How is the design margin for plate heat exchangers defined? Is it a margin for area, or a margin for the heat transfer coefficient?
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I consulted two different manufacturers, and both claimed that the heat exchangers they designed had sufficient headroom. We are given the heat transfer amount, the inlet and outlet temperatures of the hot and cold fluids; the heat transfer coefficient is provided, after which the area is calculated, with a 10% safety margin added. Another company explicitly states that a margin is taken into account in the heat transfer coefficient, and the result obtained is one that includes this margin. Which of these approaches is more reasonable or more common?
Any of these will work; it depends on how you interpret it. If we have to be specific, the increase in the number of plates is intended to provide some margin in case the operating conditions are unstable. If there is a margin in the heat transfer coefficient, it means that factors such as scaling that can affect efficiency are taken into account, so the heat transfer coefficient is set to a lower value; however, this ultimately results in an increase in the number of plates required.
Is there a theoretical or normative conclusion to this issue?
Design margin for plate heat exchangers: 1) (most common) 20% margin on the heat exchange area; 2) (considerable) a margin of up to 10% on the heat transfer coefficient specified in the heat exchange design