Exhaust/Heat Recovery

[kurbob46]

Hi All,

I'm new to the forum, and also a recent college graduate who has taken on a job as a Plant Engineer. I'm looking for some guidance on a project idea I had.

At my facility, we discharge (waste) approximately 200 million BTU/hr of hot air exhaust into the atmosphere. I was trying to come up with an idea of the best way to recover this exhaust and reuse for heat, etc. I was curious if anyone has done a project similar to this or if anyone had any ideas on how to use this exhaust.

Thanks in advance,
Joe

 

[dcasto]

waste heat recovery is always a fun one. The recovery unit typically has to have very low pressure drops to assure the waste heat vent doesn't stop flowing. I've installed both steam/water as the heat recovery fluid and oil. Steam was the cleanest.

The problem is where do you use the energy and is the vent stream at a temperature high enough to use it somewhere.

 

[Unotec]

I've researched and done quite a few projects that involved heat scavenging and, as dcasto says, watch out for pressure drop and actual temperature. 200MMBTU/hr might sound like a lot, but might actually not be hot enough.
My projects have included water heating, methanol recovery, treater feed pre-heating amongst quite a few.
Look around in your plant and the operational conditions for the processes that require heat.
Something like this might make you stand out as a hero or as a fool, depending on the sucess of the recovery (both, process wise and $$$).

 

[kurbob46]

Hero or fool? That's very true. There's no middle ground.

In our plant, we use kettles to cook the free moisture out of FGD and a kiln to dry wallboard. It might be best served to reuse this heat to keep these kettles and kiln temps up.

 

[danberry]

Only a pinch analysis can show the best heat recovery strategy. You need to identify your hot and cold streams and look at how much heat is available, or needed, and at what temperatures.

200 MMBTU/h is substantial but quite useless if you have nothing colder you need to heat.

 

[ChrisConley]

Waste heat has a thousand uses, some of which are pratical, some are 'pie-in-the-sky'.

One nice use for waste heat is absorption chilling. I'm sure you have a demand for chilled water in your facility. A waste-heat absorption chiller will generate 'free' chilled water for comfort, or process cooling.

 

[srfish]

The most common use of waste heat is to generate steam. Usually 2 inch O.D, tubing is used with 1 inch high fins and 6 to 7 fins/inch.

Here is an approximation of the maximum inlet exhaust gas temperaure that would keep the fin tube in steel material. Otherwise the fins would need to be the more expensive 409ss material. This based on the above mentioned fin configeration.
Tg = 1090 -0.23Bt
where:
Tg is maximum gas temperature in F.
Bt is boiling temperature in F.