[Gasification] heat exchangers and gas cooling.

[Ken Calvert]

O.K. you young fellers!  when it comes to gas cleaning and cooling its
already been done, by emperical methods!
     So how about someone providing the mathmatical modeling of this
situation?
    The I.I.T., in New Delhi, back in the early eighties  produced an
intregu'ing combination of cyclone gas cleaner and cooler by direct contact
of  hot gas and cooling water.  The top plate of the cyclone, as well as
taking the gas exit pipe from its centre was fitted down below the top edge
of the body casing to make an open topped tank which was kept full of water.
Around the edge of the top plate there were a series of
curved slots, which allowed a thin film of water to flow down the inside
surface of the cyclone and exit from the bottom outlet.  The inlet gas,
flowing in at right angles to the laminar flow conditions of the water,
continued to flow tangentially around the cyclone, with a prolonged and 
intimate contact with the film of water which extracted most of the
solid particles and heavier tars as well as most of the heat.  The
centrifugal force keeps the water film intact as a smooth film on the walls 
of the cyclone and flowing down through the outlet at the bottom of the 
cone.  The only down side is that even with filtration and constant 
recycling you still end up with a lot of dirty water,
loaded with PAHs and all.  With a bit of detergent added, it looks and 
smells just like the old "Jayes Fluid" in its useable form of a dirty milky 
emulsion.  The detergent helps in removing the tars.  O.K. Just keep the 
discussion going!.  Seasons Greetings . Ken Calvert.


--- Original Message ----- 
From: <gfwhell at aol.com>
To: <mark at ludlow.com>; <gasification at listserv.repp.org>
Sent: Sunday, December 30, 2007 2:02 PM
Subject: Re: [Gasification] heat exchanger / boiler design book and vortex


>
> Mark
> How would you explain the performance of gasses in a Hilsch Vortex?
> It would seem that the gas under the influence of centrifugal force
> behaves in a similar way as any fluid, the denser heavier molecules are
> thrown to the outside. These molecules would be cooler and more densely
> packed, the inner core of the Rotating column being hotter.
> The idea of being able to sort the hot from the cold in the gasifier
> output sounds rather appealing.
> There seems to be no definitive explanation of how this Vortex works?
>
> GF
>
>
>
>
> -----Original Message-----
> From: Mark Ludlow <mark at ludlow.com>
> To: 'Discussion of biomass pyrolysis and gasification'
> <gasification at listserv.repp.org>
> Sent: Tue, 25 Dec 2007 10:58 pm
> Subject: Re: [Gasification] heat exchanger / boiler design book
>
>
>
>
> Toby,
>
> "Swirling fins" do not increase residence time. Mass flux remains
> constant.
> They increase the velocity of the gases which increases their Reynolds
> Number which increases the probability that flow will be turbulent.
>
> Observe the difference in velocity profiles between laminar and turbulent
> flow. The velocity profile for laminar flow is parabolic, meaning that the
> core of the flow-stream (portion of greatest mass transfer per unit of
> time)
> has much higher velocity than the "tails" of the parabola. Therefore, much
> less heat is transferred to this part of the flow stream because it has a
> short residence time (and it is further removed from the heat-transfer
> surfaces).
>
> Fully-developed non-laminar flow streams have a flatter velocity profile.
> This is because there are fluid molecules moving in all directions at once
> and the velocity profile described is a two-dimensional average of these
> velocities. In general, higher fluid velocities imply higher rates of heat
> transfer. The faster the velocity, the thinner the boundary layer through
> which heat transfer that is purely conductive becomes.
>
> Higher velocity fluid streams also tend to result in less fouling of heat
> exchange surfaces.
>
> Mark
>
> -----Original Message-----
> From: gasification-bounces at listserv.repp.org
> [mailto:gasification-bounces at listserv.repp.org] On Behalf Of Toby Seiler
> Sent: Tuesday, December 25, 2007 6:24 PM
> To: list at sylva.icuklive.co.uk
> Cc: gasification at listserv.repp.org
> Subject: Re: [Gasification] heat exchanger / boiler design book
>
> Andrew,
>
>  Please clear up something for me.  Not long ago there was a discussion
> about heat exchanger design that I did not understand the principles
> members
> put forth.  Specifically I have believed that minimization of the boundary
> layer effect would increase the heat transfer, know to me as wiping the
> surface.  Your forth element, surface texture, would have a large effect
> as
> would flow, (ranging from laminar to turbulent) were increased or
> decreased
> (given other parameters remained the same).
>
>  I think the argument was made that turbulent flow transferred better.
> That's what I didn't understand.  I have always thought that laminar flow
> over the surface were more efficient at the transfer and that more
> molecules
> would be moved if laminar low were maintained.  Please clarify that for me
> as a general principle, if you have it at hand.
>
>  For example;  the fire tubes of a boiler will often contain swirling fins
> that cause a circular or spiraling flow.  Both residency time and surface
> contact are extended and the centrifugal force is a factor.  The wiping
> action reduces the boundary effect and also extends the flow path, giving
> more contact and transfer.  Flow would be laminar however.  It has long
> been
> my belief that making turbulent flow led to hot spots (much increased
> boundary layer in places), less flow and ultimately less system
> efficiency.
> This has led me to believe that a cyclone and heat exchanger combined
> would
> be efficient, especially with hot gas heat being transfered to a wood
> fuel.
>
>
>  Have a good New Year!
>
>  Toby Seiler, Seilertechco
>
>
> ---------------------------------
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