Heat exchanger question for novice? 1

[nabeil7]

I'm working on a heat exchanger at work. The following is what I have been given:

-Both cooling fluid and working fluid are water.
-working fluid must stay between 20 to 25 Celsius
-Cooling fluid is max of 15 celsius.
-heat exchanger is single plate, stainless steel 22 gage.
-Working water initially at 20 (ambient) Celsuis
-Water is heated in 120 usgal vat at a rate of 2 celsius per hour

I'm solving for the area of the heat exchanger plate.

At first glance this seemed simple to me but I'm encountering some problems. I'm just wondering how I should go about solving this.

Thank you for the help.

 

[vpl]

Why is the water heated in a vat that is in US gallons when all your other measurements are in SI units?

This smells like a homework problem... changing units is usually one of the things they do to trip you up.

If it's not, might I suggest you first go find an old grizzled engineer and ask him or her for suggestions on where to start.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[MintJulep]

You have a vat of water at 20C.

Your goal is to keep that water between 20 and 25 C.

Isn't the easiest solution to simply not heat the water?

 

[nabeil7]

It might sound like a homework problem but this is my frist problem at my first engineering job. They have given me the flows and volume in imperial because we are working with an american customer but we are a canadian company so temp is in celsius.

Forgot to mention that the flow is:

working water through heat exchanger: 30 -40 GPM
cooling water - a max of 3 GPM (0 to 3 using a temp regulating valve.)

Also solution of not heating water is not possible as the pump used causes the water to increase temperature.

Thank you for the help

 

[zerosum]

Looks to me as though the film coefficient on the cooling fluid side is going to set the value of U and hence control the required exchanger area. That's a result of the flow ratios being over an order of magnitude different per your given constraints.

 

[vpl]

Nabeil7

This sounds like you've been given everything you need to know to calculate the area.

Since you say you're just out of school, I recommend digging out your heat transfer book and looking in the chapter on heat exchanger design. The formulas are pretty straight-forward. So figure out your heat transfer rate and make some assumptions on the film coefficients, based on the materials being used. If you didn't keep your heat transfer book, go and borrow one from someone or there's a free one on the web. (I'd look up the reference to it, but I need to run off to a meeting.)

Post what steps you've taken, and I, or others on the fora, will chime in on your next steps.

But since it's your first assignment on your first job, it's important that you show that you can figure it out yourself rather than it just being spoon-fed.



Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[zerosum]

You're right VPL, that nabeil7 needs to figure it out, but once the problem's out on the table, enquiring minds just need to know:

Is the pumping power on the cooling liquid side free? Why so small a number compared to the process side? My intuition says the process side pump is larger than necessary, unless it's supplying the process water to another user and that's what sets the process pumping rate.

Otherwise, why not increase the cooling water flow and turn the process water pump off and on to control the vat water temp, thus saving energy, because it wouldn't need to be on long to disapate the small heat load, and the heating due to pumping obviously would be less, too.

This revision would likely also decrease the required exchanger area.

 

[zerosum]

Or just put a bayonette in the vat providing the cooling?

 

[nabeil7]

Thanks a lot guys, I really appreciate the help. Yeah, the formulas are simple enough, I'm just having a hard time determining the convection heat transfer coefficients for water and air. I realize its based on the flow and temperature of the fluids but I can't find a chart or graph that has this.

As far as increasing the flow of the cooling water, we can't do this as this is a system already in place at the customers factory.

Right now I'm trying to determine U with
U = 1 / (1 / hA + dxw / k + 1 / hB)
but I can't do this without the coef.
then the rest is easy from there.

Maybe I'm going about determining the coefs the wrong way?
Again thanks for all the help guys.

 

[zerosum]

Based on your limited system description, it's not clear why a film coefficient is needed for air?

As I mentioned, since the flow rates are so disparate, to a first approximation only the cooling side film will control the total U value. That value correlates not only to velocity (through Reynolds number), but also depends on configuration and geometry. There are lots of rough numbers out in the literarture for water-to-water U, but your case is somewhat inefficient so exercise caution.

 

[vpl]

Nabeil7

Ah, finding the heat transfer coefficients! You're right that's usually the rub.

Normally, I use h_t and h_s for tube and shell, respectively rather than A and B. This is so I can tell where my values are coming from.

To find h_t, I use Nu*k_t/d_i. For h_s, its Nu*k_s/d_o. Which then leads to finding the right Nusselt number (which is a function of the Reynolds number and the Prandl number).

While I can give you what I use, you should really check to see if your company has a standard formula it uses.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[davefitz]

Its a trick question.

The rate of heat being added to the working fluid is 54,000 btu/hr, but the max possible rate of heat removal with a 100% effective heat exchanger to the cold fluid is 27,000 btu/hr.

30 gpm * 500 lb/hr/gpm *2 F/hr*1 btu/F/lbm *1.8 F/C= 54000 btu/hr

3 gpm* (25C-15C) *1.8 F/C *500 lb/hr/gpm*1 btu/lb/F =27,000 btu/hr

 

[nabeil7]

Thank you Patricia, the thing is we work with hydrolics and hydrolic motors and ussually don't work with heat transfer, but one of our recent projects requires the working fluid to be maintained at a certain temperature (to limit bacteria growth in the water). So that is why I was having such a hard time with this problem. Theres very limited resources here for this kind of problem.

I made rough calculations for the heat transfer coefficients to give me a range of the smallest to largest required area. I am now currently talking to a heat exchanger company and I think I've found an appropriate heat exchanger that will work. Thank you all for the help. Very much appreciated.

Nabeil

 

[finman]

You could always cheat and try here;

http://flatplateselect.com

I found it via this site and it seems to work just fine