Efficiency

(a) Synthetic test dusts. The staining of walls and fabrics in rooms is not a function solely of the mass of dust present in the air, it also depends on the size of the particle, small

Particles of small mass stain surfaces just as much as larger and heavier particles. Straightforward gravimetric tests according to BSEN 779: 1993 measure arrestance, which expresses the weight of dust retained by a filter as a percentage of the weight fed into it and do not describe efficiency because large, massive particles are more readily collected than smaller ones which are the prime cause of staining in rooms. Furthermore, indoor air quality depends on contaminant particles of small size, as well as large. The discolouration test, in one form or another, has therefore been extensively adopted for the expression of efficiency whilst the ability of a filter to remove a mass of dust is referred to in terms of an arrestance test.

To achieve uniformity in the expression of arrestance test results, a standard dust has been developed in the United States and this has been adopted in Europe. In the past, simple synthetic dusts, such as crystals of sodium chloride, have been used with success as standards in the UK for testing purposes with high efficiency filters and ground aluminium oxide for gravimetric tests on less efficient filters. Although sodium chloride is retained as a European standard test dust for absolute filters, see Eurovent 4/4 (1980), general filter testing is now done according to BS EN 779: 1993 with a synthetic dust developed and used by ASHRAE (1992). It comprises: 72 per cent (by weight) of standardised air cleaner dust—fine, 23 per cent Molocco black (carbon black) and 5 per cent of No. 7 cotton linters ground in a Wiley mill with a 4 mm screen.

When testing high efficiency absolute filters it is usual to speak of penetration rather than efficiency, one being the complement of the other. Sodium chloride is adopted for this in Europe (see Eurovent 4/4 (1980)) but in the United States the practice is to use di-iso — octyl-phthalate (abbreviated DOP), a smoke-like homogeneous aerosol of particles of nominal 0.3 jxm size, chosen because it was considered the most difficult size for filters to remove. This test, for high efficiency (small penetration) filters, must not be confused with site tests that also use DOP smoke, generated locally. DOP test results are not readily comparable with sodium flame test figures.

(b) Atmospheric dust spot efficiency. The current method (BS EN 779: 1993), based on Eurovent 4/5 (1980) and ASHRAE 52.1-92 (1992), classifies filters as coarse, numbered G1 to G4, and fine, numbered F5 to F9. Coarse filters are defined as those with an initial atmospheric dust spot efficiency less than 20 per cent and are subject only to tests of arrestance. Average values of arrestance up to 90 per cent are quoted. Corresponding values of dust spot efficiency are irrelevant and not quoted. Fine filters are those with an initial dust spot efficiency greater than or equal to 20 per cent but not exceeding 98 per cent. Values of efficiency from 40 per cent to 95 per cent are given. Values of arrestance are irrelevant and not quoted. The efficiency of a filter varies over its life as it collects more dust. The Eurovent standard deals with this by specifying a series of tests of both efficiency and arrestance, over a period of time. Measured volumes of air are taken from a ducted airstream before and after the filter under test and passed through separate filter papers, termed targets, and staining them. Equal sample airflow rates are established by using critical flow nozzles. The downstream sampling is taken intermittently with an elapsed time meter to record ‘on’ time. These data are used to verify the readings on the gas flow meters in each sampling tube. There is a high pressure drop through the filter papers and vacuum pumps are required to sample the upstream and downstream air. The extent to which a beam of light passes through the filter papers is interpreted to give the filter efficiency, according to a prescribed method. Before commencing the test, the opacity, or relative light transmission, of the target paper must be established using a beam of light and a photo-sensitive cell (an ‘opacity meter’). The difference between the opacities of the

Two unused targets must not exceed 2 per cent. During the test itself the downstream (clean) sampler runs continuously for the whole of the efficiency test period but the upstream (dirty) sampler runs intermittently for a total period of time related to the anticipated efficiency and requiring some skill in its choice. The efficiency, E, is then expressed by:

(17.1)

Wherein

0 is the opacity of the dust spot on the upstream target, given by O, = (7^1 — 7’u2)/7’ul and

2 Is the opacity on the downstream target, given by 02 = (Td[ — TA2)/Tdl; Tul and Tu2 are the initial and final light transmissions as a percentage through the upstream target and and Td2 are similar transmissions through the downstream target; <2i and Q2 are total volumes sampled through the upstream and downstream targets.

When carrying out the dust spot efficiency test, expressed according to equation (17.1), ordinary air from the laboratory is used, without the addition of any synthetic dust. The intention is to get the opacities of the upstream and downstream targets, 0 and 02, as nearly equal as possible by the end of the test period. Then 02I0 is unity and the efficiency depends on the measured volumes of air, Qx and Q2, drawn through the two targets, the quantity of dirty air, Q, being much less than that of the clean air, Q2. For self-renewable filters, the tests must be done either in the period when the pressure drop across the filter is rising to its upper operating limit or during a time when there is steady-state operation.

(c) Arrestance. Arrestance tests are carried out by the injection of several increments of synthetic dust, during the total period of the test.

(17.2)

The answers obtained for arrestance or efficiency can then be graphed against the weight of dust fed into the test rig (see Figure 17.1). The filter to be tested and an after­filter—to be fitted downstream for catchment purposes—are each weighed and then mounted in the test rig. A known weight of test dust is fed into the rig and the dust passing through the filter under test is collected by the after-filter. This is then removed and reweighed, the increase in weight being used to establish the synthetic dust weight arrestance of the filter under test, as a percentage. A minimum of four measurements must be made and arrestance is then defined as

A = 100(1 — WJWX)%

Wherein Wa is the weight of synthetic dust collected by the after filter and W is the weight injected.

The average atmospheric dust spot efficiency, Em, using the notation in Figure 17.1, is defined by

(17.3)

Weight of dust fed Fig. 17.1 Arrestance or efficiency rising with a test to BS EN 779: 1993.

Wherein W], W2, etc. are the increments of dust fed into the rig and W{ is the final increment, with A], A2, … Af, the corresponding arrestances according to equation (17.2) and W is the total weight of dust injected.

(d) Dust holding capacity. For disposable and non self-renewable types of filter the dust holding capacity is the total of the dust increments, W, related to the pressure drop.

For self-renewable filters the dust holding capacity is expressed per m2 of filter medium when the filter is working in a steady-state condition.

(e) Sodium flame test. Eurovent 4/4 (1980) prescribes a test where sodium chloride solution is injected into the airstream on the upstream side of the filter under test and evaporation leaves a suspension of cubical crystals having some uniformity of shape and size: the average diagonal is 0.6 Jim, the largest 1.7 Jim and 58 per cent are less than 0.1 Jim. This test gives an instantaneous reading of the penetration of particles through the filter. A specimen of air from the downstream side of the filter is passed through a burning hydrogen flame, the intensity of the resulting yellow colour being a direct indication of the weight of sodium chloride present. The test is very stringent and therefore only used for so-called absolute filters.

Technical developments have established methods of continuous counting and sizing of particles. Most use the principles of the scattering of light, leading to the measurement of particle sizes down to 0.3 |im. With laser light sources counting and sizing is possible down to 0.1 Jim. The condensation of water vapour in the atmosphere to liquid depends on

	^2	A3
				^5
—				R* "
—			R3	—
—		R2		—
Ro	Hi			—

80

70

60

50

CD E 40

30

20

10

Q. Ј T3 Ј 3 (0 (0 Q>

Weight of dust fed

Fig. 17.2 Arrestance falling and pressure drop rising with a test to BS EN 779: 1993.

Efficiency less than 20%.

The presence of condensation nuclei and Scala (1963) has developed an instrument for their measurement. This has led to condensation nuclei counters that can deal with particle sizes of 0.01 (im. All future standards for good quality filters will be related to particle size. Ter Kuile (1997) reports developments in this direction. The effectiveness of filters can be described in terms of the ability to remove particles of a particular size, also known as their fractional efficiency. The term HEPA (high efficiency particulate air) filters has been in use for some time but with the improvement in filter standards it has become necessary to use the term ULPA (ultra low penetration air) filters. The classification in BS EN 779: 1993 then has two further classes, H10 to H14 for HEPA filters and U15 to U17 for ULPA filters. The corresponding average efficiencies are 95 per cent to 99.999 per cent at 0.3 |im for the H range and 99.9995 per cent to 99.999995 per cent at 0.12 (im for the U range. Another term used is Most Penetrating Particle Size (MPPS) for which the average efficiencies over the H and U ranges are virtually the same. Such very high efficiency filters work at lower face velocities than are usual, with capital cost implications.

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