Effect of increased water temp to mixer cooling jacket 1

[jcard]

I would appreciate any feedback regarding the negative effect of increased inlet water temperature into a cooling jacket surrounding a mixing machine.
My company uses a mixing machine that mixes together various dry powers in the manufacturing of brake shoe friction material. The mixing shaft assembly of the mixing machine is surrounded by a water cooling jacket system. The outlet temperature of the mixed material is about 180F; inlet temp is ambient.
Currently the inlet water temp to the cooling jacket is 45F; leaving water temp is 52F. Water flow is 9 gallons per minute. I can use the equation qw = 500 Qw dT to calculate the heat transfer into the water at about 31,500 Btu/hr.
qw = heat transfer rate to water in Btu/hr
Qw = water flow in GPM
dT = water temp increase across unit
This setup currently works fine. However, I have been asked if we can use cooling tower water as a the cooling medium rather than the chilled water we currently use. The temperature of the cooling tower water is 89 F maximum. Available water flow is more than 9 GPM.
Is there any method to calculate if the 89F water will provide equivalent cooling effect as the 45F water? I know the flow would have to increase, but flow can only be increased by so much.

 

[Latexman]

Is this a batch or continuous operation? How about a drawing or sketch to help us understand more? You need to calculate your overall heat transfer coefficint using Q=UA[Δ]T_LM. Your water flow is remaining the same, so U should not change much with the higher temperature. So, use the same U to calculate the new heat transfer. It will be less! About 33% less.

Good luck,
Latexman

 

[zekeman]

Agree that UA will be about the same for these flow rates.

delta T across the heat exchanger on average
(180+70)/2-(45+52/2=77 degrees
If you use 90 degree water average 93 you get a delta T of

(180 +70)/2-93=32

the average temp of the batch would have to increase 45 degrees and the outlet would rise 88 degrees, if the heat into the batch is the same.

You may not want 260 degree batch outlet temperature.

It would help to know hoe the batch is heated,to get more insight.

 

[jcard]

Thank you very much for the responses.
Please see attached drawing, which describes the process. The process is continuous for about 2 hours, then a new container of dry powders is placed on top of the mixing machine, which takes 20 minutes to complete. A new 2 hour mixing process then begins. No outside heat is added to the process as the mixing action inherently produces excessive heat that must be removed. The maximum temp of the final mixed product is 180F.

 

[zekeman]

My answer is the same as before.

Your delta T average across the jacket at the heating rate q is

hA*(Tb-Tw) for both cases and since hA is almost the same, the new Tb-Tw must be the same,or

q=hA*(180-Tw)=hA*(Tb-Twn)

(180-Tw)=(Tb-Twn)


Tw=44 old average water temperature

Twn= 93 new average water temperature


So Tb=180 +(93-44)=230

 

[jcard]

Thank you both very much for your response. One last question. Would the availability of increasing the water flow of the hotter (89F) cooling tower water above the existing 45F water flow to the mixer (9 GPM) alter these conclusions to any significant amount?

 

[willard3]

I guess you didn't read what zekeman wrote..........