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  • Heat Transfer Estimation

    Discussion in 'Calculations' started by ruzfactor, Jul 21, 2011.

    1. ruzfactor

      ruzfactor Member

      Jun 2011
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      I am working on a project involving this setup (please refer to the pic). Basically compressed air at a constant flow rate flows through a 1/4" tube to the tube shown and gets heated by the star-wound heater and, this hot air heats the small inner vessel and then leaves the system. Now I am stuck with doing a heat transfer estimation of the system. I know the ratings of my heater. The temp of the air should reach at least 300C. But I want to estimate whether the heater is capable of heating the air to 300C. Also, the heat transfer between the inner vessel (containing powdered material) and the hot air. This looks like a transient problem but I want to make a simple approximation to check whether the system is capable of generating heat and effective heat transfer is taking place.

      Need some expert help here to initiate the approximation. thanks in advance.

    3. thermal

      thermal Member

      Jul 2011
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      This is what I would do. It is a little bit swingy but after all you are asking for an estimate.

      The only air property that has an impact on convection at high pressures is the density. The dimensionless description of it involves the numbers Nu, Re and Pr. In this problem it is Re that is of interest. Inside of it there is the product of density and velocity. So rather than increasing the density you can calculate for normal air pressure and increase the velocity. In practical terms, if you have 2 m/s at 7 atm, you can calculate for 14 m/s at 1 atm.

      Now you have an angular slot. I have no idea of the dimensions but for an estimate you can for most cases unfold it and calculate it as were it a slot between parallel plates. Here:
      http://frigprim.x10.mx/online/parallel_ ... coeff.html
      (max velocity limited to 15 m/s)

      By Thermal.... A definition is by definition always correct, however it might not be purposeful.

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