Heat transfer issue with system...

[oharag]

Hello. I'm hoping someone can lead me in the correct direction for a problem my company's having with one of their systems.

The system in question uses a calrod-heating element (coiled) to supply heat. A squirrel cage fan blows air across this heating element. This fan draws in filtered ambient air from outside the system, as well as recycles the heated air inside the system. The heating element will cycle on and off to supply a specified temperature to an enclosed area, which is measured by a thermocouple. I had previously tested the max temperature dissipated by the heating coil given worst-case conditions. The max temp was approximately 107 degree F.

The problem we are having is that internal surface temperatures have a max temperature requirement of 109 degrees F. We are currently exceeding these temps by 15 degrees F. This system was developed 15-20 years ago, and I'm tasked with trying to resolve the issue.

What I would like to do is create an internal duct using thermal isolating material. This material will be easy to cut, rigid and approximately 1/4 inch thick. I would like to use either glue or tape to tie these pieces together. Any thoughts on which material is best for this application? What about the tape, or glue?

My thoughts are to control the flow of the air better than currently is the case. Also, I would like to isolate the radiated energy from the heater coil and the hot convective energy from the airflow to the outside world.

oharag

 

[pennpoint]

oharag
Insulating the duct or box (enclosure) will help keep losses to a minimun, but may not solve your overshoot problem.
You might try to limit the temperature the heater is allowed to acheive. Clamp a thermocouple on the heating element and adjust its controller setpoint to a reasonable temperature (using a separate controller). Trial and error will be required.
Another option is Cascade Control. If you have a long duct and need to control at the end of the duct run.
Still again if this system is more than 20 years old, you may only need to upgrade your temperature controller to this years model.
Maybe a combination of the above.

Good Luck

pennpoint

 

[ko99]

It does sound like a control issue, but if you prefer to solve it with insulation, try a pressure sensitive adhesive product such as:

http://www.nomacokflex.com/InsulTube/Insulsheetpsa.asp

ko (

http://www.ecooling.biz)

 

[m777182]

The problem is that the calrod heater supplies heat by radiation.So you have to prevent the IR beam to fall on the internal surface where the temperature is critical. The temperature you are measuring with a thermocouple will depend on its absorpion of IR, you have to shield it from direct IR radiation!
Try to construct a metal cylinder around calrod heater.It will absorb IR and will have lower temperatute then heater itself.To enhance heat transfer coeff. from calrod heater/cylinder to the air, use fins on the inner side of a cylinder, possibly black (mild steel painted with phosphoric acid is good) to provide large contact area.
Without a metal cylinder you would hardly isolate the surface from IR :in my case even mineral wool did not last for a long time because there is polymeric binding material in it. Ceramic wool is good but few times more expencive.But use the cylinder around the calrod h. and you will be satisfied.
M777182

 

[oharag]

Okay, a couple of clarifications.

I do not believe it's a control issue. In fact, it works like a charm. We have to heat an object in a fairly large open area (upper half), which happens to be closed to the outside environment. The heater has to work harder in some instances to satisfy the set temp for the object being measured. Are there inefficiencies in this type of scenario? Of course. Though there are other requirements in the scenario that make this the best method of heating and controlling this object. I will speak about these inefficiencies below.

The heater is situated underneath an enclosure consisting of a plastic thermoformed enclosure. A cast metal part serves as the base of the system. This cast part has irregular pockets for other features in the system. Because of this irregular volume I believe the heated air is not being distributed efficiently. I believe there are significant losses to the heated air before it exists the lower half of the system to heat the upper half. These inefficiencies lead to the heater working that much harder to control the object in the upper half. There are 2 different exit ports for the system. One port is being heated at temps that are unacceptable.

Additionally; there are Plexiglas panels that direct air once the hot air leaves this thermoformed enclosure. There is a support platform that sits on top of the thermoformed enclosure, and is isolated from the lower enclosure. The mode of heat transfer is: radiant energy from the heater (not as significant), and convective from the hot air. Conduction is not as significant. There is some isolation in the system that prevents conduction among components. Radiant energy from the heater no doubt heats up the thermoplastic enclosure, but the reradiated energy from this enclosure is minimal. The inefficiency that exists is the fan blows air across the heating element, and then it flows in all sorts of directions. I believe hotter air is exiting the rear port first, and then eventually the second port sees the heated air. The rear port is overheated, and thus surfaces in that area are being heated at temps that are unacceptable.

I would like to find a rigid thermally isolated material to create optimized ducts to deliver the appropriate air to the needed areas. I’ll try to optimize these ducts to provide equal energy to each outlet. The benefit here would be the heater wouldn’t have to work so hard to deliver optimized heated air to the upper half of the system. There would be minimal losses of this air traveling down the ducts before it had to exit the lower half. And the thermally isolated material will shield the materials near the rear port from the super heated air.

pennpoint, there is no duct per say. See above. The heater is controlled by measuring the temperature of an object in another location. It works very well. I believe by managing the flow, and eliminating losses of the heated air in the lower halve will lead to lower power requirements of the heater in the end. It may also lead to lower temperatures of the heated air before it exits the lower half, thus producing lower temperatures on surrounding materials.

ko99, I’ll look into this solution. Though it requires a rigid structure to attach to. I guess I could make the ducts out of aluminum, and then apply this to the inner walls. I was kind of hoping I could find some material that serves both purposes though.

m777182, IR energy is not the means in which we heat our object being controlled. The IR energy coming of the calrod heater heats air being past over it by a fan. This air then goes in all sorts of directions. This air then goes to the upper half, and hopefully heats the object of concern.

Here’s what I’ve found so far:

http://www.microtherm.uk.com/prod4.html

http://www.foamglasinsulation.com/technical_data.asp

I also found some stuff at Mcmaster Carr online. I was hoping you guys would have some additional solutions.

Is there any thermal tape out there you guys could offer as a solution? I thought Kapton tape was good for thermal isolation, but I believe it’s for electrical isolation. I also saw some mention of thermal paint.

Thanks
oharag

 

[MintJulep]

You state that this system was developed 15 to 20 years ago. Has it just been built, or has it been in operation for 15 to 20 years?

If the later, did it every work without exceeding your temperature limit?

If yes, then something has changed over the years. Spend your effort figuring out what changed rather than designing a band aid.

 

[m777182]

I understand that you are not heating your object directly with IR, but you have to transfer heat to the air so you need pretty large hot/warm surface to rise air temperature and prevent "leakage" of the IR rays directly onto your object because direct IR can make hot spots. Additionaly you need air flow to be completely without temperature gradients so you need an overcritical Re number of the flow. Are you sure that you do not have some back circulation or vortices around flow obstacles?
I have an experience with a steam heater: because of back flow due to vortex there was an over 20degC difference in nonhomogenious air flow;enough to spoil the product. In another case I used calrod heater: an AC motor pushed air over heater and everything seemed nice, the Re was high enough, flow was passing the winding of the motor and cooling it but the IR comming backwards burnt the winding.
m777182

 

[ko99]

I think I understand now: the part you are trying to heat is fine but some nearby parts are overheating due to the hot air intended for your target part.

If that's the case, ducting makes sense. The ducting does not need to be insulated or reflective or perfectly sealed, it just needs to redirect the hot air. It can be sheet metal or sheet plastic or whatever makes sense given other requirements.

FYI, regarding thermally insulated tape: kapton has a low thermal conductivity, but in general, any tape is too thin to be an effective insulator . However, it will prevent airflow leaks (as would duct tape or band-aids for that matter...)

ko (

http://www.ecooling.biz)

 

[oharag]

MintJulep, it has been in operation for that long. Recent stringent audit cycles are starting to find these issues, where in the past issues may have been overlooked or not identified. Because of better test methods and test apparatus I believe we are finding out the "sins" of the past. Also, what I have proposed in my initial question is not a band-aid, but more of an investigation. I never stated I saw this is a real fix. I want to perform cause and effect studies. I know why we are having the problems. I want to isolate the problem, and then perform another test to see the impact upon temps. In my scenario we may decrease the power output of the heater so significantly that temps are no longer the issue (because of efficiencies in the system).

m777182, "Are you sure that you do not have some back circulation or vortices around flow obstacles?"
No doubt we do. I want to try to isolate the IR from the heater using the duct, and then provide a laminar flow of the heated air to specific ports in the lower enclosure.

ko99, thanks. Yeah, the only reason why I wanted to isolate all the energy from the outside world is to create a highly efficient system with minimal loses. I'm intrigued on measuring not only the drop of surrounding surface, but also the power output of the heating coil.

Thanks for all your input.

oharag


m777182, "Are you sure that you do not have some back circulation or vortices around flow obstacles?"
No doubt we do. I want to try to issolate the IR from the heater using the duct, and then provide a laminar flow of the heated air to specific ports in the lower enclosure.

ko99, thanks. Yeah, the only reason why I wanted to issolate all the energy from the outside world is to create a highly efficient system with minimal loses. I'm intrigues on measuring not only the drop of surrounding surface, but also the power output of the heating coil.