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Air in thermal fluid bitumen tank coil 2

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Process Mike

Process Mike

Industrial
Joined
Feb 10, 2020
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Recently , a plant manager was suspecting that trapped air in thermal fluid bitumen tank coil could significantly impact the heat transfer performance . He might be right depending the air amount inside the heating coil :

DATA : 2 in sch 80 , A 106 coils , 500 ft lenght , butt welded with many 180 U bends to acheive the bottom tank coil
Inside coil : Therminoil heating fluid at 375 - 400 F , fluid speed 2 ft / sec , pressure 20 psig

My idea was to increase thermal fluid coil velocity ( by increasing flow) from 2 ft/s to 8 ft / sec
At this speed , any air bubbles inside the coil will be entrained into the flash tank , then air will be eliminated
We performed a bench test with water pipe and it is OK ... NO REMAINING AIR inside 2 in . pipe

MY QUESTION :
is there any empirical formula that could give me a minimal fluid speed to respect in order to make sure air bubles will be entrained out the coil , to the flash tank ? ( including viscosity, sp gravity , pipe diameter ect ...)

I read a LOTS of stuff on many forums , but found NOTHING ....

Thanks for your recommendation
 
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The usual recommendation is 1 m/sec to remove air even from any vertical down flow.

There is a lot of info on bubble flow and I'll see if I can find my research text on this.

Also I saw something recently that ASHRAE produce for hvac systems where you have sinister issues and they recommended min 0.6 m/sec for water.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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Gilles Corcos in his Air In Pipes manual recommends a Froude Number of 0,64.

Daniel
Rio de Janeiro - Brazil
 
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@ Littleinch ==== 0.6 m/s is very low , I doubt it will eliminate air bubles in horizontal pipes ...
PLZ could you be more specific about where this recommendation comes from . I need to investigate more ...

@danschwind === FROUDE number is mostly used for open gravity flow like drainage or open channel . It is not used ( from what I read) in forced flow inside pipes at medieum to high velocity

Thanks for u,r feedback

STill looking ...
 
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If you use the Search Box at the top of this page you will find many discussions on Eng-Tips regarding "Froude".

Also, download the manual "Air in Pipes" by Gilles Corcos recommended by danschwind.

Katmar Software - AioFlo Pipe Hydraulics

"An undefined problem has an infinite number of solutions"
 
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@katmar === this manual is for gravity driven drinking water pipes = not for forced under pressure flow

Thanks ciao

Process Mike
 
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In my understanding, the Froude parameter is valid for both cases.

Daniel
Rio de Janeiro - Brazil
 
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@ProcessMike Yes, the particular application in this manual is drinking water, but the hydraulics are the same. The fluid does not "know" what is causing it to flow - so it makes no difference if the flow is caused by gravity or a pump.

Katmar Software - AioFlo Pipe Hydraulics

"An undefined problem has an infinite number of solutions"
 
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0.6 came from this post and download the link in the OP.

Try this also below.

There is a calcualtion vc = 1.23 SQR rt (gD sin theta). But do it even for vertical pipes and you get about 1m/sec (lower for smaller diameter pipes)



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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We are goin to make a real test tomorrow in the client' installation with water at different speed and see if we can correlate with empirical calculation

Thanks

 
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Coil piping should be continously sloping upwards from inlet to exit to initially prime the coils, else demolish and reconstruct to remove all pockets.
 
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@georgeverghese === demolition is not an option ( $$) . We can flush air ( once a year) with medium fluid speed . We try to calculate what spped would be optimal
 
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Final test done with 2 in pipe sch 80 A 106 , with many high points artificilally created on a bench test , we could fluh air out the system (the artificial coil )with fluid speed ( cold water ) around 1 meter / second .

This will give us good milestone to go ahead with our project , thanks all for your help

Process Mike
 
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Dear Process Mike,

Instead of demolishing, the coil supports could be modified. The swivel joints in the coil piping (if any) could be refurbished. But you don't seem to be taking a shutdown soon.

Regards.

DHURJATI SEN
Kolkata, India

 
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each coil is constructed with 25 pieces of 7 m long piping with U bends to have back and forth flow direction , (se description from the beginning of this post ) so if we slope in one direction we counter slope in the other ... this would be useless at the end ...
 
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Good to know what your results were. Thanks for the update and also that 1m/sec has been validated ([thumbsup2])

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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@Process Mike
Thanks very much for the feedback on the results achieved. It is very rare to get that type of feedback here, but it greatly adds to the value of the thread as a learning resource in future.

As a strong believer in the value of the Froude number in flow anlysis I would like to add a couple of comments that will hopefully reinforce what your practical investigation has proven.

The Froude number does not define specific flow regimes the way that the Reybolds number does, so it is not possible to give many specific guidelines. The most specific point that I am aware of is that for Fr < 0.31 in vertical downflow air bubbles are able to escape upwards against the liquid flow. This is important in the design of drains where air or vapor can become entrained in the liquid. As Fr increases above 0.31 some air bubbles will be entrained downwards, even in vertical downflow.

When Fr reaches 1.0 syphons form readily and bubbles are "rapidly" flushed out. (Ref: Larry Simpson, Chem Eng, June 17, 1968, Fig 12) Between 0.31 and 1.0 the rate at which bubbles are swept out increases, but there are no easily identifiable changes in flow pattern. In horizontal or sloped piping the recommendations in the literature vary quite widely. As mentioned above, Corcos recommends for horizontal piping a Fr of 0.64 to ensure that bubbles are swept out "reasonably quickly". For piping that includes high points and vertical sections a Fr of 1.0 or even 1.2 is probably a safe design point.

For your Sched 80 2" pipe, a velocity of 2 ft/s gives a Fr of 0.88 and a velocity of 1 m/s (3.3 ft/s) gives a Fr of 1.44. In your 500 ft long coil it will take between 150 and 250 seconds for the liquid to pass through. The air is going to be dragged along more slowly than the liquid so I would not be surprised if it took 20 minutes or more to flush all the air out. The timing will of course also depend on any high points and the general slope of the pipes.

Katmar Software - AioFlo Pipe Hydraulics

"An undefined problem has an infinite number of solutions"
 
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@katmar
thanks for very usefull information . We performed our test with cold water . Not to forget that real situation will be with thermal fluid with viscosity around 80 SSU ( 14 cSt) while visosity of water is around 30 SSU ( 1 cSt)
So internal liquid shear tension should create a force that will push air a little quicker than what to said ...
For sure we will be consevative but at least we have data to start working

Thanks a lot

Process Mike
 
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