How large should the heat exchanger area be? ? Help needed
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This post was last edited by Anye Weiyang on 2018-7-27 at 17:35. It’s my first time designing the area of a heat exchanger, and I’m not very confident about it; I would like to ask for advice from fellow experts. The existing process flow is as follows: Heat stream – ethanol at a rate of 400 kg/h, with a vapor fraction of 1 and a temperature of 80°C. The temperature is required to be reduced to 60°C, with a vapor fraction of 0. The cold stream consists of water, with an inlet temperature of 32°C. I have designed the outlet temperature of the cooling water to be 40°C; the overall heat transfer coefficient is K=250 W/(m2·°C). The calculated heat exchange area is 11 square meters, so I chose a size of 18 square meters. However, an engineer from another company also calculated 18 square meters, while suggesting 25 square meters. Since I cannot get in touch with that person, I would like to seek advice from those with more experience here. Thank you! !
Using the information you provided, I calculate that the area is 11 M2; if we multiply this by a design margin of 1.25, the area comes to around 14 M2. Of course, it’s possible that the manufacturer cannot create a space that small, which is why 18 M2 is suggested as the alternative value. PS.LMTD is 31.98 when calculated in the forward flow direction and 33.64 in the reverse flow direction; basically, the area remains 11 M2 in all cases. PS. The unit for the overall heat transfer coefficient (U-value) you provided should be W/M2 K; this is for reference only
For a small heat exchanger, it doesn’t matter if it’s a slightly larger size. You didn’t specify the pressure for ethanol; my calculations show that only around 10 square units are needed. Going with a slightly larger size has no impact. For such a small heat exchanger, there isn’t much difference in cost, so it’s better to be cautious
Based on the calculation of C1m1t1=C2m2t2=KAtm, if the results differ, is it due to different values of C?
The difference in C values is also not significant: gaseous state: 74.21, liquid state: 71.12; unit: KJ/KMOL*K
Has the heat of vaporization been calculated as well?
The heat of vaporization is also taken into account. 80(G) → 78(G) → 78(L) → 60(L); the heat absorbed at these three temperature points all needs to be considered. PS. Basically, if there is a phase change, a simpler calculation method can be used by simply applying the heat value associated with that phase change, as the differences are quite significant; the differences resulting from simple temperature changes are minimal (the proportion is too small). The above information is provided for sharing
Thank you for the calculation; my unit was written incorrectly
This process was taken into consideration, and it was calculated several times – it’s 11 square meters