|
Some of the more frequent technical questions ...
Which are the main parameters of a ventilator?
The main parameters, characteristic to a
fan, are four in number:
Capacity (V) Pressure (p) Efficiency (n)
Speed of rotation (n min.-1)
What is the Capacity?
The capacity is the quantity of fluid moved by the
fan, in volume, within a unit of time, and it is usually expressed in m3/h,
m3/min., m3/sec.
What is the Total Pressure and how may I calculate it?
The total pressure (pt) is the sum of the
static pressure (pst), i.e. the energy required to withstand opposite
frictions from the system, and the dynamic pressure (pd) or kinetic
energy imparted to the moving fluid (pt = pst + pd). The dynamic
pressure depends on both fluid speed (v) and specific gravity (y). .
|
|
Where:
pd= dynamic pressure (Pa)
y=specific gravity of the fluid (Kg/m3)
v= fluid speed at the fan opening worked by the system (m/sec)
|
|
|
Where:
V=
capacity
(m3/sec)
A= gauge of the opening worked by the
system
(m2)
v= fluid speed at the fan opening worked
by the system
(m/sec)
|
What is output and how may I calculate it?
The efficiency is the ratio between the energy
yielded by the fan and the energy input to the fan driving motor
|
|
Where:
n=
efficiency (%)
V= capacity (m3/sec)
pt= absorbed power (KW)
P= total pressure
(daPa) |
What is speed of rotation?
The speed of rotation is the number of revolutions the fan impeller has
to run in order to meet the performance requirements.
What happen changing the number of revolutions?
As the number of revolutions varies (n), while the fluid specific
gravity keeps steady (?), the following variations take place:
The capacity (V) is directly proportional to the speed of rotation,
therefore :
|
Where:
n=
speed of rotation
V= capacity
V1= new capacity obtained upon varying of
the speed of rotation
n1=
new speed of rotation |
|
|
Where:
n=
speed of rotation
pt= total pressure
pt1= new total pressure obtained upon
varying of the speed of rotation
n1=
new speed of rotation |
The absorbed power (P) varies with cube of rotation ratio, therefore:
|
|
Where:
n=
speed of rotation
P= abs. power
P1= new electrical input obtained upon
varying of the speed of rotation
n1=
new speed of rotation |
How the specific gravity may be calculated?
The specific gravity (y) may be calculated with the following formula
|
|
Where:
273=
absolute zero (°C)
t= fluid temp (°C)
y= air specific gravity at t C
(Kg/m3)
Pb= barometric
pressure (mm Hg)
13.59= mercury
specific gravity at 0 C (kg/dm3) |
For ease of calculation, the air
weight at various temperatures and heights a.s.l. have been included in
the table below:
|
|
Temperature |
|
-40C |
-20C |
0C |
10C |
15C |
20C |
30C |
40C |
50C |
60C |
70C |
Height
above
sea level
in meters |
0
|
1,514 |
1,395 |
1,293 |
1,247 |
1,226 |
1,204 |
1,165 |
1,127 |
1,092 |
1,060 |
1,029 |
| 500 |
1,435 |
1,321 |
1,225 |
1,181 |
1,161 |
1,141 |
1,103 |
1,068 |
1,035 |
1,004 |
0,975 |
|
1000 |
1,355 |
1,248 |
1,156 |
1,116 |
1,096 |
1,078 |
1,042 |
1,009 |
0,977 |
0,948 |
0,920 |
| 1500 |
1,275 |
1,175 |
1,088 |
1,050 |
1,032 |
1,014 |
0,981 |
0,949 |
0,920 |
0,892 |
0,866 |
|
2000 |
1,196 |
1,101 |
1,020 |
0,984 |
0,967 |
0,951 |
0,919 |
0,890 |
0,862 |
0,837 |
0,812 |
| 2500 |
1,116 |
1,028 |
0,952 |
0,919 |
0,903 |
0,887 |
0,858 |
0,831 |
0,805 |
0,781 |
0,758 |
|
|
Temperature |
|
80C
|
90C
|
100C
|
120C
|
150C
|
200C
|
250C
|
300C
|
350C
|
400C
|
70C
|
Height
above
sea level
in meters |
0
|
1,000
|
0,972
|
0,946
|
0,898
|
0,834
|
0,746
|
0,675
|
0,616
|
0,566
|
0,524
|
1,029
|
| 500 |
0,947
|
0,921
|
0,896
|
0,851
|
0,790
|
0,707
|
0,639
|
0,583
|
0,537
|
0,497
|
0,975
|
|
1000 |
0,894
|
0,870
|
0,846
|
0,803
|
0,746
|
0,667
|
0,604
|
0,551
|
0,507
|
0,469
|
0,920
|
| 1500 |
0,842
|
0,819
|
0,797
|
0,756
|
0,702
|
0,628
|
0,568
|
0,519
|
0,477
|
0,442
|
0,866
|
|
2000 |
0,789
|
0,767
|
0,747
|
0,709
|
0,659
|
0,589
|
0,533
|
0,486
|
0,447
|
0,414
|
0,812
|
| 2500 |
0,737
|
0,716
|
0,697
|
0,662
|
0,615
|
0,550
|
0,497
|
0,454
|
0,417
|
0,386
|
0,758
|
|
