Cooling coil optimisation in hot and humid climates for IAQ and energy considerations

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1 Cooling coil optimisation in hot and humid climates for IAQ and energy considerations Chandra Sekhar 1, Uma Maheswaran 2 1 Department of Building, National University of Singapore 2 Jurong Consultants Singapore * Corresponding bdgscs@nus.edu.sg ABSTRACT In hot and humid climates, the primary challenge is cooling and dehumidification, which needs to be achieved in an energy efficient manner in order to provide adequate ventilation and maintain good indoor air quality. This necessitates a careful consideration of all the related design parameters both on the air handling unit side as well as the chiller side. In this regard, the design of cooling coil plays an important role in obtaining optimal performance. In this paper, the water-side parameters of the cooling coil are studied using a standard coil selection programme with the view of fulfilling the design requirements of airflow, cooling capacity and indoor environmental conditions. Conventional design practices of 6 and 7 C chilled water supply (CHWS) temperatures and 6 and 7 C chilled water Ts are used as reference cases and the following three variations are explored a) increasing CHWS temperatures and reducing chilled water Ts b) larger chilled water Ts with constant CHWS temperature and c) increasing CHWS temperature and constant CHW T. Energy simulations are also done to estimate the overall energy performance in these various combinations. KEYWORDS Energy Performance; HVAC and mechanical systems; Climate Specific Designs INTRODUCTION Cooling and dehumidification is of utmost importance in air-conditioning designs in hot and humid climates for good thermal comfort and Indoor Air Quality (IAQ). This is equally critical from energy considerations as the dehumidifying process would normally impose significant energy penalties. Hence, the design of cooling coils plays an important role in addressing these issues in an optimal manner. For a given amount of design air flow rate (ventilation and recirculated air flow quantities) and cooling capacity, there are several cooling coil parameters of significance that include chilled water supply (CHWS) temperature, chilled water temperature rise ( T), chilled water flow rate, face velocity through the coil and coil geometry. The causes and mitigation of degrading chilled water plant T have been studied before (Taylor, 2002). Higher chilled water T has been suggested as a means of improving overall chiller performance (Fiorino, 1999). In the present study, conventional design practices of 6 and 7 C chilled water supply (CHWS) temperatures and 6 and 7 C chilled water Ts are used as reference cases and the following three variations are explored a) increasing CHWS temperatures and reducing chilled water T b) larger chilled water Ts with constant CHWS temperature and c) increasing CHWS temperature and constant CHW T. METHODOLOGY

2 A standard coil selection programme (SPC 2000, 2011) is used to analyse the cooling coil performance for three different sets of studies: Set A involves increasing CHWS temperature and constant CHWR temperature. This results in decreasing CHW T Set B involves increasing CHW T and constant CHWS temperature Set C involves increasing CHWS temperature and constant CHW T In all these studies, the design criteria of thermal comfort and IAQ are dictated by ensuring that the same off-coil conditions are achieved at all times. For the energy analysis, a standard energy simulation tool is employed (IES 2011). A 1000 m 2 single storied office building is modeled for the annual energy simulation. Lighting and equipment power density as well as envelope designs that conform to minimum requirements of Singapore codes are used. Typical office schedules of 10 hours/day and 5 days/week are employed for occupancy, lighting and equipment. RESULTS AND DISCUSSION The cooling coil performance analysis for the three different sets of studies is shown in Table 1. In Set A, the chilled water return temperature (CHWS) from the cooling coil is kept constant at 13 C and the chilled water supply temperature is varied from 6 C through 10 C for the 5 cases. For a given coil duty, this will result in increasing chilled water flow rate as the chilled water T is reduced from 7 C through 3 C. Whilst this would imply reduced chiller energy consumption, it would result in raising the water pressure drop through the coil and would also result in significant increase in pump energy consumption in the chilled water cycle between the chiller and the Air Handling Unit. In order to be able to compare these 5 cases, it is important to ensure that the design off-coil conditions (13 C DBT and 12.9 C WBT) are always achieved, as seen in Table 1a. This governs the design criteria of good thermal comfort and IAQ. It is observed from Table 1a that the 5 cases of coil selections are made possible by choosing coils with different coil geometries. Whilst the number of rows (4) of the coil and the finned height (1185 mm) is the same for all the 5 cases, the finned length progressively increases from 1350mm through 1550 mm, as the chilled water T is decreased from 7 C through 3 C. It is seen that Set A results in reducing face velocities through the coil in the range of 2.19 m/s through 1.91 m/s. Consequently, the air pressure drop through the coil is reduced in the range of Pa through Pa, which implies significant energy saving potential on the air side. In Set B, the chilled water supply temperature (CHWS) to the cooling coil is kept constant at 7 C and the chilled water T is varied from 6 C through 11 C for 6 cases. For a given coil duty, this will result in reducing chilled water flow rate as the chilled water T is increased from 6 C through 11 C. This not only results in reducing water pressure drop through the coil but would also result in significant reduction in pump energy consumption in the chilled water cycle between the chiller and the Air Handling Unit. In order to be able to compare these 6 cases, it is important to ensure that the design off-coil conditions (13 C DBT and 12.9 C WBT) are always achieved, as seen in Table 1a. This, again, governs the design criteria of good thermal comfort and IAQ. It is observed from Table 1a that the 6 cases of coil selections are made possible by choosing coils with different coil geometries. Whilst the number of rows (4) of the coil and the finned height (1185 mm) is the same for all the 6 cases, the finned length progressively increases from 1400mm through 2500 mm, as the chilled water T is increased from 6 C through 11 C.

3 PHYSICAL COIL DATA FLUID SIDE DATA AIR SIDE DATA Table 1a : Cooling coil performance analysis SET A SET B Increasing CHWS Temp & Constant CHWR Temp Increasing CHW T & Constant CHWS temperature Air On Dry Bulb Temperature ( C) Air On Wet Bulb Temperature ( C) Air Off Dry Bulb Temperature ( C) Air Off Wet Bulb Temperature ( C) Coil Duty (kw) Standard Air Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Air Density (kg/m³) Face Velocity (m/s) Air Pressure Drop (Pa) Air Volume (m³/s) Sensible Heat Ratio Fluid On Temperature ( C) Fluid Off Temperature ( C) Fluid (Chilled Water) T Fluid Flow Rate (l/s) Max Fluid Pressure Drop (kpa) Actual Fluid Pressure Drop (kpa) Fin Material and Type Coil Type Water Water Water Water Water Water Water Water Water Water Water Tube Diameter 12mm 12mm 12mm 12mm 12mm 12mm 12mm 12mm 12mm 12mm 12mm Tubes High Finned Height (mm) Finned Length (mm) Fin Density (FPI) (Fixed) Circuit Type (Fixed) F F F F F F F F F F F Number Rows

4 PHYSICAL COIL DATA FLUID SIDE DATA AIR SIDE DATA It is seen that Set B results in reducing face velocities through the coil in the range of 2.11 m/s through 1.18 m/s. Consequently, the air pressure drop through the coil is reduced in the range of Pa through 61.7 Pa, which implies significant energy saving potential on the air side. Table 1b : Cooling coil performance analysis SET C Increasing CHWS temperature & Constant CHW T Air On Dry Bulb Temperature ( C) Air On Wet Bulb Temperature ( C) Air Off Dry Bulb Temperature ( C) Air Off Wet Bulb Temperature ( C) Coil Duty (kw) Standard Air Yes Yes Yes Yes Yes Air Density (kg/m³) Face Velocity (m/s) Air Pressure Drop (Pa) Air Volume (m³/s) Sensible Heat Ratio Fluid On Temperature ( C) Fluid Off Temperature ( C) Fluid (Chilled Water) DT Fluid Flow Rate (l/s) Max Fluid Pressure Drop (kpa) Actual Fluid Pressure Drop (kpa) Fin Material and Type Coil Type Water Water Water Water Water Tube Diameter 12mm 12mm 12mm 12mm 12mm Tubes High Finned Height (mm) Finned Length (mm) Fin Density (FPI) (Fixed) Circuit Type (Fixed) F F F F F Number Rows In Set C, the chilled water supply temperature (CHWS) to the cooling coil is varied from 6 C through 10 C and the chilled water T is kept constant at 6 C for all the 5 cases. For a given coil duty (design cooling coil load), this will result in the same chilled water flow rate for all the 5 cases. In order to be able to compare these 5 cases, it is again important to ensure that the design off-coil conditions (13 C DBT and 12.9 C WBT) are always achieved, as seen in Table 1b. As mentioned earlier, this governs the design criteria of good thermal comfort and IAQ. It is observed from Table 1b that the 5 cases of coil selections are made possible by choosing coils with different coil geometries. Whilst the number of rows (4) of the coil and the finned height (1185 mm) is the same for all the 5 cases, the finned length progressively increases as the CHWS temperature is increased from 6 C through 10 C. This, in effect, results in reducing face velocities through the coil in the range of 2.36 m/s through 1.3 m/s.

5 Consequently, the air pressure drop through the coil is reduced in the range of Pa through 71.8 Pa, which implies significant energy saving potential on the air side. This is, however, to be weighed in the context of an increase in the water pressure drop through the coil in the range of 38.6 kpa through 53.3kPa as the CHWS temperature is progressively increased. An annual energy analysis is done for the chiller, pumps and the cooling tower for Set A and Set B. The energy analysis is performed for two different chiller COP : 5 & 6.4. For a chiller COP of 5, the summary data for Set A and Set B are shown in Table 2a and 2b respectively. It is seen that Set A offers the most reduction in chiller energy consumption and despite significant increase in pump energy, the total plant energy savings of 3.6% is still the largest. It is to be borne in mind that the simulation model is for a small single storey office building with not much implication for pump energy. With increased base pump energy, as would be the case with high rise buildings, this could become even more significant. In the case of Set B, there is no chiller energy reduction while the pump energy consumption does reduce due to reducing chilled water flow rate as the CHW T increases and this results in a total plant energy savings of 1.1%. Table 2a : Annual energy analysis (MWH) of chiller, pumps and cooling tower based on a chiller COP of 5 for Set A (Increasing CHWS Temp & Constant CHWR Temp) CHWS/CHWR 6/13 7/13 8/13 9/13 10/13 Chiller Pumps Cooling Tower Total % Table 2b : Annual energy analysis (MWH) of chiller, pumps and cooling tower based on a chiller COP of 5 for Set B (Increasing CHW T & Constant CHWS temperature) CHWS/CHWR 7/13 7/14 7/15 7/16 7/17 7/18 Chiller Pumps Cooling Tower Total % CHWS : Chilled Water Supply Temperature; CHWR : Chilled Water Return Temperature Table 3a : Annual energy analysis (MWH) of chiller, pumps and cooling tower based on a chiller COP of 6.4 for Set A (Increasing CHWS Temp & Constant CHWR Temp) CHWS/CHWR 6/13 7/13 8/13 9/13 10/13 Chiller Pumps Cooling Tower Total %

6 Table 3b : Annual energy analysis (MWH) of chiller, pumps and cooling tower based on a chiller COP of 6.4 for Set B (Increasing CHW T & Constant CHWS temperature) CHWS/CHWR 7/13 7/14 7/15 7/16 7/17 7/18 Chiller Pumps Cooling Tower Total % CHWS : Chilled Water Supply Temperature; CHWR : Chilled Water Return Temperature For a chiller COP of 6.4, the summary data for Set A and Set B are shown in Table 3a and 3b respectively. It is seen that Set A offers some reduction in chiller energy consumption and despite significant increase in pump energy, the total plant energy savings is still 2.6%. In the case of Set B, there is no chiller energy reduction while the pump energy consumption does reduce due to reducing chilled water flow rate as the CHW T increases and this results in a total plant energy savings of 1.3%. CONCLUSION It is seen that CHWS temperatures up to 10 C may be viable for a given chilled water T of 6 C and this leads to improved chiller performance. For a given CHWR temperature, increasing CHWS temperatures resulting in reducing chilled water T are also found to be interesting strategies for both thermal comfort and overall plant energy performance. For a given CHWS temperature, it is further seen that chilled water T up to 11 C may be viable and this leads to reduced chilled water flow rate resulting in lower pumping energy consumption, although there is nothing significant in terms of chiller energy reduction. It is to be noted that there are several combinations of plant (chiller, pump and cooling tower) and system (coil and fan) parameters that are feasible but their combined performance in terms of thermal comfort, IAQ and energy need to be carefully evaluated. ACKNOWLEDGEMENT The authors gratefully acknowledge the contribution of Mr Jean-Baptiste Noel, ESD Engineer, Jurong Consultants Singapore, in performing the energy simulations in this paper. REFERENCES Fiorino, Donald P., Achieving High Chilled-Water Delta Ts. ASHRAE Journal, November Issue, pp IES Integrated Environmental Solutions Ltd. Taylor, Steven T., Degrading Chilled Water Plant Delta-T : Causes and Mitigation. ASHRAE TRANSACTIONS, V. 108, Pt. 1. SPC 2000 (2011). S&P Coil Product Limited, Self-Selection Software, Product Information and Professional Development Download,