LECTURE – 1
THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS:
1.0 BASIC TYPES OF FANS USED FOR VENTILATING
UNDERGROUND MINES
2.0 CENTRIFUGAL FAN
2.1 Eye in a Centrifugal Fan
2.2 Single Inlet and Double Inlet Centrifugal Fans
2.3 Types of Blades
2.4 Evasee and Scroll/Volute
2.5 Classification of Centrifugal Fans
2.5.1 Based on intake and discharge
2.5.2 Based on location of drive/motor
3.0 Axial Flow Fan
3.1 Classification Based on Attachment of the Blade
3.2 Types of Blades
REFERENCES
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1.0 BASIC TYPES OF FANS USED FOR VENTILATING UNDERGROUND MINES
Fans are used in mines to cause flow of air. They are basically of two types:
-
Centrifugal Fan
-
Axial Flow Fan
It is important to note that, centrifugal fans deliver low quantity but at higher head
whereas axial flow fans deliver higher quantity at lower head.
2.0 CENTRIFUGAL FAN
The word centrifugal means “moving away from the center”. Anything including air,
if revolved, will have the tendency to leave the center. There are two rings in the
impeller of a centrifugal fan. In between these two rings are the blades of the
centrifugal fan. The rings are connected to the shaft by means of spokes. Fig. 1
shows the line diagram of a centrifugal fan with various details.
2.1 Eye in a Centrifugal Fan
When a centrifugal fan is rotated, air is drawn parallel to the shaft into the open
ends of the impeller which is called the eyes. It is then thrown out in a radial
direction through the blades.
2.2 Single Inlet and Double Inlet Centrifugal Fans
Air can enter the impeller of a centrifugal fan either from one side or from both the
sides. When air enters the impeller from one side only, then it is called single inlet
fan. If the air enters the impeller from both the sides, then it is called double-inlet
fan.
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Fig. 1 Schematic line diagram of centrifugal fan (after Hartman et al.,
1982)
2.3 Types of Blades
The blades of the centrifugal fan can be either laminar or of aerofoil shape. Each of
these can be again either radial, backward inclined or forward inclined (Fig. 2).
Sometimes, centrifugal fans also have arrangement for making variation in the
blades tip-length to have flexibility of performance. In this type of arrangement, to
the existing fixed blades, extensions of different lengths can be bolted.
When a centrifugal fan is rotated in a direction opposite to that of its actual
rotational direction, air will still enter at the eye and leave in a radial direction.
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Therefore the flow of air is same as before (actual direction of rotation). However,
there is reduction of air quantity and efficiency.
Fig. 2 Different types of centrifugal fans (after Le Roux, 1972)
2.4 Evasee and Scroll/Volute
A centrifugal fan has an evasee. There is a casing around the impeller of centrifugal
fan which is shaped like a spiral curve. This is called the scroll or volute of the fan.
The point where the scroll is nearest to the impeller is called the cut-off.
Sometimes, centrifugal fans are fitted with adjustable inlet control vanes. It is
possible to change the angle of these vanes so that the fan output changes.
2.5 Classification of Centrifugal Fans
2.5.1 Based on intake and discharge
It is important to note that in case of centrifugal fans, the air is turned through 90˚
on passing through the centrifugal fan. This may increase the amount of space
required for its installation. Based on the intake and discharge, different variants of
centrifugal fan are:
-
Top Horizontal Discharge
-
Bottom Horizontal Discharge
-
Top Vertical Discharge
-
Bottom Vertical Discharge
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2.5.2 Based on location of drive/motor
Sometimes, centrifugal fans are also classified based on the location of drive/motor
as:
-
Left Hand Drive
-
Right Hand Drive
When a person stands behind the fan and face the direction in which the air is
discharged, then in left-hand drive fan, the motor is on the left and in case of righthand drive, the motor is on the right.
The fan can sometimes be direct-driven i.e., the motor shaft is in line with and
directly connected to the fan shaft. But in most of the cases, it is indirectly driven
through gears, belts, clutches etc.
3.0 AXIAL FLOW FAN
It consists of a shaft with a hub (Boss). A number of blades are attached to the
hub. The salient constructional features of axial flow fan is shown in Fig. 3.
Fig. 3 Basic components of axial flow fan (after Le Roux, 1972)
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In case of axial flow fan, the flow of air is parallel to the shaft or axis. That’s why its
name has come as axial flow fan.
In case of axial flow fan, the air is given a twist because of which the air leaves the
impeller with a spiral motion. To remove this spiral motion of air, and also to
improve fan efficiency, a set of stationary blades is installed on either the inlet or
outlet of the impeller. We call these fixed blades as inlet or outlet guide vanes.
Sometimes it is observed that we have two or three impellers in a fan, one behind
the other. Each of these impellers have their own set of guide vanes. We call such
fans as two stage and three stage fans respectively. Sometimes, we have two
impellers, driven by separate motors and they are made to revolve in opposite
direction. In such cases, guide vanes are not required. We call such fans as “contrarotating fans”.
Axial flow fans are also provided with casings. However, this is not the case with
small fans used in rooms. The casing has three sections:
-
A cylindrical section round the impeller
-
An inlet section provided with a bell type of mouth or cone type to reduce the
entrance losses
-
A diverging outlet section or diffuser or evasee to reduce shock losses by
allowing the air to slow down gradually
For improving the efficiency of the fan so that the flow of air is smooth through it,
the axial flow fan is provided with a short snub-nosed inlet fairing towards the inlet
side and a long pointed outlet fairing towards the outlet side of the fan.
3.1 Classification Based on Attachment of the Blade
Based on the attachment of the blades, they are classified as:
a. Fixed blades – In this case, the blades are permanently attached to the hub
at a certain angle.
b. Adjustable pitch blades – In this case, the angle of the blades can be
changed by loosening the nuts and screws at their base.
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c. Variable pitch blades – In this case, the angle of all the blades can be
changed with a suitable mechanism while the fan is rotating.
3.2 Types of Blades
The blades of axial flow fan are of two types:
a. Laminar blades – The fan blades are made from flat steel plates.
b. Aerofoil blades – The fan blades are casted into aerodynamic shape.
In case of aerofoil type of blade, the leading edge is rounded and thick whereas the
trailing edge is thin and sharp.
If the direction of rotation of an axial flow fan is reversed, the direction of air
current will also be reversed. But in this case, the trailing edges of the blades
become leading edges. Also, the evasee and outlet fairing are on the inlet side. Due
to this reason, the fan will handle less air and will also be less efficient.
REFERENCES
Banerjee S.P. (2003); “Mine Ventilation”; Lovely Prakashan, Dhanbad, India.
Deshmukh, D. J. (2008); “Elements of Mining Technology, Vol. – II”; Denett & Co.,
Nagpur, India.
Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (1982); “Mine Ventilation and Air
Conditioning”; John Wiley & Sons, New York.
Le Roux, W. L. (1972); Mine Ventilation Notes for Beginners”; The Mine Ventilation
Society of South Africa.
McPherson, M. J. (1993); Subsurface Ventilation and Environmental Engineering”;
Chapman & Hall, London.
Misra G.B.
(1986); “Mine Environment and Ventilation”; Oxford University Press,
Calcutta, India.
Vutukuri, V. S. & Lama, R. D. (1986); “Environmental Engineering in Mines”;
Cambridge University Press, Cambridge.
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