In the last issue we talked about how volume and static pressure requirements for primary and booster fans are determined. The next step is to select the type of fan.
General guidelines that have been around in the industry recommend that for a fan operating with a Static Pressure (SP) of 8¨ w.g., an axial seems to provide the best value. The design is inherently simpler and as it is in line with the ducting there will be some advantages with installation.
When the pressures tend to go higher ~ 18 inches or greater, a centrifugal is the better selection as it can more easily develop higher static pressures. In between cases can be made for either fan.
Other technologies such as mixed flow fans also have their place, especially when considering capital cost or resistance to erosion.
When a fan’s performance is measured in a test lab, it is under idealized conditions. The entrance to the fan will be preceded by a straight length of duct 10 times the diameter of the fan’s impeller diameter and a smooth inlet bell. These optimal design characteristics for the ducting allows the air streamlines to approach the fan as laminar flow leading to the best fan performance. Generally though, in the field we often have restrictions on real estate and compromises are made. Generally the rule of thumb is that at least six diameters upstream be straight without any changes to diameter.
The same rules also apply to the fan discharge. Any abrupt change such as an elbow can have highly detrimental effects.
For a centrifugal fan, selecting the discharge and rotation of the fan to eliminate additional changes in duct direction will help reduce pressure losses. If you need to use an elbow, the use of a turning vane helps to maintain the velocity profile while mitigating pressure losses.
In order to develop useful static pressure, a fan rotates and therefore expels the air at a much higher velocity than will eventually travel down the mineshaft. In most fan literature, Fan Total Pressure is specified. Fan Total Pressure is the sum of the fan’s Velocity and Static Pressure.
However useful work is achieved by the static pressure, not the velocity pressure and the velocity pressure must therefore be converted to static pressure. To do this fan engineers will use a discharge cone (or evase) to gradually allow the velocity pressure to be converted into static pressure. A well-designed evase avoids turbulence that creates pressure losses while it slows down the air.
A well-designed evase can save 10% of the energy of the fan.
Where the air goes from there we will cover in the next article.