Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 100

Another M&C Tech Group external dilution probe design is shown in Figure 3‐23. This second‐generation design by M&C is more modular and robust than the design shown in Figure 3‐22. It is again compact and easily heated and has been successful especially in mercury monitoring applications. In this design, two modular heated blocks, located on the top of the assembly, enable the flue gas flow and sample dilution using two ejector pumps. In operation, the bypass eductor contained within the first block draws the sample through the probe filter. As in the external probe shown in Figure 3‐22, the dilution eductor in the second block draws a slipstream from the bypass eductor flow line at a lower flow rate. Again, clean, dry, conditioned air from the dilution air cleanup system provides the motive force for the pump vacuum and also serves to dilute the flue gas that it draws in from the slipstream. The flow rate of the sampled gas is controlled by a glass capillary serving as a critical orifice under the sonic flow conditions.

Figure 3‐22 External dilution system with cross‐piece dilution unit.

Figure 3‐23 Dilution system with modular block dilution unit.

In the daily calibration verification of the CEM system, the calibration gas is sent through the calibration/purge gas line (shown in Figure 3‐23 at the bottom of the assembly) at a flow rate sufficient to flood the annulus outside of the filter, expelling the flue gas. The calibration gas is then extracted by the CEM system similarly to the sample gas, being conditioned in the same manner as the sample gas.