There are various methods to calculate pressure drop in heat exchangers. I recommend you KERN'S METHOD.
One of the factors in the overall heat transfer coefficient for a heat exchanger is velocity in the exchanger. The higher the velocity usually the higher the heat transfer coefficient. The higher the velocity, the higher the pressure drop. In other words, the higher the velocities and pressure drops, the more effective the exchanger is. For a given exchanger, the greater the flow rate, the higher the pressure drop is likely to be. Also the greater the flow rate the larger the exchanger will need to be to keep the pressure drop within allowable limits.
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