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In a more widely applied extractive system design, the gas is conditioned before it enters the analyzer. The gas temperature is reduced to ambient temperature and moisture is removed, so that the sample is both cool and relatively dry. The conditioning may be conducted either at the probe location (Figure 3‐2) or at the analyzer shelter (Figure 3‐3). Conditioning at the probe location offers the advantage of using unheated sampling line; however, preventive maintenance of the conditioning system at the probe may be inconvenient. Conditioning at the shelter or CEM room enables the CEM system operator to check the system performance conveniently. However, the necessary heat‐traced lines must be maintained at a proper temperature over their entire length.

Figure 3‐2 A cool/dry CEM system with conditioning at the probe.

Figure 3‐3 A cool/dry CEM system with conditioning at the CEM system shelter.

Extractive systems that condition the flue gas allow greater flexibility in the choice of analyzers and are commonly used when emission calculations are performed on a dry basis or when monitoring a number of different gases is required. Although this type of system is not as sophisticated as some others, it is flexible enough to accommodate engineering changes when application problems arise. In problem applications, the system components can be readily modified or replaced so that the system can meet performance specifications.

A source‐level extractive system is made up of a set of basic components: probe, sample line, filters, moisture removal system, and pump. Because the operation of an extractive system is dependent on the design and quality of each component, as well as on their arrangement in the system, it is necessary to review these characteristics.