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Abstract
A two-dimensional theoretical model has been carried out to predict the performance and temperature distribution inside the cross flow plate-fin type gas fired air heater associated with natural gas burner at different operating conditions. The gas-fired air heater obtains hot air at 300 °C from ambient air in a safe operation under a complete control.
The heater is designed as a cross-flow heat exchanger type, and the air is forced to flow through its three-passes horizontally while the hot gases are flowing vertically upward in one pass. According to this construction of the gas-fired air heater, the temperature profiles of both the hot gases and the process air are treated in the model as two dimensional. The rate of increasing of the air temperature in the flow direction, i.e. X-direction, is expected to be much higher than that in the Y-direction (vertical direction). The reverse is also true with respect to the hot gases temperature which suffered large decrease in the vertical direction, i.e. Y- direction, compared to that in the X-direction (horizontal direction).
The model was validated with experimental measurements. The outlet temperature of heated air predicted by the model has close agreement with the experimental results. Although, the exhaust gases exit temperature is at acceptable deviation, the total flue gases temperature drop across the heater is in good agreement with the measured one for all operating conditions.
As a case study, the effect of heated air and fuel mass flow rates variation on the heat exchanger performance was explored. The increase in fuel mass flow rate from 0.4 g/s to 0.6 g/s, i.e. 50%, causes a decrease in the heater efficiency by 10%. On the other hand, the increase in air mass flow rate from 0.08 to 0.2 kg/s, i.e. 150%, causes an increase in the heater efficiency by 10%. The simulation results showed that, the maximum heat gained by the air is obtained from the lower section (third air-pass), which gives about 48 % from the total transferred heat, and the least heat gained is occurred at the top section (first air-pass).
The wall temperature had been checked to be sure that the operation of the gas-fired air heater is safe and no thermal stresses occur at the plates during the heat transfer, it is found that, the maximum average wall temperature occurs at the end of the third air-pass, near the exit section of the heated air, where all the air and gases temperatures are maximum there. So, the maximum wall thermal stresses occur in this air-pass. The maximum wall temperature is found equal to 436 ˚C giving an indication that the operation of the heat exchanger is in the safe temperature side, as the maximum allowed temperature of the Carbon-Steel 35 is 700 ˚C.
The model can be used for the prediction of the heat exchanger performance depending on the construction design as well as the air passes number. It is found that, the maximum wall temperature for the three air passes apparatus is lower than that for an air heater constructed with two and one air pass, which makes the operation safer.