Page 101 - Advanced Mine Ventilation

82 Advanced Mine Ventilation

Expressing the head into inches of water gauge,

WV 2 t
H ¼ (6.3)
5:2g
where W is the density of air and g (acceleration due to gravity) ¼ 32.2 ft/s 2

6.1.2 Backward Bladed Fans

Blade angles for backward bladed fans seldom exceed 45 degrees. In the velocity
diagram, q ¼ 90 þ q.
Hence,

WV t Q
H ¼ V t cot q (6.4)
5:2g A
if q ¼ 45 degrees; cot q ¼ 1.00

WV t Q
H ¼ V t
5:2g A
6.1.3 Forward Bladed Fans
Here, q is an acute angle. It seldom exceeds 60 degrees.
Hence,

WV t Q
H ¼ V t þ cot q (6.5)
5:2g A
It is to be noted that for the same diameter, speed and air flow; forward bladed fans
develop the most pressure and the backward bladed fans the least.
An example:
Given the following conditions, calculate head developed by each of the above type
blades in a centrifugal fan:

Q ¼ 50,000 CFM; n (rpm) ¼ 500; D (diameter) ¼ 8 ft, Width ¼ 2 ft at outlet; q ¼ 45
degrees; W (air density) ¼ 0.075 lb/ft 3
Calculate:

P Dn
V t ¼ ¼ 209.4 ft=s
60

A ¼ P Db ¼ 50:26 ft 2