Submerged cooling coil system 3

[MIkkelPoulsen]

Hi

I'm having troubles finding out how to calculate a cooling coil system that is submerged in a water tank.
This is what i have of data.

I have a water tank that is filled with none agitated water - this is then is coming in contact with some gasses that makes the water getting up to 70deg C. ¨
This is to hot - so i need to cool this down to under 40 degC.

For this i have available cooling water of 37 degC, so the idea from the process department is to make a cooling coil inside the water tank.
But i'm unsure how to calculate this,

How do i setup a formula so that I can calculate the pipe size and length?
I have the following data:
Tank size: ø1450mm inside
Water volume: 2400L
Water temp: up to 70 deg C
Cooling water temp: 37 degC
Water only has normal conviction - so no moving parts of the water.

I hope somebody has some ideas on formulas or theory i can read on this - i have not made this kind of system before.

I know that it is very inefficient to have the water standing still - but it is not possible right now to get it moving.
But if i make the formulas correct i will be able to calculate the difference if it is agiated or not.

Thanks in advance,
Br
Mikkel

 

[EdStainless]

If the water isn't circulating or being agitated your heat transfer will be very very low.
Besides as the water heats it will be less effective.
Where will the heated water go?
You need flow on both sides to have any real heat transfer.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[IRstuff]

Warm water is ostensibly buoyant, so a MUCH larger tank, with the coil near the bottom, could possibly work. Otherwise, you need constantly refreshed cooling water supply or heat exchanger, like a cooling tower.

What is the maximum air temperature? If it's sufficiently low, you could just run your gases through a fan-blown radiator.


TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[MIkkelPoulsen]

Yes i will be very ineffective, also what i'm trying to explain to the guys that made the proposol - but i need some data to prove how much.
Because then I can maybe make another system.

it is not the gas that are hot - it is the chemical reaction in the water that makes my stading still water rise in temperature.

I have cooling water from another place availible at this 36 deg C max - it is normally much cooler than that, so this is a constant flow I have that I can cool with.

 

[Compositepro]

It is mostly the small 3C delta T that I would be concerned with.

 

[chicopee]

Assuming you had courses in thermodynamics, and transfer and physics, you will first assume no heat transfer to the surrounding of the tank and no gain in the heat absorb by the tank wall. The equations to consider include the mass flow rate of the fluid entering and exiting the coil, the first law of thermodynamics for the fluid flow and internal energy of the bath fluid and the heat transfer between the fluid within the coil and the bath fluid; keep in mind that the temperature of the bath fluid is bulk temperature and not stratisfied. A spread sheet should be used since the fluid temperatures within the coil and that of the bath fluid will change during a time stepp evaluation.

 

[EdStainless]

The hang up with this plan will be the tank side boundary layer heat transfer.
The nearly stagnant water in the tank will create a situation where you will get heat transfer by conduction only.
Just look at heat transfer by conduction in water compared to forced convection.
Off the top of my head I am guessing that the difference is a factor of 100 or so.
After all there is reason that heat exchangers are designed with flows in the range of 2-5m/s (depending on materials, fluids and pressure drop).
If you don't want to pump the fluid in the tank through a heat exchanger then you need to look at other ways to create some velocity across the coils. Maybe a mixer, gas sparging, or an in-tank jet pump.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

Design guidelines for in tank cooling coils in batch mode cooling operation are to be found in pages 11-18 onwards in Perry Chem Engg Handbook, 7th edn. For non agitated operation, table 11-2 states a U of 105-180 Btu/hr/ft2/degF, which seems rather high to me - you can derive this from fundamentals to confirm / verify. You will also have to compute pressure drop through the coils I think, if you have cooling water returning to some common CW return header.
This is not a task for the un initiated graduate engineer - ask for help from senior process engineers. All too often, managers develop a sadistical weakness for throwing young engineers in at the deep end and watching them choke in the water.

 

[pierreick]

Hi,
Let you consider this resource.
Pierre

 

[georgeverghese]

Mean cooling water temp in the coil = 40degC
Delta T between CW temp and bath water temp = 5-8degC
Bath water temp = 45-48degC
So trying to get down to < 40degC, especially with non flowing water over the coils, will require much longer time for each batch.

 

[MIkkelPoulsen]

Yes i fully agree, that making the water standing still is not an option - and I actually have 3 cases you can say. I then just need to find the best and most economically one.

1. Case - the water is actually not standing still all the time. When the gasses are pushed into the water it will be through a gas sparger submerged in the bottom. I just do not know how often, and the water needs to be under 40 deg C. when this happens. The water will only change to high temperature during this sequence. So that is why i have choosen to not look at this a away of agitating the water.

2. Case - i have a pump for emptying the system - this can be used in 2 ways. One way - make a cooling coil inside the tank, use the pump to pump the water out in the bottom of the tank and into the top and by this keeping it in motion.

3. case - use the pump to pump the water through a heat exchanger and then back into the tank.

I would think that case 3 would be the easiest to make - also here I do not need to make the tank larger to have the coil inside.

Any ideas on these 3 cases.

And thank for the feedbacks - i will look into the formulaes.

 

[IRstuff]

How often does this need to happen? Removing 33C from 2400L of water is going to take time.

Even at once per hour, that's 92 kW of heat removal.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[georgeverghese]

Agreed, case 3 may be the only practical option. Use a pure countercurrent flow HX to get the tank contents down to less than 40degC. Since thermal duty may be small, a double pipe HX from Brown Fintube may be a choice. Else use a plate frame HX or a compact welded plate compact HX.

 

[pierreick]

Hi,
Definitely an external HX is the best option, in your case (3).
Pierre

 

[LittleInch]

Any reason you can't use an air cooler if you're going for option 3?

Min delta T realistically is about 7C between any two substance or your time is too big or the HX gets too big.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.