Heat transfer area issue of heat exchangers
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When calculating the heat transfer area of a heat exchanger, should the overall heat transfer coefficient be based on the clean condition (with a fouling coefficient of 0) or the fouled condition? If a 15% design margin is required for the area, should this 15% margin be added based on the area calculated under the fouled condition or under the clean condition?
If the fouling coefficient is taken into account, a margin of about 10% in area is usually sufficient. If the fouling coefficient is not considered during calculations, it is normal for the area margin to exceed 15%. Sometimes, for safety reasons, manufacturers even use a margin of 40%. Reputable large clients will provide you with the fouling coefficient and specify what percentage margin is required. An excessive area margin results in a low demand for coolant, which in turn lowers the flow rate. When the flow rate of water is below 1 m/s, the lower the speed, the faster fouling occurs.
First, considering the dirt factor, it’s fine to add another 10% to the area margin. Second, without considering the dirt factor, an area margin of over 15% is also considered normal – is there any basis for this? Third, the area margin is too large; why is the flow rate of the cooling medium so low, or in other words, why has the flow rate decreased?
When designing a heat exchanger without taking dirt into account, either the fluid is very clean, or the customer uses chemical agents to prevent dirt from forming, or the dirt is cleaned regularly. haha It’s not a problem if the margin for area is set small; firstly, software calculations use empirical formulas, which inherently include some margin. Secondly, the heat exchanger is part of an entire system, and the other heat exchangers help maintain dynamic balance. If a margin is considered for the heat exchanger, it should also be considered for the entire system. Finally, this is just an assumption~ Under the condition that the process of cooling 2000 kg of water from 60°C to 50°C remains unchanged, the customer can supply at most 1000 kg of water at 20°C. If there is no margin in the design of the heat exchanger, and all 1000 kg of water at 20°C is used for cooling, then the 2000 kg of water will drop from 60°C to 50°C. But if there is a large margin, what temperature will the 2000 kg of water reach after using all 1000 kg of water at 20°C for cooling? Either raise the cooling water temperature by 20 degrees or reduce the cooling water flow rate to maintain the process conditions.