Sizing a heat exchanger and boiler

[edwardmp2009]

I am experiencing problems in sizing a heat exchanger and boiler for the following duty:

We intend to produce 92 % v/v ethanol from 9.25 % v/v fermented juice using a still. the juice must be heated in a tank from 25 deg C to about 85 deg C (not quite sure about the optimum temp) using a heat exchanger in a closed circulation loop implying that the heated juice returns back to the tank. once the temp is attained, the heat exchanger will simply maintain that temp to allow production of vapors into the still; Once the desired temp is attained, I expect a drastic drop in steam requirements. It is desired to use saturated steam at 145 kPa absolute for this duty. the flow rate of fermented juice is 11.11 kg/s.

I sized the shell and tube heat exchanger and came up with this: U = 53 W/m2.K; A = 176 m2; total number of tubes = 560; number of passes = 8; tube I/O dia = 16/20 mm; tube length = 5 m; shell internal dia = 764 mm. One problem I have noted with my design is that steam has a low heat transfer co-efficient which results in low overall heat transfer co-efficient way out of the expected range for steam-water heaters (500-1500 W/m2.K).

I also need to size the boiler to produce steam at 145 kPa absolute at the desired flow rate. Your assistance is highly appreciated.

 

[MJCronin]

ed....

"One problem I have noted with my design is that steam has a low heat transfer co-efficient.."

???????/

Hmmmmmmmmm...... I don't buy it.......Your existing saturated Steam system is damn good stuff for heating.

I have a feeling that you do not need so much surface or so many passes in your design

Are you considering the latent heat of steam when sizing your exchanger ?

http://en.wikipedia.org/wiki/Latent_heat

 

[macmet]

Why don't you just go to a vendor and say you need X amount of btu/hr? Why are you sizing the number of passes, etc?

Sure, you should check the math so that you are comfortable that the surface area, boiler output, etc, but it sounds like you are going overboard. I've sized boilers and heat exchangers before and I've never told them how many passes I want.

If I were you, I would say I need X btu/hr, given them Temps in and out, expected flow rate and let them do the rest.

 

[edwardmp2009]

Thanks for your responses. This is just a research project and I do not intend to purchase the equipment. I am attempting to follow the procedure in Richardson and Courlson Chemical Engineering text book, chapter 12 in which you estimate U from data given in literature say for Steam-water systems U = 1500-4000 W/m2.K. Then calculate surface area based on power rating calculated from:
Q = m*Ls where m is the steam flow rate (kg/s) and Ls is the latent heat of condensation (J/kg). To verify that the initial U is correct, you then specify number of passes, dia and length of tubes so as to calculate number of tubes required. You then calculate the individual heat transfer co-effs from the Nusselt number formula and calculate U from these indivudial co-effs. If final U is lower the initial U, you restart (iteration) until final U >= initial U.

Looking at my problem I thought, a small heat exchanger can do this duty but the iterations end up giving a small U=53 W/m2.K which then results in large surface area (unless if there is a significant error on my calculations). This is because the individual heat transfer co-eff of steam is tending to be low (abt 58 W/m2.K). But given that the juice will be circulating through the exchanger in a loop, I think the major issue will be that of the heating up time given that a smaller heat exchanger is used.

Your comments are welcome

 

[Latexman]

Based on U=53 W/m2.K for steam/water, without a doubt, something is wrong in the calculations/conversions. Since this is a virtual project, just assume 1500 (min.) or 2750 (ave.) W/m2.K and get on with it.

Good luck,
Latexman

 

[MJCronin]

I agree with Macmet.......have an HX fabricator use an in-house sizing program to come up with his sizing and cost estimate.

Compare his calculated surface area with yours......I believe that your numbers are high

Suggest that you specify tube/shell materials, max pressure drops (on both sides) and desired tube size to the fabricator.

Spirax Sarco, a fine US company, has much information on steam heat exchangers....

http://www.spiraxsarco.com/us/