Transient analysis of heat exchange 1

[skuntz]

I am looking for resources to help me to determine the time to heat up air circulating in a closed loop by passing it through a fin & tube exchanger where the heating medium is a glycol loop that is picking up waste heat from a generator. Both the air and glycol start out at the same temperature and the generator specs give me the heat rejection and coolant temperature from the engine. I have been tasked with determining the temperature of the air as a function of time.

 

[zekeman]

Who tasked you? And not enough information.
Why would anyone want a transient solution?

 

[skuntz]

What I want to know is, given the heat rejection of the engine, how long will it take for the coolant to reach an equilibrium temperature and likewise if the coolant is transferring it's heat to a moving air stream through an exchanger, how long will it take the air to reach an equilibrium temperature.

 

[zekeman]

Sounds like a poorly stated homework problem without any thought of the consequences of calling the air in a "closed loop".
If so, I will let the administrator decide whether we should offer any further assistance.
In my experience, only steady state values are of interest and vastly simpler to solve; therefore nobody would waste their time to get a transient solution, except for a professor who is teaching differential equations .
I too like some challenges but this one, while easily tractable to me, is not the stuff of real engineering.

 

[skuntz]

For your information "zekeman", I work for a $1B+ company that designs and builds systems for the defense & aerospace industry. The system we are currently developing is used for the environmental conditioning of sensitive military systems where thermal shock is a concern and therefore we need to predict the temperature gradient over time.

Due to the sensitive nature of this work I cannot go into more detail and I've responded in more detail than you frankly deserve considering your rather rude attitude.

 

[IRstuff]

If it's really closed loop to the air, there is no practical steady state temperature, since there is nowhere for the heat in the air to go. The temperatures of both the coolant and air will rise until the generator stops working.

TTFN

FAQ731-376

 

[skuntz]

Yes, dissipation of heat will only come from thermal losses through all the ducting. Wouldn't it be true that the maximum outlet temperature of the air would be the inlet temperature of the coolant from the engine?

Now that I think about it the real problem may be a thermal runaway in the coolant unless I can expel the heat after it runs through the coil.

 

[IRstuff]

Unless you can lose ALL the heat generated, there will be thermal runaway, until the generator seizes. I don't think you want to get the air temperature anywhere near that condition, since it'll probably mean that the coolant will be boiling internally within its plumbing, which is a BAD idea.

TTFN

FAQ731-376

 

[gwolf2]

> I work for a $1B+ company that designs and builds systems for the defense & aerospace industry.

I appreciate that you have a problem here, and I can solve it for you using standard equations but why would I unless your company gives me some of it's $1B?

On a more serious note - if this is such a big company then they will have departments of specialists to do this - why are you not using the usual internal departments, or subbing the job out to a consultancy?

Maybe this is a grad question after all..........

 

[zekeman]

"For your information "zekeman", I work for a $1B+ company that designs and builds systems for the defense & aerospace industry. The system we are currently developing is used for the environmental conditioning of sensitive military systems where thermal shock is a concern and therefore we need to predict the temperature gradient over time"

Well, if yur company needs a thermal shock solution why would you ask for a time to "heat up air" as you do in your OP? They have nothing to do with each other.

I also strongly suggest that you use the resources of your $1B company or hire a consultant to get the answer you need. And BTW "sensitive military systems" is a joke. If the program is classified, certainly a properly framed problem that requires a solution based on physical theory is NOT classified.

 

[skuntz]

IRstuff said:

"Unless you can lose ALL the heat generated, there will be thermal runaway, until the generator seizes. I don't think you want to get the air temperature anywhere near that condition, since it'll probably mean that the coolant will be boiling internally within its plumbing, which is a BAD idea."

You're right. The system we are developing is a modification of a system that was originally open loop (the airflow that is). The air was heated by diverting the generator engine coolant (with a 3-way valve) from the radiator to a heat exchanger which - if you think about it - is just another radiator. So we are simply swapping one radiator for another. The problem is we are not blowing fresh air though it anymore but recirculating the heated air. (Ouch).

Thanks - your observation led me to reframe the problem in a way that I can explain the dilemma to my project manager.
Perhaps the key is the control of the amount of coolant diverted for the engine.

 

[IRstuff]

If I understand your description, that won't help at all. The generator has a certain heat output, that must be removed or it will die.

There are only 3 possible outcomes:
> Run briefly, off for a LONG time to let the heat out
> Run slightly longer, store the heat somewhere, but eventually, you'll need to shut down for a LONG time to remove the heat
> Run continuously, with continuous heat removal.

Actually a 4th outcome:
> Run continuously, engine gets dangerously hot and melts or seizes.

TTFN

FAQ731-376

 

[skuntz]

OK, this leads me to examine the problem of dissipating heat from the engine. I have some specs on the generator that say that the heat rejection to the coolant system is 2616 BTU/min. That is when the coolant system that is shipped with the generator is used. Does this figure represent the minimum amount that must be rejected in addition to heat rejection into the exhaust?

 

[IRstuff]

Most likely. Note that it represents a 46 kW heat load.

A 1000 gal tank of water with that much heat inflow will start to boil after 7 hrs

TTFN

FAQ731-376

 

[skuntz]

I've presented the dilemma to my manager. In fact I used your example of the 1000 gal tank of water and it was a real eye-opener. This has now turned into a priority project in our organization. Thanks for taking me seriously and providing your observations.