Heat exchanger vapourizing of water to steam 1

[Sycorax]

I have a counter flow air- air heat exchanger for which I know the heat transfer coefficients for various flow rates and inlet temperatures . This data is empirical data got from testing.

Now on one side I want to model hot air(~800C) and the other side I want to model water(~25C). Basically I am trying to vapourize the water to 100C steam.

How do I go about modeling this as I do not have any data about the evaporation process of water or the heat transfer coefficients.

Appreciate your help.

 

[IRstuff]

Water evaporation simply requires raising the bulk temperature to 100ºC and then supplying the latent heat of vaporization. I would think there are enough resources on the web on this and similar subjects for you to swag the required parameters.

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[MintJulep]

I have a counter flow air- air heat exchanger

and then

Now on one side I want to model hot air(~800C) and the other side I want to model water

Water is not air.

 

[chicopee]

I think that he meant that the original HX was an air-air system and now he wants to modify it for air- water. There are a lot of technical literature about heat transfer coefficients for the air-water HX. Research is the key to many engineering jobs.

 

[25362]

Sycorax doesn't tell anything about the operation of the present unit and the purpose behind the switch to water boiling.

Heat Transfer by JP Holman (McGraw-Hill), Table 9-3, brings heat transfer coefficients for air-water boiling on horizontal and vertical surfaces and for various heat fluxes.

 

[Sycorax]

@Chicopee- You are right, the original HX is a compact air to air and I want to run it with air-water.

@25362- I haven't mentioned the purpose as it has nothing to do with the answer to my question.

A few more details, I already have a model setup to predict air-air performance and it is based on the Kays and London text on compact heat exchangers.

I have been reading Incropera text on heat transfer and it says for evaporation or condensation, the specific heat goes to infinity of the fluid which is being evaporated or condensated.

I want to incorporate this into my model and somehow be able to predict the heat transfer for the air-water ( a rough estimation is good enough).

Any help appreciated.Thanks.

 

[25362]

Keep in mind that the overall heat transfer coefficient would be lower than the hot air h. t. convection coefficient you already have. Uses of atmospheric-pressure steam? If for heating purposes, may be warm water is a better option.

 

[racookpe1978]

Why would he (or why should we) assume that an air-air heat exchanger would be water-tight - capable of holding high-pressure steam safely - when air flows are not usually high pressure systems?

 

[crshears]

Er, ahem...

Sycorax wrote: "I am trying to vapourize the water to 100C steam."

According to the steam tables in my possession, water will vapourize to saturated steam @ 100°C only at a pressure equivalent to that of one standard atmosphere, ergo, there will be no 'high-pressure steam' anywhere at all in the application under consideration....whatever it is.

And Sycorax, you are right; we don't NEED to know the reason why you want to use an air-to-air heat exchanger as a boiler; it simply appears to me 25362 has one of those 'rerum cognoscere causas' minds...just as do I.

Side note: if you're going to use an air-to air HXR off-design, make sure the feed water doesn't cause you insoluble [pun intended!] scaling problems.

 

[IRstuff]

" water will vapourize to saturated steam @ 100°C only at a pressure equivalent to that of one standard atmosphere"

It's pretty obvious that the OP is not sufficiently knowledgeable to understand that nuance. Given his scenario, he could produce saturated steam at some pressure well above atmospheric.

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[racookpe1978]

His "hot air" that is heating the water is 800 deg C. Which is why I assumed he could not "limit" his water side jury-rigged ex-air side to only 100 deg C.

Unless it were "vented" and his steam was escaping to atmosphere. That is, if he were literally only interested in boiling off the water.

But the air-air HX I've seen were not waterproof.

 

[EdStainless]

In reality the way that you would do this is to run at pressure (if the HX can handle it).
Heat to a temp above 100C.
The install a flash drum to get your steam when the pressure drops.
and recycle the hot water in the bottom of the drum.

You don't want to boil in teh HX
1. You will probably get a lot of fouling -unless your water treatment is very good.
2. The heat transfer at boiling is very low, very very very low.

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[crshears]

Hi EdStainless,

I like your approach! I'm guessing that if a circ pump were harnessed to maintain a steady flow through the HX/flash drum loop, it would simplify water treament by enabling conductivity cells etc. to be tapped into the circ line, and blowdown via a separate HX could be used to heat the incoming feedwater...

What would be the recommended circ/feedwater ratio to hold down the delta T across the HX to preclude boiling?

CR

 

[EdStainless]

You would have to analyze the overall energy balance to see.
A lot would depend on the incoming water temp. If it is cold then you could recerc a lot without worry.
Think of it as a simplified (non-fired) boiler.

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