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Sample conditioning 2 - heated filters and hoses
Sample conditioning starts at he point of sample and not at the conditioning unit , and here is a brief decription of the problems involved in this.
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Sample conditioning 2
The problem of sample conditioning does not start at the gas dryer or cooler, it really starts at the stack itself. The purpose of the exercise is to get a continuous stream of fluid from an environment where it is flowing rapidly at a high temperature to an instrument which does not tolerate high rates of flow, pressure surges, dirt or water.
The judicious use of reductions, resonance chambers and pumps should solve all problems caused by pressure surges and flow rates, but we still have to deal with the high temperature and, dirt and water. In the stack itself and at the extraction point, the water does not present a problem, since it is simply another gaseous component.
The problem of heat can only be solved by using the correct materials. Steels can be used up to about 1200°C provided the gas is not too corrosive and above this temperature there are ceramic materials that will survive temperatures well in excess of anything likely to occur in a normal industrial process.
The dirt must, however, be removed somewhere along the route. Where these impurities are extracted depends basically on a number of factors:
1. How large they are.
2. What is to be done with the gas afterwards.
3. In what concentration they are present.
4. Whether they have any adhesive tendencies.
A filter of some heat-resistant material can be placed inside the stack to remove everything at source, so to speak. This will, however, not work with particles that are very small, numerous or adhesive. A filter inside the stack is limited in size by the necessity to place it inside. This requires a hole to be made, which will have to be sealed afterwards during sampling and operation. At high levels of contamination such filters will become clogged very rapidly and require frequent cleaning or replacement, making longer term measurement difficult or impossible. One possible solution is to reverse the flow and blow the filter clean, but adhesive particles will make this impossible, although the system is quite common in applications where this is possible.
The simplest solution is to place the filter outside the stack. Here, it can be of any size required for long-term operation without replacement. Dual filter systems, where one is used and the other at least partially cleaned by reverse flow, are in use for permanent installations and will give good service for extended periods of time. The only real drawback is the requirement to keep the filter above the dew point of the sample gas at all times. Condensing water will remove the soluble components of the gas or form a highly effective cement with the dirt particles, blocking the filter permanently. It will also combine with the acidic products of combustion to corrode the filter housing amazingly quickly. The high temperatures and constant exposure to even weak acids will produce an accelerated effect that is surprising.
We now have in our possession a stream of clean hot gas, free of solid impurities. Everything is still in the vapour state, so it may be viewed as a homogenous mixture. This can be passed directly to a sample cooler to remove the water vapour, but it is more common that it must be transported for some distance first. The tubing used for this purpose must be capable of withstanding the temperatures involved and the corrosive effects. PTFE has proved very popular for this purpose in the past and will probably remain so for some time. It has the advantage, too, of being flexible enough for portable applications where the tube will be bent a number of times every day. A piece of tubing is not the whole solution to our problem, however. This will require heating and thermal insulation to deliver the sample in the original condition to the sample conditioning unit. The simplest form of heating is electrical power, which is generally used. Roughly speaking, 100 to 150 Watts will be needed per metre of hose length to keep the sample above the dew point at all times. Good thermal insulation will reduce the power consumption and prevent the operating personnel from incurring burns during sampling. Thermal regulation is not essential, but it can reduce the power requirement in cases where the local supply is limited or the maximum temperature not needed.
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