Controlling the temperature drop in a duct

[jegan02]

Hi All,
I have to design a test jig for temperature aging of the plastic duct. The important criteria of this system is, the temperature drop between inlet and outlet should be less than 10° Celsius when the maximum temperature level of the duct is 240°C and the maximum allowed pressure inside the duct is 2.5 bar. Currently, I have a heating system (Consist of a heater and a fan) which can be used to heat the duct up to 350°C using hot air flow. But the temperature drop is 25-35 °C. So, How to accelerate the hot air flow in the duct to control the temperature drop?. I am also thinking about to send back the portion of the exhaust from out let to inlet to save the energy consumption. Is there any method to control the temperature drop or special device to accelerate the hot air flow? Does anybody has the experience related to this problem? Please help me to find a solution.

Thank you.

 

[FredRosse]

The duct is treated as a simple heat exchanger, I am assuming it is cooled by natural convection of ambient air on the outside, and heated by forced circulation electrically heated air on the inside?

Increasing the mass flow of the heated air will decrease the temperature drop in the duct. Adjusting the energy input,and recirculation of the hot air back into the heater section will conserve energy. The forced flow hot air inside the duct is not enough mass flow for only 10C temperature drop, a forced flow several times larger is needed.

After establishing a forced fan sized to give a temperature drop of less than 10C, fan flow can be adjusted to exactly match the 10C temperature drop you want by reducing flow. This is done with a throttling of mass flow with a variable damper, or a variable speed fan.

 

[georgeverghese]

The specification requires that you have no more than 10degC drop when hot air feed supply is at 240degC, but you now have hot air at 350degC, so obviously the temp drop will be much more than 10degC for 350degC feed.

To me, it looks like you have to first extrapolate backwards to see whether this duct meets this 10degC temp drop for a 240degC feed hot air. Else is there not some way you can reduce feed hot air temp to 240degC to see what the corresponding temp drop is ?

If you are now operating at 240degC feed, then one way to reduce heat loss / reduce temp drop would to apply external hot insulation. Heat insulation thickness can be calculated to bring the temp drop down to 10degC - see any heat transfer text book for theory and procedures.

 

[willard3]

A very obvious solution is to insulate the duct to control the delta t.

 

[berkshire]

Or place the duct in a heated room heated to 10 degrees below the inlet temp of the duct.
B.E.

You are judged not by what you know, but by what you can do.