CONDENSING HEAT EXCHANGERS
The conventional design of economizers and air heaters ensures that cold end corrosion due to condensing sulfuric acid or water vapor does not occur because the minimum tube wall temperature is maintained above the dew points. However, owing to this design philosophy, a significant amount of energy is lost or not recovered in boilers and HRSGs. The condensing heat exchanger is designed to allow for the condensation of acid and water vapor over the heat transfer surfaces, thus recovering a significant amount of sensible and latent heat from the flue gases. The efficiency of a boiler plant with a condensing heat recovery system can be close to 99%. With natural gas firing, the partial pressure of water vapor is about 18%, whereas with oil fuels it is about 12%. With the condensation of this water vapor, significant improvement in efficiency can be obtained by using oil-fired boilers as shown in Fig. 3.29. Due to the improvement in the overall efficiency of the boiler or HRSG, the emissions of CO2, NOx and CO are also reduced.
Unlike conventional economizers and air heaters, which maintain temperatures above 270-300°F to prevent condensation, the condensing exchanger can operate with water or air at ambient temperatures. Hence condensate or makeup water at 60-80°F or so can be directly used to be heated up by the flue gases, whereas in a noncondensing exchanger the lowest feedwater temperature would vary from 230 to 270°F. Hence the exit flue gas temperature can be around 100- 130°F versus 270-300°F. Because the exchanger tube surface and the exhaust section of the exchanger are below the dew point of water vapor, a rain of condensate is produced through dropwise condensation of the water vapor. This condensate passes around the tube array, carrying particulates and acids that have been scrubbed and washed from the tubes. A few designs handle the problem of heat recovery and scrubbing at the same time to remove particulates and acid gases from the waste gas stream from incineration plants.
The condensing exchanger consists of specially designed tubes coated with a 0.015 in. extruded layer of FEP Teflon. The inside surfaces of the heat exchanger are covered with a 0.06 in. thick sheet of PTFE Teflon. During fabrication, the tubes are pushed through extruded tube seals in the Teflon — covered tube sheet to form a resilient Teflon-to-Teflon seal. This ensures that all heat exchanger surfaces exposed to the flue gases are protected against acid corrosion. To protect the Teflon, the inlet gas temperature is limited to about 500°F. The tubes are generally made of Alloy C70600, which protects them
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Figure 3.29 Efficiency improvement in oil and gas firing using a condensing exchanger.
Against acid corrosion. The tube sheet and casing are coated with Teflon to prevent corrosion. The sub dew point condensing exchanger uses bare tubes due to the coating required and hence is larger than a finned tube bundle for the same duty.
Potential applications also include recovery of water from the gas turbine exhaust for recycle, reducing the amount of fresh makeup water required. The water could be redirected with proper treatment into the steam-water injection system for reducing NOx emissions. Cheng cycle systems, in which a large amount of steam is injected into a gas turbine, are also candidates for condensing exchangers.