Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 87

Another approach to moisture removal is to use a compressor‐run refrigerator to cool the gas. Either laminar‐flow heat exchangers such as that shown in Figure 3‐10 or coils of Teflon, glass, or Kynar such as that shown in Figure 3‐11 are cooled by the refrigeration system. The condenser can be immersed in a liquid such as an antifreeze solution or water to facilitate better heat exchange. To avoid freezing condensate in the coil or impinger, temperature is typically not allowed to decrease to below 3 or 4 °C. Condensed water vapor is again continuously removed using a peristaltic pump. If condensate is removed only periodically or manually, failure to perform this operation may cause the trap to fill and overflow into the sample line. Most systems will incorporate a thermal conductivity detector or other means to detect or prevent moisture breakthrough into the sample line and analyzer.

Another condenser can be added to further reduce the water content, but a more efficient technique is to place the sample pump after the first coil and transport the gas from the chiller, under pressure, to a second chiller as shown in Figure 3‐11. A gas under pressure will condense more readily than a gas under vacuum. Since it is more difficult for water molecules to escape the liquid surface to vaporize when the gas is under pressure, this pressurization will, in effect, reduce the moisture content to a level lower than the content under atmospheric pressure.

Figure 3‐11 Refrigerated condenser moisture removal system with a secondary chiller under positive pressure.

In refrigerator systems, Freon sometimes escapes, pumps wear out, or algae start growing in the liquid. Another cooling option of using vortex coolers avoids these problems by using a counterflow of high‐pressure plant air to cool the sample gas. However, the power requirements necessary to generate the high flow rates of air needed may be excessive in some applications.