Why Isokinetic?

While we understand that many of our clients are concerned with the results of the emissions compliance testing only, and not how the testing is done, we thought that we would explain more about the isokinetic methodology for those who are interested.

As you will have noted under the tab entitled “Legislation” above, the South African Air Quality Legislation for boilers larger than 14 t/hr requires only Particulate Matter (PM) and Sulphur Dioxide (SO2) to be measured. A different machine is used for each of these two measurements. This raises the first question: Why are different machines used?

The flue gas stream exiting from a boiler consists largely of Nitrogen (N2), Oxygen (O2), Water (H2O) and Carbon Dioxide (CO2) gases. Entrained in this flue gas stream are small quantities of SO2 which are also a gas. Because the SO2 is a gas within a gas stream, the SO2 is well mixed and can be sampled at any point in the stream. This is also true of NOx, CO and any other pollutants that one may wish to measure. Hence these concentrations are all measured by one machine. It is very important, however, that this machine remove the water vapour prior to measuring the pollutant concentrations to ensure that results are reported on a dry basis. Always check that your emissions tester uses a machine with a peltier cooler that condenses out the water vapour.

The measurement of Particulate Matter (PM) is a much more complicated process. This is because the concentration of the tiny particles of ash and carbon varies throughout the gas stream in which it is entrained, because these particles are solids in a gas stream. For example, if the flue gas flows through a bend in a duct, the momentum of the particles will carry them towards the outer edge of the duct, and there will be a higher concentration at the edge of the duct. For this reason scientific procedures have been written to ensure that sampling of PM is representative. One of the best known standards was written by the United States, Environmental Protection Agency. This procedure says that in order for sampling to be representative several things must be done:

1. The sampling point must be greater than a certain distance away from any disturbances to flow,
2. The sampling must be conducted at a number of representative points across the duct’s diameter and not at a single point. These points must represent equal cross sectional areas of the duct.
3. The sampling must be performed in two dimensions.
4. One of these dimensions should be in the plane of the nearest disturbance to flow.
5. The testing must be isokinetic.

The first four points are easy enough to understand, and pertain to the way in which the testing is done, but the fifth point pertains to the equipment used. What does “isokinetic” mean?

Isokinetic means that the sample that is extracted must be extracted at the same speed as the flow of gas through the duct. Why is this necessary? The diagram beneath explains why this is necessary by showing two non-isokinetic conditions.

In the picture on the right, the sampling nozzle is extracting a sample at a slower flow rate than the flow up the chimney stack. As a result some of the gases (shown in blue) are forced to flow around the nozzle. Particle A was entrained in this gas that has now flowed around the nozzle, but because of the particle’s momentum, it still enters into the nozzle in spite of the gases spreading around the nozzle. For this reason, less gas is drawn into the nozzle, and more particles are drawn into the nozzle, resulting in a falsely high concentration being measured.

Conversely, in the diagram on the left, the nozzle is drawing a sample at a higher flow rate than the flue gas flow through the duct or stack. As a result, gases that would normally have flown passed the nozzle are pulled from their orginal course and drawn into the nozzle. However, the heavier particles that had greater momentum, such as particle B, are not drawn into the nozzle along with the air that surrounded them, because their momentum carries them past the nozzle. Naturally, larger particles are more susceptible to this problem than smaller particles. In this scenario, one ends up with a lower concentration of PM.

The first problem that arises in non-isokinetic conditions, is that the concentration of Particulate Matter is not correct, but there exists a second problem too. Non-isokinetic sampling means that larger particles are either favoured or rejected, and so the sizing analysis of samples becomes skewed. The new legislation in South Africa pertaining to boilers larger than 14 t/hr only requires that the concentration of the PM be measured and no size analysis is required. However, in countries where a size analysis of the PM is required, then the testing needs to be isokinetic for this reason as well. Why is sizing important? Smaller particles are known to be more hazardous to human health by damaging the lungs, and in addition, smaller particles have a longer residence time in the atmosphere before falling out.

So how does an isokinetic emissions testing machine work? An isokinetic machine continuously measures the speed of flow in the chimney stack or duct, and ensures that the pump, which is extracting the sample through the nozzle, speeds up or slows down to match the flue gas flow in the stack. This process is not perfect, but as long as the sampling is ± 10% isokinetic, that is acceptable. It is important that your testing company state in the report how isokinetic the testing was. Feel free to send us any questions that you may have. Our details are available on the contacts page. We would love to hear from you. The environment is our passion, and we are excited to serve your business.