5 replies to this topic

[Tong weng]

How to know if its worth it to install an exhaust system for a continuous drying oven for humidity removal? The exhaust hot air can be used to reclaim back some of the energy in the form of hot water by using air to water heat exchanger. Is the system worth to invest and get good ROI? How to determine if the humidity in the oven is too high and this causes the temperature in oven to increase to compensate the high humidity to cure the product? 

[thorium90]

Would using the hot exhaust air to preheat the incoming cool air be a better way to recover the heat?

[Tong weng]

Would using the hot exhaust air to preheat the incoming cool air be a better way to recover the heat?

The current design of the oven doesn't allow this

[thorium90]

Recovering the oven exhaust heat is a question of the design of the proposed heat exchanger, not about the oven's current design isn't it? Why does the oven's current design not allow it? Might you clarify?

[Art Montemayor]

You want to know if it is economically worthwhile to recover the heat content in hot, moist air exiting an oven in the form of hot water.

 

The answer lies in your domain: how expensive is the energy in your sector of the world?  How economically necessary is it to recover the heat input?  What is the capital worth - in other words, how do you gauge the minimum rate of return (or Return on Investment)?  Only you can address and answer those questions.

 

I’ve done this type of heat recovery before - but for different reasons and I can share my experience as follows:

• You will find it is extremely expensive and uneconomical to try to recover any heat between a gas (or vapor) and a liquid fluid in a typical heat exchanger.  The gas film heat transfer coefficient is depressingly very low and controlling.  Consequently, a very large heat exchanger working in true counter flow is required.
• A more rational and simple device is usually applied to hot gaseous streams: a direct-contact, packed bed water scrubber tower.  This type of apparatus is a true counter-current device and fully meets basic process conditions such as compatibility, availability, conventional and available water, no contamination, and easy controls, and low pressure drop for the hot humid air.

The amount of heat available in the hot air exhaust determines the amount of water required to directly capture the heat in the inlet air.  A simple heat and mass balance will determine this for you.  The height and diameter of the packed bed will determine the pressure drop across the scrubber and the pressure imposed on the oven.  The smaller the scrubber diameter, the better the water and hot gas contact with less channeling - but the greater the pressure drop.  Hot water exits the bottom of the scrubber tower and cool humid air exits the top and out to atmosphere.  How hot the water exits is determined by the cool water inlet rate and how hot you can tolerate it for proper heat transfer operation.

 

You can design this type of direct heat transfer based on theoretical calculations and empirical data.  A pilot plant would help in obtaining empirical data.

[breizh]

hi .

a brochure with some examples of heat recovery systems .

 

You may check with them ?

 

Breizh