Re: GAS-L: More about steam efficiencies

["Jim Bland" <enecon@ozemail.com.au>]

Peter,

For your analysis of the spec for your 8-stage steam turbine, your heat in
is too high.  You need to subtract the enthalpy of steam turbine condensate,
as this is returned to the system.  This is 93 Btu/lb.  So heat in is 42,530
x (1389 - 93) = 55.1 million Btu/h (16 145 kW - note: not kWh) So your
efficiency is 3750/16145 = 23.2%.  Who is the manufacturer of this turbine -
it looks pretty good?

For your second question, the enthalpy of the turbine exhaust is not 1116
Btu/lb, because the exhaust is wet.  From the information provided by the
turbine manufacturer, I calculate an isentropic efficiency of 70.3% (pretty
good for a small turbine, and quite achievable with 8 stages), and that the
exhaust steam will be 97.4% dry, and will therefore have an enthalpy of 1089
Btu/lb.  Work extracted is then 1389 - 1089 = 300 Btu/lb.  Efficiency is
then work extracted/net heat in = 300/(1389-93) = 23.1%

The difference is probably due to rounding somewhere.

When are you guys going metric?  These Btus drive me nuts.  Thank goodness
my steam properties software can do the conversions.

Regards,

Jim Bland
Enecon Pty. Ltd.
Melbourne, Australia

 ----- Original Message ----- From: Peter Singfield <snkm@btl.net> To:
<gasification@crest.org> Sent: Thursday, March 16, 2000 1:04 PM Subject:
GAS-L: More about steam efficiencies


> > Picking up where I left off --
> >
> > We have steam of 1500 psi, 1000 F, 1491 btu/lb "in" and 1 psi, 101.74F,
> > 1106 btu/lb "out".
> >
> > Question -- what is the ultimate thermal efficiency???
> >
> > OK:
> >
> > To make that quality steam the boiler invests 1491 btu for every pound
of
> > steam of that quality produced.
> >
> > The cooling tower has to absorb 1106 btu/lb to achieve a vacuum of 1 psi
> > (absolute) pressure.
> >
> > 1491 - 1106 = 385
> >
> > 385/1491 = 25.8%
> >
> > Pitiful but true! 25.8% is all the "heat" that is available in a form of
> > energy that can be used to make mechanical power. So even at 100%
> > mechanical efficiency -- one cannot get better than 25.8% efficiency in
> > converting heat invested in making steam of those conditions to power
> > coming out.
> >
> > The "mechanical" efficiency of the device put in between boiler and
> > condenser will be power out over btu in.
> >
> > In this case -- power is electricity.
> >
> > An example: (from one such plant I am working on now)
> >
> > ITEM I
> >
> > One (1) eight stage  steam turbine designed for 400 PSIG, 750oF steam
> inlet
> > and 4" Hg abs. exhaust.  The turbine will produce 3,750 KW with 42,530
PPH
> > steam flow and a turbine speed of 4,750 RPM.  The turbine will be
complete
> > with NEMA "D" governor, Gimbel T&T valve, labyrinth gland seals,
> insulation
> > and lagging, vibration probes, bearing RTD's and magnetic pick up.
> Turbine
> > complies with NEMA SM23 standard.  Turbine shall be equipped with a
single
> > inlet valve. Turbine inlet shall be 6"/600# and turbine exhaust shall be
> > 36"/125#.
> >
> > OK -- using these figures supplied by the manufacturer --
> >
> > 400 PSIG = 415 psi Absolute at 750F = 1389 btu
> >
> > So 42,530 PPH steam of this quality = 42,530 * 1389 btu =
> >
> > Total of 59,074,170 "in"
> >
> > Converting this to KWH's  -- that is divide by 3414
> >
> > 17,303.5 KWH's in and  3,750 KWH's out.
> >
> > Gross efficiency = 21.6%
> >
> > We have two types of mechanical efficiency numbers. One is the
> theoretical.
> > The other is after subtracting process "costs". A large process cost
would
> > be operating the cooling tower -- for example.
> >
> > This is not over all plant efficiency. That is heat in over power out.
> This
> > is just engine/turbine efficiency.
> >
> > But what confuses me in the above example is this.
> >
> > We are given an exhaust vacuum of "4" Hg abs."; which is 1116 btu/lb of
> > condensing.
> >
> > So: 1389 btu in - 1116 btu out = 273 btu
> >
> > 273/1389 = 19.65%
> >
> > Yet the manufacturer is claiming, what in effect, works out to 21.6%
> > efficiency! The turbine has to be a little bit of a perpetual motion
> machine!
> >
> > So where have I gone wrong??
> >
> > Now -- if anyone can find errors in the above logic -- please point them
> out!
> >
> >
> > Peter in Belize
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> >
>
>
>

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