Hybrid Car – More Fun with Less Gas

Looking for some guidance for my own power generation - Page 2

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Posted by daestrom on February 11, 2009, 1:09 am
Ken Maltby wrote:

Keep in mind too that 'hot oil' will not be as hot as an open flame.  This
means a given size boiler will probably not develop as much steam for a
given size when 'fired' with just hot oil instead of a fire.  May have to
look at a much larger boiler to get the output the OP decides he needs.


Posted by Ken Maltby on February 11, 2009, 3:57 am


  I would think that you can look at both a Fire Tube Boiler and
a Water Tube Boiler as a heat exchanger.  With the traditional
designs, the exchange is between a hot gas (the fire) and the water
that we are trying to turn into steam.  If we replace the hot gas
with a hot oil/liquid, we should be able to make a few
optimizations and simplifications (we don't have to worry about
maintaining the conditions needed to support the combustion of
the fule, for instance).  Not that you could totally make up for a
large difference in specific temperture.  Still it might not be as big
an issue, as you might expect.  I don't know for sure, of course
but that would be something that could be established in the
modelling/ experimentation.  Properly designing the heat
exchanger/boiler to have the right amount of exchange surface
and hot oil flow, waterjacket/pressure vessel volume, superheater
area, ect is most likely going to have to be aproached as a new
discovery or rediscovery.  I doubt you will find any source for
the art/design of such steam generating hot oil based exchangers.

  It might be that a water tube based design could work better
than the fire tube approach.  It might make a better "flash
boiler" with less of the pressure vessel issues.


Posted by daestrom on February 11, 2009, 11:52 pm
 Ken Maltby wrote:


Indeed.  Water tube designs are used for almost anything more than a couple
hundred psi.  Old fashioned steam locomotives were fire-tube and never
operated more than about 300 psi (UP's "Big Boy" was a 300 psi boiler).
Higher pressure marine and stationary boilers are water-tube.

The question I was raising was the amount of heat transfer one could get.
With a much lower temperature difference between water side and 'fire side',
it would be a lot lower.  OTOH, hot oil probably has a higher thermal
conductivity than hot flue gases.  But again, it's undoubtedly more viscous,
meaning a thicker boundary layer on the heat transfer surface.  In my
experience, it is the boundary layers on each side of the surface that
make/break heat exchanger design.

I just wouldn't get an 8 hp fire-tube boiler and expect to be able to get 8
hp using hot oil in the fire tubes instead of a real fire.  I'm sure there
is some data out there for using various oils in a heat exchanger, so it
should be possible to get an estimate of the true capacity of such a design.


Posted by vaughn on February 12, 2009, 2:00 am

This immediately brings to mind the steam generator designs of a typical
pressurized water reactor (PWR).  In that case, the hot water (around 500
degrees average temp) runs through the tubes because the pressure of the
water is much higher than the steam pressure.

The boundary layer that you mentioned would only be an issue on the oil
side, because the steam bubbles tend to scrub away the boundary.  (If I
remember properly after more than 40 years)


Posted by Ken Maltby on February 12, 2009, 3:02 am

  From what I've read, there are a number of specially engieneered
fluids that are classed for use as a "hot oil" in various manufracturing
processes and some even tailored for thermal solar collection systems.
Some are not vary viscous.  They all seem to be designed to transfer
their heat easily, they are also called "Heat Transfer Fluids".


  The collector performance I was envisioning would provide such a
"hot oil" at ~750 F but could be tuned up to around 1200 F., not
that I would want to go so high.  I think that ~750 F is close to a
sweet spot in terms of materials handling and the cost of high temp.
rated equipment.

  The pressure generated in the water tube boiler as the steam is
generated would not be contained to any extent.  The greatest
build up of pressure would be at the initial turbine spin up, after
that there would be much less restriction.  The design factor for
the strength of the tubes would relate to that.  Unless it turns out
that loading on the turbine has a greater effect  (but I was
envisioning the need to gear down the turbine speed providing
plenty of torque).

  I haven't tried to make any of this yet so this is all just conjecture
on my part.


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