BTU/hr for heat exchanger 1

[jbasin0]

I'm trying to figure out how many BTU's I can get out of exhaust gas from an engine going into a pre-heat oven (essentially a heat exchanger). I know the mass flow and temperatures in and out. Is there an equation I can use?

I have seen the following equation and I'm wondering how the constants came about and do they change for different conditions (if this equation is even correct):
BTU = (lbs/hr)(0.24)(T2-T1) , (lbs/hr)/4.5 = scfm for exhaust

Thanks in advance.

 

[ivymike]

Basic heat transfer... mass flow times delta t times heat capacity.
Is this a homework problem?

 

[ivymike]

C_p for exhaust is about 1.06 kJ/kg.K if memory serves. In other words, ever kg/s that drops by 1C gives you about 1.06 kJ.

 

[ivymike]

* kJ/s, that is

 

[jbasin0]

Although it was a very basic question this is not homework. I am trying to find ways to maximize heat recovery for an engine by pre-heating pre-heat ovens. I'm just kinda green when it comes to heat transfer, as I have a Civil/Structural background. Thanks for your help.

 

[Lou Scannon]

The ideal heat transfer theory is pretty straight forward, and you can find many tools online to work the problem. The difficulty will be assessing the coefficients of resistance that limit the heat transfer rate in your actual heat exchange mechanism.
Another practical problem will be dealing with condensates from the engine exhaust, assuming your plant is able to extract enough heat from the exhaust so as to cause the exhaust to reach its dew point. In that case, not only will you have to have solutions for disposing of the condensate, but also for withstanding the probable corrosivity of the liquid.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

Surely the exhaust going into a continuously downhill spiral should solve the condensation issue.

Also the total heat available should be about 1/3 of the heat content of the fuel burned. Maybe a little less, especially for a diesel.

Heat exchange calculations are far from my area of expertise, but I would take a real wild guess that you won't do much better than 60% of the potential heat energy in the exhaust being recovered, so 60% of 30% or 18% at best.



Regards
Pat
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[Lou Scannon]

I agree, capturing the condensate isn't difficult. The next problem is what to do with it once you have it.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

I would have thought to just dump it down a drain, or into a pit to evaporate away.

It will be no more toxic than normal exhaust emmissions.

Regards
Pat
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[Lou Scannon]

OK, is that standard practice?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

I have no idea what std practice is for exhaust from a heat exchanger, however I think it is std practice for an engine exhaust pipe to discharge to atmosphere albeit via some scrubbing devices like a catalytic converter.

Regards
Pat
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[Lou Scannon]

A little rooting around on the web yielded a formula for Synthetic Muffler Condensate Acid Solution; .06 N comprised of 0.05N H2S04 and 0.01 N HBr.
I gather this is used for testing corrosion resistance of post-catalyst exhaust component materials.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[catserveng]

In general for large industrial gas and diesel engines we figure half of the rejected exhaust heat is usable, in practise on several site we've done much better. Also in practise a lot of variables come into play.

First a gas or diesel engine? Heat recovery on either is fairly commom in a lot a of areas, but a few things to consider,
Diesel units will have higher fouling factors, how much depends on things like fuel you are using, load factor and ambient humidity. Gas units on pipeline fuels usually have the lowest exhaust side deposits and best long term overall heat recovery results, at least in my experience. Gas engines on biofuels, like landfill and digester gas have differing deposit levels and can be as bad or worse than some diesels. Engines on HFO are really tough, but can also provide usable heat recovery.

Your engine manufacturer/dealer should be able to provide you with a heat rate table for your engine to use as a starting point. We used to use "rules of thumb" with pretty good results for years, but newer engine designs to meet reduced emissions made a lot of old estimates not so good.

In most cogeneration applications I've worked with you can take heat out of the exhaust and reduce it's stack outlet temp down to 350 degrees F without causing problems with mild steel exhaust system and generating lots of condensate. On the extreme side I've worked at some gas engine based district heating plants in Europe where the exhaust stack outlet temp is less than 100 degrees F, but they had to add condensate recovery and seperation systems and use stainless steel exhaust piping and silencers. However if you any kind of exhaust aftertreatment what ever you do for heat recovery can not have a detrimental affect on emissions reduction, can still be done but takes more effort.

I've worked with companies such as Cain, Maxim and Envirokinetics on heat recovery for larger recip engines, maybe yuo could see if they can provide assistance in your application.

Depending on your process you might just be able to dump your exhaust into your preheat oven, done that on a few brick making sites. Check with your engine manufacturer, he may have an installation or applciation type guide that may address your heat recovery needs, or appliciation engineering help that may have already done what you're looking to do.

Hope that helps, Mike L.