In addition to the information yesterday which gave the efficiency of a
steam pumping engine but excluded boiler efficiency (ie efficiency from
steam to mechanical effort) - I can add the following on the efficiency of
a typical boiler (ie efficiency from fuel to steam).
A modern oil fired shell boiler has a typical efficiency of about 76% at
full load (based on measurements I have made in the past). The main
losses in a modern boiler are from waste heat in flue gases and account for
18%. Shell radiation losses, blowdown losses are relatively low.
Most books on boilers and fuels tell you how to measure efficiency but do
not give typical figures.
A fairly inefficient oil-fired Lancashire boiler operating at full load
measured at the same period had an efficiency of around 45% and I believe I
have seen expected efficiencies quoted of under 60%.
If the boiler is working at less than full load, the constant heat loss
factors will be the same and the efficiency will reduce, particularly for a
low efficiency boiler.
In a Lancashire or Cornish boiler the flue gas losses are high in that
there is poor heat transfer to the boiler shell by virtue of the design.
Also, draught is often manually controlled and depends very much on the
skills of the boilerman. Coal boilers can be expected to be 5-10% less
efficient at best because of heat losses in ash and added difficulty of
controlling draught, particularly with hand firing. Also coal requires
a larger excess air than oil to achieve good combustion (again on a modern
boiler 10% oxygen excess against 4-5% for oil or 2% for gas. Multiply
by 5 to get approx. excess air).
I'm not sure that a single tube Cornish boiler would be very much different
in efficiency from a twin-tube Lancashire. The main differences would be
in the design of the flues which recirculate smoke around the outer parts
of the boiler shell and would be a result of a particular manufacturer's
design.
Based on the above, I would expect a Cornish, coal-fired boiler with hand
stoking and manual draught control to have an efficiency of around 40%.
If the efficiency of the pumping unit was about 20%, the overall
efficiency of the assembly would be 0.4 X 0.2=0.08.
Richard Smith
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