0,03 20,2 44-0,05 20,2 44-1,80 0,06 21, ,06 21, ,05 21,3 46-0,03 21,4 42-0,04 21,4 52-1,50 0,08 21, ,06 21, ,04 21,1 43-0,04 21,2 38-0,04 21,1 47-1,10 0,08 21, ,05 20,9 66-0,04 20,8 41-0,04 20,8 40-0,04 20,9 34-0,60 0,04 20,5 53-0,03 20,5 42-0,03 20,5 49-0,03 20,4 46-0,03 20,5 40-0,20 0,04 20,4 41-0,05 20,5 45-0,06 20, ,08 20, ,08 20, ,10 0,04 20,2 52-0,10 20, ,11 20, ,10 20, ,12 20, Temperature gradients with and without internal loads with max heating capacity 2,00 1,50 1,00 0, with internal loads 8 with internal loads 13 with internal loads 18 with internal loads 23 with internal loads 0,00 20,0 21,0 22,0 23,0 24,0 25,0 Temperature Figure 6. Temperature gradients are lower than 2 O C throughout the whole floor area. Measurement point 3 is nearest from the window and 23 is close to the door. All the points are in the middle of the room halfway between the chilled beams. With internal loads the room air temperature is a. 2 O C warmer, if water flow rate is not reduced.

8 CONCLUSIONS Integration of the heating mode in the actie chilled beam system is a challenging task. The main problem has been to control the temperature gradient in the space and to creating thermal comfort conditions in an energy-efficient way. On the other hand, meeting the heating capacity requirement with chilled beam is not a problem in most cases. The key factor is the temperature leel of the inlet water. Lower the inlet water temperature is, lower is also the temperature gradient in the space. Based on the measurements, a temperature leel of 40 C or below, gies a reasonable gradient for the room space. For design practice, this also means the possibility to use low temperature leel heating sources. During working hours, there is normally enough or een too much of a heat loads to keep indoor temperature within acceptable comfort leels. This means that this worst case exists in the beginning of the working day when the external temperature is also near design conditions. In the case study of exposed chilled beams in heating mode, the right design alues (inlet water temperature of 35 O C) with relatiely low heating output (33 W/m 2,floor) demonstrated excellent thermal comfort and good energy efficiency. It was also showing, that earlier gien design guidelines are a good basis for design of actie chilled beams in heating. LITERATURE Kosonen, R., Horttanainen, P, Dunlop, G. Integration of heating mode into entilated cooled beam. Proceedings of Rooment Virta, M., Butler, D., Gräslund, J., Hogeling, J., Kristiansen, E. Reinikainen, M., Sensson, G. Chilled Beam Application Guidebook, Reha Guidebook n:o 5. International Standard ISO 7730, Moderate thermal enironments- Determination of the PMV and PPD indices and specification of the conditions for thermal comfort, 1990.