An Experimental Investigation of Water Vapor Condensation from Biofuel Flue Gas in a Model of Condenser, (1) Base Case: Local Heat Transfer without Water Injection

Abstract

Waste heat recovery from flue gas based on water vapor condensation is an important issue as the waste heat recovery significantly increases the efficiency of the thermal power units. General principles for designing of this type of heat exchangers are known rather well; however, investigations of the local characteristics necessary for the optimization of those heat exchangers are very limited. Investigations of water vapor condensation from biofuel flue gas in the model of a vertical condensing heat exchanger were performed without and with water injection into a calorimetric tube. During the base-case investigations, no water was injected into the calorimetric tube. The results showed that the humidity and the temperature of inlet flue gas have a significant effect on the local and average heat transfer. For some regimes, the initial part of the condensing heat exchanger was not effective in terms of heat transfer because there the flue gas was cooled by convection until its temperature reached the dew point temperature. The results also showed that, at higher Reynolds numbers, there was an increase in the length of the convection prevailing region. After that region, a sudden increase was observed in heat transfer due to water vapor condensation.

An Experimental Investigation of Water Vapor Condensation from Biofuel Flue Gas in a Model of Condenser, (2) Local Heat Transfer in a Calorimetric Tube with Water Injection

“…The experimental setup (Figure 1) which was used for the investigations was the same as described in [1]. However, some minor changes necessary for water injection were introduced and are described further.…”

“…The test section was made of stainless steel. It was composed of an internal calorimetric tube (length x ≈ 5.8 m, inner diameter d = 0.034 m, wall thickness δ = 2 mm, x/d ≈ 170) where condensation takes place on its internal surface and an outer tube (length x ≈ 5.9 m, inner diameter D = 0.108 m) [1].…”

“…This is the second paper of the series on water vapor condensation from the flue gas in a long vertical tube-model of the condensing heat exchanger. Therefore, the review of the literature related to water vapor condensation from flue gases has already been presented in [1]. The results presented in [1] revealed that, under certain inlet flue gas conditions, the initial part of the condensing heat exchanger is not used efficiently for condensation heat and mass transfer due to the cause that the flue gas has to be cooled down until its temperature reaches the dew point temperature.…”

“…Therefore, the review of the literature related to water vapor condensation from flue gases has already been presented in [1]. The results presented in [1] revealed that, under certain inlet flue gas conditions, the initial part of the condensing heat exchanger is not used efficiently for condensation heat and mass transfer due to the cause that the flue gas has to be cooled down until its temperature reaches the dew point temperature. Usually after that, a significant increase in heat transfer was determined due to condensation of water vapor from the flue gas.…”

“…The results presented in [1] showed that there is a necessity to reduce the flue gas temperature prior to the condensing heat exchanger in order to have condensation in the heat exchanger from its very beginning. A possible way to reduce the flue gas temperature is the injection of water into the flue gas flow [2,3].…”