4. Introduction
to Equivalent
Orifice
Mechanical definition:
An expression of fan performanc
e as the theoretical sharp-
edge orifice area which would of
fer the same resistance to flowas
the system resistance itself.
The equivalent orifice is a
conception which gives a
convenient visual presentation
of the relative ease of a
ventilating through a mine
opening.
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5. Cont….
The equivalent orifice of a mine is defined
as;
An opening A in a thin plate through
which same amount of air passes under
the effect of a pressure difference
between the sides of the plate, as would
pass through the mine opening under the
same head.
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6. Determination of area of equivalent
Orifice
Let us assume;
Q (m3 /sec ) is the quantity of air flowing through the orifice.
A (m2 ) is area of orifice [fig 8.8]
Let p1 be the pressure on the left side of the plate
And p2 be the pressure on the right side of
the plate (Kg/m2)
So mine head in that case is given as .
h = p1 - p2
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7. Determination of area of equivalent
Orifice
For section 1-1,
As it far away from the surface so v1 is neglected
i.e. V1 =0
And for section 2-2,
At a point in the stream, where stream lone is parallel,
We will use Bernoulli’s equation i.e.
p1 + (v1
2/2g ) ϒ = p2 + (v2
2/2g) ϒ --------- (1)
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8. Determination of area of equivalent
Orifice
Now , as we know that.
V1 = 0
Equation (1) becomes,
p1 - p2 = h = v2
2 ϒ /2g
And consequently,
v2 = 2𝑔ℎ/ϒ m/sec
Now for quantity flowing through section 2-2, We know
Q = v2A2
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9. Determination of area of equivalent
Orifice
Where, A2 is the cross section of stream in plane 2-2
But A2 = φ A where,
φ = the coefficient of discharge which on average has
a value of 0.65
Thus,
A = A2 / φ
=> Q/ φ. V2
=> Q/ φ. 2𝑔ℎ/ϒ
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10. Determination of area of equivalent
Orifice
=> (1/0.65 19.62/1.2 )(Q/ ℎ)
=> 0.38Q/ ℎ
The dimension of the equivalent orifice are;
[A] = Q / 𝑔ℎ/ϒ (sqr.meter)
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11. Result;
The simple dimension of the equivalent orifice. Sqr.meters
enables the value of the orifice to visualize very simply and
thus to judge the degree of ease or difficulty of ventilating a
colliery or metal ore mine.
The larger the value of A, the easier it is to ventilate
For metal ore mines,
If Equivalent Orifice Area A = 1 sq m, the mine mine will
narrow
If A = 1 or 2 sq m, it is medium mine
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12. Results cont….
If A > 2 sq m, it means the mine wide or large.
So all we need to do is fine out the mine head h and quantity
of air flowing the through the mine.
As the equation shows .
A is directly proportional to the quantity of air Q flowing
through the mine.
And Area A is inversely proportional to the sqr.root of mine
head h.
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