An Engineer's Choice

  • Friday, 15 April 2016

    [How To] Calculate Time-Cycle required for Heating/Cooling

    Hello ,Good day Readers.......!!!
    Some of the engineers who were working in technical services will be well aware of the theoretical calculations that were performed to get an brief idea before taking some trials,while coming to the process engineers those who will be focusing on the product launches or technology transfer activities ,need some sound knowledge regarding the heat load calculations, Steam consumption,  pump and its sizing's that are used while going for equipment mapping,and many of us will be going as per thumb rule,which will support upto some extent, and we can take this thumb and apply to every case,Lets say that,

    "If somebody is asked a question about what is your condenser capacity needed for a 10KL distillation reactor,I bet within 5Secs,20 sq.m will hit my ears.And Usually while selecting a condenser the major required parameters were the Amount of vapours to be condensed,Nature of vapours to be condensed and the temperature of the vapours to be condensed,Without these all I'l be getting an answer"

    So, here my intension is only one thing,and that,it is to mention you guys that Thumb rules are tolerate only upto some extent,it can't be applied to every case,so learn the theoretical one's perfectly,If you want to take heating trial for any heat exchanger then we should be aware of the range of the Temperature in which it lies,so for that we should the mechanical and thermal properties of heat exchanger on which the rate of heat transfer depends, and along with sometimes fouling/scaling factors, Dirt resistance.

    Usually the Heat transfer rate strongly depends on the material of construction[MOC] and the thermal conductivity of Material[MOC], Area available for Heat transfer.

    The Rate of Cooling/Heating depends on the Utility that we are providing for the operation and the Heat transfer rate.

    And whenever we need to calculate the Time-Cycle of heating or Cooling of a mixture, we will have two choices,

    1) Calculation based on MOC physical/Thermal properties,
    2) Calculation based on the Utility we are using,

    So here, you may get a doubt, How can we calculate based on thermal properties, Actually this is a simple way of determining the Time-Cycle, and i'll explain it later in a Separate post.

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    Coming straight away to the point, Calculating Time-Cycle based on the Utility that we are using:

    For this we should be knowing the following data:

    1) Temperature of Utility ( Hot water/Hot oil/ Brine/Chilled water/Steam/Liquid Nitrogen ) [T],

    2) Initial temperature and Final temperature and Volume of the mixture ( t1, t2, V ),

    3) Specific heat capacity of the mixture to be heated ( C p ),

    4) Average Overall Heat Transfer Co-efficient of the system ( U ),

    5) Heat Transfer Area of the System ( A ).

    [How To] Calculate the Heat Transfer Area of a Reactor?

    So, Initially we need to calculate the load over the Utility, Whether its a cooling load or heating load,
    That Can be calculated by our traditional formula,

    Q= M x C p x dT
    M  - Mass flowrate - Volumetric Flowrate x Density,
    C p - Specific Heat Capacity,
    dT  - Temperature difference of the Mixture 
          ( t2-t1) for Heating, ( t1-t2 ) for Cooling.

    Now the Heat Load based on the vessel Overall Heat transfer co-efficient to be calculated,

    Q= U x A x (dT)ln

    U  - Overall Heat Transfer Co-efficient,
    A  - Heat Transfer Area,
    (dT)ln - Log Mean Temperature Difference - (( T - t1) - ( T - t2 ))/Log[( T - t1 )/( T- t2 )],

    So equating the both of the Heat loads, I.e., Supply heat load = Required heat Load.

    M x C p x dT  =  U x A x ( dT )ln  =  U x A x (( T - t1)-( T - t2 ))/Log[( T - t1 )/( T- t2 )],

    Time Required for Heating/Cooling = [ M x C p x Log(( T - t1 )/( T- t2 )) ] / ( U x A ). 

    That's it......... Cheers,

    About The Author

    Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a technoholic geek by passion, and a Chemical Process Engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
    Follow Me on Twitter AjaySpectator & Facebook

    Pharma Engineering


    1. dt is temp boss..............not time

      1. Thats what i mentioned boss, have a clear look

    2. Ajay I didn't got at final step. Can you explain me clearly.


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