[Gasification] heat exchanger / boiler design book

[Mark Ludlow]

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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