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Laboratory measurements to quantify the effect of bypass on filtration efficiency.

Article Excerpt
INTRODUCTION

In the HVAC industry, the focus has migrated towards improving respiratory health and protection of building occupants from contaminants. This has lead to the development and marketing of higher efficiency filters and equipment. However, the fact is that no matter how efficient the filter is, without proper installation, it will not perform up to its full potential. One of the key components to proper installation is the elimination of filter bypass. It is widely known that filter bypass can have a major effect on efficiency, but minimal effort has been exerted on quantifying this effect. Ward and Siegel (2005) developed an analytical model to predict the effects of bypass on filter efficiency. This study employed a mathematical model developed to estimate pressure driven air flow through cracks in building envelopes. A range of gap sizes, geometries, and filter efficiencies were analyzed, but model validation using field studies was not performed. In this work, we provide validation experiments for a wide variety of gap geometries and filter efficiencies. Furthermore, a critical assumption used when applying the model is that the pressure drop and volumetric airflow through the filter is the same with and without bypass. Although this assumption allows the model to be directly applied to existing data, it does not replicate what is found in many field installations. For this reason, a second group of tests were performed where fan speed remained unchanged during the test, regardless of the presence of bypass. This work presents the results from the constant pressure drop and constant fan speed tests and analyzes the differences between the two types of tests.

Experimental Method

To conduct this research on filter bypass, a test apparatus designed to ASHRAE Standard 52.2-2007 was used. This standard provides a methodology for testing standard 24 in. x 24 in. (0.61 m x 0.61 m) filters for initial efficiency as well as efficiency after five incremental dust loadings. The methodology for measuring the efficiency of the test filter consists of injecting potassium chloride (KCL) particles into the test apparatus and measuring the concentration of the particles upstream and downstream of the test filter using a laser particle counter. These concentrations were then used to determine the efficiency of the test filter at twelve individual particle size ranges from 0.3-10 [micro]m. The efficiency was also measured after injecting ASHRAE dust upstream of the filter. The dust was used to increase the resistance of the filter and simulate the loading effect that particulate has on a filter over time. Dust was injected five times throughout the test until the final pressure drop of the test filter was attained.

The standard also provides a means of quantifying the performance of a filter with a single value. This corresponds to the minimum efficiency reporting value or MERV. This value is determined based on the calculated efficiencies obtained after performing a full ASHRAE 52.2 test (initial efficiency and five dust loading efficiency measurements). The minimum efficiencies from the six efficiency measurements are averaged for each range shown below in Table 1. The three average minimum efficiencies for each of the ranges are then used to determine the MERV value of the test filter using the lookup table below.

Table.1 Minimum Efficiency Reporting Value Table from ASHRAE Standard 52.2-2007 Standard 52.2 Average Particle Size Efficiency, % in Size Range Minimum Efficiency Reporting Value (MERV) Range 1 0.30-1.0 Range 2 Range 3 3.0-10 [mu]m 1.0-3.0 [mu]m [mu]m 1 n/a n/a [E.sub.3]

Four filter efficiency values MERV 2, MERV 7, MERV 11, and MERV 14 were selected for this research. These were chosen to cover a range of filters commonly found in HVAC applications. The bypass gaps that were chosen were 0.25, 0.75, and 1.25 in. (6.4, 19.1, 31.8 mm). Based on Ward and Siegel (2005), it was decided that any bypass gap smaller than 0.25 inch (6.4 mm) would most likely cause a smaller change in efficiency than the uncertainty of an ASHRAE Standard 52.2 test.

In order to incorporate bypass into the duct using standard filters, modifications to the test rig were implemented. The main modification involved replacing one of the test filter access doors with a custom designed filter bypass door which provides adjustable bypass on only one side of the filter. This door contains a pocket for the filter to slide into resulting in a...

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