Heat Exchanger Cooling Temperature

[sjs83]

Hi Everyone,

I currently have the following issue that I need help with:

I have a tube in tube heat exchanger.
On the inside of the tube I have a hot liquid (assume to be the same as water) flowing at 0.11 kg/s.
This enters at 90 deg.C and leaves at 60 deg.C. That is 13.9kW of cooling.
On the outside of the tube I have cooling water entering at 20 deg.C and leaving at 30 deg.C at a flowrate of 0.33kg/s.

Now in the future, for the cooling I will need to change to another cold water supply where the water is supplied at 5 deg.C and should not rise above 10 deg.C. I still need to cool the hot stream from 90 deg.C to 60 deg.C at the same flow hot flow rate of 0.11kg/s.

What do I need to take into account in order to make this change to my system?

Thanks in advance.

 

[Gr8blu]

Most fluids have reasonably constant thermal capacity (maybe not the correct term, but keep reading and you should figure out what I mean). All else being equal, a fluid will be able to absorb a constant heat load per unit of mass flow. If the intent is to limit the temperature rise of the fluid to 50 percent of what it presently is, the flow rate must (more-or-less) double.

If that happens - now check on the conditions within the exchanger. Will the higher flow be more turbulent or laminar, compared to the original value? How does that affect the efficiency of heat transfer from one medium (inside loop) to the other (outside loop)? Might need a bit more than 2x the original flow ... or perhaps a bit less.

Converting energy to motion for more than half a century

 

[IRstuff]

The zero-order answer is double the flow, but you might not have sufficient surface area contact to make that happen. Your flow rates are relatively small, and your exchanger might be sized for that load, at that flow rate. There would be some operating margin to account for things like fouling, over time, but it's typically not the case that the designer would have designed it to handle 200% of the design load at end of life.

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[horacio Torres]

You must do the heat exchanger calculations for each case,, It will said to you what care must to be considered.


Horacio

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[LittleInch]

I'm no HX expert, but your initial conditions seem to imply a 100% efficiency to me.

30C x 0.11 kg/s vs 10C x 0.33 kg/s

That sounds quite high.

So change one parameter and its difficult to see efficiency starting the same.

Also external temperature is being ignored.

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[sjs83]

Thanks to everyone that answered, very helpful!

One follow-up question, if I need to increase my hot flow by 50% (i.e. from 0.11 kg/s to 0.165 kg/s) and keeping the same hot inlet and outlet temperatures, can I simply increase my cooling flow by 50% (with the same cooling inlet and outlet temperatures)?
Or is there anything else I need to take into account?

Thanks. Best regards

 

[IRstuff]

The question is whether your convection coefficient is likewise increased; that requires detailed analysis on information you have not provided.

see

https://www.researchgate.net/public...OEFFICIENT_IN_A_SHELL_AND_TUBE_HEAT_EXCHANGER

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[LittleInch]

At least 50%. All sorts of things happen which we can't judge which reduces the efficiency of the overall heat transfer.

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[MedicineEng]

As pointed above, it seems that you are working out of a theoretical model. There is not way that a heat exchanger (and basically anything in reality) is 100% efficient.
In a heat exchanger there are always losses: thermal to environment, fouling, laminar/turbulent flow, just to name a few that will drop your systems efficiency, some of it in dramatic fashion.

Having said that, you can theoretically calculate your flows assuming no loss, 100% efficient system. This will give you your very minimum in a perfect world. Reality is fairly different from that.

P.S. This is not homework, is it?

 

[Christine74]

Heat exchangers are routinely assumed to be adiabatic, i.e. that 100% of the heat lost by the hot fluid gets absorbed into the cold fluid. Not an unreasonable assumption in this case in my opinion.


-Christine

 

[IRstuff]

Heat exchangers are routinely assumed to be adiabatic, i.e. that 100% of the heat lost by the hot fluid gets absorbed into the cold fluid. Not an unreasonable assumption in this case in my opinion.

The efficiency isn't in the amount of heat transferred, but in the delta temperature between inlet and outlet. For example, if the HX is fouled, then flow is reduced, so while the same amount of heat is transferred, the temperature change must increase to accommodate the loss of coolant flow.

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[LittleInch]

And it implies that every single tube is the same in terms of heat loss.

If you only have one tube then maybe, but once you go to multiple tubes, nothing is equal.

Most HXs have some form of control also to regulate the key process input ( usually the outlet temp) to allow for changes and some element of margin.

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Also: If you get a response it's polite to respond to it.