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Required Time To Bring A Heat Exchanger To Temperature


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#1 SCOTTRR

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Posted 03 September 2019 - 10:02 AM

Question was answered!


Edited by SCOTTRR, 04 September 2019 - 01:58 AM.


#2 Pilesar

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Posted 03 September 2019 - 12:10 PM

Please use the student section of the forums if you are a student. I suggest you find a worked example problem as a go-by. Review the definition of the terms in your equation and their units of measure.



#3 MrShorty

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Posted 03 September 2019 - 02:34 PM

I'm not sure what sort of explanation you are looking for. We know that heat always flows from high temperature to low temperature. Among all of the other variables, the rate of heat flow is related to the temperature difference.

 

Perhaps this tutorial will provide a refresher of the basics of heat transfer: https://www.physicsc...f-Heat-Transfer



#4 SCOTTRR

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Posted 03 September 2019 - 02:43 PM

I'm not sure what sort of explanation you are looking for. We know that heat always flows from high temperature to low temperature. Among all of the other variables, the rate of heat flow is related to the temperature difference.

Perhaps this tutorial will provide a refresher of the basics of heat transfer: https://www.physicsc...f-Heat-Transfer


Thank you for the resource.

In terms of the overall heat transfer coefficient, will this remain constant if the air stream remains at constant temperature and the area for heat transfer remains constant?

#5 MrShorty

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Posted 03 September 2019 - 03:00 PM

I'm not sure that I have the expertise to give you a definitive answer. From what I know of heat transfer coefficients (basically this page: https://www.engineer...ient-d_434.html), heat transfer coefficients are functions of several different variables -- some of which (like thermal conductivity) can be functions of temperature. How much a heat transfer coefficient might change in a scenario where one side's temperature is constant and the other side is approaching that temperature, I cannot say. The math seems a lot simpler if I can assume it doesn't change (and I wouldn't be surprised if there are many simple scenarios where engineers make that assumption). You would need to know enough about your heat exchanger to know if it is safe to assume a constant heat transfer coefficient or if you need to vary the coefficient as the cold side warms up.



#6 PhilippM

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Posted 03 September 2019 - 03:23 PM

Asides from the issues that Pilesar and MrShorty already mentioned: How large is the heat exchanger and how much water is inside it? Even if the heat flow was known and constant (which it probably will not be) and you used the formulas correctly (which you didn't) you couldn't calculate the time it takes to heat up the system, because you don't know how large the system is.






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