Parameters Affecting Efficiency

The efficiency of the unit has little meaning if the contaminated air does not get to the air cleaner. Therefore, unducted air cleaners must be sized and strategically placed to create air patterns that will assure that the air in the room passes through the air cleaner. Unless this is accomplished, the air cleaner, no matter how efficient, will do a poor job of cleaning the air. Source capture units depend on hooding to direct contaminants to the air cleaner, assuring successful cleaning of the air. (For details, see the section on Source Capture.)

Efficiency vs. Airflow Rating As System Design Criteria

Air cleaners have a certain efficiency for a given airflow rating. If this rating is exceeded, efficiency will drop. Some units, as shown in the there test data, were evaluated at their nominal rating, then at values 20 percent under and 25 percent over nominal. Common practice among air cleaner manufacturers is the use of terms like "up to 99% efficient," with no reference to air handling capacity. To a designer of a system for which air cleaning efficiency is critical, more accurate evaluations of efficiency are necessary.

Efficiency Test Evaluation

There are five fundamentally different methods used to evaluate efficiency:

1. The Particle Count Method
2. The Weight Method
3. The Atmospheric Dust Spot Efficiency Method
4. The Cold DOP Method
5. The Hot DOP Method

The Particle Count Method

In this method, actual particle count per unit volume of air is determined through microscopic analysis of the air sample. This procedure is extremely tedious and is susceptible to human error. The dust concentration must be quite low (or the sampling time must be unreasonably short) because the sample cannot be allowed to become too dense to count. Few authorities view this as a practical efficiency test.

The Weight Method

The weight affestance, or efficiency, indicates the weight of the dust removed by the filter as a percentage of the weight of dust in the air before filtering. This method is very popular and easy to use. However, it is open to criticism because weight measurements give predominantly the weight of the largest particles in the sample. This is generally true of filter test samples regardless of the absolute size of the largest particles. Little credit is given for the removal of very small particles because they contribute insignificantly to the total weight of the sample. Implications of the weight method are very important. Most, perhaps all, impingement-type filter manufacturers claim more than 80% efficiency for their products. They may be right, but only from one point of view. If the weight of the particulate matter collected by their filters is compared with the total weight of the particle samples from unfiltered air, they honestly obtain 80% efficiency or more by weight. Perhaps the filter traps only the 300 largest of the 300,000 particles actually in the air, but these 300 captured particles weigh enough to account for 80% of the total weight.

If all airborne particles were of uniform, filterable size, measuring filter efficiency by weight evaluation might prove satisfactory. Obviously, particles are not of uniform size. A filter rated at 90%-plus efficiency by weight is very often leaving the majority of the inhalable and respirable particles in the air.
The primary job of air cleaning equipment must be to remove some large particles and a multitude of small particles that account for the soiling and health effects of air pollutants. By weight analysis, electronic air cleaners remove large particles very well, more than 95% of them by weight. In fact, weight analyses are so repeatable in this range, with almost no change from air cleaner to air cleaner, that it is not even beneficial to use them anymore. Small particles can only be accounted for in the last 1% by weight. Dust Spot or some other method is used to study fine particle efficiency.

The Atmospheric Dust Spot Efficiency Method

In the Atmospheric Dust Spot Efficiency Test 1, ambient outdoor atmospheric air is passed through the unit being tested and samples are taken at the inlet and outlet of the unit to evaluate its collection efficiency on the dust particles suspended in the atmosphere. The samples are drawn, at a defined rate, through filter blanks which are 100% efficient on atmospheric dust. The effect is to soil the filter blank to a specified change in opacity. The inlet sample time is controlled in relation to the efficiency of the product under test. The efficiency is then expressed as a function of the ratio of the inlet sample volume/time to the outlet sample volume/time required to produce an equal opacity change in both upstream and downstream filter blanks. The justification for using such a test is that it is based on one of the observable effects of air pollution-the soiling effect.
The nature of soiling effect depends upon the light scattering ability and light-absorbent quality of soiling agents. Walls, ceilings and furnishings appear dirty because their original, clean appearance has been obscured by particles which scatter the transmitted or reflected light by which they are seen. Particles of 0.25 to 1 micron in diameter have greater soiling effect because of their greater light scattering ability. On the other hand, larger particles, (1 to 10 microns in diameter and larger), cause relatively little light scattering and contribute very little to soiling effect. It is this obscuring, soiling, light scattering effect which is measured in the Dust Spot Evaluation made with a light source and a photocell. This method shows that the electronic air cleaner effectively removes particles in the range of those that cause most soiling and, further, those of the size that may be inhaled and retained in the respiratory tract to cause injurious health effects. Also the Dust Spot Test provides a basis for comparison which excludes the weight effects of the large, heavy particles. Thus it demonstrates the true small particle effectiveness of various air cleaning devices, particularly electronic air cleaners and commonly used filters.
In considering the characteristics of various air contaminants, one must remember that, for a given contaminant weight, the smaller the average particle size, the greater the relative surface area for a given volume of air. Microscopic particles, with far greater total surface area, are a significantly larger factor as a light scattering agent. They have the ability to cover far more wall area with a light absorbing film.
One drawback to the Dust Spot Test is that it uses atmospheric air. Because this air changes constantly, it is difficult to obtain repeatability for verification. As a result, many tests have to be run and the data averaged.

The Cold DOP Test

To overcome the drawback of the Dust Spot Test, the Cold DOP (Di-octyl Phthalate) test can be used. This test method is described in Government Standard SF209 for leak testing. Cold DOP generators produce aerosol at room temperature, with particles ranging in size from 0.2 to 1.2 microns and with a mean diameter of 0.7 micron. The aerosol is introduced to the unit being tested and light scattering, due to particle concentration, is measured at the inlet and outlet of the unit. Because light scattering varies in direct proportion to particle concentration, the collecting efficiency of the unit can be expressed as a function of the difference in light scattering measured at the inlet and outlet at any given time.
This test, unlike the Dust Spot Test, is repeatable. However, it must be noted that the efficiency measured in this test is for the particle size generated by the Cold DOP generator. Because particle size is larger in the Dust Spot Test, results will normally indicate higher efficiencies. This test has been used because the particle size is reasonably representative of most applications.

The Hot DOP Test

In this test, DOP is evaporated by heat and condensed to form 0.3 micron particles with very little variation in size. This particle size is the most difficult for all kinds of air cleaners to collect and will normally produce a slightly lower efficiency on all kinds of air cleaning devices than the Cold DOP Method.