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Help with heat transfer calculations

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jimbob20

Mechanical
Joined
Feb 26, 2008
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4
Location
US
The project I am working on at work is determining the cooling system requirements for wire drawing machines. This involves the wire being pulled through sets of dies to reduce the diameter. This process is done while submerged in the "bath" of the machine. The bath liquid has to stay under a certain temperature, or it will break down and the dies will wear out prematurely. There are cooling coils in the bath, and tower water flows through the coils to "cool" the bath. Also, the bath is agitated from the movement of the machine. Our test unit is in place of the tower water supply and it is a closed loop heating/cooling system with a data logger that records the temperatures and flow every 8 minutes. The first thing I did was run the machine so it kept the bath temperature constant. From that I could calculate the heat transfer rate from Q=M*c*deltaT. I said this was the heat that the wire drawing process creates since the bath temp. stayed constant. Then I turned the cooling system off and recorded the rise in the bath temp. over a fixed amount of time. From this, and the process heat transfer rate, I calculated a constant for the bath that includes the mass and specific heat since those are secret and unknown.(units are kJ/deg. Celsius) The problem I have starts with all my other data where the bath temperature starts out at a lower temperature and gradually increases througout the day. The heat transfer book I am looking at only has equations for constant external fluid temperature(the bath in our case), not varying temp. What I want to calculate is the heat removed by the flowing water and add it to the excess heat that raised the bath temperature using the bath constant I found earlier. I thought this would add up to be the process heat transfer rate I found earlier when the bath temp. was constant, but it's not. Obviously the varying bath temperatures is affecting my calculations and if anyone can help me I would appreciate it a lot. The multiple varying delta T's is throwing me off. If there needs to be clarification and/or more info, i'll try to supply it. Thanks.
 
From what I can tell the mcp delta T will not work. Your tempeartures are varying with time. You need to use heat transfer equations that take into acount time. For example if I add a contant heat to a solid with a starting temp fo 100C, it will heat up to a higher temperature. I do not have my heat transfer text in front of me but I think your missing parameter is time.

Cvanvoerbeke
 
You say the problem is that your bath water gets slowly hotter throughout the day.
You are putting in more heat than your cooling system is able to reject?

But, you were able to make the system balance under testing, but it still climbs in operation?? Are you reading the same gauge when you determine this?

My guess is that the bath has stagnation points, and you are not getting good mixing or flow past the machine and/or temp sensor.

 
KiwiMace: Sorry for the unclear explanation. While I was testing, the bath started at 30 degrees Celsius one day because the tower water that I switch off was warm on that day and with our test unit running, the bath temperature stayed constant so I was able to use the equations for constant external temperature. However, while testing on the other days, the bath temperature started out around 22-24 degrees and kept rising all day (to around 26-28 degrees) but never reached a constant temperature by the end of the day when I stopped recording data. I'm reading from the temperature display on the machine which uses an RTD temperature probe.

Cvanvoerbeke: Yeah, that's what I thought, there must be a time variable or something else that I am missing, however my book I have only deals with the case of a constant external temperature.

I thought that the log mean temperature difference would take care of the different temperature differences between the supply and the bath, the return and the bath, and the supply and the return, but my calculations for the process heat keep rising throughtout the time period, but I would think it should be very close to constant since the wire is being drawn at the same speed to the same diameter every time.

Any help is greatly appreciated. Thanks.
 
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