We have a kettle (with agitator) of 700 gallons of canola oil with our target temp of 180°F. Using a 50/50 water glycol mix and low pressure (<40psi). Heat transfer is great till oil reaches 140°F (rise about 1°/min in 1-1 1/2 hours). Then it takes 3-4 hours for the oil to go from 140 to 180. At this point, the glycol is running 218 to process and basically 218 on the return side and the heaters are only running about 10%. It seems the heat transfer has basically stopped. Why? the only thing I can reason is that we are creating a vapor with the water, thus, lossing the heat transfer as the energy is being used to create the vapor.
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Heat transfer problem with 50/50 water glycol solution
- Thread starter vonbad
- Start date
How about air being trapped over the tank glycol?
When the oil/glycol is cool there is still thermal contact between the oil and glycol with say fifty % air at the top of the heat exchanger surface BUT after heating the air expands so that if is covering 100 % of the heat exchanger surface --so no(relatively) heat transfer occurs.
My WAG -(wild assed guess).
When the oil/glycol is cool there is still thermal contact between the oil and glycol with say fifty % air at the top of the heat exchanger surface BUT after heating the air expands so that if is covering 100 % of the heat exchanger surface --so no(relatively) heat transfer occurs.
My WAG -(wild assed guess).
VONBAD: Despite glycol being called a "coolant", it can actually inhibit heat transfer. I had a 1966 Chevy with a 327 engine. In the winter with "anti-freeze" it ran at at 190 deg F. In the summer when I drained the "Anti-freeze" and added rust inhibitor it ran at 180 deg F. Glycol is an alcohol that lowers the specific heat of the water slightly. Do you have a corrosion inhibitor in the system? You may want to check the inside for the presence of corrosion and sclae. So-called film coefficients can severely inhibut heat transfer, particularly when the temperature difference gets small. Is this new or has it been doing if for some time. If it is a new problem of has gradually gotten worse, I suspect that corrosion and scale are forming.
Regards
Dave
Regards
Dave
vonbad
Typically these jackets the fluid enters at the bottom and exits near the top if this arraingement is reversed you will not purge the jacket of trapped air or vapors and coverage of fluid will not be uniform.
If possible attach thermocouples to the jacket at intervals from the bottom-up and you should be able to detect a delta T.
This will show you where you have vaporlock.
Good Luck
Typically these jackets the fluid enters at the bottom and exits near the top if this arraingement is reversed you will not purge the jacket of trapped air or vapors and coverage of fluid will not be uniform.
If possible attach thermocouples to the jacket at intervals from the bottom-up and you should be able to detect a delta T.
This will show you where you have vaporlock.
Good Luck
- Thread starter
- #7
Gentleman,
Thank you all for you responses. It gives me a place to start looking for the problem.
Let me clarify the application. First, it's a relatively new system. The jacketed kettle measures 60" diameter, 78"high; top is open. The tank and piping is insulated. System is running at 110 GPM with 2" pipes.
As it is new, I do not believe that it is an issue with scaling or fouled surfaces.
The jackets are a little more involved. First, the original jacket (primary)was made incorrectly (it was not dimpled). The only solution was to place another dimpled jacket(secondary) above the primary. The system runs from the bottom of the primary then into the bottom of the secondary jacket. This definitely increased the heat transfer but did not solve the problem 100%. This is where we are now.
I was told that the system was purged of all air.
I did some research into the boiling point of the HTF. At a 50/50 mix the boiling point is 221°F. Although, the system pressure is 30-40 psi it would increase the boiling point, but I am sure how much. I am thinking it's possible that the heaters are boiling the solution creating the vapor then shutting off. This would account for the heaters only operating for 10% of the time and the very low heat transfer at this point.
Any advice would be appreciated.
Thank you all for you responses. It gives me a place to start looking for the problem.
Let me clarify the application. First, it's a relatively new system. The jacketed kettle measures 60" diameter, 78"high; top is open. The tank and piping is insulated. System is running at 110 GPM with 2" pipes.
As it is new, I do not believe that it is an issue with scaling or fouled surfaces.
The jackets are a little more involved. First, the original jacket (primary)was made incorrectly (it was not dimpled). The only solution was to place another dimpled jacket(secondary) above the primary. The system runs from the bottom of the primary then into the bottom of the secondary jacket. This definitely increased the heat transfer but did not solve the problem 100%. This is where we are now.
I was told that the system was purged of all air.
I did some research into the boiling point of the HTF. At a 50/50 mix the boiling point is 221°F. Although, the system pressure is 30-40 psi it would increase the boiling point, but I am sure how much. I am thinking it's possible that the heaters are boiling the solution creating the vapor then shutting off. This would account for the heaters only operating for 10% of the time and the very low heat transfer at this point.
Any advice would be appreciated.
- Thread starter
- #10
Hacksaw,
the system was designed with an expansion tank for the water/glycol. The pump was designed for 40 psi and 250°F operating 110 gpm. I will check the total head.
Amorrison4
I would like to be able to get my hands on the system, but it's 1500 miles away. Working on applications without being able to see them is rather difficult and the information you get is not always accurate.
Thanks for the help
the system was designed with an expansion tank for the water/glycol. The pump was designed for 40 psi and 250°F operating 110 gpm. I will check the total head.
Amorrison4
I would like to be able to get my hands on the system, but it's 1500 miles away. Working on applications without being able to see them is rather difficult and the information you get is not always accurate.
Thanks for the help
<))I am not that familiar with this application, but I would investigate the following two area as a sousrce for your differences in heat up time, let me know if this helps:
1. The jacketed system could be building up condensate if it is not completly free draining or if other systems create a backpressure on the condensate system with time.
2. In addition to vented gases, you may also want to look at the potential for rollover of fluids as you heat up which will change the amouint of vented vapors substantially.
The more you learn, the less you are certain of.
1. The jacketed system could be building up condensate if it is not completly free draining or if other systems create a backpressure on the condensate system with time.
2. In addition to vented gases, you may also want to look at the potential for rollover of fluids as you heat up which will change the amouint of vented vapors substantially.
The more you learn, the less you are certain of.
At 180 to 219, you have a 40 degree approach, compared to an 80 degree approach when you started at 140. The basics of heat transfer given simplisticlly as TTT, Time, Temperature, and Turbulence. Heat transfer rates are a function of the driving force (delta T) and the turbulence not only in the vessel caused by your stirrer, but in the jacket as well, and the time of contact, your overall heat transfer rate for this type equipment may have approached equilibrium.
The factor of time is not in play here as I see it.
And, remember, what is in the kettle is losing heat to the surrounds at the surface by the same processes.
Therefore anything you can do to have an effect on the temperature of the glycol, (probably not very doable, or you would have done it) or the turbulence in the vessel or the jacket will help you over the hump. That might involve stirrer speeds, and/or pump rates for the glycol.
Plus, do you actually need 50/50 water/glycol. As pointed out by the poster above, water glycol mixtures are not as good a heat transfer agent as pure water. True pure water has the highest specific heat, but the automotive world has to accept and design around the limitations of antifreeze mixtures so that we can park our cars outside in the wintertime. (A problem ignored in the temperate climates.)
If you have no freeze problems with your coolant, why not reduce the ratio of glycol to water. The specific heat of the fluid increases as you go towards pure water. Undoubtedly you are using a mixture for some reason, probably a combination of freeze protection and corrosion protection. Investigate changing the mixture.
Now, addressing your configuration. When you say made incorrectly, not dimpled, with a secondary jacket that is dimpled above it?? Does above it mean outside it?? Is the dimple plate clamped over the original non-dimpled jacket? Is the original jacket integral, or clamped on? Did you use heat transfer mastic between any clamped jacket to the surface to which you want to transfer heat?
And then, as I understand your explaination, you flow your hot water/gyclol mixture first into the original jacket, at the top, and then to the secondary. If this is understood correctly, the hot fluid in the original jacket will transfer heat both ways. It will transfer some heat into the kettle, and cool off in the process, and then as it flows through the secondary jacket, it will pick up some heat from the primary jacket, reheating itself, (at the kettle's expense) until you see the inlet and outlet temperatues being equal as you state.
You have to pipe this so that your heat is always flowing form the hottest to the coldest. Right now you have a competition between where the coldest is for the heat in the primary jacket to flow to. When the oil is cool, the kettle wins. When the oil is warmer, the secondary jacket wins. Early in the process, the secondary jacket is acting like a heated insulation blanket. The heat flows more easily to the oil, and your transfer rate is good to the oil. Later, the oil is hot, and the heat would rather flow back into the secondary jacket, with it's superior HTC than the oil in the kettle. What little heat that is going into the kettle is being lost to the atmosphere at the top.
That is just the way I see it.
rmw
The factor of time is not in play here as I see it.
And, remember, what is in the kettle is losing heat to the surrounds at the surface by the same processes.
Therefore anything you can do to have an effect on the temperature of the glycol, (probably not very doable, or you would have done it) or the turbulence in the vessel or the jacket will help you over the hump. That might involve stirrer speeds, and/or pump rates for the glycol.
Plus, do you actually need 50/50 water/glycol. As pointed out by the poster above, water glycol mixtures are not as good a heat transfer agent as pure water. True pure water has the highest specific heat, but the automotive world has to accept and design around the limitations of antifreeze mixtures so that we can park our cars outside in the wintertime. (A problem ignored in the temperate climates.)
If you have no freeze problems with your coolant, why not reduce the ratio of glycol to water. The specific heat of the fluid increases as you go towards pure water. Undoubtedly you are using a mixture for some reason, probably a combination of freeze protection and corrosion protection. Investigate changing the mixture.
Now, addressing your configuration. When you say made incorrectly, not dimpled, with a secondary jacket that is dimpled above it?? Does above it mean outside it?? Is the dimple plate clamped over the original non-dimpled jacket? Is the original jacket integral, or clamped on? Did you use heat transfer mastic between any clamped jacket to the surface to which you want to transfer heat?
And then, as I understand your explaination, you flow your hot water/gyclol mixture first into the original jacket, at the top, and then to the secondary. If this is understood correctly, the hot fluid in the original jacket will transfer heat both ways. It will transfer some heat into the kettle, and cool off in the process, and then as it flows through the secondary jacket, it will pick up some heat from the primary jacket, reheating itself, (at the kettle's expense) until you see the inlet and outlet temperatues being equal as you state.
You have to pipe this so that your heat is always flowing form the hottest to the coldest. Right now you have a competition between where the coldest is for the heat in the primary jacket to flow to. When the oil is cool, the kettle wins. When the oil is warmer, the secondary jacket wins. Early in the process, the secondary jacket is acting like a heated insulation blanket. The heat flows more easily to the oil, and your transfer rate is good to the oil. Later, the oil is hot, and the heat would rather flow back into the secondary jacket, with it's superior HTC than the oil in the kettle. What little heat that is going into the kettle is being lost to the atmosphere at the top.
That is just the way I see it.
rmw
Why not put in a couple bottles of Redline's 'water wetter'.
If the problem is heat transfer, this will help but if not it will not hurt as it is designed for aluminum radiators and should not corrode your new equipment. Use statistics to track the results, because you can not understand where you are going no matter how much you know about where you want to go without knowing where you have been. Also look into stabilized paraffin heat transfer fluid.
If the problem is heat transfer, this will help but if not it will not hurt as it is designed for aluminum radiators and should not corrode your new equipment. Use statistics to track the results, because you can not understand where you are going no matter how much you know about where you want to go without knowing where you have been. Also look into stabilized paraffin heat transfer fluid.
I would also ask from experts dealing with vegetable oils and their lipids, whether the canola's relatively high content of unsaturated fatty acids, when exposed to heat and air may induce a sudden chemical change, at about 140 deg F, that may markedly reduce its individual HTC, by, for example, forming an adhesive film on the warm wall of the kettle ?
As a sequel to my previous post, the individual HTC for the canola oil-side is very strongly dependent on the Re number. Anything that may affect the Re would reflect on the heat transfer.
Would you consider increasing the rotating speed of the agitator ?
- Thread starter
- #19
Gentleman,
Thank you all for your assistance. Right now I am waiting on a response as to how the system is working.
I am also taking your advice and looking into the Re number of the canola oil and the pump suction.
To clarify the jacket situation. The original jacket was not dimpled. The glycol/water flows from the bottom of the jacket to the top, then into the bottom of the dimpled jacket and out the top. The dimpled jacket is above (vertically) the original jacket and attached to the kettle itself. It is not over the original jacket.
We did increase of the speed of the agitator on the canola oil, but I do not know the specifics.
As for increasing the glycol/water ratio, we have not done this. Although the system is pressurized, I do not know at point the water would boil. Does anyone know how to calculate what the water boiling point would be under pressure? I did find a couple of things on the net, but they are obviously wrong.
Thank you all for your assistance. Right now I am waiting on a response as to how the system is working.
I am also taking your advice and looking into the Re number of the canola oil and the pump suction.
To clarify the jacket situation. The original jacket was not dimpled. The glycol/water flows from the bottom of the jacket to the top, then into the bottom of the dimpled jacket and out the top. The dimpled jacket is above (vertically) the original jacket and attached to the kettle itself. It is not over the original jacket.
We did increase of the speed of the agitator on the canola oil, but I do not know the specifics.
As for increasing the glycol/water ratio, we have not done this. Although the system is pressurized, I do not know at point the water would boil. Does anyone know how to calculate what the water boiling point would be under pressure? I did find a couple of things on the net, but they are obviously wrong.
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