REPORT
TOPIC 1
PRELIMINARY DESIGN OF A CENTRIFUGAL BLOWER CAPABLE OF DELIVERING 900 CFM AT STANDARD TEMPERATURE AND PRESSURE
TABLE OF CONTENTS
|
Sr. No. |
Name of content |
Pg. no. |
|
1. |
ABSTRACT |
3 |
|
2. |
INTRODUCTION |
4 |
|
3. |
LITERATURE REVIEW |
5 |
|
4. |
BRIEF ABOUT THE CENTRIFUGAL BLOWER |
6 |
|
5. |
DESIGN AND CALCULATIONS |
9 |
|
6. |
ANALYSIS AND RESULTS |
17 |
|
7. |
SUMMARY/ CONCLUSION |
21 |
|
8. |
REFERENCES |
21 |
1. ABSTRACT
There are various kinds of fan/blower used in the industries. Depending upon the application, they are selected and designed. The centrifugal blower/fan is mostly used because of its high efficiency. But, its cost is little bit high. Such pumps are quiet, sturdy, reliable, and operates over a wide range of parameter variations. The main aim of this report is to design centrifugal blower which is capable of delivering the 900CFM of fluid (considering air) at standard pressure and temperature conditions. The focus of the report will be towards the fluid mechanics side of the blower. The application to be taken as an example shows the design aspects as per the fluid mechanics concepts only. This will provide us the preliminary design with calculations of the centrifugal blower having discharge of 900 CFM.
2. INTRODUCTION
The machine that circulates the fluid (air, gas etc) on the specific or pointed area is known as blower. High pressures are used for the production of large volumes of gas. It provides air in a specific location or point. There are mainly two kinds of industrial blower. First, the centrifugal blower and the second one is positive displacement blower. The centrifugal blower consists of two main parts: an impeller and the casing. The impeller is the moving part while the casing is the stationary part. The air enters axially from center of the impeller eyes and leaves the blower radially. This is how centrifugal blower works. The specific pressure ratio produce by centrifugal blower is 1.1 to 1.2. Nowadays, the selection of the right size of the blower is very important. Since, there are many things depends on that like energy efficiency, space, cost, manufacturing time etc. Also, the performance plays the vital role here. The design of blower should meet the requirement fully. There 4 different types of centrifugal blowers: radial, forward curves blades, backward curve blades, and airfoil type. They used in boilers, conveyors, huge machineries, etc.
During extensive research on design and performance evaluation of centrifugal blowers and fans, it is seen that work has been in the area of general basic physics, aerodynamics, energy transfer and fluid mechanics etc. The design of any turbo machine requires computation of coefficients and variables. It is also seen that the method of designing the blowers/fan differ from place to place or researcher. The desired performance can be revealed by explicit design.
The aim of this report is to make a preliminary design of a centrifugal blower having 900 CFM discharge.
After the detailed literature review, focusing on suitable design methods calculations are performed to get an optimum solution.
3. LITERATURE REVIEW
There are many fans laws which are used to design the centrifugal blower. Hence, a lot of importance has been given to learn about the basic of the theory of Fans, their type and its working. We are mainly going to focus on the centrifugal blowers. The selection of various parameters is very necessary for the performance of the blower. During the process of design optimization of centrifugal fans, blowers mass flow variation is controlled and not by speed variation of the electrical motor.
Fan Laws
The fans operation is governed by the laws of physics, which links speed, with pressure and thus to power. A change in fan RPM changes rises in the pressure and power consumption at new RPM.
The Fan Laws provides parameter which can be used as dynamically similar series of fans at the same point of rating on the performance curve. The variables are fan size, D; rotational speed, N; gas density, p; volume flow rate, Q; pressure, p; total efficiency and power (shaft), P.
• Fan Law No. 1 Vary speed of operation, size, or fluid density on volume flow, pressure and power level.
• Fan Law No. 2 focuses on the effect of size variation, density, pressure or volume flow rate, power.
• Fan Law No. 3 governs the size
variation for power variation, volume flow or density on speed.
Source: ASHRAE Handbook, 1988 Equipment Volume
4. BRIEF ABOUT THE CENTRIFUGAL BLOWER
Velocity vector technique is used to study the blower performance. The height of the diagram – indicated by the relative radial velocity vector Vr – is based on the volume of air flowing through the fan. The air velocity relative to the blade – indicated by Vb is nearly tangential to the blade as some slip occurs due to boundary layer effects. The velocity component near the tip “wr” is perpendicular to the radius of wheel. Here “w” is impeller rotational speed in radians per second. The “r “is the radius of the impeller at blade tip. Because the speed of the wheel is the same for each case, the vector “wr” remain same at all sections. The absolute velocity indicated by Vs is the resultant of Vb andwr.The relative tangential velocity vector indicated by Vt is projected from Vs in the direction of wr.If volume decreases, the vector Vr decreases and as the vector Vbdoes not change for a given blade, Vt increases with BI blades, remains constant with R blades and decreases with FC blades. Since the pressure near the fan surfaces is proportional to Vt * wr, the pressure rises with fall in volume. These vector diagrams illustrated in fig2, fig3 and fig4, at a given speed, the smallest fan selection will be a forward curved fan. Oppositely, the largest will be an airfoil. (Sources: Fan Selection Guides)
Fig.1 General Components of Centrifugal blower
Source: Air Movement and Control Association from Publication 201–90
Centrifugal fans are of 4 kinds:-
Based on impeller type:
1. Airfoil
It the most efficient centrifugal fan designs, but the most expensive. Air leaving the wheel of the impeller is at a lower velocity compared to tip speed. Further deep blades permits efficient air expansion within the blade passages.
2. Backward-Inclined
Backward-inclined or backward curved blowers have an impeller with uniform thickness inclined or curved away from the direction of rotation. There are typically 10 to 16 blades on the impeller.
The velocity diagram of backward inclined blower is:-
Fig2.vector velocity diagram of backward inclined blower
3. Radial
Radial type centrifugal blowers have an impeller wheel with high mechanical strength.There are typically 6 to 10 blades of heavy gauge material radiating out from the hub.
The velocity diagram of radial centrifugal blower is:-
Fig3.vector velocity diagram of radial centrifugal blower
4. Forward-curved
Forward-curved blowers have an impeller wheel made of light gauge material. There are typically 24 to 64 shallow blades with both the heel and the tip curved forward. The exit velocity at blade is greater than the velocity at the tip where kinetic energy gets transferred to the air.
The velocity diagram of forward curved centrifugal blower is:-
Fig4.vector velocity diagram of forward curved centrifugal blower
5. DESIGN OF CENTRIFUGAL BLOWER
The designing a centrifugal blower involves lots of parameters. The different type of centrifugal blower has different formulas and their assumptions. It involves design of impeller, design of casing, its mounting position, its type and position of discharge, the motor specification etc. So covering all the points may not be possible. By taking radial type of centrifugal blower, the below assumptions and formulas are taken. And the rest of the design will be done based on that only.
1. Design of Impeller
The design of impeller depends upon the basic flow rate and rotational speed of the blades. For the single stage of centrifugal blower, some essential formulae’s are given:-
Overall pressure ratio:
εp = Pd/Pi (1)
Where Pd is air pressure at discharge and Pi is inlet air pressure.
Total adiabatic head:
Had= 1/g x(RTi/ 0.287) x(εp0.287-1) (2)
And then, the weight flow of gas is;
w= Qρi/ 60(3)
Where, ρi is design of air and it is expressed by
ρi = Pi/ RTi (4)
Thus, the adiabatic horsepower is determined by,
a.hp= wHad / 746 (5)
The shaft diameter at the hub section:
Ds
=
Where, T is the torsional moment and it can be estimated by,
T = 60×b.hp / 2πn (7)
V0is the velocity at the eye of the impeller which is slightly greater than the velocity near suction flanges. The velocity at suction flange depends on pipe size chosen. Thus the velocity at impeller eye is given by;
H0=V02 / 2g (8)
For the pressure ratio between impeller inlet and impeller eye,
εp0.287-1=0.287×H / RTi (9)
The pressure at impeller eye is:
P0 = Pi/ εpi-0 (10)
The temperature at impeller eye is:
T0 = Ti/ ε0.287pi-0(11)
The density of the impeller eye is:
ρ0= P0/ RT0(12)
Volume flow through impeller eye is;
Q0 = w / ρ0 (13)
The hub diameter;
DH=Ds + (19.05 to 50.88) (14)
The vane inlet diameter D1 can be made slightly greater than the eye diameter D0. The impeller inlet speed;
U1 = πD1 n / 60 (15)
Inlet angle β1;
Tanβ1 = V1/ U1 (16)
Relative inlet velocity:
The inlet area of the impeller:
A1 = Q0 / V1 (19)
Impeller inlet width:
b1 = A1 / πDε1 (20)
Where, the inlet vane thickness factor ε1 (Assume 0.85 to 0.95)
D2
= 60×
Where, K' is the pressure coefficient which has a value between 0.5 and 0.65 depending on the type of impeller.
The outlet vane angle of impeller is 90 ̊.
Blade number:
Z=6.5× (D2 + D1) /(D2-D1) ×sin (β1 +β2/2) (22)
But, the usual number of vanes varies between 15 and 30in most blowers. Larger the number lower will be circulatory flow effect, and friction effects get increased.
The impeller tip speed at the outlet:
U =πD2 n / 60 (23)
Wz =U2x πsinβ2/z(24)
Outlet gas velocity along the radial direction is Vr2. This velocity is made lower than the inlet absolute velocity V1.
U’2=U2–Wz (25)
V2=
V’2=
Absolute outlet angle at impeller:
tanα’2 = Vr2 / U2 (28)
Virtual pressure head:
Hvir.∞
p= 1/2g (
The effective head is;
Heff=η overall×H vir.∞ p(30)
Pressure ratio between outlet and impeller eye is;
εp 0.287 -1= 0.287 x Heff/ RT0 (32)
Thus, impeller outlet pressure is;
P2 =εp×P0(33)
The friction and turbulence losses with be transformed into heat which raises the temperature of the gas. The outlet temperature can be based upon the adiabatic head in the impeller neglecting losses.
εp 0.287 -1= 0.287 x Hvir.∞ p / RT0 (34)
Then, the impeller outlet temperature is;
T2 =T0 ×εp0.287(35)
The outlet density is;
ρ2 = P2/ RT2 (36)
Thus, the flow leaving the impeller is;
Q2 = w / ρ2(37)
Where, Q2 is the flow leaving the impeller. Assuming the vane thickness is constant. The outlet vane thickness factor is;
Where, t is the blade thickness which the vane is chosen as3.175mm. Thus, the outlet area of the impeller:
A2 = Q2 / Vr2(39)
The impeller outlet width:
b2 = A2 / π D2 x ε2 (40)
2. Design of volute casing
The steady state relationship between the flow energy at inlet and exit:
Neglecting potential difference,
Casing outlet Velocity (V4):-
Q= Av x V4
Where Av is Exit area of volute casing = Av = bv (r4 – r3)
Allowing for 5 mm radial clearance between impeller and volute tongue,
r3 = (D2 / 2) + 5
D3 = r3 x 2
In design the width of volute “bv” is in the range of 2 to 3 times b1
We can take the mean value as 2.5. Hence,
bv = 2.5 b2
The volute angle as function of the radius of casing is
Radius of Volute Tongue rt = 1.075 x r2
Angle of Volute Tongue
3. Hydraulic, leakage and power Losses
a) Leakage loss
Here, Ps = 2/3 ΔPs
And Cd is discharge coefficient and its values range between 0.55 to 0.69,
δ = clearance between impeller eye inlet and casing = 2mm as per fabrication requirements,
b) Suction pressure loss
Here the loss factor is “ki”, and is in the range of 0.49 to 0.81.
c) Volute pressure loss
kiii is close to a value of 0.4 and is function of the design conditions
d) Disc friction loss
The material friction “f” is 0.005 if we chose mild steel or sheet metal.
4. Efficiencies
a) Hydraulic efficiency
b) Volumetric efficiency
c) Total Efficiency
Ideal shaft power required to run the fan
Pideal
Shaft Diameter
Blade profile
6. RESULTS AND ANALYSIS
As we have to design a centrifugal blower of 900 cfm discharge at standard temperature and pressure. So, several assumptions need to be made for the design and analysis to obtain the optimal solution. Suppose the fluid passing the blower is air.
So, Air flow rate (Q) = 900 cfm = 1529.10 m3/hr = 0.4247 m3 / sec.
Assuming, it the direct drive type of blower (no belt and pulley) with 4 pole motor
N = 1440rpm
Atm. air temp. Tatm = 20ᵒC
Atm. air pressure, Patm = 1.01353 MPa = 1.013 x 105 pa or N/m2
Static pressure P = 76mm of water column
Discharge air Pressure, Pd =306 mm of water column = 300 pa or N/m2
Static pressure gradient ΔPs = 700pa or N/m2
Acceleration due to gravity, g = 9.81m/s2
Air constant, R = 0.287 kJ/kg k
Density air ρ1 = 1.165 m3 / sec
Fig.5 velocity diagram of inlet and outlet for radial type centrifugal blower
Results data in tabular as per the formulas given in above chapter are given below:
|
Sr. no. |
Name of parameter |
Value |
Unit |
|
1. |
Vane inlet diameter, D1 |
199 |
mm |
|
2. |
Peripheral speed at inlet, U1 |
15 |
m/s |
|
3. |
Velocity at vane inlet, V1 |
13.64 |
m/s |
|
4. |
Relative vane velocity W1 |
20.27 |
m/sec |
|
5. |
Impeller inlet width, b1 |
52 |
mm |
|
6. |
Inlet vane angle, β1 |
42.28 |
degree |
|
7. |
Number of vanes, Z |
15 |
|
|
8. |
Outside diameter of impeller, D2 |
433 |
mm |
|
9.. |
Radial outlet velocity, Vr2 |
33.24 |
m/s |
|
10. |
Impeller width at outlet, b2 |
52 |
mm |
|
11. |
Impeller outlet tip speed, U2 |
40.84 |
m/s |
|
12. |
Vane outlet angle, β2 |
90 |
degree |
Table 1 shows calculations of design of impeller
|
Sr. no. |
Name of parameter |
Value |
Unit |
|
1. |
Casing outlet Velocity (V4) |
52.05 |
m/s |
|
2. |
Width of casing (bv) |
2.5 |
mm |
|
3. |
Scroll radius at inlet (r3) |
221.5 |
mm |
|
4. |
Scroll radius at outlet(r4) |
284 |
mm |
|
5. |
Radius of tongue (rt) |
232 |
mm |
|
6. |
Angle of tongue (θt) |
22.06 |
degree |
Table2 shows calculations of involute casing design
|
Sr. no. |
Name of parameter |
Value |
Unit |
|
1. |
Suction loss(dpsuc) |
65.02 |
pa |
|
2. |
Impeler pressure loss (dPim) |
1.471 |
pa |
|
3. |
Casing pressure loss (dPvc) |
8.9 |
pa |
|
4. |
Leakage Loss (QL) |
0.0212 |
m3/sec |
Table3 show various losses values
|
Sr. no. |
Name of parameter |
Value |
Unit |
|
1. |
Hydraulic Efficiency |
90.02 |
% |
|
2. |
Volumetric Efficiency |
95.2 |
% |
|
3. |
Total Efficiency |
85.72 |
% |
Table 4 shows the values of various efficiencies
|
Sr. no. |
Name of parameter |
Value |
Unit |
|
1. |
Blade Profile radius (Rb) |
23.9 |
mm |
|
2. |
Shaft Diameter(Ds) |
11.6 |
mm |
|
3. |
Power required to run the Fan (P) |
403.43 |
W |
Table 5 shows the values of other necessary parameters
For the reference to be taken after the design calculations:
|
Sr. no. |
Static Pressure (in mm of water column) |
Fan Speed (rpm) |
Motor Rating (HP) |
|
1 |
76 |
1440 |
0.5 |
|
2 |
102 |
1440 |
1 |
|
3 |
152 |
1440 |
1.5 |
|
4 |
203 |
1440 |
2 |
|
5 |
305 |
1440 |
3 |
|
6 |
381 |
1440 |
5 |
|
7 |
406 |
1440 |
7.5 |
Table 6 direct drive static pressure with motor rating
(Source: - Various industrial blowers catalog)
7. SUMMARY/ CONCLUSION
The preliminary design of centrifugal blower has been done. The above calculations were done for the radial type centrifugal blower in which it requires 0.4 W (0.5 HP) power to operate the blower. From table 6 we can cross check design is correct or not. As the static pressure taken by us was 76mm of water column. For this pressure, the required power of motor is 0.5HP. Hence our design is correct.
8. REFERENCES
1. Ye Man Aung, Mg, 2013 Design and Flow Analysis of Radial Type Centrifugal Blower for Coal Plant. Mechanical Engineering Department, Mandalay Technological University.
2. Arkar Htun, Mg, 2007 Design Calculation of Centrifugal Blower Used in Super Charger Engine, Mechanical Engineering Department, Mandalay Technological University.
3. Ebara Corporation, 1998. Essential of Fans and Blowers. Ebara Hatakeyama Memorial Fund, Tokyo.
4. Austion H.Church, 1972. Centrifugal Pumps and Blowers, John Wilely and Sons, Inc, New York.
5. The Design of a Closed-Type-Impeller Blower for a 500 kg capacity Rotary Furnace. Engineering Materials, Development Institute, Akure, Ondo State, Nigeria.
6. “Fluid Mechanics”, 4th Edition Frank M. White University of Rhode Island.
7. http://www.fluidedesign.com/.
8. Www. Cincinnatifan.com.
9. Air Movement and Control Association (AMCA). Fans and Systems. Publication 201-90. 30 West University Drive, Arlington Heights, Ill. 60004-1893, (708)394-0150, 1990.
10. AMCA. Field Performance Measurement of Fan Systems. Publication 203-90. Arlington Heights, Ill., 1990.
11. American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). Handbook, l988 Equipment Volume. Atlanta, 1988.
12. ASHRAE. Handbook, 1989 Fundamentals Volume. Atlanta, 1989.
13. ASHRAE. Handbook, 1991 HVAC Applications Volume. Atlanta, 1991.
14. ASHRAE. Metric Bin Weather Data, Toronto International Airport. Atlanta, n.d.
15. ASHRAE. Simplified Energy Analysis using the Modified Bin Method. Report TC 4. I . Atlanta, 1983.
16. Greenheck Fan Corp. Industrial Fans – Open Radial & Radial Tip. Catalogue IF 1-86 M. Schofield, Wis., 1986.
17. Jorgensen, R. (ed.) Fan Engineering. 8th ed. Buffalo, NY, Buffalo Forge Company, 1983.
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