Resultant temperature of mixed streams

[NPC5]

If I have multiple streams of cooling water exiting exchangers, combining and returning to a cooling tower, what would be the quickest way to estimate the temperature it would be when it reaches the cooling tower. Is it sufficient to simply do a ratio type thing

i.e. [(temp1 * flow1) + (temp2 * flow2)] / (flow1 + flow2)

Thanks.

 

[zdas04]

As long as the fluids are the same, then the sum of the product of volume flow rate times temp, quatnity divided by the sum of the flows should be pretty close. If the fluids are not the same then you have to use mass flow rate instead of volume flow rate.

It doesn't matter if you use absolute temp units or common temp units.

David

 

[sheiko]

NPC5,

Theoretically, your equation is OK when no phase change occur and the bodies in contact are of the same nature.

If the bodies differ, i would add the thermal capacities:
[(temp1 * Cp1 * flow1) + (temp2 * Cp2 * flow2)] / (flow1 * Cp1 * + flow2 * Cp2).

N.B.: Flow will be molar (mass) if you use molar (mass) thermal capacities.

"We don't believe things because they are true, things are true because we believe them."

 

[NPC5]

Thanks for the help zdas04 and sheiko!

 

[sheiko]

zdas04 ,

Could you please explain your concept of using the volume flowrates when the fluids are the same, and of using the mass flowrates when the fluids differ, because i don't know it.

"We don't believe things because they are true, things are true because we believe them."

 

[zdas04]

Volume flow rate is mass flow rate divided by density. If the density is the same on every term in the equation and constant, then it can be divided out without changing the answer. It ain't science, it's algebra.

If any term has a density that is materially different from the rest, then the technique can still work (by using specific gravity) but it usually isn't worth the effort.

Same way with your recommendation to add cp into the equation. If they are all the same then they cancel. If you have different fluids comming together (not a common cooling system problem) then you want to use both mass flow rate and cp.

David

 

[sheiko]

Thanks David for your explanation,

However, if you use molar Cp in the basic equations (different fluids), then you have to use molar flowrates also. As a result, the density becomes irrelevant...

"We don't believe things because they are true, things are true because we believe them."