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December 2005
library  >  PAPERS  >  Design

Specifying filters for forced convection cooling


introduction

 

the importance and utility of air filters is often underestimated since it is typically an after-thought in the design cycle. the wrong filter is often used for an application which may compromise the electrical and thermal performance of the system as a result of contaminant build-up. in this article, different filter designs and the selection process is reviewed to assist the design engineers in selecting a suitable filter for their specific application.

 

filter purpose

 

the three main reasons air filters are used in forced convection cooling systems are:

 

  1. to remove particulate contaminants from the air
  2. to create a laminar air flow
  3. to attenuate electromagnetic interference (emi).

 

particulate contaminants

 

the importance of the removal of corrosive materials is well appreciated, but the reason for removing more benign ones (dust and dirt) may not be as obvious. these particles can accumulate on and in between electronic components, resulting in an electrical short and shrouding. the latter can alter air flow distribution and thus adversely affect thermal performance.

 

the particles removed by air filters range from normal indoor dust and lint to corrosive elements such as solvents, humidity and salt. the size of the particles depends upon the material. dust and bacteria particles are 50 microns in diameter and tobacco smoke is about 0.5 microns. the removal of particles is obviously a function of their diameter and larger ones are easier to remove.

 

laminar air flow

 

flows with less turbulence are better in many applications because they have less friction and subsequently require less fan work. the latter is very desirable since fan generated acoustic noise is a point of contention in the forced convection cooling of electronic systems.

 

electromagnetic interference

 

some electronic components are susceptible to electromagnetic interference, which can cause a malfunction (giving distorted signals), or a break down. mounting electronics in a metallic enclosure would provide adequate protection against emi, if it were not for the openings created to allow air to flow for cooling. therefore, metallic ventilating barriers are coupled with air filters to attenuate electromagnetic signals.

 

operating characteristics

 

there are a variety of materials used as filter media. these include coarse glass fibers, metallic wools, expanded metals and synthetic open cell foams. each type of filter medium will have its own filter efficiency and pressure drop at a given air flow.

 

filter efficiency

 

filter efficiency is the percentage measure of the air borne particulates that a filter is able to remove from the flow at a given velocity. some of the standards and their definitions are shown in table 1. hence, filters with higher efficiency have a larger pressure drop for a given air velocity. also, the smaller the particle a filter can collect, the greater the pressure drop across the filter for a given air velocity.

 

table 1.
ashrae std 52-76 synthetic dust weight arrestance measures the weight of test dust retained by the filter as a percentage of the total weight of dust used.
ashrae dust spot efficiency compares the discoloration effect of filtered air containing normal dirt particles with that of non-filtered air.
mil-std-282 dop measures the percentage of di-octyl phthalate smoke retained by the filter. the particle tested is 0.3 microns.

 

pressure drop

 

filter pressure drop is a measure of the force required to move air through the filter at a given velocity. each component in the system contributes a resistance to the air flow, which results in a pressure drop across itself. the total system resistance is the sum of all the pressure drops along the air flow path (including the filter).

 

the air filter pressure drop is a function of the velocity of the air and the filter type (medium). each filter medium will have a unique pressure versus air velocity characteristics. performance curves showing these characteristics are used by designers during the filter selection process.

 

filter selection

 

several specifications are required before an appropriate filter can be selected. a minimum list of specifications would be:

 

  1. the available area for the filter.
  2. the system's volumetric flow rate.
  3. the maximum pressure drop allowed for the filter.
  4. agency specifications (ul, csa.)
  5. filter efficiency
  6. type of contaminants to be filtered
    a). size of particles
    b). corrosive or non-corrosive

 

the choice of filter types is then limited to those which meet the list of specifications.

 

a suggested procedure is to first narrow the field of possible choices to those which meet the application's agency, efficiency and contaminant removal requirements. for example, an application may require the filter to remove solvents and meet ul (underwriter's laboratory) specifications for flammability resistance. therefore, filters meeting those specifications could be used (aluminum or coarse glass fiber would be appropriate).

 

second, choose the filters which meet the pressure drop requirement of the application. performance curves are used to determine the pressure drop across a filter for a given air velocity.

 

air velocity through a filter can be calculated using:

 

v = r/a (1)
where    
v = velocity  
r = volumetric flow rate (ft3/min)  
a = filter planar area.  

 

it would not be appropriate to select a filter with a pressure drop equal to the maximum allowed by the system. such a filter would have a short operating life. due to the fact that as soon as the filter started to collect particles, the pressure drop would increase beyond the allowed maximum.

 

a good rule of thumb is to select a filter that has 1/2 or less than the maximum pressure drop allowed by the application. for example, if the blower can deliver the required flow rate at 1.5 in h2o (3.8 cm h2o), and the system itself will have a pressure drop of 1 in h2o (2.54 cm h2o) at that air flow, then the maximum allowable pressure drop for the filter would be 0.5 in h2o (1.25 cm h20). therefore, the selected filter should have a pressure drop of 0.25 in h2o (0.63 cm h2o) or less.

 

if a filter cannot be found that meets the allowable pressure drop for the application, then a redesign of the system is required. either the pressure drop across the filter has to be reduced or a blower capable of meeting the pressure drop would have to be selected.

 

if it is necessary to reduce the pressure drop across a filter for a given r, then the filter area has to be increased. this may be accomplished by increasing the actual space available for the filter, and/or pleating the filter medium. the increase in filter area will decrease the air velocity through the filter (eqn 1). this in turn decreases the pressure drop across the filter.

 

example:

 

a designer calculates the required volumetric flow rate of air to be 300 ft3/min (1.52 m/s), and the maximum allowable filter pressure drop to be 0.1in h2o (0.254 cm h2o). the filter medium chosen is a 2 inch (5 cm) thick aluminum mesh with the desired filter area 0.59 ft2 (548 cm2). figure (1) shows the performance curve for this filter.

 

 


figure 1

 

 

for this system to work properly the filter pressure drop will have to be 1/2 or less than the maximum filter pressure drop allowed by the system (0.05 in h2o, 0.13 cm h2o). therefore, the pressure drop must be calculated from the air flow. referring to eqn (1), the air velocity is given by:

 

v = r/a = (300 ft3/min)/0.59 ft2 = 508 ft/min (2.58 m/s)

 

the performance curve shows that the filter pressure drop for a velocity of 508 ft/min is 0.12 in h2o (0.31 cm h2o). this is greater than the maximum pressure drop desired for the filter.

 

therefore, a more powerful blower needs to be used or the filter area has to be increased. it is generally more practical to increase the filter size rather than use a higher capacity blower, due to the associated cost of higher capacity blowers.

 

before the appropriate filter area can be calculated, the air velocity which results in the proper pressure drop must be determined. this can be accomplished by using the filter performance curve. once the velocity is determined eqn (1) can be used to calculate the filter area.

 

the filter performance curve shows that the air velocity for 0.05 in h2o (0.13 cm h2o) should be 325ft/min (1.65 m/s). by rearranging eqn (2), the proper filter area can be found as follows:

 

a = r/v = (300 ft3/min)/(325 ft/min) = 0.92 ft2 (854.7 cm2)

 

this new filter area can be accomplished by increasing the height of the original desired filter area by 3 inches (7.5 cm2) approximately.

 

selection of a filter to create laminar air flow is straight forward. choose a filter medium that has a uniform porosity, such as a non-woven polyester filter. even more important than the filter selection is the orientation of the filter with respect to the fan or blower. the filter should be placed several inches away from the blower or fans via a mixing plenum. the filter should also be oriented perpendicular to the desired air flow direction, figure (2).

 

 


figure 2: a custom cooling system for the telecommunications market manufactured
by mclean engineering, showing an example of air filters used to create a laminar air flow.

 

in many applications, metallic barriers are used in conjunction with air filters to reduce electromagnetic interference. although these barriers do not remove contaminants from the air, they are combined with filters often enough to be discussed here.

 

there are four choices of metallic barriers available for use with filters for emi protection. they are as follows:

 

  1. perforated sheet
  2. random mesh of round fibers
  3. square mesh of round wire
  4. waveguide-beyond-cutoff

 

the most applicable choice for forced convection cooling is the waveguide-beyond-cutoff, otherwise known as a honeycomb mesh, figure (3). this is because of its low pressure drop at high air velocities. the name "waveguide-beyond-cutoff " comes from the fact that emi frequencies higher than the "cutoff" frequency are guided to the metallic enclosure holding the filter assembly. these emi frequencies are then grounded by the enclosure. frequencies less than the cutoff are attenuated so that they are not disruptive to the electronic equipment in the enclosure. the cutoff frequency and the attenuation values are determined by the dimensions of the cells making up the honeycomb, specifically the length and width. here, a cell is one hexagonal tube within the honeycomb mesh, (cells can come in other shapes, e.g. square). the equations relating the cutoff frequency and attenuation level to these cell dimensions for an ideal waveguide are as follows:
figure 3: a standard packaged blower from mclean engineering showing a honeycomb mesh used for emi protection.

 

attenuation = 27.3 x length width, db (2)

 

width = (1.5 x 1010)/cutoff frequency, cm (3)

 

attenuation levels are usually around 100db for adequate emi protection.

 

designers therefore can use eqn (2) and (3) to determine the cell size based on the emi specifications of the application.

 

summary

 

it is easy to see why there may be a compromise to the cooling application if the filter is not included as part of the design process. the benefits of filter use can only be assured by understanding why they are used and knowing the correct selection procedure.

 


 

alan woolfolk
mclean engineering

 

references


1. thomas c. ottney, determining filter requirements can be a challenge, assn. responds, air conditioning , heating & refrigeration news, april 24, 1995.
2. thomas c. ottney, air filter efficiency, air conditioning, heating & refrigeration news, october 14, 1991.
3. an accelerator pedal for cooling fans, reprint from machine design, september 26, 1994.
4. air filtration components, chapter ii of the 1991 version of the hvac systems applications manual, air conditioning, heating & refrigeration news, october 14, 1991.

 

 

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