shell-end-tube heat exchanger 3

[KZ001]

I am designing a shell-end-tube heat exchanger. My shell is a rectangular water tank and my tubes are 1.5in stainless steel pipes. In my design I will not use baffles. The only problem I am facing is calculating the shell-side heat transfer coefficient h(o) (the heat transfer coefficient outside the tube bundle). I used a different sources to calculate h(o) (one of them is Wolverine data book) and they all give me a different results. The calculated value of h(o) is too low which leads to tubes that too long. Can anyone help me with the correct equation for determining h(o). Thank you!

 

[insult2injury]

How do you know the calculated h(o) is too low?

I2I

 

[KZ001]

The calculated value of h(o) is too low because at the end when I calculate the length of single pass the result is not realistic. I am getting too long tube length then expected. Equations I used:

(h(o)*D(e))/k = 0.36*Re^0.55*Pr^0.33*(M(b)/M(w))^0.14

Re = (D(e)*G(s))/M(b)

D(e) - equivalent shell diameter
G(s) - shell side mass velocity
M(b) - viscosity at bulk fluid temperature
M(w) - viscosity at pipe wall temperature

Equation for single pass length:

L = q/(U*pi*d*deltaT(lm)*N(p)*N(t))

q - heat transfer rate
U - overall heat transfer coefficient
d - tube diameter
deltaT(lm) - log mean temp. difference
N(p) - number of passes
N(t) - number of tubes

 

[25362]

Since you are not using baffles, how do you generate turbulence on the shell side ?

 

[srfish]

The equation used above is based on using an equivalent diameter. The term "water tank" implies a large space compared to the 1.5 inch pipe. This would result in a large equivalent diameter which in turn would result in a low heat transfer coefficient for the water side. If the tube side temperatures are quite high, a free convection heat transfer equation could give you a higher heat transfer coefficient.

 

[KZ001]

srfish,
Thank you for the reply. Water temperature in the shell is T(i)=85F, T(o)=106F (approximately); water temperature in the tube is T(i)=120F, T(o)=95F. I am not sure if the tube side temperature is high enough to use free convection heat transfer equation. Also, can I use free convection heat transfer equation since water flow is caused by a pump? Please advice! Thank you!

 

[vesselguy]

What you're doing is designing a tank heater coil (immersion coil). This is not a shell and tube exchanger. What have you put in for the outside stream velocity? If you no mixing or some induced velocity, of course your ho is low. You should put in a mixer or a tangential inlet nozzle next to the coil to cause some kind of cross flow and reevaluate your design as a tank immersion coil. Better yet, send your data to a tank heater vendor and let the experts size it.

 

[KZ001]

Vesselguy,

Could you please recommend a book providing coil-in-tube design equations?
Thank you!

 

[SirCepxat]

Hi,
I work in process heating department in my company. Also i have done similar applications here in the TURKIYE. There is a direct heating immersion burner with the immersion tubes. And i can calculate the heater capacity, and the length of pipe.
Also i need the datas; What is the liquid ? (water , Zinc-poshpat, etc. ) And what is entrance temperature of liquid, and the what is the process teperature which is wanted. What is the tank's dimensions?

 

[KZ001]

I am designing a heat exchanger consisting of rectangular water-holding tank and four tubes making 28 passes each. Initially I approached the problem by considering shell-and-tube heat exchanger. My shell is the rectangular water-holding tank and my tubes are 1.5 inch stainless steel pipes. In my design I will not use baffles. The only problem I face is calculating the shell-side heat transfer coefficient h(o) (the heat transfer coefficient outside the tube bundle). I used different sources to calculate h(o) (one of them is Wolverine data book) and they all give me different results. The calculated value of h(o) is too low which leads to tubes that are too long. Now, I am wondering if my initial approach is correct. I should probably consider the exchanger as coil-in-tank type. The problem is that I did not find any information regarding coil-in-tank design calculations. What is the best source/guide book which explains and provides coil-in-tank design calculations?
The cooling fluid inside the tubes is de-ionized water and the water in the tank is CSO (Combined Sewer Overflow) water from Chicago Sanitary Canal.
The heat transfer rate each unit generates is 70kW. I have six units but they might be reduced to 4. Water temperature in the shell is T(i)=85F, T(o)=106F (approximately); water temperature in the tube is T(i)=120F, T(o)=95F. Flow rate is 22GPM. Tank dimensions 8.67ft x 9ft, hight=6ft
If you need any more information please let me know. Thank you!

 

[Skuhlke]

@ Zumbulev,

If you get help answering your question, can you please advise? I am working on a very similar problem and unfortunately have a lack of information where I am doing work. My fundamentals are around, but it has been a while since I have worked in thermodynamics.

Please let me know, thank you!

 

[KZ001]

Skuhlke,

Unfortunately I am not getting much help so far but as soon as I gather more info I will post it here. In my last message I answered SirCepxat’s questions. I hope he would help.

 

[nickelkid]

GO TO:

PROCEDURE 6-8 DESIGN OF PIPE COILS FOR HEAT TRANSFER

PRESSURE VESSEL DESIGN MANUAL, 3rd EDITION, DENNIS MOSS
Gulf Professional Publishing

 

[vpl]

Zumbulev

Unfortunately, one of the reasons you're not getting a lot of responses is that calculating the shell side heat transfer coefficient is one of the hardest things to do in heat exchanger design. Used to be, it was "back calculated" by putting together a heat exchanger based on empirical knowledge, figuring out all the other parameters and solving for it. The formulas are somewhat complex and I, for one, don't want to spend a lot of time typing in the methods I've used. It's one thing when someone provides all the data and somebody can just plug it into their purchased software and another to go through all the steps, not knowing when your question's been answered.

Ok, having finished my rant, I see that no one has directed you to Lienhard's heat transfer book. It probably has the information you need, if you don't already have a good heat transfer book.

The Lienhard textbook can be downloaded (legally and free) here:

http://web.mit.edu/lienhard/

http://www/ahtt.html

Patricia Lougheed

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