However, the gases must always be maintained above their dewpoint. The cooling effect of a cold wind on an exposed stack cannot be underestimated, and probes which are unheated and not inserted fully to the stack without lagging are particularly vulnerable. It is a misconception that any condensation doesn’t matter as the fi lter is at 160-180ºC and the condensation will all change straight back to a gas. The latent heat of water is high and the residence time of the gases in the fi lter housing is short, plus the chemical nature of the gases may well have changed and aqueous acids which have formed have an even higher latent heat of evaporation, so once the water has condensed it generally “stays condensed”.

Multi-hole probes give good mixing of the sample across larger diameter ducts and they ensure that representative samples are obtained from the process especially where reproducible distribution of gases occurs in a duct, however these are typically restricted to an operator’s permanent installations as these have to be custom made.

It is a common practice to inject calibration gases into a T-piece connected either in front of the fi lter or behind the fi lter at a slight overpressure to provide a system calibration and/or test for any leaks. This is an excellent practice but the pressure and fl ow of calibration/test gas must be such that it is only applied very, very slightly above the sample fl ow and pressure. Too much pressure and any leaks may be overcome and can give a false impression of the integrity of the system. It should be recognised that calibration and leak checking are different processes even though they may be conducted at the same time. Ideally calibration and leak checking are supposed to be conducted under a similar set of conditions as the sampling but even manufacturers can recommend applying calibration gases at too high a pressure. Sometimes this is to overcome back-fl ow of sample gases through check valves but it can give false integrity checks. Likewise, excessive pressure can mask issues associated with fi lters starting to block and blind due to moisture. In quality rack mount systems, sample pressure is often monitored and displayed to give an indication of whether fi lters need cleaning. However, with the move towards simpler more portable equipment diagnostic displays start to disappear or be replaced by pass fail/indicator LEDs, if present at all.

Gas Conditioners

These devices are used to remove water from gas streams under analysis and they operate in very different ways. The main reasons for gas conditioners are:

a. To minimise the effect of cross interferences (from water) particularly where Non-Dispersive Infra-Red (NDIR) analysers or electrochemical cells are used.

b. To protect the analyser system optics from water damage

c. To prevent the removal of water soluble gas phase components into the water phase and their subsequent loss in the analysis.

d. To allow measurement on a dry gas basis to be reported

There are four key ways to condition gases:

1. Using a Thermoelectric Chiller (can be refrigerative compression but usually too heavy for Source Emission Testers) 2. Using a PermapureTM

type drier

3. Using a chemical absorbent 4. Using a Dilution probe or bench

Many driers operate differently depending on the ambient temperatures they are exposed to. Some Thermoelectric effect or Peltier chillers have warning lights to show if/when the ambient temperature is inappropriate, others show a failure and others just don’t dry the gases properly and run the risk of damaging the equipment downstream of them. If a warning light appears it does not suggest that removing the casing is an acceptable action, as was suggested by one sampling team to their client, rather the chiller needs to be relocated to a cooler location so it can operate properly.

Dilution systems are relatively exempt from temperature effects although even these systems may need temperature control of the dilution venturi or orifi ces. This is because dimensional changes may affect the dilution being experienced and hence the analysis concentrations recorded where if these are different from the temperatures experienced during calibration. Well- designed venturi temperature compensate and the changes due to temperature should be minimal.

Even a chemical absorbent is affected by temperature and will become less effi cient at high ambient temperatures, and of course this is how many are regenerated through heating.

A Thermoelectric cooler needs to discharge the condensed water out of the cooler channels freely, and this can be highly acidic if there are high levels of nitrogen dioxide, sulphur dioxide or other acid gases present. Over the years I have seen Gas Conditioners (Chillers) with the outlet pipe from peristaltic pumps blocked, the silicon/VitonTM

tubing in the peristaltic

pump so deformed that air ingresses back into the gas stream causing leaks in the sampling system or the pumping out of water is so ineffi cient as to be useless and hence the gas conditioning compromised.

The air ingress back into the gas stream can sometimes be masked where there are two peristaltic pumps in series but the pumping of water will still be poor, leading to absorption of gases into water which builds up in the condensation chambers of the chiller. This leads to loss of water soluble components from the gas stream such as HCl, NO2

, SO2 , etc.

Different chillers have different effi ciencies not least of which is due to what the condensate chambers are constructed from as this affects the heat transfer and the amount of heat that needs to be dissipated by the chiller. If the cooling fi ns (hot side of the cooler) become too hot due to ambient air temperatures then the dewpoint of the gases will be compromised. Most chillers are designed to achieve a 4ºC dewpoint, but why is this important? At 4ºC the moisture content is minimal. Likewise, the residence time in the chiller is a trade-off between effi cient cooling (and heat dissipation) and absorption of components on the walls of the chiller chamber. The chiller hence must be of an appropriate size for the fl ow rates needed for the analyser system and its heat dissipation characteristics.


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