Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 99

In a system designed by M&C Tech Group, an assembly, close‐coupled to the stack, is unique by cleverly incorporating both the dilution eductor and a dilution capillary within a Swagelok cross fitting (Figure 3‐22). This makes for a compact design where the temperature can be easily maintained.

In locating the dilution system outside of the stack at the end of the probe, the characteristic low flow rate of a dilution system would decrease the response time of the system to real‐time flue gas concentration fluctuations. This is in contrast to an in‐stack dilution probe, where the flue gas is diluted essentially at the probe tip and the diluted sample is transported relatively swiftly to the analyzers. A solution to this problem in external dilution probes is to add a venturi bypass eductor to transport the flue gas to the analyzer at a relatively high flow rate of 3–5 l/min. A slipstream from this flow is then extracted by the dilution eductor at a lower flow rate on the order of 30–50 ml/min. The flue gas sample is then diluted after the critical orifice (see the inset in Figure 3‐22) by mixing with the motive air flowing through the dilution eductor. It is necessary that the filter and other system components connected prior to sample dilution be heated to avoid condensation by flue gas moisture.

Using a bypass eductor solves a response time problem; however, by bringing the flue gas sample in at a high flow rate, the low flow advantage of the in‐stack dilution probe is lost. Higher flow rates will transport higher levels of particulate matter through the probe to the probe filter than the lower flow of the dilution eductor in transport of the flue gas. This will require that the probe filter and a probe be periodically inspected and cleaned, and may require the installation of a blowback system.