There are several portable sample conditioners available on the market. Other types of sample conditioning system are also available for other purposes, which may be more suitable for specific cases.
These range from a simple panel containing a knock-out pot and silica gel drier, (only suitable for very short term operation without the risk of damage to the analyser and not at all suitable for the measurement of soluble components such as NO2, SO2 and HCl) to mains powered units which combine Peltier coolers, filters, sample and condensate pumps and simple sample probes. More sophisticated units are becoming available which include heated sample probes and heated sample lines. The permeation dryer is also becoming interesting, since measurements with infrared sensors require that the water be completely removed from the system. Even low levels of water vapour will produce false peaks on the sulfur dioxide and nitric oxide measurements.
Most of these sample conditioners, with good portable flue gas analysers, are very useful tools for checking permanent installations, for use where permanent analysers are not justifiable, or technical problems exist in a particular process or furnace prohibiting the installation of a permanent system.
The success of sample systems and their associated analysers is entirely dependent on good maintenance. Any analyser system requires a maintenance schedule which must be adhered to. Experience shows that over 97% reliability is achievable with good preventive maintenance. The fuel saved because of that can alone often pay for the analyser and its maintenance, not to mention the positive effects on our environment.
Analyser maintenance is part of the cost of ownership.
In the conventional flue gas system the sample is cooled rapidly and transported to the analyser. This is satisfactory for oxygen, carbon monoxide and nitric oxide analyses.
When the gases being analysed include sulphur dioxide, nitrogen dioxide, hydrogen chloride or ammonia the sample is transported hot (heated sample line) to the sample cooler and then rapidly cooled. There are various types of sample conditioner available for this purpose. These can be separated roughly into two types: refrigerant and solid-state Peltier coolers. Thsiis not the place for a discussion about sample conditioners. Both types work satisfactorily, but the refrigerant type is lees portable and therefore seldom used with the portable equipment.
It is preferable, but not essential, that the temperature of the sample line is capable of being controlled. It is only necessary to remain above the dew-point of the components of the gas stream. Prevention of condensation is essential and a temperature above the sample dew-point aids good reproducibility of analysis as well as accuracy. Where ammonia is to be analysed, a temperature control must be used and kept at a higher temperature. One main advantage of a temperature control is that the temperature can then be reduced when not needed. Thsi is not really meant as a means of saving power costs, but of reducing the loading on power supply and extension cables when operating far from the high capacity cables and power sockets of the main supply. Gas analysis with portable instruments is carried out at intervals, so there is no impetus to provide a complete infrastructure for this.
The cost of these sample lines is high and may be considered a luxury. This is a mistake, and their omission may cost more in maintenance than their initial cost. Analysis errors will also result which may be the subject at a later date of litigation due to breach of legislation.
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