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Heat Transfer Fluid with the Free Convection of Water

10/15/2013 7:43 PM

Looking for a heat transfer fluid with the fast free convection currents of water and a specific heat of 1/2 that of water. Easy enough to find the second requirement but not the first. Have found a fluid with slower free convection currents than water but too slow for my application. It must also have the usual anti-oxidation and non-corrosive properties of heat transfer fluids.

If anybody has come across such a fluid please let me know what it is and who supplies it.

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#1

Re: heat transfer fluid with the free convection of water

10/15/2013 7:55 PM
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#12
In reply to #1

Re: heat transfer fluid with the free convection of water

10/16/2013 10:38 PM

No I haven't looked at fluorosilicone fluids lyn. Who would be a manufacturer of these?

I probably should have mentioned that to get good free convection you need a low viscosity fluid. I see that some of the replies seem to misunderstand the meaning of viscosity. High viscosity means a high resistance to flow like honey and also most oils. There is a scale of measurement "cst" in which water has a viscosity of 1.

I have found a heat transfer fluid with a viscosity of 1.3 at 50c and a specific heat 1/2 that of water. I want this low specific heat so that I can use less heat energy. The problem I found with this fluid was that the convection currents were slower than with water and therefore did not support a fast dissipation of heat.

Therefore I am searching for a htf which supports the efficient convection currents of water but also has a specific heat 1/2 that of water. I have come close to it with the above mentioned fluid but not quite close enough for 1 of my applications.



Ideally the fluid you recommend will also have been already tested for its efficient free convection properties. Please note that free convection does not mean that the htf is circulated by artificial means such as pumps. The circulation is by convection currents only as in a pot of water on a heating element.

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#13
In reply to #12

Re: heat transfer fluid with the free convection of water

10/16/2013 11:00 PM

I understand viscosity.

I don't understand exactly what you are trying to do.

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#14
In reply to #12

Re: heat transfer fluid with the free convection of water

10/16/2013 11:39 PM

"I want this low specific heat so that I can use less heat energy."

'use less heat energy' is not at all clear! You say you want low viscosity so natural convection currents will flow, yet you want to transfer less heat energy?

If you want to transfer less energy, then a higher viscosity fluid of the same or lower heat capacity will do just that!

Just what do you mean by use?

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#17
In reply to #14

Re: heat transfer fluid with the free convection of water

10/17/2013 11:43 PM

I tried to give you a clearer explanation but the system has not accepted my reply for some reason.

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#18
In reply to #17

Re: heat transfer fluid with the free convection of water

10/18/2013 12:20 AM

CR4 currently does not allow more than 5 or 10 minutes to submit an answer. They supposedly are working to correct the problem, but until that happens, do one of the following:

1. Compose and edit your reply in some form of word processing program, such as µsoft Word. Once it is finished, Select All (Control-A, on a PC; Command-A on a Mac), Copy (Control-C, on a PC; Command-C on a Mac), then go back to your browser and press the Reply button, click in the edit window, and Paste (Control-V, on a PC; Command-V on a Mac), then click Preview and finally Submit.

2. Press the Reply button as usual and compose and edit your reply. When you are finished with the reply, Select All and Copy as above, then click the page back arrow in your browser (more than once, if necessary) until you can again see the Reply button. Click Reply a second time, Paste your text, then press the Preview and Submit button.

WOW! I didn't have to follow the above to submit this. Perhaps they've fixed it...?

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#21
In reply to #18

Re: heat transfer fluid with the free convection of water

10/18/2013 5:04 PM

NO they haven't, everytime I try typing in a long question or answer( more than 2 paragraphs) I can't post....very frustrating!

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#22
In reply to #21

Re: heat transfer fluid with the free convection of water

10/18/2013 5:05 PM

Before trying to post it, copy it to the clipboard.

Like I remember to do that every time.

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#19
In reply to #14

Re: heat transfer fluid with the free convection of water

10/18/2013 6:22 AM

I will give you an example that should clarify this for you. I know its not much fun being left in the dark on some things.

This is a test I did with water and a heat transfer fluid( I don't think I am allowed to mention the fluid as it might be construed as advertising) with a viscosity of 1.3 and specific heat 1/2 that of water. Water in each case is 1 . I used 750ml and heated it with a heating element of 460 watts power for 2 minutes. The energy used is watts x time and this is what my electricity bill is for from the electricity supplier. The results for water and the unnamed fluid were;

start temperature end temp

water 18.7c 35c 35c
unnamed fluid 16.6c 57.9c

The result shows that for a fluid with 1/2 the specific heat of water the energy used is the same as with water but the temperature of the htf is raised much higher than water. So if you use the htf for a heating application this is a good way to save power.

The only problem I encountered with the htf was that the free convection currents were slower than with water so that in one of my applications the dissipation of the heat was faster than the convection currents could keep up with.

I therefore came on to the forum to see if an engineer has come across a htf in which the free convection currents are equivalent to water. The makers of these fluids don't seem to test their fluids for this property. As several makers have told me they usually design their fluids for forced circulation with a pump.

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#20
In reply to #19

Re: heat transfer fluid with the free convection of water

10/18/2013 11:46 AM

No, they have not fixed the timeout problem!

∆T1 = 35.0°C - 18.7°C = 16.3C°.

∆T2 = 57.9°C - 16.6°C = 41.3C°.

41.3C°/16.3C° = 2.53:1 ratio.

Yet you say you have a 1:1/2 or 2.0:1 specific heat ratio. Since the temperature ratio is higher than the specific heat ratio, some other factor must be involved.

I suspect this is the result of doing the experiment in an unstirred and uncovered container, while leaving the thermometer tip sitting on the bottom of the container. The higher viscosity of the (probably glycol) fluid will make it less efficient at carrying heat away from the heating element, and the lower evaporation rate of the mixture will reduce losses at the surface, both leading to a higher indicated temperature.

You could achieve the same higher temperature difference by reducing the quantity of water from 750ml to 296ml. This might involve finding (or fabricating) a container with a closer match to the object(s) being heated, and/or placing some blocks to fill voids. Chunks of PTFE (Teflon) would be close to ideal as fillers.

Since you are concerned about the energy used, I presume this process will either be continued or repeated extensively. In either case, the cost of good insulation will have a short ROI time.

Finally, pumping is not the only method of increasing circulation. Placing the heating element off-center will automatically increase convection, although it may cause an uneven heat distribution in the object being heated. Many laboratory heaters include a rotating magnetic field so a Teflon-coated magnet placed in the bottom of the container will spin and stir the fluid.

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#2

Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 8:40 PM

Well it's going to depend on what temperature range we're talking about....Just about any light oil would fit your given specifications as stated....If by convection currents you're talking about viscosity, then just about any oil will become more viscous than water above 50ºC...

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#3
In reply to #2

Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 8:45 PM

More?

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#4
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Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 9:02 PM

Well it would also depend on the flow characteristics....Working environment.....

have I gone too far?

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#5
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Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 9:20 PM

No. Maybe I don't understand:

"then just about any oil will become more viscous than water above 50ºC..."

Isn't "oil" already more viscous than water?

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#6
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Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 9:45 PM

Depends on the weight of the oil and the temperature of each, generally speaking if they are at the same temperature then yes.....the exception would be a heavy oil (40w+)at a low temp.....say close to 32ºF....

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#7
In reply to #6

Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 9:58 PM

You've lost me.

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#8
In reply to #7

Re: Heat Transfer Fluid with the Free Convection of Water

10/15/2013 11:06 PM

Oh sorry, I got that backwards....need some sleep I guess.....

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#11
In reply to #4

Re: Heat Transfer Fluid with the Free Convection of Water

10/16/2013 10:02 AM

have I gone too far?

Nope..... farther......

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#9

Re: Heat Transfer Fluid with the Free Convection of Water

10/16/2013 3:14 AM

There are several heat transfer fluid suppliers across the planet. Please list the ones that have been contacted without success, so the forum can list a few more without the premise of them becoming an advertisement..

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#26
In reply to #9

Re: Heat Transfer Fluid with the Free Convection of Water

10/20/2013 8:05 PM

The manufacturers of heat transfer fluids I have tried are Duratherm and Paratherm. The Duratherm XLT50 I have tried with samples from this firm is getting close to the fast free convection currents of water but not quite close enough for 1 of my applications. XLT50 has a specific heat of 0.495 of water at 30c and a viscosity of 1.9 at 30c.

A contributor from CR4 has suggested that it may be not be as good as water regarding its free convection currents because its density may be lower than water. It turns out that this is the case as its density is 0.832 of water.

Paratherm has told me that they know the free convection currents of their htfs and none are as efficient as water. Therefore I will take your advice and ask the forum for further suppliers of htfs across the globe. It was difficult even to get the XLT50. I live in Australia and there were no suppliers here. I had to import from America.

Looking forward to help from the forum

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#10

Re: Heat Transfer Fluid with the Free Convection of Water

10/16/2013 5:58 AM

How about supercritical steam?

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#15

Re: Heat Transfer Fluid with the Free Convection of Water

10/17/2013 9:42 AM

Most hydrocarbons have specific heat about 1/2 of water. Acetone, ethanol, ethylene glycol would seem like some candidates (gasoline if you're sporty). Some of them you might could fine-tune by mixing with water (or some other fluid). I'm not sure about their convection currents compared to water as that is dependent of Δ density/Δ temperature, viscosity and other factors.

I'm having trouble mentally visualizing a heat transfer device where you want the heat transfer fluid to control the operation of the device.

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#16
In reply to #15

Re: Heat Transfer Fluid with the Free Convection of Water

10/17/2013 8:42 PM

The equations of convection don't care about the specific heat.. What you are looking for is a factor called "heat transfer coefficient" or simply h with comes out as a result of the solution these equations using the boundary layer scaling analysis.. The specific heat comes into play when you are worried about transient heat conduction but I can see from your problem, you are worried about steady state.. So I think you can safely use water...

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#23
In reply to #15

Re: Heat Transfer Fluid with the Free Convection of Water

10/19/2013 1:23 AM

THE HEAT TRANSFER FLUID I USED WAS FROM A COMPANY WHERE ALL THEIR HEAT TRANSFER FLUIDS HAD A SPECIFIC HEAT OF ABOUT 1/2 THAT OF WATER SO PERHAPS THEY ARE ENGINEERED FROM HYDROCARBONS AS YOU SUGGEST. THEY HAVE THEIR OWN NAMES FOR EACH PRODUCT BUT THERE IS NO MENTION OF THE PRODUCTS YOU MENTION.

DO YOU KNOW IF I AM ALLOWED TO MENTION THE PRODUCT I HAVE USED? I HAVE NO AFFILIATION WHATSOEVER WITH THE COMPANY SO I CAN'T SEE HOW IT CAN BE CONSTRUED AS ADVERTISING. ALTHOUGH I THINK I MENTIONED THE PRODUCT NAME IN A POSTING AND THE POSTING WAS REJECTED. IT MAY HAVE BEEN REJECTED BECAUSE OF THE TIME LIMIT AS I CAN'T SEE THE SOFTWARE OF CR4 BEING THAT GOOD.

I PICKED THE LOWEST VISCOSITY OF ALL THEIR PRODUCTS AS I KNOW VISCOSITY IS ONE OF THE FACTORS IN FREE CONVECTION. I KNOW THAT I HAVE CHOSEN THE RIGHT PROPERTY WITH LOW VISCOSITY BECAUSE I AM GETTING CONVECTION CURRENTS BUT NOT AS FAST AS WATER. PERHAPS EFFICIENT WOULD BE A MORE APPROPRIATE WORD.

THE HEAT TRANSFER FLUID DOES NOT CONTROL THE OPERATION OF THE DEVICE BUT IS PART OF A HEAT EXCHANGER.

THE COMPANY DOES SAY THAT THEIR FLUIDS ARE DESIGNED FOR FORCED CIRCULATION SO PERHAPS I SHOULD BE LOOKING FOR FLUIDS ENGINEERED SPECIFICALLY FOR FREE CONVECTION. HOWEVER THERE DOES NOT SEEM TO BE ANYTHING IN THIS CATEGORY OF HEAT TRANSFER FLUIDS.

I AM ABOUT TO SUGGEST TO THE COMPANY THAT THEY TEST THEIR RANGE OF HTFS AGAINST THE STANDARD OF THE FLUID I HAVE TESTED TO SEE IF ONE OF THEIR FLUIDS HAVE MORE EFFICIENT CONVECTION CURRENTS. I HAVE DEVISED A TEST THEY CAN USE. HOWEVER I DON'T KNOW IF THEY WILL DO THIS. I HAD TO GET THE 2 FLUIDS I TESTED FROM AMERICA. THE COMPANY DID NOT CHARGE ME FOR THE SAMPLES BUT I HAD TO PAY $117 IN POSTAGE CHARGES.

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#24
In reply to #23

Re: Heat Transfer Fluid with the Free Convection of Water

10/19/2013 8:29 AM

Sure, you can mention the product you used. I suspect you didn't get as much convection because the density is probably lower than water. Alcohols tend to have higher density, and may be more readily available. But they are flammable!

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#25
In reply to #24

Re: Heat Transfer Fluid with the Free Convection of Water

10/19/2013 9:21 PM

The product was duratherms xlt50. specific heat of 0.495, viscosity of 1.9 at 30c but a density of 0.832 of water. I have suspected the same, the density is lower than water.

I have listed the properties of all of duratherms htfs and their duratherm s has a higher density of 0.956 but unfortunately a much higher viscosity of 37.5 times water.

So the search is still on.

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