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larly careful to use the full cylindrical contents
of the fan as the volumetric efficiency will be abnormally
high on account of not using the full
cylindrical contents. All fans at low or no resistance
will put-out cent per cent, of their displacement
volume every minute, or every revolution
when run at proper speed, but in Guibal fans
proper and their many modified forms the diminution
of out-put is an inverse ratio somewhat
greater than the increase in the water gauge or
lesistance.
THE MECHANICAL EFFICIENCY
of the fan is obtained by carefully ascertaining the
power developed by the engine driving it, on the
one hand, and the power given out by the fan in
delivering air in the mine and overcoming resistance
on the other hand. The proportion these
two powers bear to one another is the mechanical
efficiency. Thus it is we have a plant in which
the engine developed 20 horse power and the fan
it drives gives 60,000 cubic feet of air per minute;
at one and 3-10 inch water gauge, this will give
a mechanical efficiency of 62.35. The indicated
horsepower of the engine is generally used in the
calculations of the fan's duty and gives results
against the fan by the amount of power used to
overcome the resistance of the engine itself, and
in the best constructed fans by the additional
power lost in transmission.
"One very important point in obtaining the
manometrical efficiency of the fan, is the placing
and reading of the water gauge and the notation of
other necessary observations at a point not less
than 20 feet from the inlet, if an exhaust fan and
whether an exhaust or blowing fan it should be so
placed that the pipe leading to the gauge be out
of the flow of the air, and not within the influence
of an eddy. Many eminent German authorities
place the gauge pipe in a recess and place over its
mouth several thicknesses of flannel in order to
obtain the true statistical gauge. Long pipes
and pipes of small diameter leading from fan drift
to gauges placed in offices or at distant points are
not to be recommended as friction or leakage may
render them unreliable. Under no condition
should a water gauge be placed upon the fan case.
A difference of as much as 400 per cent, has been
noted from this cause: the gauge showing higher
reading the nearer it is placed to the center. This
remark applies more particularly to single inlet
fans."
The manometrical efficiency of fans is found by
the gauge obtained in a preceding formula divided
into the gauge read, thus, by calculation a 10"
water gauge is obtained, and we note a difference
of 6 inches in the legs of the water gauge, then
the result obtained would show a manometrical
efficiency of 60 6-10 per cent.; manometrical ef­
THE COAL TRADE BULLETIN.
ficiency is high in many fans which are low in mechanical
efficiency and still lower in volumetric
efficiency.
The practical application of the law and formula
enumerated above is to enable us to intelligently
select a fan for the duty to be performed, to blow
equal quantities through equal areas in equal
times. The pressure varies inversely as the
fourth powers of the diameters of the orifices, and
to prove the statement, let the quantity be 150,000
cubic feet of air per minute, and let the pressure
for an orifice 10 feet in diameter be 3 inches of
water gauge, then the pressure per square foot
required to blow the same volume of air per minute
through an orifice 5 feet in diameter is equal
to the fourth power multiplied by three inches of
water gauge, equaling 448 inches water gauge.
To obtain the best results from the ventilating
fan the depression necessary for the entry of the
air should, if possible, not exceed one pound per
square foot; hence the velocity should
NOT EXCEED 18 FEET PER SECOND.
for by formula where V equals velocity and P equals
pressure, the square root of the pressure multiplied
by 18 equals the velocity in feet per second;
then by using quantity the port of entry may be
found by the following formula.
Where Q equals quantity and D equals diameter
of the port of entry, .0343 a factor proven by experiments,
we have the following equation:
.0343 multiplied by the square root of the quantity
equals D. or in other words, the square root of the
quantity multiplied by .0343 equals the diameter of
the port of entry.
Then by the square of the volume obtained and
the resistance by which it is obtained, we are enabled
to calculate the dimensions of the fan we
want and what speed we must drive it to obtain
a desired larger volume. For the benefit of the
practical members present who have not had the
opportunities of an education, we will go through
the steps required to determine tip speed and
increased quantities. Suppose we have a fan
producing 72,000 cubic feet of air per minute at
1.75 inch water gauge, and we want to increase
that volume to 200,000 cubic feet per minute. Our
first step would be to find the water gauge due to
the increased volume. This may be found by the
square of the quantities and we find that if it required
1.75 inch water gauge to produce 72,000
cubic feet per minute, it will require
200
2x1.75=13.5
72
water gauge to produce 200,000 cubic feet per minute,
all other conditions remaining the same. In
our next formula we will show how to find the tip
speed from the water gauge. By an equation we