1、OR-05-17-1 Modeling Filter Bypass: Impact on Filter Efficiency Matthew Ward ABSTRACT Current models and test methods for determining filter eficiency ignore filter bypass, the air that circumvents filter media because ofgaps around thefilter orjlter housing. In this papel; we develop a general model
2、 to estimate the size-resolved particle removal eficiency, including bypass, of HVACfilters. The model applies the measuredpressure drop of the filter to determine the air-ow through the bypass cracks and accounts for particle loss in the bypass cracks. We consider a particle size range of 0.01 to 1
3、 O ,um, nine typical commercial and resi- dentialfilters in clean and dust-loaded configurations, and a wide range of bypass gaps typical of those found in realfilter installations. The model suggests that gaps on the order of 1 mm around well-seatedfilters have little efect on the perfor- mance of
4、mostfilters. For high pressure dropfilters, small gaps decrease filter performance and large gaps substantially decrease filter performance. Because higher eficiencyjlters also typically have a largerpressure drop, bypass tends to have a larger effect on highperformancejlters. The resultsprovided he
5、re suggest that bypass can dramatically affectfilterperfor- mance. INTRODUCTION Filtration in HVAC systems is the most widely used method for protecting people and equipment from airborne particulate matter. To aid in filter selection, there are several standards that address HVAC filtration efficac
6、y, including ASHRAE Standard 52.2, Method of Testing General Ventila- tion Air-Cleaning Devices for Removal Eficiency by Particle Size (ASHRAE 1999) and ASHRAE Standard 52. I, Gravimet- ric and Dust-Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate
7、 Jeffrey Siegel, PhD Member ASHRAE Matter (ASHRAE 1992). The result of an ASHRAE Standard 52.2 test includes the Minimum Efficiency Reporting Value (MERV), which classifies filters according to their efficiency. Standard 52.2, as well as most other filter test methodologies, are tests of the filter
8、media, rather than the installed filter system. When applied to real systems, filter test results implicitly assume that no bypass exists around filters. Exam- ination of most residential and commercial HVAC systems suggests that this is not a good assumption: both small and large gaps are common. T
9、he purpose of this paper is to simu- late the effect of filter bypass on common filters. HVAC filtration has been widely studied, and severa1 studies have measured particle-size resolved efficiencies for a variety of filters (e.g., Hanley et al. 1994; Raynor and Chae 2003). Filter efficiency curves
10、are typically U-shaped with very small particles ( 5 pm) removed by inertial mechanisms. Although most measurements have been made with filter bypass intentionally sealed, there are numerous anecdotal reports of particle bypass. Braun (1 986) reported that catastrophic filter bypass led to fouling o
11、f an evaporator coil. Ottney (1 993) and several others suggest that eliminating filter bypass is an important component of achieving accept- able indoor air quality. Siegel (2002) simulated filter bypass and suggested that even moderate amounts of filter bypass can dramatically increase HVAC heat e
12、xchanger fouling. Despite its obvious importance, we know of no existing mathematical models for filter bypass, and decision makers have limited information available on the effect of bypass. In this paper we present a model of filter bypass that predicts the amount of air that will bypass a filter
13、and the effect on overall filter efficiency. The most important independent parameters Matthew Ward is a graduate student and Jeffrey Siegel is an assistant professor in the Department of Civil Engineering, The University of Texas at Austin. 02005 ASHRAE. 1 o91 are the size (i.e., gap width) and geo
14、metry of the gaps around the filter and the efficiency and pressure drop of the filter. We report several parameters including the volumetric airflow that bypasses the filter (QB) and the effective filter efficiency as a function of particle diameter (qe$ for the filter system (filter + bypass). We
15、apply our model to a variety of commonly used HVAC filters in order to understand the interplay between filter efficiency, pressure drop, and bypass. From these simu- lations, we calculate the effective MERV (MERV Mosley et al. 2001; Carrie and Modera 2002). To account for PB, we adapted the model o
16、f Liu and Nazaroff (2001) for particle penetration efficiency through a building envelope crack. As shown in Equation 4, Liu and Nazaroff (2001) modeled parti- cle penetration through a rectangular crack as the product of penetration due to individual particle removal mechanisms. PB = PgXPdXPi“PgXPd
17、 (4) Pg, particle penetration due to gravitational settling, is assumed to be independent of Pd, particle penetration due to dimision, since these two particle removal mechanisms are significant for different sized particles. Pi, particle penetration due to impaction, and Pg are not independent, and
18、 particles with enough inertia to be removed by impaction usually are removed by gravitational settling. Therefore, we have neglected Pi in order to avoid overestimating the removal of larger particles in the gaps. The model of Liu and Nazaroff was intended for cracks in buildings where AP is less t
19、han 10 Pa, whereas the AP across an HVAC filter can be greater than 100 Pa. However, Liu and Nazaroffs reasoning should extend to HVAC filter gaps because it is based on the Baker et al. (1987) relationship between QB and AP, which was vali- dated for AP up to 100 Pa and applies theoretically for hi
20、gher AP. Liu and Nazaroff (2003) later experimentally validated their model. Model Parameters The model was applied to ten different HVAC filters with particle size, pressure drop, and gap shape varied. The face velocity (and, hence, QF) was held constant for each filter. Table 1 describes each filt
21、er. Effective particle removal efficiency, qeP was compared for each filter with five gap shapes, while AP and QF were held constant. The gap configurations were characterized as 1092 ASHRAE Transactions: Symposia Table 1. Filter Characteristics U-Shaped Straight-Through U-Shaped 1 mm gap 1 mm gap 1
22、0 mm gap Dimension No Bypass 2 bends O bends 2 bends H O 1mtl-I lmm 10 mm L O W O Filter perimeter Filter perimeter Filter perimeter Filter depth + 2x20 m Filter depth Filter depth + 2x20 mm Filter Name Furnace filter* Self-charging panel filter* Pleated panel filter* Panel electronic filter* Pleate
23、d paper-media filter* Pocket filter* MERV t MERV 1 1 MERV lSt Straight-Through 10 mm gap O bends 10 mm Filter depth Filter perimeter Filter Depth (m) 0.025 0.025 0.025 0.025 O. 150 0.560 0.127 0.102 0.051 Filter Face Area (m2) 0.372 0.372 0.258 0.372 0.372 0.372 0.315 0.330 0.372 Face Velocity his)
24、1.30 1.30 1.87 1.30 1.30 i .30 1.50 2.50 2.50 Display Element Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10 follows: the first was the no bypass case; the second, H= 1 mm and n = 2, was chosen to represent the lower bound on QB in which a filter is well seated around its perimeter
25、in a U-shaped slot; the third, H = 1 mm and n = O, was chosen to represent a well-seated filter with a straight-through crack; the fourth gap configuration, H= 10 mm and n = 2, was chosen to represent a poorly seated filter with a U-shaped gap; the final, H = 10 mm and n = O, was chosen to represent
26、 the upper bound on QB in which the filter is poorly seated against a flange with no bends in the path of the air bypassing the filter. For all gap configurations, Wis equal to the distance around the perimeter of the filter, and L (the distance a particle travels as it bypasses the filter) is equal
27、 to the depth (short dimension) of the filter plus 20 mm added for each bend (each flange adds 20 mm to L.). Table 2 summarizes the bypass gap dimensions for each case considered. RESULTS This section presents model simulation results for each of the nine filters described in Table 1. Crack height (
28、H), pressure drop (U), and, to a lesser extent, the number of bends (n) significantly affected the bypass flow rate (QB). The penetra- tion fraction (PF) and QB significantly affected the effective filtration efficiency (qeri 0.7 Z 0.6 .s 0.5 4 0.4 o o L 1.0 i$ 0.9 -3 0.8 3 0.7 ss G Nobypass 1 inm 2
29、 bends 1 mm I O mni 2 bends IO mm - - . 3 3 0.6- .s 0.5.- 4 0.4.- 0.3.- .e 8 0.2- I= w 0.1- O o Lri Y n nl i . .O1 .O2 .O5 .1 .2 .5 1 2 5 10 .O1 .O2 .O5 .1 .2 .5 1 2 5 10 Particle Diameter (pn) Particle Diameter (pm) (a) (b) Figure 4 Effective particle removal eficiency for a clean (a) and dust-load
30、ed (6) pleated paneljlter with pressure drops of 68 und 125 Pu, respectiveb. 1 min 2 bends 10 mm 2 bends . 3 0.6- *G 0.5- 2 0.4- $ 0.3- g 0.2- rtt w 0.1- n n, O o Fr, .* c ,., , .O1 .O2 .O5 .I .2 .5 1 2 5 10 Particle Diameter (e) (4 1 min 2 bends 10 mm 2 bends .i .O1 .O2 .O5 .I .2 .5 1 2 5 10 Partic
31、le Diameter (pm) (b) Figure 5 Effective particle removal eficiency for a clean (a) and dust-loaded (b) panel electronicjlter with pressure drops of 50 and 125 Pa, respectively. self-charging panel filter with a large gap, qeff is zero for the most respirable range of particle size. This observation
32、indicates that bypass could negate most protection to indoor air quality afforded by this filter. Like the furnace filter (Figure 2) and the self-charging filter (Figure 3), the pleated panel filter (Figure 4) offers no protection from most respirable particles when large bypass gaps are present. Be
33、nds begin to play a significant role in this filter with a difference of five percentage points for 0.02 pm particles. The number of bends in the bypass gap is important for the panel electronic filter (Figure 5). Two bends decrease 1096 ASHRAE Transactions: Symposia 10 mm 2 bends .O1 .O2 .O5 .1 .2
34、.5 1 2 5 10 Particle Diameter (pn) (a) o. o .O1 .O2 .O5 .I .2 .5 1 2 5 10 Particle Diameter (p) (b) Figure 6 Efective particle removal ejciency for a clean (u) and dust-loaded (6) pleated paper-media jlter with pressure drops of 40 and 125 Pa, respectively. 1.0- 0 0.9- i? 0 0.8- G 8 0.7.- g 0.6- *c
35、0.5- $ 0.4- - O o CL( 0.3 0.2 w 0.1 o.otq , I .O1 .O2 .O5 .I 1 nini 2 bends -_- i0 mni 2 bends 10 mm . .2 .5 1 2 5 10 Particle Diameter (pn) (4 _-I-. +- .”.- -_-_-i- Ti 0.6 -_ . a - i .F: o o* 5 . u 2 0.4 $? 0.3 0 0.2 w 0.1 o. o Fr, .3 c, G .O1 .O2 .O5 .I .2 .5 1 2 5 10 Particle Diameter (pn) (b) Fi
36、gure 7 Effective particle removal efficiency for a clean (a) und a dust-loaded (b) pocketjlter with pressure drops of 50 and I25 Pu, respectively. efficiency by two to three percentage points for a clean filter with small gaps to about six percentage points for large gaps. Bypass decreases efficienc
37、y much more for the smallest and largest particles than for the middle range for this filter. Figures 6 and 7 show the effective efficiency of the pleated paper-media filter and the pocket filter, respectively. ASHRAE Transactions: Symposia Bypass has a similar impact on both of these filters. The 1
38、 mm gap causes almost no change in the effective efficiency, and the number of bends is unimportant. For the 10 mm gaps, the effective efficiency degrades by 20 to 40 percentage points for the clean pleated paper-media filter and 30 to 40 percentage points for the pocket filter. When loaded to 125 P
39、a, the 1097 1 mm 2 bends . 1omm u 2 0.3 .L + 0.2 : 0.5 0.4 O Fr, $2 0.3 o.oi+-.-_ .I .2 .5 Nobypass 1 mm 2 bends I mm _-_ 10 mm 2 bends 10 nim _-_ . If 12 5 10 Particle Diameter (pi) (4 1.0 8 0.9 -2 0.8 3 3 0.7 h $2 0.3 pJ 0.1 o.oL-.- .I .2 .5 t Nobypass 1 mm 2 bends 1 mm 1 O mrii 3 bends 10 m1 _-_
40、_-_ . 12 5 10 Particle Diameter (pi) (b) Figure 10 Effective particle removal eficiency for a clean (a) and dust-loaded (b) MERV 15$lter with pressure drops of 92 and 156 Pa, respectively. economic analysis seeking to optimize the cost-effectiveness of filtration must either include costs for minimi
41、zing bypass or account for reduced efficiency caused by bypass. The data presented in this paper can provide a basis for such analyses. The results also show that respirable particles are not appreciably removed in the gap, which means that bypass is significantly detrimental to indoor air quality.
42、An HVAC design that employs high efficiency filters to prevent health problems associated with indoor fine particles may fail to perform as intended due to bypass. The results presented in this paper can provide a basis to quanti% the effect of bypass on indoor air quality. For all of the simulation
43、s, we assumed that volumetric flow through the filter (eF) was constant. In some HVAC systems, it would be more correct to hold the total flow (Q) constant. The analysis of bypass would thus involve an itera- ASHRAE Transactions: Symposia 1 o99 Table 5. Effective MERV Ratings with Bypass Included Fi
44、lter I I I I I 1 mm gap, 2 bends 1 mrn gap, O bends 1 10 mm gap, 2 bends 10 rnm gap, O bends MERV 6 MERV 11 6 6 5 4 11 11 8 8 MERV 15 tive procedure where the flow is allocated between the filter and the bypass crack until the pressure drop through both flow paths was equal. We did not complete this
45、 procedure because we did not have efficiency data for the reduced filter face velocities that would result, but this effect should be included in future measurements of bypass. While the model simulations presented in this paper provide a quantitative account of bypass, they do not substitute for e
46、xperimental data, and the results should be verified exper- imentally both in a laboratory apparatus with controlled parameters and in real HVAC systems. Also, the bypass results coupled with full HVAC deposition models can provide a comprehensive accounting of HVAC systems influence on indoor parti
47、culate matter with an ability to relax the usual assumption that the particle removal efficiency is equal to the rated filter efficiency. Finally, the authors hope that this work will motivate methods to detect bypass in the field and to create HVAC designs that reduce bypass. 14 14 8 8 REFERENCES A
48、SHRAE. 1992. ANSI/ASHRAE Standard 52.1-1992, Gravi- metric and Dust-Spot Procedures for Testing Air-Clean- ing Devices Used in General Ventilation for Removing Particulate Matter. Atlanta: American Society of Heat- ing, Refrigerating and Air-conditioning Engineers, Inc. ASHRAE. 1999. ANSUASHRAE Stan
49、dard 52.2-1999, Method of Testing General Ventilation Air-Cleaning Devices for Removal EfJiciency by Particle Size. Atlanta: American Society of Heating, Refrigerating and Air- conditioning Engineers, Inc. Baker, P.H., S. Sharples, and I.C. Ward. 1987. Air flow through cracks. Building and Environment, Vol. 22, pp. 293-304. Braun, R.H. 1986. Problem and solution to plugging of a finned-tube cooling coil in an air handler. ASHRAE Transactions, 92( 1 B): 3 85 -3 89. Came, F.R., and M.P. Modera. P. 2002. Experimental inves- tigation of aeros
