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Another approach to diluting the flue gas is not to dilute it in the stack, but outside of the stack in a dilution assembly contained in a protective housing close‐coupled to the stack. External dilution systems can be more easily serviced and temperature controlled than an in‐stack dilution probe; however, in principle, their response time is somewhat slower due to the length of time necessary to transport the sample from the probe tip to the dilution orifice. External dilution probes do offer several advantages over in‐stack dilution probes. They can be heated relatively easily to maintain a more constant temperature. In wet scrubber applications, they can resolve droplet and aerosol condensation problems that may be more difficult to address with an in‐stack critical orifice. For example, by sloping the stinger of an external system, droplets can run off into the stack before they reach the critical orifice. The external systems also offer a significant maintenance advantage by not requiring the probe to be extracted to examine or replace the filter or critical orifice.

Figure 3‐21 An in‐situ dilution probe extractive system.

Several external dilution system designs are available. Each design is associated with a particular manufacturer and different industrial sectors, such as coal‐fired power plants or pulp and paper plants, tend to favor one particular design over others. In CEM system upgrades, external dilution systems often replace in‐stack dilution probes, not so much for any inherent faults in the in‐stack probes, but more often for not having to remove the probe from the stack for maintenance or because the designs are newer and “trendy.”