Centrifugal Fan

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Chapter 10. 10.1. 10.2. 10.3.

FANS

INTRODUCTION FAN PRESSURES IMPELLER THEORY AND FAN CHARACTERISTIC CURVES 10.3-l. The centrifugal impeller 10.3.1.1. 10.3.1.2. 10.3.1.3. 10.3.2. Theoretical pressure developed by a centrifugal impeller Theoretical characteristic curves for a centrifugal impeller Actual characteristic curves for a centrifugal fan

The axial impeller 10.3.2.1. 10.3.2.2. 10.3.2.3. Aerofoils Theoretical pressure developed by an axial impeller Actual characteristic curves for an axial fan

10.4.

FAN LAWS 10.4.1. 10.4.2. Derivation of the fan laws Summary of the fan laws

10.5.

FANS IN COMBINATION 10.5.1. 10.5.2. Fans in series Fans in parallel

10.6.

FAN PERFORMANCE 10.6.1. Compressibility, fan efficiency and fan testing 10.6.1.1. 10.6.1.2. 10.6.2. The pressure-volume method The thermometric method

Fan specifications

APPENDIX A10

Derivation of isentropic temperature-pressure relationships

10-l

10.1.

INTRODUCTION

A fan is a device that utilizes the mechanical energy of a rotating impeller to produce both movement of the air and an increase in its total pressure. The great majority of fans used in mining are driven by electric motors, although internal combustion engines may be employed, particularly as a standbye on surface fans. Compressed air or water turbines may be used to drive small fans in abnormally gassy or hot conditions, or where an electrical power supply is unavailable. In Chapter 4, mine fans were classified in terms of their location, main fans handling all of the air passing through the system, booster fans assisting the through-flow of air in discrete areas of the mine and auxiliary fans to overcome the resistance of ducts in blind headings. Fans may also be classified into two major types with reference to their mechanical design. A centrifugal fan resembles a paddle wheel. Air enters near the centre of the wheel, turns through a right angle and moves radially outward by centrifugal action between the blades of the rotating impeller. Those blades may be straight or curved either backwards or forwards with respect to the direction of rotation. Each of these designs produces a distinctive performance characteristic. Inlet and/or outlet guide vanes may be fitted to vary the performance of a centrifugal fan. An axial fan relies on the same principle as an aircraft propeller, although usually with many more blades for mine applications. Air passes through the fan along flowpaths that are essentially aligned with the axis of rotation of the impeller and without changing their macro-direction. However, later in the chapter we shall see that significant vortex action may be imparted to the air. The particular characteristics of an axial fan depend largely on the aerodynamic design and number of the impeller blades together with the angle they present to the. approaching airstream. Some designs of axial impellers allow the angle of the blades to be adjusted either while stationary or in motion. This enables a single speed axial fan to be capable of a wide range of duties. Axial fan impellers rotate at a higher blade tip speed than a centrifugal fan of similar performance and, hence, tend to be noisier. They also suffer from a pronounced stall characteristic at high resistance. However, they are more compact, can easily be combined into series configurations and may have their direction of rotation reversed, although at greatly reduced performance. Both types of fan are used as main fans for mine ventilation systems while the axial type is favoured for underground locations. In this chapter, we shall define fan pressures and examine some of the basic theory of fan design, the results of combining fans in series and parallel configurations, and the theory of fan testing.

10-2

10.2.

FAN PRESSURES

A matter that has often led to confusion is the way in which fan pressures are defined. In Section 2.3.2. we discussed the concepts of total, static and velocity pressures as applied to a moving fluid. That section should be revised, if necessary, before reading on. While we use those concepts in the definitions of fan pressures, the The relationships between the two are not immediately obvious. following definitions should be studied with reference to Figure 10.1 (a) until they are clearly understood.l

Fan total nressure. FTP, is the increase in total pressure, ptf (measured by facing pitot tubes) across the fan, (10.1) FTP = pt2 - ptI Fan velocity oressure, FVP, is the average velocity pressure the fan outlet only, pv2 = pt2 - ps2 &

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Fan static Pressure, FSP, is the difference between the fan total pressure and fan velocity pressure, or FSP = FTP - FVP (10.2) (10.3) = pt2 - pt1 - (Pt2 - Ps2) = Ps2 - Pt1

The reason for defining fan velocity pressure in this way is that the kinetic energy imparted by the fan and represented by the velocity pressure at outlet has, traditionally, been assumed to be a loss of useful energy. For a fan discharging directly to atmosphere this is, indeed, the case. As the fan total pressure, FTP , reflects the full increase in mechanical energy imparted by the fan, the difference between the two, i.e. fan static pressure has been regarded as representative of the useful mechanical energy applied to the system. Notwithstanding that historical explanation, it is important to remember that in subsurface ventilation planning, it is total pressures that must be used to quantify frictional pressure drops in airways. Accordingly, it is fan total pressures that should be employed in ventilation network exercises. The interpretations of fan pressures that are most convenient for network planning are further illustrated on Figure 10.1. In the case of a fan located within an airway or ducted at both inlet and outlet (Figure 10.1 (a)), the fan static pressure, FSP, can be measured directly between a total (facing) tube at inlet and a static (side) tapping at outlet. A study of the diagram and equation (10.3) reveals that this is, indeed, the difference between FTP and FVP. Figure 10.1 (b) shows the situation of a forcing fan drawing air from atmosphere into the system. A question that arises is where to locate station 1, i.e. the fan inlet. It may be considered to be

10-3

Figure 10.1 Illustrations of fan pressures.

Ps2m

2 - 0 fan ptl42

-

?i2 Fvp= P2

(a) Fan with inlet and outlet ducts.

FSP

(3) Fan and inlet unit in a forcing system.

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-

1

O

-

.

0

2

FTP and FSP

FVP=

pV2 = 0

(c) Fan and outlet unit in an exhausting system.

(i)or

immediately in front of the fan

(ii) at the entrance to the inlet cone (iii) in the still external atmosphere

These three positions are labelled on Figure 10.1 (b). If location (i) is chosen then the frictional and shock losses incurred as the air enters and passes through the cone must be assessed separately. At location (ii) the fan and inlet cone are considered as a unit and only However, if the shock loss at entry requires additional treatment. location (iii) is selected then the fan, inlet cone and inlet shock losses are all taken into account. It is for this reason that location (iii) is preferred for the purposes of ventilation planning. Figure 10.1 (b) shows the connection of gauges to indicate the fan pressures in this configuration. The same arguments apply for a fan that exhausts to atmosphere If the outlet station is taken to be in the still (Figure 10.1 (c)). external atmosphere then the fan velocity pressure is zero and the fan In this configuration total and fan static pressures become equal. the fan total (or static) pressure takes into account the net effects of the fan, frictional losses in the outlet cone and the kinetic energy loss at exit. During practical measurements, it is often found that turbulence causes excessive fluctuations on the pressure gauge when total pressures are measured directly using a facing pitot tube. In such cases, it is preferable to measure static pressure from side tappings and to add, alaebraicallv, the velocity pressure in order to obtain the total pressure. The mean velocity can be obtained as flowrate Particular care divided by the appropriate cross-sectional area. should be taken with regard to sign. In the case of an exhausting fan, (Figure 10.1 (c)) the static and velocity pressures at the fan inlet have opposite signs. Another practical problem arises when fan manufacturers publish characteristic curves in terms of fan static pressure rather than the This is fan total pressure required for ventilation planning. understandable as such manufacturers may have no control over the types of inlet and outlet duct fittings or the conditions at entry or exit to inlet/outlet cones. Where fan velocity pressures are quoted then they are normally referred to a specific outlet location, usually either at the fan hub or at the mouth of an evasee. It is clear that care must be taken in using fan manufacturer's A simple guide characteristic data for ventilation network planning. for main surface fans is that it is the total pressure of the airflow on the inbve (system) side that should be employed for network exercises. This takes into account cone and entry/exit losses.

10-4

10.3.

IMPELLER THEORY AND FAN CHARACTERISTIC CURVES

An important aspect of subsurface ventilation planning is the specification of pressure-volume duties required of proposed main or booster fans. The actual choice of particular fans is usually made through a process of perusing manufacturer,s catalogues of fan characteristic curves, negotiation of prices and costing exercises (Section 9.5). The theory of impeller design tha