Indoor Air 2008, 17-22 August 2008, Copenhagen, Denmark - Paper ID: 741 Optimisation of cooling coil performance during operation stage for improved humidity control S.C. Sekhar * and L.T. Tan National University of Singapore, Singapore * Corresponding email: bdgscs@nus.edu.sg SUMMARY In hot and humid climates, indoor humidity levels are usually high and this is a matter of concern from Thermal Comfort and IAQ perspectives. This phenomenon can be attributed to the inadequate dehumidifying performance of the cooling coil. Amongst other factors, poor dehumidifying coil performance results from the selection and installation of an oversized coil and the conventional strategy of modulating the chilled water flow rate as the only available control method. It is not uncommon to find that operating conditions demand a reduced chilled water flow rate not just during part-load conditions but even during the actual peakload condition. Based on a simulation method that involves selection of coils for different conditions, this paper presents the findings of the performance evaluation of an oversized coil at different conditions during the operation stage. The findings show that the dehumidifying performance of the oversized coil at reduced loads during normal operation can be significantly enhanced by changing the effective surface area of the coil through a simple manipulation of the effective number of rows of coil operation. KEYWORDS Cooling coil, Dehumidifying performance, Design and operation, Thermal comfort and IAQ, Hot humid climates INTRODUCTION In hot and humid climates, the performance of the cooling and dehumidifying coil has a strong bearing on the eventual indoor environmental conditions, which in turn, has a tremendous impact on the indoor air quality. It is important to recognize that the primary function of the cooling coil is not just cooling but the ability to cool and dehumidify in the correct proportion of sensible and latent loads in which they occur in a dynamically changing operation stage (Sekhar et al., 1989). Thus, the room sensible heat ratio (RSHR) and the sensible heat ratio (SHR) of the coil are key design considerations. Notwithstanding the design of the coil, its operation is typically controlled by attempting to match the actual cooling load in operation with the required coil capacity. The only way to achieve this is by modulating the chilled water flow rate. Some control strategies include the use of air-bypass around the coils or temperature reset (Mathews et al., 2001). Often, the cooling coils are oversized due to an over estimation of the anticipated cooling loads and this would have a detrimental effect on the cooling and dehumidifying performance at part load operating conditions. Inevitably, conventional HVAC control strategies would inhibit the ability of the coil to dehumidify exactly when the opposite is desired. This paper deals with the issues associated with overdesigning, commonly prevalent in typical design practice, and recommends a solution that can result in improving the cooling and dehumidifying performance of the oversized coils in a dynamically fluctuating operation stage.
METHODS A simulation approach is adopted in this study that involves an evaluation of the coil performance for various operating conditions and through the use of various coil geometries. A standard coil selection program, freely downloadable from the internet, is used in performing the simulations (SPC 2000, 2007). A hypothetical building with an actual maximum cooling load of 100 kw supplied with a 200 kw design capacity (oversized) cooling coil is considered. The required indoor air condition for the entire simulation is set at 24.5 C with an expected relative humidity of 60%, resulting in the upstream coil conditions of 26 C DBT and 65% RH. The psychrometric state points, associated with the design conditions of 200 kw, are shown in Figure 1. The simulations at other anticipated operating conditions are then performed and this process involves the use of the oversized coil as well as coils with other geometries that are customized to the prevailing cooling loads. Based on the evaluation of the various simulation results, a solution is recommended that could result in an improved performance of the oversized coil during the normal operation stage. No Parameter Values 1 Outside Air 32 C DBT & 75% RH 2 Enteringcoil condition 26 C DBT & 65% RH 3 Return air condition 24.5 C DBT & 60% RH 4 Space condition 24 C DBT & 63% RH 5 Leaving coil condition 13 C DBT & 12.5 C WBT 6 Chilled water supply temperature 6 C 2 1 DBT Dry Bulb Temperature WBT Wet Bulb Temperature RH Relative Humidity Coil Condition Curve 6 5 4 3 Room Sensible Heat Ratio Figure 1. Psychrometric state points for the base-case simulation of the oversized coil. RESULTS AND DISCUSSION Coil selected for design capacity Operating at design capacity Since the simulation exercise aims at comparing between the various cooling coil operating conditions, the first step is to simulate the coil at design capacity (also known as base case). Both Table 1 and Table 2 show the configuration of the cooling coil at its design capacity (base case). Table 1 shows the conditions that are required to remain constant throughout the entire simulation while Table 2 shows the required conditions, with some having the ability to be manipulated to simulate the effectiveness of the various operating conditions. The different simulated conditions can then be used to determine the difference in coil performance as compared to the base condition.
Table 1. Fixed configuration of the cooling coil at design capacity. Air Side Air on DB Temp 26 C RH (%) 65% Standard Air Yes Fluid Side Fluid on Temp 6 C Max Pressure Drop 50kPa Physical Fin Material/type Aluminium 0.15 rippled Coil Type water Tube Diameter 16mm Tubes High 30 Finned Height 1218mm Finned Length 2100mm No. of sections 1 Surface Margins 1 Table 2. Adjustable configuration of the cooling coil at design capacity. Air Side Air Flow (m3/s) 6.45 Face Velocity 2.52 Air off DB 13 Air off WB 12.5 Capacity ( kw) 200 SHR (%) 52% Physical Rows 6 Fin Density 9 Circuit type F Fluid Fluid on Temp 6 Side Fluid off Temp 12 Fluid flow rate (l/s) 7.96 Actual PD 56.5 Oversized coil - Operating at actual maximum peak load While the coil design capacity is 200 kw, the actual maximum peak load is only 100 kw and thus, the coil is now oversized. Series A1 simulates the oversized coil operating at actual peak load condition. To understand the effectiveness of operating the design capacity coil operating at half its design load, a comparison between base condition (Design load) and Series A1 (Actual Peak load) is shown in Table 3.
The basic performance criterion for any typical cooling coil is often judged by its cooling capacity (sensible) and its dehumidification (latent) capability, which is measured in terms of SHR. As seen in Table 3, operating a coil at half its design capacity (from 200 kw to 100 kw) increases the SHR by 10%. Kavanaugh (2006) elaborated that coils with higher SHR may reduce temperature too quickly for adequate moisture removal to occur, while coils with low SHR may dry out the air too much and indoor air may become too dry. This 10% increase in SHR could affect the air quality that is produced as the efficiency of the dehumidification process is reduced, i.e. the coil is able to achieve the required temperature but at a higher humidity. This higher humidity could affect both the indoor air quality and thermal comfort. Table 3. Comparison between coils operating at design and peak load. Base Series A1 Air Side Air Flow (m3/s) 6.45 6.45 Face Velocity 2.52 2.52 Air off DB 13 18.3 Air off WB 12.5 17.2 Capacity ( kw) 200 100 SHR (%) 52% 62% Physical Rows 6 6 Fin Density 9 9 Circuit type F F Fluid Side Fluid on Temp 6 6 Fluid off Temp 12 14.8 Fluid flow rate (l/s) 7.96 2.7 Actual PD 56.5 13.1 In order to improve the dehumidifying performance through a reduction of the SHR, a simulation was performed to match the actual maximum cooling load with the correct heat transfer surface area. This, naturally, leads to a reduction of the overall heat transfer surface area, and therefore, leads to the exploration of 4-row and 3-row coils. The simulated series A1, B1 and C1 in Table 4 were simulated while keeping the coil duty and air flow rate constant at 100 kw and 6.45 m 3 /s respectively. Meanwhile, the effective number of rows in the coil was reduced from 6 to 4 and eventually to 3. Figure 2 shows the analyzed results from Table 4, and it can be clearly seen that by reducing the effective number of operating coil rows, from a 6-row to a 4-row and finally to a 3-row configuration, SHR could be reduced thus increasing the coil efficiency.
Table 4. Simulation of coils operating at peak load with reduced coil rows. Base Series A1 Series B1 Series C1 Air Side Air Flow (m3/s) 6.45 6.45 6.45 6.45 Face Velocity 2.52 2.52 2.52 2.52 Air off DB 13 18.3 18.4 18.8 Air off WB 12.5 17.2 17.1 17.1 Capacity( kw) 200 100 100 100 SHR (%) 52% 62% 61% 58% Physical Rows 6 6 4 3 Fin Density 9 9 9 9 Circuit type F F F F Fluid Side Fluid on Temp 6 6 6 6 Fluid off Temp 12 14.8 13.2 10.8 Fluid flow rate (l/s) 7.96 2.7 3.33 4.95 Actual PD 56.5 13.1 17 17.8 Reduction of SHR with a reduction in number of Coil Rows 62% 61% 60% SHR (%) 59% 58% 57% 56% Series A1 Series B1 Series C1 6 row s 4 row s 3 row s Simulated Conditions Figure 2. Coil SHR performance with respect to reducing rows. Oversized coil - Operating between actual maximum and part loads If the actual maximum cooling load ie the actual peak load is only 100 kw, the part load operating conditions would experience an even further reduction in coil capacity. In order to explore the performance of the oversized coil at actual part loads, a simulation is performed at a part load capacity of 50 kw (Series A3). A simulation is also performed to evaluate the performance of a 3-row coil with reduced total surface area (Series C3). It is seen from Table
5 that the lower capacity of 50 kw increases the SHR by well over 30% for the oversized 6- row coil. The performance, in terms of SHR, is significantly improved with a 3-row coil. It is, therefore, recommended that a simple chilled water by-pass strategy be adopted to reduce the effective surface area of the coil from a 6-row coil to a 4-row or a 3-row coil during operation stage. Table 5. 6-row and 3-row coils operating at a part load of 50 kw. Base Series A3 Series C3 Air Side Air Flow (m3/s) 6.45 6.45 6.45 Face Velocity 2.52 2.52 2.52 Air off DB 13 20.6 21.3 Air off WB 12.5 19.2 19.1 Capacity ( kw) 200 50 50 SHR (%) 52% 87% 75% Physical Rows 6 6 3 Fin Density 9 9 9 Circuit type F F F Fluid Side Fluid on Temp 6 6 6 Fluid off Temp 12 14 11.5 Fluid flow rate (l/s) 7.96 1.5 2.18 Actual PD 56.5 10 6.9 CONCLUSIONS This paper has addressed an important practical challenge related to the operation of an oversized cooling and dehumidifying coil. The impact of such a coil at various operating conditions including those at design, actual peak and part load has been evaluated through a simulation approach. The Sensible Heat Ratio has been used as an indicator of the dehumidifying performance of the coil and, for the worst case scenario of 50 kw cooling load, it is observed that changing the effective surface area of the coil from 6-rows to 3 rows results in a reduction of SHR from 87% to 75%. This is a significant improvement in the dehumidifying performance of the oversized coil.
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