THE EFFECT OF HEAT LOAD ARRANGEMENT ON THE PERFORMANCE OF RADIANT PANELS

Transcription

1 THE EFFECT OF HEAT LOAD ARRANGEMENT ON THE PERFORMANCE OF RADIANT PANELS Panu Mustakallio 1, Arsen Melikov 2, Marcin Duszyk 2, Risto Kosonen 1 1 Oy Halton Group Ltd., Helsinki, Finland. 2 International Centre for Indoor Environment and Energy, DTU Civil Engineering, Technical University of Denmark, Lyngby, Denmark. Abstract The objective of this study was to identify the importance of heat load distribution on the cooling power of radiant panels. A full-scale test room was equipped with two top insulated ceiling radiant panels installed near the long walls. Radial multi-nozzle supply air diffuser was installed in the middle of the ceiling so that the supply air jet was flushing the radiant panels. The test room was constructed according EN-standards for accurate measurement of cooling power of the radiant unit. The cooling power of the panels was studied in two situations: 1) without supply air to compare two standardized testing methods for chilled beams in the case of radiant panels when the are conducted through test room walls or when using uniformly distributed heated dummies in the test room, and 2) with supply air in the similar cases as earlier and also in the case when the internal heat loads are located only in one end of the test room. The air temperatures and black ball temperatures were measured in the test room, and supply air jet pattern was visualized with smoke. Two standardized testing methods for the chilled beams gave nearly the same cooling output for the radiant panels. The most significant effect of on the performance of the radiant panels occurs when are located unevenly and their convection flow turns or weakens the supply air jet flushing the radiant panels. This should be taken into account in the design when supply air is used to enhance the operation of the radiant panel. Keywords: radiant panel, cooling power, heat load 1 Introduction The cooling power of radiant panels is affected by the arrangement of in the room because the operation of the panels is based on free convection and radiation. It is important to account for this effect during the design of radiant panel systems because often their cooling output is the limiting factor for the use in practice. The objective of this study was to identify the importance of heat load distribution on the cooling power of radiant panels. The radiant cooling panels are tested normally according EN (2004) standard in test room under uniform distribution of internal heat sources. In this study the cooling power of radiant panels determined by two methods namely with presence of uniform heat load distribution in test room and when the heat load is conducted through the walls of test room. Both methods are used in the standardized testing of active and passive chilled beams. The situation when are located unevenly and located on one end of the room was studied as well. It is similar to the conditions in rooms in the external zone of building where heat loads are located unevenly near warm window façade. 1

2 2 Methods A full-scale test room 4.7 m (L), 3.0 m (W) and 2.5 m/ 2.8 m (false ceiling H/ H) was equipped with two top insulated ceiling radiant panels with dimensions 3 m x 0.6 m. The panels were installed near to the long walls of the room. The multi-nozzle supply air diffuser was installed in the middle of the ceiling so that the supply air jet was flushing the radiant panels. Test was performed first without supply air, and then with 25 l/s (1.8 l/s/m 2 floor) and 16 ⁰C supply air. The test room was constructed according EN-15116/EN standards to allow for accurate measurement of the cooling power of the chilled beams and following also guidelines in chilled ceiling testing standard EN (2004). The water flow rate was measured with electromagnetic flowmeter and air flow rate with orifice plate flowmeter. Temperatures were 4.7m Radiation panel 3m x 0.6m Room height 2.5m / 2.8m (false ceiling / total height Radiation panel 3m x 0.6m 3.0m Heated dummy (cylinder) Case 1 Case 1B Case 2 Case 2B Case 2C Figure 1. Location of the radiation panels, air distribution unit and heat sources in the ceiling Figure 2. A) Overall view of the setup with heated dummies positioned symmetrically; B) heated dummies positioned unevenly, i.e. only in one side of the test room, C) supply air jet smoke visualizations in case 1 and case 2C. 2

3 measured with PT-100 temperature sensors. The measurement systems are designed for maintaining very stabile conditions and are calibrated regularly for maintaining the measurement tolerances required by the EN standard. The accuracy requirement for water flow rate is 1% (+/- 0.5%) and for water temperature 0.05 ⁰C. The test cases are listed in Table 1 and are shown in the Fig. 1. Photos of the actual room and smoke visualizations are shown in the Fig. 2. In the first case, case 1, water temperature for the radiant panels was 15/17 ⁰C and the room air temperature was adjusted to 26 ⁰C by compensating heat load conducted through the walls of test room. In the second case, case 1B, the compensated were located symmetrically inside the test room. Isothermal ambient temperature was used by adjusting the external temperature to the same as average room temperature at 1.3 m height. Supply water temperature and mass flow rate were kept same as in the case 1. The cases 2, 2B and 2C were done similar way as cases 1 and 1B, but with supply air. In the last case, the effect of uneven heat load arrangement was measured. The case 1B is done mostly according to the chilled ceiling testing standard EN (2004). The test room fulfils the accuracy criteria and standardized heated dummies are used. There are only some minor exceptions, e.g. the heated dummies are located correctly, but there should be 10 instead of 8 of them, and at least 70% of ceiling should be covered with radiant panels, now only 26% as in the present study. Room air temperature (T) was measured at four locations and vertical temperature difference at one location. Black ball temperatures (Tbb) were measured at two locations. Supply air flow pattern was visualized with smoke. The measurement locations and heights are shown in the Fig. 3. Cooling power of the radiant panels was calculated from water temperature difference and mass flow rate. Conductances were calculated by dividing the cooling power with the difference of mean water temperature and average room air temperature at 1.3 m height (from four locations measured). Exponent based on radiant panel s manufacturer data was used for the temperature difference. Case parameters and measured cooling powers are shown in the Table 1. Cooling power of 16ºC supply air is approx. 22 W/m 2 floor and of radiant panels 26 W/m 2 floor. This supply air temperature level is often required during summer design day due to the drying of the supply air in the air handling unit Temperature measurements in the test chamber (T=Air temperature, Tbb=Black ball temperature) 1. T1 at 1.3m height Tbb1 at 1.1m height T5 at 0.1m height T6 at 1.8m height T7 at 2.3m height 2. T2, Tbb2 3. T3 4. T4 Figure 3. Locations of the temperature measurement Table 1: Experimental conditions during the tests and measured cooling power CASE ROOM T. SUPPLY AIR WATER IN PANELS TOTAL COOLING [degc] [l/s] [degc] [W] [W/m2_fl.] [kg/s] IN [degc] OUT [degc] [W] [W/m2_fl.] [W] [W/m2_fl.] 1 No supply air, wall B. No supply air, symmetric dummy Multi-nozzle air supply, wall B. Multi-nozzle air supply, symm C. Multi-nozzle air supply, uneven

4 3 Results The measured horizontal and vertical temperature distribution in the studied cases are shown in Fig. 4 and Fig. 5 respectively. The results in Fig. 4 show that the difference between black ball temperature and air temperature was less than 0.5 ºC. The difference between the operative temperature calculated from the black ball temperature following the recommendations in standard ISO 7726, and the air temperature was approx. 20% lower. The largest difference of air temperature in horizontal and vertical direction was measured in the case 2C where the convection flow of the heated dummies centralized to one side of the test room causes higher temperature readings locally. Otherwise the vertical and horizontal temperature difference is less than 0.5 ºC in all cases. Fig. 6 compares the results on the cooling power of the radiant panels determined for the studied cases. The cooling power of the radiant panels was decreased by 7% when the heat load was simulated by heated dummies (case 1B) compared to the case when the heat load was conducted through the walls (case 1). In the case 2 with supply air, the cooling power was increased 10% when comparing to the case 1 without supply air, but this disappeared when comparing the case 2B, with dummy heat loads and supply air, to the case 1. Still when comparing the case 2B with the case 1B (in both cases dummies were used as heat source) there is 5% increase because of the supply air flushing the radiant panels. In the case 2C with uneven heat load distribution (dummies located on one side of the room) the increase in cooling power of the radiant panels disappears. This is caused by the colliding local convection flow from dummies with the ventilation flow that can be seen in the smoke visualization shown in Fig Room temp. (1-4) at 1.3m height / Black ball temp. (bb1-bb2) at 1.1m height Temperature [ºC] T1 Tbb1 T2 Tbb2 T3 T No supply air, wall 1B. No supply air, symmetric dummy 2. Multi-nozzle air supply, wall 2B. Multi-nozzle air supply, symm. 2C. Multi-nozzle air supply, uneven Studied case Figure 4. Room air temperatures and black ball temperatures in the occupied zone 4

5 Height [m] Temperature [ºC] Vertical room temperature in studied cases 1. No supply air, wall 1B. No supply air, symmetric dummy 2. Multi-nozzle air supply, wall heat loads 2B. Multi-nozzle air supply, symm. 2C. Multi-nozzle air supply, uneven Figure 5. Vertical temperature stratification measured in the studied cases Radiation panel cooling conductances with relative increase on top -6.7 % +9.6 % +5.1 % -1.8 % to case 1 to case 1 to case 1B to case 1B 30.0 Conductance [W/K] No supply air, wall 1B. No supply air, symmetric dummy 2. Multi-nozzle air supply, wall 2B. Multi-nozzle air supply, symm. 2C. Multi-nozzle air supply, uneven Studied cases Figure 6. Radiation panel conductances in cooling mode in the studied cases. Comparison of the cases 1B and 2 to the case 1, and of the cases 2B and 2C to the case 1B is shown. 5

6 4 Discussion When comparing the cooling power of the radiant panels in the cases without supply air, the case 1B with dummy used as heat load gives a bit smaller cooling power. Although the difference is not large, the result is not consistent to the assumption that internal, more local should increase the power of the radiant panels. This same effect can be seen more clearly in the cases 2 and 2B when air supply was used. The room temperature in the case 1B was higher than in the other cases for this reason and due to too high dummy heat load comparing to the cooling capacity of the radiant panels. The cooling capacity used to define the dummy heat load was based on the manufacturer s product data that was not detailed enough for this kind of installation at that time when the measurements were performed. The relative effect of that was the biggest in the case without the cooling power of the supply air. Still when the conductances are calculated, the performance of the radiant panel can be compared in different cases. The ratio of radiant and convective heat transfer of radiant panels in cooling conditions is approx. 50/50 according to Babiak et al. (2007). The or supply air jet can effect the convective heat transfer but the radiant part stays nearly the same. When comparing the cases with wall and with dummy, there is most probably somehow non-optimal flow field near the radiant panels in the case with dummy, and it has small effect on the operation of the radiant panels. This should be further studied with CFDsimulations, which could give detailed information on the difference in the flow field between these cases. Radiant panels were located near the long side wall in order to obtain longer distance from the supply air diffuser. This would allow increase of the supply air temperature (which was 16 ºC and close to the water temperature 15/17 ºC cooling the panels) and thus to increase the convective heat transfer between the air and the radiant panel. In the case 2C where heated dummies were located only on one side of the room the generated thermal plums interact with the ventilation flow resulting in its discharge towards opposite end of the room. For that reason the supplied ventilation air does not flush the radiant panel as efficiently as in the case with symmetrical. The effect of the heat load arrangement on the cooling power of the radiant panels is biggest when the thermal plums generated by the heat sources effect significantly the distribution of the supply air jet used to enhance the cooling output of the radiant panel. This can be seen when comparing the cooling power of the cases 2 and 2C. 5 Conclusions The heat load arrangement in room affects the performance of radiant cooling panels. It can be concluded that two standardized methods for testing the performance of chilled beams, namely heat load simulation by dummies and by heat load conducted through walls, give nearly the same cooling output for the radiant panels. The most explicit guideline for the design of the radiant panels when considering the effect of the heat load arrangement is to ensure that the supply air jet flushing the panels is as less as possible influenced by the convective flows generated by the heat load in the room. 6 Acknowledgement The study is supported by Technology Agency of Finland (TEKES). The authors wish to thank Mr. Risto Paavilainen for his participation in the measurement work, and Itula Oy for providing the radiant panels and for all other co-operation. 6

7 7 References European standard EN 14240:2004 Ventilation for Buildings. Chilled ceilings. Testing and rating, European committee for standardization CEN, January 2004, Brussels, Belgium European standard EN 14518:2005 Ventilation for Buildings. Chilled beams. Testing and rating of passive chilled beams, European committee for standardization CEN, June 2005, Brussels, Belgium European standard EN 15116:2008 Ventilation in Buildings. Chilled beams. Testing and rating of active chilled beams, European committee for standardization CEN, April 2008, Brussels, Belgium International standard ISO 7726:1998 Ergonomics of the thermal environment- Instruments for measuring physical quantities, International Organization of Standardization ISO, November 2008, Geneve, Switzerland, p , Babiak, J., Olesen, B.W., Petras, D. (2007) Low temperature heating and high temperature cooling, Rehva guidebook no 7, Finland, ISBN , p