Ground Fog from Heat Exchanger

[Rhodrons]

I have a large vaporiser (heat exchanger) that is used to supply oxygen gas to a facility from a Liquid oxygen tank. It is an ambient air vaporiser which I understand draws heat from a downdraught of air passing through the vaporiser by convection. My problem is that we are using so much gas that the air is cooling below dewpoint and causing ground fog around the vaporiser. Although it looks nice and surreal, the fog is moving across a patch of open ground and then spilling into a dip in the road under a nearby railway bridge. Needless to say the affected motorists are not finding it a pleasing experience suddenly hitting a bank of fog! Can anyone suggest some sources of information where I can find out how to overcome this problem? One option would appear to be to provide extra heatload to the vaporiser, another would be the use of a fan to drive a larger volume of air through the vaporiser. Obviously(?) such additional energy would only be needed on days when the relative humidity was such as to cause fog.

 

[pmover]

some thoughts to ponder . . .

by natural means, the fog will migrate to lowest point in surrounding area - railway bridge.

perhaps a barrier or some device to disperse the fog upstream of railway bridge can be installed. any source of turbulent air in area to help disperse the fog . . .

any possibility to use "conditioned" air with less moisture for the vaporizer - just a thought. . .

interesting situation . . .

this situation is very similar to the ice fog here in the AK interior. can be nasty . . .

good luck!
-pmover

 

[StoneCold]

Rhodrons
There is a power plant in Green Bay Wisconsin that has a similar problem. Only it is fog from the cooling tower that condenses on a raised road near by and coats it with ice. Nice. You might go with a fan or break down and install an electric heater. Control the heater so that it comes on when the ambient temperature is low or humidity is high. It will be a nice break for your vaporizer allowing it to thaw out as well. Probably just as cheap to add the heater as the fan when you look at the total lifecycle costs.

Good luck.

 

[Rhodrons]

Thanks for the responses. I am sure I recall seeing a TV programme some time ago in the UK. It described how a village in a particularly dry part of the world had rigged up screens of some sort on the top of a hill to "catch" the hill fog that occurred on frequent occasions. I seem to remember that the screens were made of an absorbent fabric so as the mist moved down the hill the water was caught on the fabric. A series of gutters and then pipes routed the water down the hill to the village. I don't think I'm making it up but it strikes me that such a permanent solution may perhaps work if only I could find the details. Can anyone help?

 

[Montemayor]

Rhodrons:

The TV program you refer to was a NOVA (public TV in the USA) episode which featured the engineering application of surface condensation on the arid, coastal slopes of Northern Chile. The coast of Peru and Chile are notoriously dry (the Sechura desert is drier than the Sahara!) although they are constantly exposed to cold, saturated air that originates in the cold Humboldt Current that flows in front of these coasts. The winter in Lima, Peru for example, consists of approximately 50 oF and 100% humidity. The challange to South American engineers has always been to efficiently irrigate the mineral-rich coastal lands and the Chileans have succeeded in finding a Polymer sheet screen application that condenses the water from the saturated air and the product is used to irrrigate. I seriously doubt you will succeed in trying to employ the same technique for your purposes.

Your situation stems from the basic engineering theory that each favorable process has its trade-off. You are applying an ambient evaporator to generate gaseous O2 from liquefied Oxygen - without paying the price for use of a heating fluid. Your trade-off is the "fog" (100% saturated air) temporarily created due to its sub-cooling. I have installed many of these evaporators in Hospitals and clinics using piped-in medical Oxygen and the resultant "fog" obtained on humid, wintery days was a price well worth the while when compared to the cost of electrical or water energy when using forced air or circulating water.



Art Montemayor
Spring, TX

 

[EdStainless]

Install larger vaporisers. Spread the heat (cold?) load out so that the air isn't cooled as much.
The only other easy solution is to use heat tapes or fans, both will cost you a bunch in the long run. And you will need a control system to sense when there is a risk of fog to turn them on.

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Corrosion never sleeps, but it can be managed.

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[25362]

Fogs form when the heat-transfer rate is greater than the mass-transfer rate. If possible, heating a bit the foggy stream until it is not visible any more (ie, to bring it just below its critical supersaturation) may be a not-so-expensive solution. Nothing comes gratis.

 

[Rhodrons]

OK, so let's see where we go from here! We actually have four vaporisers that cycle in pairs to allow the ice to melt between cycles. One idea I have is to put all four into simultaneous service if the weather conditions are such as to cause fog with just two in service. My reasoning here is that the temperature drop of the cooling air will be less (halved?) if more vaporisers are in use.

Does anyone know how to go about calculating the temperature drops you can anticipate in the cooling air that passes through an Ambient Air Vaporiser. It seems to me that there must be a theoretical limit to the heat transfer capability that will be related to the structure and materials of construction. Where would I look to find some worked examples?

Anyway thanks for helping me along so far!

 

[25362]

As long as the temperature difference, [Δ]T, between warm moist air and cold pipe, as the driving force, remains constant, the ratio of heat transfer to mass transfer rates also keeps unchanged, and the fogging effect wouldn't disappear.

A way should be found to reduce the heat transfer rate, for example, by reducing the [Δ]T or the OHTC, or, alternatively, to bring down the moisture content of the circulating air.

If, as proposed by Rhodrons, more surface (in parallel) is made available for heat exchange, assuming the same air movement, heat transfer would be enhanced, which is the opposite of what is wanted.

Kindly correct if I'm wrong.

 

[EdStainless]

My thought concerning adding area is that you want to use more air, so that the air isn't cooled as much.
More volume, less dT, less fog.

The toatl amount of heat transfered will stay constant. This is set by the gas demand.

As I think about this, it would seem that you want more surface area, but with lower heat transfer capability, such as smaller fins.
As you know once ice forms there is a lot less heat transfer, maybe this can be used to your advantage.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm