One of my regular readers asked me to provide some information on managing heat exchanger cleans using the approach temperature. Using this simple technique, a cooling system heat exchanger can be checked to determine when cleaning is required. The approach temperature used in a shell and tube heat exchanger is simply the difference between the outlet and inlet temperature of the cooling water stream minus the difference between the inlet and the outlet temperature of the process stream.
This can be shown as:
AT = (CTout-CTin)/(PTin-CTin)
Where AT = Approach temperature, CT = cooling water temperature and PT = Process temperature. AT will be a co-efficient without units.
This calculation gives rise to the simple view that a decrease in approach temperature over time indicates that the heat exchanger is becoming scaled or fouled and then a decision can be made as to when to clean it. As the approach temperature decreases then more energy is required to cool the medium being cooled and hence an increase in operating costs also results which means a clean will improve performance and decrease operating costs.
This can be seen visually by graphing the approach temperature over time.
As the heat exchanger fouls then the cooling water temperature in and out will become closer as less heat is transferred from the heat exchanger to the water, thus (CTout-CTin) reduces in value. If the flow rate of cooling water and medium being cooled remain the same, then (PTin-CTin) should remain the same. Thus, a reducing number divided by a static number gives a reducing co-efficient.
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