Use of “Approach Temperature” Monitoring for Cooling Systems

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.

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6 Comments

1. A new technic which is now largely used in Europe & USA is installation " Hydrpath" a gadget which keeps water under suspension and restrict to form any limescale/slime & biofouling hence noscale is formed in the tubes/Ht.Exchanger plates so U can the system for a prolong period but still Delta T of the system must be monitored.

    M.ILYAS KHAN

2. Instead of Shutting down the whole system the stand bye unit to be put in ops and faulty to be checked for cleaning or carryout on line cleaning through Chemicals or reducing Ph of the system. Daily back wash is also a good practical approach.

   Regards

   M.ILYAS KHAN

    

    

3. Check over formula. Thanks for the concept for determining when to clean the exchanger , keep it coming.

    

4. If the deposit builds up slowly, it may show up as a barely different Delta t  but the temperature of the cooling water can increase.  In other cases the Delta t barely changes but the process temperature is higher.

5. Text differs from formula:

   "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" 

   To be correct should the "minus" be "divided by" 

6. AT = (CTout-​CTin)/(PTin-​PTin)