Heat Transfer in Jacketed Vessel

[dlewis]

I am looking to calcualte the heat transfer in a process vessel with a cooling jacket on vessel outside diameter. Vessel internal temperature and cooling water temperature are known. Jacket is "conventional spiral design". Will heat transfer be improved by increasing the number of "spirals" around vessel outside diameter?

 

[gk2001]

are these half coil limpets on the outer shell

 

[phex]

heat transfer is basically influenced by the following things:

1. Area of heat transfer. the bigger the area, the more heat is transfered.

2. temperature difference. this is the driving force of the process. the bigger, the better. =)

3. heat transfer resistance. (implemented through the use of a "heat transfer coefficient&quot this is largely dependant on thermo properties of involved substances.

getting back to your question with these 3 influences in mind, we can deduct the following:

does the area increase? this is the case if your spiral is a tube/pipe (whatever is the correct word wound around the vessel. more windings from the same pipe means the spiral has less free (unused) space, area is getting bigger, heat transfer is improved. you will also get a different LMTD with a longer coil (which is essentially the effect of more windings - longer coil). as the pipe/tube will be the same nonetheless, the flow regime doesn't change, provided the heating/cooling fluid flow doesn't change. so you have better heat transfer.

if you have a double-walled-vessel, and the "spiral" is made by a spirally wound obstruction between the two walls, changing the spiral would not result in bigger active area. It would change the fluid properties, once again, provided the cooling/heating fluid flow is not changed. due to the change in cross area of the flow path with the new spiral, the flow regime, expressed through reynolds, changes, which influences heat transfer - again to be better than before. the more turbulent your flow is, the higher is - generally speaking - your heat transfer coefficient. this will once again have an effect on LMTD, as more heat will be transfered over the same area. so in this case, more coils will once again help you.

the things said above are from a purely heat transfer side of view. if dp in the cooling/heating jacket is of importance, this will certainly change with modification, and to the worse. (you will have more dp).
also, the increase in coils will have no real effect if your overall LMTD is too low. at least the effect will not be worth the effort.
in addition, try to raise the heating/cooling fluid throughput to give yourself better performance without neccessarily changing the apparatus itself. this seems to be a more economical solution.

hth,
chris

 

[235zzzo]

Hi, Dlewis

As a first approach, the answer is afirmative, as long you consider counter-flows configuratons in order to assure the maximal LMTD, to work on the heat transfer surface you have for the job.

Keep in mind, that to maximize Q in the balance Q = S*U*LMTD = flow rate* cp* TD (Tout-Tin), you should maximize all the components you can. However, some naturally are fixed. You have to work on the others left.

In this case, to cool the process vessel, you should start "to spiral" the cooling fluid from the bottom to the top.

2nd Approach:
Increasing the number of spirals, you improve the turbulence, thus heat-transfer mean coefficient, however you can face limitations about the flow rate you can apply.

This means the necessity to balance your global solution in order to choose the best fits you. In some conditions, you need to go for thermal simulations to optimize the best solution for your specific case. The problem can be very simple, or very complicate, it depends on the several particular conditions you have.

I hope this can help. Please, give feed-back.
Cheers
zzzo

 

[235zzzo]

Just to mention to the forum that Phex and 235zzzo, wrote and sent, imagine! at the same time their posts, which tell about the same considerations as well, may i say. Please notice.
zzzo

 

[gk2001]

dlewis

As mentioned in the above posts, heat transfer will definately increse with the no of spirals as the area increases.
However the eqns to use are not straightforward of the type q = UA LMTD as the LMTD changes as the vessel temp decreases.
The approach to calculate the heat transferred is therefore that of a batch cooling process as outlined in Kern

gk2001