Calculation of the heat transfer coefficients for compressed air and water
Thread Content
The shell side of the existing heat exchanger is fed with compressed air (with semi-circular baffle plates), while the tube side contains cooling water. The design conditions for the air are an inlet temperature of 120 degrees, an outlet temperature of 40 degrees, and a working pressure of 7 bar； The design conditions for the cooling water are 30 degrees at the inlet and 35 degrees at the outlet ； The diameter of the copper tubes used for heat exchange is 1/2” ； The pipe spacing is 16 mm ； After preliminary calculations, the overall heat transfer coefficient is approximately 270, but according to relevant empirical data, the heat transfer coefficient between air and water is not that high. So I would like to ask everyone how to calculate a high accuracy for this approach.
Generally speaking, this heat transfer coefficient is considered acceptable. The heat transfer coefficient is influenced by various factors, especially the temperature difference; since liquid water is used here, the value is relatively higher. If there is concern that the heat exchange area is insufficient, a certain margin can be taken. Accurate calculation requires professional software, such as HTFS.
Under normal circumstances, it shouldn’t be this high. I think the reason for such a high value is probably an excessive pressure drop in the design. Using internal tube reinforcement will significantly increase the heat transfer coefficient, but you don’t have this type of structure, do you?
Water flows inside the pipe, so no further reinforcement is needed; it’s mainly the coefficient outside the pipe that matters. My calculations are based on the principles of chemical engineering; specifically, the coefficient on the shell side is first determined using the formula Nu=0.36Re^0.55*Pr^1/3*0.95, and then the convective heat transfer coefficient is calculated. (The cylinder is initially planned as DN450) ； Length 3m ； Shutter distance: 370 mm)
According to what you said above, if the heat exchange coefficient of a heat exchanger that uses air and water is around 120 W/(m²•°C), compared to ordinary copper tube heat exchangers
Under this pressure, the heat transfer coefficient of a conventional gas-water heat exchanger is approximately 150 W/(m2°C); common methods for enhancing heat transfer outside the tubes include using threaded tubes or external fins (rectangular fins). It can be strengthened to 2 times under normal conditions.
270 units is W/m2.K, right? Water flows inside the pipe; under normal conditions, the heat transfer coefficient within the pipe is above 1000. Under normal circumstances, the heat transfer coefficient of gases is very low, but since your air pressure is relatively high, it is possible to reach a flow rate of over 200 if the conditions are right. However, I think using copper tubes is a waste; the heat transfer resistance lies mainly on the air side. It would be necessary to find ways to increase the heat transfer coefficient on that side, and I believe finned tube heat exchangers could be considered.