Have you ever heard about the **Five fingers rule** when sizing and designing a **heat exchanger**? There are in fact some important data that are required in order to properly design a heat exchanger.

These data are: the **fluid to be heated or cooled** and its flow rate; the **inlet temperature** of the fluid, which is the temperature of the fluid when it arrives from the process; the expected **outlet temperature** of the fluid, which is the temperature of the fluid required to get back into the process; the **kind of working fluid** and how much of cooling or heating fluid is available, expressed in flow rate (kg/hour, cubic meters/hour); the **temperature of the working fluid** available.

Five data, five fingers: having these five parameters we can calculate a thermal balance, finding out all of the required data to properly size the exchanger. Let’s clearly start taking for granted that we already know the kind of fluid employed, working for example with a water/water exchanger, or water/oil or steam/water.

Someone could object that **also the pressure drop** is needed. But let’s suppose we have a fluid at a certain defined pressure, that allows to define a certain pressure drop value. Usually this is a value provided by the customer, or anyway we can indicate the **ideal pressure drops in order to ensure the best performances of the exchanger**. Otherwise, the customer will ask to recalculate the pressure drops available based on the pump or the pressure of the available fluid.

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