The impact of air filter pressure drop on the performance of typical air-conditioning systems

Transcription

1 BUILD SIMUL (2012) 5: DOI /s The impact of air filter drop on the performance of typical air-conditioning systems Nabil Nassif ( ) Department of CAAE Engineering, North Carolina A&T State University, 455 McNair Hall, 1601E. Market Street, Greensboro, NC 27411, USA Research Article Abstract Filters are used in heating, ventilation, and air-conditioning (HVAC) systems for both commercial and residential buildings to protect the equipment and improve indoor air quality in conditioned spaces. Although there are many benefits of using the air filter in an air-conditioning system, the resistance associated with it can increase fan energy use and may adversely affect air-conditioning system performance and efficiency. The paper explores the impact of air filtration on energy consumption for a typical air-conditioning (AC) system with constant- or variable-speed fan. A whole building simulation model is used to simulate the annual energy consumption for various air-conditioning system capacities, different levels of filter cleanliness, and various filter minimum efficiency reporting values (MERV). The results indicate that with a constant-speed fan, the cooling energy use increases as the filter gets over time and the energy use in the fan may increase but this depends heavily on the investigated fan performance curve. With a variable-speed fan, the fan energy use increases with a filter but the cooling and heating energy uses are slightly affected. The fan energy use rise due to the filter depends mainly on air system capacities, filter MERV ratings, and the degree of the filter cleanliness. Keywords air filters, HVAC system, MERV, energy consumptions, fans Article History Received: 3 April 2012 Revised: 2 June 2012 Accepted: 19 June 2012 Tsinghua University Press and Springer-Verlag Berlin Heidelberg Introduction Filters are typically used in both commercial and residential building systems. They are located in the main airstream to protect the equipment and improve indoor air quality in conditioned spaces (ASHRAE 2008). Without filters, particles may accumulate on fans and heat exchanger coils adversely affecting heat transfer (Yang et al. 2007a, b; Siegel and Nazaroff 2003). Although there are many benefits of using the air filter in an air-conditioning system, the resistance associated with it can add extra energy power in the fan operation and may affect adversely on air-conditioning system performance (Nassif 2012; Stephens et al. 2009). As the filter gets over time, the drop across it raises and causes not only an increase in fan power but a reduction of air-conditioning system performance and air distribution system efficiency. With a typical constant-speed fan motor, as filter gets the static increases and airflow rate drops. The reduced airflow rate leads to reduce the system capacity and sensible heat ratio (James et al. 1997; Palani et al. 1992; Parker et al. 1997). With variable-speed fan, as the resistance increases due to a filter, the fan speed increases, and thereby the fan power, in order to maintain the same airflow rate and meet space sensible load requirements. Thus, the study will investigate these effects on whole system performance that includes fan and air-conditioning system. The impacts of air filters on energy consumption for typical air-conditioning (AC) systems equipped with constant- or variable-speed fan are investigated. The study considers a small commercial building (small retail) with various system capacities and locations. 2 Methodology ANSI/ASHRAE Standard (ANSI/ASHRAE 2007) uses the minimum efficiency reporting value (MERV) to rate the effectiveness of air filters. The scale is designed to represent the worst case performance of a filter when dealing with particles in the range of 0.3 to 10 micrometers ( to inches). The MERV rating is from 1 to 16. Higher MERV ratings correspond to a greater percentage of particles captured on each pass, with a MERV 16 filter Building Systems and Components nnassif@ncat.edu

2 346 Nassif / Building Simulation / Vol. 5, No. 4 capturing more than 95% of particles over the full range. Two MERV rating filters MERV 8 and MERV 12 are considered, mostly used in residential and small commercial systems (Table E-1 of ASHRAE Standard ). In Table 12-1 of the Standard , the Standard recommends a minimal final resistance that should be at least twice the initial resistance through filters. The filter is assumed to be clean when the filter resistance is equal to the initial value in Table 12-1 of Standard 52 and to be very when the filter resistance is equal to the final resistance value. The filter is then assumed to be in the middle of that range. Table 1 and Table 2 show the filter resistance ( drop across the filter), total, and airflow rate used in this study. Several field testing has been performed by air filter manufactures for several types and size of filters. These tests showed that the drop through clean filters with different air velocities and filter depths is in the range of 25 Pa 100 Pa. These results are consistent with the data for the clean filters provided in Table 1 and Table 2. For a constant-speed fan, only 3-ton and 5-ton AC systems are considered with MERV 8 and fan sizes of 0.16 kw and 0.26 kw, respectively, whereas the 5-ton, 7.5-ton, and 10-ton systems are considered for the variable-speed fan with MERV 12 and fan sizes of 0.3 kw, 0.45 kw, 0.6 kw respectively. With a constant-speed fan, when the static increases, the airflow rate drops and the amount of flow reduction depends on fan performance data. In addition, the flow and variations change the fan efficiency. The airflow rates in Table 1 and efficiency considered in this study are for certain fan characteristics collected from particular manufacturer s data. As example, for the 3-ton AC system, when the total increases from 157 Pa to 207 Pa, the Table 1 and data for constant-speed fan AC system with different MERV8 filter cleanliness 3-ton system Filter 5-ton system Filter Clean Dirty flow rate drops from 595 L/s to 472 L/s. The flow rate for clean filter is based on 198 L/s per ton. The study discusses a typical single storefront building located in the following locations: Greensboro (NC), Orlando (FL), New York (NY), and San Francisco (CA). The building area varies to get various system capacities ranging from 3 ton to 10 ton. In this paper, for small systems (3 ton and 5 ton), the conditioned air is provided by a split system single zone with furnace and MERV 8 filter. For relatively larger systems (5, 7.5, and 10 ton), the conditioned air is provided by a packaged variable-speed variable-temperature system with furnace and MERV12 filter. To evaluate the annual energy consumption for air-conditioning systems, a whole building energy simulation model equest is used. The software uses DOE 2.1-E hourly building energy simulation software to simulate energy use. In addition, to investigate the effect of reduced airflow rate on cooling performance, the direct expansion AC model (ACDX) is used (Brandemuehl et al. 1993). The ACDX model is reliable for airflow rates between L/s and 236 L/s per ton (e.g., L/s and 708 L/s for a 3 ton system). 3 Impact of filters on fan performance For a constant-speed fan, as the filter becomes or very, the head of the fan increases and airflow rate drops. Figure 1 shows fan and system curves for clean,, and very filters. In Fig. 1, point 1 represents the conditions when the filter is clean. In this example, the fan operates at total static of 75% of maximum and airflow rate of 80% of maximum flow. With an increase of filter drop and consequently total static, the airflow rate decreases (point 2 and point 3), leading to an increase in fan energy use as the fan is required to run longer to meet the same sensible cooling load and with a higher operating static. Table 2 and data for variable-speed fan AC system with different MERV12 filter cleanliness Filter 5-ton system 7.5-ton system 10-ton system Clean Dirty Fig. 1 Performance curves for constant-speed fan with different levels of filter cleanliness

3 Nassif / Building Simulation / Vol. 5, No For variable-speed fan, as the filter becomes or very, the variable-speed fan will react to operate at an increased speed because of higher through-filter resistance in order to maintain the same airflow rate. Figure 2 shows fan and system curves for clean,, and very filters. The point 1 represents the conditions when the filter is clean and the fan operates at a speed of n 1 and uses power of hp 1 to maintain for instance, an airflow rate of 60% of the design value. However, the point 2 represents the conditions when the filter is somewhat and the fan operates at a higher speed of n 2 and consumes more power (hp 2 ) to maintain the same airflow rate. Similarly, the point 3 repents the condition when the filter is very. As the filter becomes (moving from point 1 to point 2 then to point 3), the fan speed increases from n 1 to n 2 and then to n 3 and the fan power from hp 1 to hp 2 and then to hp 3 to keep the same airflow rate. As the airflow rate varies in variablespeed systems, the potential operating points are then located on system curves shown in Fig. 2. Fig. 2 Performance curves for a typical variable-speed fan with different levels of filter cleanliness filter, the fan speed increases to maintain the same airflow rate and meet the space sensible load requirements. As there is no change in the airflow rate, there is no change (or slightly change) in the cooling and heating energy use. The cooling energy use slightly increases and heating energy use slightly decreases due to increased fan power and heat released from the fan motor to supply air stream. In addition, the elevated in the air duct increases the air leakage and effects negatively on cooling and heating system performance. In this paper, the air leakage is not considered. The drop through the filter depends mainly on certain factors such as airstream velocity, filter type, MERV rating, area, and cleanliness. As shown in Table 2, for a typical 5-ton system, the drop across the filter varies from 50 Pa to 100 Pa and the total static varies from 182 Pa to 232 Pa. The drop associated with the air filter leads to higher total static and higher fan power. Figure 3 shows the annual fan energy consumption for the packaged variable-speed AC system with various static s (Greensboro, NC). For the 5-ton system located in Greensboro, the annual fan energy use increases from 285 kwh to 360 kwh when the total static increases from 182 Pa to 232 Pa. Due to the assumptions presented in Table 2, the drop through the clean filter is 50 Pa and becomes 100 Pa when the filter gets very. Table 3 shows the annual fan energy consumption for the variable-speed AC system with clean,, and very filters. The clean,, very filters are corresponding to the total 182 Pa, 207 Pa, and 232 Pa as presented in Table 2. The fan energy consumption increases as the filter gets. As example, for the 10-ton system located in San Francisco, the annual fan energy consumptions are 510 kwh, 585 kwh, and 660 kwh for clean,, and very filters, respectively. This result indicates that the annual fan energy consumption increases 4 Results and discussion The analysis considers different sizes of a small commercial building (small retail storefront) with different AC system capacities and filter drops. It is assumed that the store opens at 9 AM and closes at 8 PM. The store area is varied from 93 m 2 to 279 m 2 depending on the locations to vary the AC system capacities, ranging from 3 ton to 10 ton. For a particular location and system and building size, the input variables of equest model such as airflow rate, total static, and fan efficiency are varied to simulate the annual energy consumptions. In this section, a variable-speed fan motor AC system (packaged variable-speed variable-temperature system) with MERV12 is considered with three different capacities (5 ton, 7.5 ton, and 10 ton). As the resistance increases due to a Fig. 3 Annual fan energy consumption for the packaged variable speed AC system with various static s (Greensboro, NC)

4 348 Nassif / Building Simulation / Vol. 5, No. 4 Table 3 Annual fan energy consumption for the variable-speed AC system Greensboro New York San Francisco Orlando 5-ton system 7.5-ton system 10-ton system Clean Dirty Clean Dirty Clean Dirty Clean Dirty kwh with the filter and 150 kwh with the very filter as compared to that for the clean filter, an increase of 14.7% and 29.4 %, respectively. The annual fan energy use rise could be up to about 30%, depending on locations considered and the degree of the filter cleanliness. In the following sections, a constant-speed fan motor AC system (split system single zone with MERV 8 filter) is considered with two different capacities (3-ton and 5-ton systems). As the filter gets, the static increases and airflow rate drops. This causes not only an increase in the fan power but a reduction of air-conditioning system performance and air distribution system efficiency. For specific fan performance data as presented in Table 1, the airflow rate drops from 595 L/s to 472 L/s when the total static increases from 157 Pa to 207 Pa due to an increase of the drop through the filter from 25 Pa to 75 Pa. The fan power is influenced by both airflow rate and static variables, whereas the AC cooling performance is influenced by only the value of airflow rate. The direct expansion AC model (ACDX) is used (Brandemuehl et al. 1993) to study the effect of reduced airflow rate on cooling performance. The simulations were done on the 3-ton AC system with various airflow rates and the following conditions; entering dry temperature of 26.6, humidity ration of 0.011, and outdoor air temperature of 35. The resulted cooling capacity and airflow rates are depicted in Fig. 4. The results are represented as a percentage of the selected design value, e.g., 212 L/s per ton, total airflow rate of 637 L/s, total cooling capacity of W, and sensible cooling capacity of W. The total cooling capacity and sensible cooling capacity both drop with a lower airflow rate, whereas the latent capacity increases. For instant, when the flow rate drops to 80% of design value, the total capacity drops to 96.1% and the sensible capacity drops to 92.1% of design value (i.e., W). The reduced airflow rate makes the fan and air-conditioning system run longer to meet the same cooling load. These effects of reduced airflow rate on cooling performance were similarly investigated and discussed in details elsewhere (Parker et al. 1997). The energy simulations run for the constant-speed 3-ton and 5-ton AC systems with MERV8 and with various combinations of flow rate and static to estimate the annual energy use. Figures 5 and 6 show the annual cooling and fan energy consumptions for the system located in Greensboro. If the flow rate drops from 713 L/s to 401 L/s for 3-ton system, the cooling energy consumption increases from 2930 kwh to 3490 kwh. The increase in cooling energy consumption is because of a lower airflow rate and associated lower cooling capacity as shown in Fig. 4. Other effect but not considered here is the increased air leakage in the return side duct or plenum due to the high with a filter. During the heating season, there is a slight change in the heating energy consumption due to the reduced airflow, the increased return duct leakage, and the increased heat generated by elevated fan power. The change in the cooling consumption of a constant-speed fan varies only with the airflow rate. However, when the static increases and at the same time, the airflow rate drops, the fan energy varies depending on both values and fan efficiency as well. For example, for the 3-ton system located in Greensboro, the fan energy increases from 640 kwh to 830 kwh when the increases from 107 Pa to 232 Pa and the flow rate drops from 713 L/s to 410 L/s. This change in the fan energy consumption depends on the investigated fan performance curve that represents the relationship between the and flow rate including the fan efficiency. Fig. 4 System capacity as a function of airflow rate

5 Nassif / Building Simulation / Vol. 5, No Fig. 5 Annual cooling energy consumption for the constant-speed split system with various airflow rates (Greensboro, NC) Table 4 summarizes the results for the clean, and very MERV 8 filters. The clean,, very filters are corresponding to the total of 157 Pa, 182 Pa, and 207 Pa as presented in Table 1. When the clean MERV 8 filter becomes very, the total static is assumed to rise from 157 Pa to 207 Pa and the flow consequently drops from 595 L/s to 472 L/s for the 3-ton AC system and from 991 L/s to 779 L/s for the 5-ton AC system due to fan performance curve obtained for those specific units. This change causes (1) an increase in the fan energy consumption from 790 kwh to 820 kwh (an increase of 3.8%) for the 3-ton AC system and from 1310 kwh to 1360 kwh (an increase of 3.8%) for the 5-ton AC system located in Greensboro, Fig. 6 Annual fan energy consumption for the constant-speed split system with different combinations of airflow rate and static as presented in Table 1 (Greensboro, NC) and (2) an increase in the cooling energy consumption from 3210 kwh to 3460 kwh (an increase of 7.8%) for the 3-ton AC system and form 4540 kwh to 4870 kwh (an increase of 7.3%) for the 5-ton AC system. In dry climate such in San Francisco, due to the relatively low latent load, the cooling energy use is slightly affected by the airflow rate variations. As a summary, for a constant-speed AC system, the increase in the cooling energy use is in the range of kwh per ton (except San Francisco) and the increase in the fan energy use is in the range of kwh per ton. It should be noted that the change in the cooling energy use Table 4 Annual fan and cooling energy consumption for the constant-speed AC system Greensboro New York San Francisco Orlando 3-ton AC system 5-ton AC system Fan Cooling Fan Cooling Clean Dirty Clean Dirty Clean Dirty Clean Dirty

6 350 Nassif / Building Simulation / Vol. 5, No. 4 depends only on the variation of airflow rate but the change in the fan energy use due to the filter strongly depends on the fan characteristics curve used. For a variable-speed fan, no significant change in the cooling and heating energy use is occurred. However, the increase in the fan energy is in the range of kwh per ton. The results indicate that the filter can cause higher energy use and show the important of replacing the filter frequently and in optimal manner. 5 Conclusions The whole building energy simulation model equest is used to investigate the impacts of air filter on fan energy consumption in air-conditioning systems with constant or variable-speed fan. The study takes into consideration two MERV filters and system capacities, ranging from 3 to 10 ton. The simulation results showed that the annual energy consumption increases as the filters get over time. For a variable-speed fan AC system, the annual fan energy consumption increases with very filter up to 30%, depending on the locations and the degree of the filter cleanliness. The fan energy use increase could be in the range of kwh per ton. There is no change or slightly change in the cooling and heating energy uses due to the change in fan power, the heat released to the supply air, and air duct leakage. For a constant-speed fan AC system, the annual fan energy consumption also increases but less than with the variable speed fan and it could reach up to 5%, depending strongly on the fan performance data considered. The annual cooling energy use increases with very filter up to 9%, depending on locations. In relatively dry climate as in San Francisco, the cooling energy use increases relatively less than other areas due to the lower latent load. There is no change or slightly change in the heating energy use due to the increased fan power and air duct leakage. The increase in the cooling energy use could be in the range of kwh per ton (except San Francisco) and the increase in the fan energy use could be in the range of kwh per ton. The results indicated that the air filter can be important source of energy waste if the filter is not replaced in an optimal time manner. References ANSI/ASHRAE (2007). ANSI/ASHRAE Standard Method of testing general ventilation air-cleaning devices for removal efficiency by particle size. Atlanta, USA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. ASHRAE (2008). ASHRAE Handbook-HVAC Systems and Equipment, Chapters 9 and 28. Atlanta, USA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. Brandemuehl MJ, Gabel S, Andersen I (1993). A Toolkit for Secondary HVAC System Calculation. Atlanta, USA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. James P, Cummings J, Sonne J, Vieira R, Klongerbo J (1997). The effect of residential equipment capacity on energy use, demand, and run-time. ASHRAE Transactions, 103(2): Nassif N (2012). Impacts of air filters on energy consumption in typical HVAC systems. Paper presented at the 2012 Annual ASHRAE Meeting, San Antonio, USA. Palani M, O Neal D, Haberl J (1992). The effect of reduced evaporator air flow on the performance of a residential central air-conditioner. In: Proceedings of the 8th Annual Symposium on Improving Building Efficiency in Hot and Humid Climates, Dallas, USA. Parker D, Sherwin J, Raustad R, Shirey III D (1997). Impact of evaporator coil airflow in residential air-conditioning systems. ASHRAE Transactions, 103(2): Siegel JA, Nazaroff WW (2003). Predicting particle deposition on HVAC heat exchangers. Atmospheric Environment, 37: Stephens B, Novoselac A, Siegel AJ (2009). Impacts of HVAC Filtration on air-conditioner energy consumption in residences. In: Proceedings of Healthy Buildings 2009 (paper 474), Syracuse, USA. Yang L, Braun JE, Groll WA (2007a). The impact of evaporator fouling and filtration on the performance of packaged air-conditioners. International Journal of Refrigeration, 30: Yang L, Braun JE, Groll WA (2007b). The impact of fouling on the performance of filter-evaporator combinations. International Journal of Refrigeration, 30: