Objective
1. The objective of this experiment is to obtain a performance and an efficiency curve of a
centrifugal fan.
2. To determine the functions and applications of centrifugal fan and rotary blower.
3. To be able to explain the performance test of centrifugal and rotary blowers.
4. To identify the parts and function of centrifugal and rotary blowers.
5. To identify the factors in selecting the types of blowers.
6. To develop professional work ethics including precision, neatness, safety and ability
to follow instruction.
Centrifugal Fan Parts
Centrifugal Fan Diagram
Experimental Set-Up
The experimental apparatus is shown in Fig. 1 and schematics of flow passage . Air is drawn
from the atmosphere through a centrifugal fan and passes through rectangular duct with
varying cross section and exits to the atmosphere through a thin plate orifice at the end of
the flow passage. Pressure of the air is measured at two locations: one near the fan and the
other at the orifice. Temperature of the air in the duct is also measured.
Figure 1. Experimental Apparatus
One dimensional energy equation between any two points in the flow passage is
𝑃1 𝑣1 2 𝑃2 𝑣2 2
+ + 𝑧1 + ℎ𝑠 = + + 𝑧𝐿 + ℎ𝑀 Eq(1)
𝛾 2𝑔 𝛾 2𝑔
Let the flow conditions of the atmosphere are denoted by subscript ‘1’, and the conditions
after passing through the fan by subscript ‘2’. Neglecting elevation change and all losses,
application of Eq. (1) to the air flow passing through the fan results in
𝑃2 − 𝑃1 𝑣1 2
ℎ𝑠 = + Eq(2)
𝛾 2𝑔
Multiplied by the specific weight of the air ( 𝛾 = 𝜌𝑔 ), Eq. (2) becomes
1
ℎ𝑠𝑦𝑠 = ∆𝑃𝑓𝑎𝑛 + 𝜌 𝑣𝐴1 2 Eq(3)
2
where the velocity is the average air speed at area A1 pressure rise is measured by a
manometer.
Eq. (3) shows that the energy added to a unit volume of air by the fan is the summation of
static pressure rise through the fan and the kinetic energy of air leaving the fan. The total rate
of energy added to the air by the fan is then
𝑃𝑓𝑎𝑛 = 𝑄 ℎ𝑠𝑦𝑠 Eq(4)
where Q is the volume flow rate of air. The volume flow rate is measured by a thin orifice
method.
The efficiency of the fan is defined by
𝑃𝑓𝑎𝑛
𝜂= Eq(5)
𝑃𝑜𝑤𝑒𝑟
where the ‘Power’ is the electrical energy supplied to the motor and is directly measured by a
watt meter.
The static pressure rise through the fan is measured directly by a manometer. The velocity is
however, measured indirectly using the volume flow rate obtained by a thin orifice attached
at the end of the duct
Applying Eq. (1) on the air stream passing through the orifice, we obtain the air flow speed at
the orifice (see Figure 2 for symbols.)
2 ∆𝑃𝑜𝑟𝑖𝑓𝑖𝑐𝑒
𝑣𝑜𝑟𝑖𝑓𝑖𝑐𝑒 = Eq(6)
√𝜌 [1−(𝐴𝑜𝑟𝑖𝑓𝑖𝑐𝑒)2]
𝐴2
where A2 is the duct area near the orifice and the pressure drop across the orifice is
measured by a manometer. Eq. (6) gives an ideal flow speed since all losses in the flow are
neglected. The orifice correction coefficient is obtained by experimental correlation in terms
of Reynolds number and diameter ratio,
91.71𝛽2.5
𝐶0 = 0.5959 + .0312𝛽2.1 − .184𝛽8 + Eq(7)
𝑅𝑒 0.75
The diameter ratio is defined by
𝑑 𝑜𝑟𝑖𝑓𝑖𝑐𝑒 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟
𝛽= = Eq(8)
𝐷ℎ ℎ𝑦𝑑𝑟𝑎𝑢𝑟𝑖𝑐 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑑𝑢𝑐𝑡
Reynolds number is based on the flow conditions in the duct area A2
𝜌 𝑣𝐴2
𝑅𝑒 = 𝐷ℎ Eq(9)
𝜇
Where flow speed in the larger duct is calculated by using mass conservation
𝐴𝑜𝑟𝑖𝑓𝑖𝑐𝑒
𝑣𝐴2 = 𝑣𝑜𝑟𝑖𝑓𝑖𝑐𝑒 ( ) Eq(10)
𝐴2
For the most of turbulent flow, 𝐶0 is close to 0.6.
The total air flow rate is then calculated by
𝑄 = 𝐶0 𝑣𝑜𝑟𝑖𝑓𝑖𝑐𝑒 𝐴𝑜𝑟𝑖𝑓𝑖𝑐𝑒 Eq(11)
Flow speed in the duct near the fan, needed in Eq. (3), is then obtained by
𝑄
𝑣𝐴1 = Eq(12)
𝐴1
1. Select a fan speed to the desired constant rpm using a tachometer.
2. Mount an orifice securely. Start with smaller size first.
3. Measure ΔPfan and ΔPorfice. Make sure the manometer is leveled and the manometer.
fluid level indicates zero reading before taking the measurement.
4. Measure the air temperature. Also measure the power going to the fan motor.
5. Repeat steps 2, 3 and 4 for each orifice diameter. Record the raw data in Table 1.
6. Perform a sample calculation for a selected orifice case to obtain Q, hsys and . Include
the results in your report. Assume C0=0.6.
7. Use the spreadsheet program and calculate the remaining cases.
8. Plot Q vs. , hsys and Q vs η
9. Attach the spreadsheet and the plot to your report.
𝐴1
Figure 2. Schematics of Flow Passage
𝐴2
