CENTRIFUGAL BLOWER TEST RIG
Aim: To conduct a test on a centrifugal blower and determine its efficiency. Description: Blowers are used to discharge high volumes of air at low pressures and are used in blast furnaces, cupolas, mines, air-conditioning plants, drying plants, etc. The test blower is a single stage centrifugal type driven by an electric motor. Air is sucked from the atmosphere by the rotating impeller through the inlet. Due to the centrifugal action of the impeller, kinetic energy is imparted to the air and it exits the impeller with high velocity. The air then passes through the spiral casing, where a portion of the kinetic energy is converted into pressure energy before it comes out through the outlet. The pressure rise across a blower is small and is typhically measured in cms of water column. Hence the air can be treated as incompressible as it flows through the blower. The experimental set up consists of a centrifugal blower directly driven by a 5 HP motor. At the blower outlet, a buterfly valve is used to control the discharge. An orificemeter is fixed in the outlet pipeline to measure the actual discharge. A sets of pitot tube and thermometer is provided at the outlet to measure the velocity and temperature. The pitot-tube can also be used to measure the velocity profile at the blower inlet. U-tube manometers are provided to measure the pressure difference across the orificemeter, in the pitot tubes and the delivery pressure. An energy meter is provided tom calculates the input to the blower. Three types of interchangeable impellers - impellers with radial vanes, backward curved vanes, and forward curved vanes are provided with the est rig to study the effects of different vane types.
Altech Industries, Coimbatore 641 006
Experimental Procedure: 1. Ensure the manometers connecting the orificemeter, outlet pressure tap and pitot tubes contain manometric fluid (water) upto half-way mark. 2. Close the delivery valve completely 3. Start the motor 4. Open the valve slowly and for various stages of opening observe the following manometer readings: a. Delivery pressure manometer readings - h1 and h2 cm of water b. Orificemter manometer readings - h3 and h4 cms of water c. Energy meter reading for 10 revolutions.
Altech Industries, Coimbatore 641 006
Calculations: a. Delivery pressure - one limb of the manometer is open to atmosphere and the other limb connected to the delivery side pressure tap. If h1 and h2 are the water column levels, Delivery pressure = (h1 - h2) cm of water column Ha = 861x(h1-h2)/100 m of air column = 8.61x(h1-h2) m of air column Where, 861 is the ratio of densities of water and air. b. Discharge - The two limbs of the manometer are connected to the upstream and downstream pressure taps of the orificemeter. If h3 and h4 are the water column levels in the manometer, Pressure drop across orificemeter dh = (h3-h4) cm of water Actual discharge of air Q = K.(dh)0.5 cu.m/sec Where orificemeter constant K = 0.0743 Note: Q = (Cd.A.B2 ) ((2gH )/ (1-B4))0.5 Here, Cd = Orificemeter discharge coefficient = 0.62 A = delivery pipe area (0.131m dia)= 0.01348 sq.m B = orificemter diameter ratio = 0.75 g = 9.81 m/sq.sec H = 861x(h3-h4)/100 = 8.61xdh m of air column c. Output of the Blower: Output = (Ha x Q x D )/102 KW ( D = density of air - 1.162 kg/cu.m at 30 degC) d. Input to the blower: Energy meter constant N Time for 10 revolution Input to blower Efficiency of motor Motor output Blower input
f. Velocity measurement with pitot tube: Rotate the pitot tubes such that the probe is aligned in the flow direction. This can be verified by observing the water in the manometer limbs - the difference is maximum when the probe is aligned in the flow direction. This difference in water column, gives the dynamic pressure of the air stream directly and velocity is calculated. If 'dp' is the difference in cms of water column in the manometer, then Velocity V = (2xgxdpx1000)/(100x1.16) m/sec. = 13.0 dp m/sec g. Temperature rise across the blower: The thermometer fixed at the outlet of the blower displays the blower outlet temperature. Inlet temperature is the room temperature and the difference between the two is the rise in temperature of the air across the blower. Inlet temperature = T1 deg C Outlet temperature = T2 deg C