Selection of heat exchangers – experts on the case
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Hey everyone~~~ Below are the selection parameters for the two heat exchangers. I have two questions (marked in red): 1. Can the temperature difference between the cooling water temperature and the temperature required by the process be kept at just 3 degrees? By how much can it generally differ at the lowest? How to calculate it? 2. Why does a higher heat transfer power require less plate area? Is it just because of the large temperature difference? How to calculate it? Thank you~~~ Hot side Cold side Fluid Water Water Density kg/m³ 989.7 992.8 Specific heat capacity kJ/(kg*K) 4.18 4.18 Thermal conductivity W/(m*K) 0.634 0.623 Viscosity inlet cP 0.432 0.767 Viscosity outlet cP 0.721 0.631 Volume flow rate m³/h 2.4 7.2 Inlet temperature °C 65.0 32.0 Outlet temperature °C 35.0 41.9 Pressure drop kPa 1.21 10.0 Heat exchanged kW 81.80 Heat transfer area m² 4.3 Relative flow direction Countercurrent Number of plates 33 Effective plates 31 Number of passes 1 1 Expansion capacity 4 Hot side Cold side Fluid Water Water Density kg/m³ 973.4 991.8 Specific heat capacity kJ/(kg*K) 4.18 4.18 Thermal conductivity W/(m*K) 0.667 0.627 Viscosity inlet cP 0.314 0.801 Viscosity outlet cP 0.432 0.586 Volume flow rate m³/h 4.7 7.2 Inlet temperature °C 90.0 30.0 Outlet temperature °C 65.0 45.9 Pressure drop kPa 13.1 22.0 Heat exchanged kW 131.8 Heat transfer area m² 1.4 Relative flow direction Countercurrent Number of plates 12 Effective plates 10 Number of passes 1 1 Expansion capacity 25 Last edited by bijindu on 2008-1-31 09:43]
I edited it twice; it looked neat every time, but once it was published, its format changed~~~ And they call it the “what you see is what you get” mode~~~! I’m frustrated. Experts, please note: if there are two data entries under a data name, one corresponds to the hot side and the other to the cold side~~~
Paste it in picture mode directly, otherwise it won’t be visible.
I can’t see your data clearly, but a temperature difference of 3 degrees refers to the difference in outlet temperature; it’s possible that there is backflow, so there should be no problem. The key temperature difference is likely the logarithmic mean temperature difference. The second issue is the reduction in heat exchange area due to a significant increase in the logarithmic mean temperature difference between the two fluids. Because the heat transfer area is related to the logarithmic mean temperature difference, the heat transfer coefficient K, and the heat load. For similar materials (water) in the design of plate heat exchangers, it is estimated that the K value remains relatively constant, while the heat load varies by around a factor of 1.5. However, the logarithmic mean temperature difference is much greater than 1.5. As a result, the heat exchange area, which should handle a high heat load, becomes smaller.
It’s better to present it in a table; taking a screenshot will make it clearer, as looking at it this way is confusing
When the temperature difference between the hot and cold streams is less than 10 degrees, the heat exchange area becomes too large. :lol :lol :lol
The key is to decide on the type of heat exchanger to use. I’m not entirely sure about the details, but for shell-and-tube exchangers with counterflow arrangement, a temperature difference of 25 degrees is the most economical option; plate heat exchangers can achieve a temperature difference of 5 degrees, while finned plate heat exchangers can reach 3 degrees if necessary. Of course, the shell-and-tube type can also reach 5 degrees, but the area required will be very large, making it uneconomical
Thanks for the analyses from the 4th and 7th floors; they make sense, I understand now