Determination of fan characteristics.
From the fan testing facility provided in the mine ventilation laboratory, development of total, static, and fan efficiency curve, for the axial flow fan for the given operating speed.
Fan rating tests are conducted for the determination of fan characteristics. The rating test aims at finding out the fan static and total pressures, power input to the fan and total efficiency of the fan for various flow rates at the rated speed so that the fan characteristics can be drawn.
The standard test setup existing in the laboratory confirms to the AMCA, USA recommendations. The necessary measurements are the
Ø Static and velocity pressures at the duct at the traverse section
Ø The RPM and the power input to the fan
Ø Barometric pressure and the dry wet bulb temperatures on the path of inflowing air, and dry bulb temperature at the traverse section.
The characteristic curves are drawn for
ü Fan total pressure
ü Fan static pressure, and
ü Fan and motor efficiency as a function of fan inlet quantity.
The fan total pressure, which is the total pressure at the fan outlet, is an algebraic summation of the fan static pressure and the velocity pressure. The fan total pressure is obtained by adding the (friction) pressure loss in the intervening duct, and in the straightener, to the static and average velocity pressures measured in the duct. If Pvo is the average velocity pressure in the duct, then the friction pressure loss in the duct (P1) is given by:
P1= 0.02 * (L/D) Pvo
Where L is the length of the duct from the fan outlet to the traverse section and D is the duct diameter. The pressure loss in the straightener (P2) is given by:
P2 = 0.08 Pvo
The fan velocity pressure is given by:
Pv = Pvo (A0/A)2
where A0 and A refer to the cross section areas at the duct and fan outlet respectively.
The fan inlet quantity Q is found by:
Q = (r0/r) * A0 * (2 * Pvo/r0)1/2
The density of the air in the duct (r0) is related to the inlet air density (r) as follows:
r0 = r * (T/T0) * ((B+ Ps)/B)
where T and T0 are the dry bulb temperatures at the fan inlet and in the duct respectively, B is the barometric pressure and Ps is the static pressure measured in the duct Pa. For fan pressures below 1 kPa, the may be assumed to be constant.
The input power to fan is equal to the input power to the motor multiplied by the motor efficiency. Air power is the product PQ, where P is the fan total pressure.
The fan efficiency = Air power/ Fan input power
The fan RPM is measured with a Stroboscope to within a limit of 1 % and all measurements of quantity, pressure and power are converted to rated RPM using the following relations:
Quantity a speed
Pressure a speed2
Power a speed3
AMCA standard fan testing setup, Askania minimeter, inclined tube manometer, pitot static tube, wattmeter, Stroboscope, Assmann Psychrometer, Aneroid barometer, Scale and calipers.
(1) at the traversing section in the duct plane the pitot static tube with the nose at center, facing the air in the air stream
(2) To the Pitot tube connect the Askania to measure velocity pressure, and inclined tube manometer to measure the static pressure. Level these two instruments and take their initial reading.
(3) Make wattmeter connections and start the fan. Use the symmetrical conical throttle provided at the duct outlet to vary the quantity in the duct.
(4) For each throttle bottle measure the
ü Motor input power
ü Fan RPM
ü Static and velocity pressures at the traverse section
(5) Measure the L and D values, obtain wet and dry bulb temperatures and barometric pressures at the fan inlet. Obtain a representative of dry bulb temperature inside the duct.
The average velocity pressure in the duct (Pvo) is obtained by multiplying the central velocity pressure measurement with the method factor computed in the other experiments. Fom the relationships explained in the “introduction” section compute fan total, and static pressures, fan quantity, and fan efficiency values for each throttle condition. Plot the pressure and efficiency values as functions of quantity.