| 1)
Fan Laws
The performances of all types
of fans follow certain laws which are useful in predicting
the effect upon performance of changes in the conditions
of operations, the duty required of the installation,
or the size of the equipment due to the space, power,
or speed limitations. In the following laws, groups
1 to 6, Q = air volume and P = static, velocity or total
pressure. The laws pertaining to fan size apply only
to fans geometrically similar, i.e., those in which
all dimensions are proportional, it may be used; otherwise,
fan diameter is commonly used as a size criterion.
(1) Variation in Fan Speed:
(RPM)
Constant Air Density -
Constant System
(a) Q: Varies as fan speed ratio
(b) P: Varies as square of fan speed ratio.
(c) Power: Varies as cube of fan speed ratio.
(2) Variation in Fan Size:
(but same number of blades)
Constant Tip Speed - Constant
Air Density
Constant Fan Proportions - Fixed Point of Rating
(a) Q: Varies as square of fan diameter.
(b) P: Remains constant
(c) RPM: Varies inversely as fan diameter.
(d) Power: Varies as square of fan diameter.
(3) Variation in Fan Size:
(but same number of blades)
At Constant RPM - Constant
Air Density
Constant Fan Proportions - Fixed Point of Rating
(a) Q: Varies as cube of fan diameter.
(b) P: Varies as square of fan diameter.
(c) Tip Speed: Varies as fan diameter.
(d) Power: Varies as fifth power of diameter.
(4) Variation in Air Density:
Constant Volume - Constant
System
Fixed Fan Size - Constant Fan Speed
(a) Q: Constant.
(b) P: Varies as density.
(c) Power: Varies as density.
Note: The cooling tower is designed to handle the process
requirement on a design summer day.
As the inlet air
wet bulb temperature drops, the tower will produce colder
discharge water temperatures and consume slightly greater
fan horse power.
(5) Variation in Air density:
Constant Pressure - Constant
System
Fixed Fan Size - Variable Fan Speed
(a) Q: Varies inversely as square root of density.
(b) P: Constant.
(c) RPM: Varies inversely as square root of density.
(d) Power: Varies inversely as square root of density.
(6) Variation in Air density:
Constant Weight of Air
- Constant System
Fixed Fan Size - Variable Fan Speed
(a) Q: Varies inversely as density.
(b) P: Varies inversely as density.
(c) RPM: Varies inversely as density.
(d) Power: Varies inversely as square of density.
2) Fan Performance Variables
A change in fan efficiency
will have a direct bearing on cooling tower performance.
Reduced fan efficiency may be caused by high wind velocity,
high exit temperature, poor fan blade balance, incorrect
track, dirty blade surfaces, and below major factors.
(1) Airflow Rate: This factor
is a primary variable in the design and operation of
axial flow fans. It is basically an independent variable
for most types of towers, but there are exceptions,
for example natural draft towers and towers with constant
speed &fixed-pitch fans.
Most mechanical towers are
equipped with adjustable pitch fans. Often the blade
pitch angle is adjusted on a seasonal and/or load basis
to prevent the unnecessary use of fan power and
to help maintain fairly uniform outlet water temperature.
Many towers are also equipped with multi-speed fan drives
to further increase air-handling flexibility.
(2) Fan Speed: The effect
of fan speed at constant fan power depends precisely
on the effect of fan efficiency. Generally speaking,
an increase in fan speed at relatively high static pressure
and relatively low fan speed will result in an increase
in fan efficiency. For the opposite case, that is, a
relatively low static pressure and high fan speed, the
speed increase could likely reduce fan
efficiency. Specific
predictions of the effects of fan speed should be made
from a study of appropriate fan curves.
(3) Air Density: Since the
density of the air varies with temperature and pressure
(altitude), it is necessary to evaluate the effect of
air density on the system design and fan performance.
The system designer must evaluate the actual air density
that will be handled by the system in order to properly
determine the volume of flow required and the actual
pressure losses in the system. Since fan is essentially
constant volume machine, the volume of air handled by
the fan will remain constant regardless of the density,
but the total pressure developed by the fan and the
power required by the fan will vary in direct proportion
to density.
(4) Inlet Conditions: Fan
stack design can have a significant effect on fan performance.
A poorly designed inlet bell is a potential cause of
poor air distribution and low fan efficiency. The fan
stack should have smooth interior surface, and should
be shaped from inlet to outlet to prevent sudden air
direction changes or sudden contractions or enlargements.
The flow pattern at the fan tip is of major importance.
The losses fall into two categories: first Vena Contracta
(The point at which the flow area reaches its minimum
is called Vena Contracta), or increase in velocity pressure
and second Starvation at fan tip. Below Figure gives
actual loss in terms of Total Pressure and Efficiency
for a fan running in short duct.
(5) Velocity Recovery Stacks:
As cooling towers become larger, velocity stacks become
more common. The diverging nozzle at the fan discharge
is used to save energy by converting a portion of the
velocity pressure to static pressure. The regain of
static pressure appears at the fan inlet as additional
suction pressure. For high velocity recovery designs
the normal height/diameter ratio is from 0.6 to 1.0.
A well designed stack will enable recovery of from 70
to 90% of the theoretical maximum velocity pressure
recovery.
An appreciable amount of
the energy spent for the achievement of air flow through
the cooling tower is wasted in the form of the kinetic
energy of the exit air. Particularly in the case of
towers in which the fan velocity pressure is very high
in comparison with the stack pressure differential,
much affection is directed toward the gradual reduction
in air velocity from the fan plane to the discharge
plane. The resultant reduction in the exit air kinetic
energy results in substantial power savings.
For example, a cooling tower
with fan rings (designed with a height/diameter ratio
of about 0.2) capable of only negligible kinetic energy
reduction from fan to exit is operating at the following
conditions:
- ● Fan driver - output horsepower:
147.2 hp
- ● Static pressure, inch
water gage: 0.5100
- ● Net velocity pressure,
inch water gage: 0.1963
Then, if the fan ring is
replaced by a velocity pressure recovery design that
reduces the net velocity press. to 0.1687 inch water
gage, assuming no change in fan efficiency or air flow,
the fan power is reduced as ff.: HP2 = HP1
x (TP2 / TP1) = 147.2 x [(0.5100
+ 0.1687) / (0.5100 + 0.1963)] = 141.4 HP
(6) Blade Tip Clearance:
The clearance between the tip of the blade of the axial
flow fan and the fan stack is an often neglected parameter
which influences fan and cooling tower performance.
Large clearances allow the shedding of a vortex from
the upper surface of the blade back to the low pressure
area beneath the fan; this produces lowered air flow
rates and reduced fan efficiency.
Fans are often installed
in cooling towers with tip clearance of up to two (2)
inches because of the manufacturing tolerances inherent
in large fiberglass stack segments. In addition, clearances
also vary by as much as an inch due to eccentricity
of the stack. Fan manufacturers recommend that tip clearance
be minimized to insure proper fan performance. To achieve
a small clearances is difficult in cooling tower installations
without very careful attention to the design and installation
of the fan stack. In addition, small clearances less
than about 1/2?are often undesirable and impracticable
in large cooling tower applications due to differential
thermal expansion between the fan blade and the stack.
The tip clearance of many
factors reducing the actual fan performance must be
carefully studied. Losses up to 20% of fan efficiency
are possible with excessive clearance. Since most of
the work is done by the outer third of the fan blade,
excessive tip clearance allows "spillover"
of the air flow from the high -pressure region to the
low-pressure region in the inlet side. "Excessive"
tip clearance means greater than about 0.3% of fan diameter
for cooling tower fans. This would be no more than 1
inch clearance for 28 feet diameter fan or about 0.32
inch for a 9 feet diameter fan. Sometimes this is not
always possible to attain a small tip clearance in a
practical sense especially for the large diameter cooling
tower fans due to the thermal expansion differences
and the constructional accuracy of the fan stack. The
followings are, however, Hudson's recommendation for
tip clearance and they have been successfully used.
| Fan
Size |
Minimum |
Maximum |
| up
to 14 ft |
3/8
inch |
3/4
inch |
| 16
- 20 ft |
1/2
inch |
1
inch |
| 22
- 30 ft |
3/4
inch |
1-1/4
inch |
The reasons that the fan
efficiency is improved with the reduced blade tip clearance
are basically due to the followings;
- ● By minimizing air tip
losses between the blade tips and fan stack, the average
vertical air velocity and, therefore, the volumetric
air flow rate is increased.
- ● The exit air, vertical
velocity profile is more uniform so that the air flow
is more evenly distributed across the fan stack.
If the average tip clearance
between the fan and fan stack is larger than above the
maximum
values, a pressure loss due to the increase
of fan casing sectional area will occur. There will
be a
rapid decline in the efficiency due to the decrease
of total pressure and airflow, and will be a slight
decrease in the brake horsepower due to the decrease
of fan efficiency. If this cannot be avoid,
consult
with Chungrok about an accurate calculation of pressure
loss and efficiency. The following
multiplying factors
are in general being used to correct the fan total efficiency
given in the fan
performance curve.
The power consumption is
generally decreased as much as the tip clearance is
increased, since
the volumetric air flow rate is significantly
decreased. The efficiency at the larger tip clearance
is
decreased, because the input power is not reduced
as much as the airflow is decreased.
| Tip
Clearance |
Multiplying
Factor |
Tip
Clearance |
Multiplying
Factor |
| <=
0.10% to fan dia. |
1.000 |
0.50%
to fan dia. |
0.950 |
| 0.20%
to fan dia. |
0.990 |
0.60%
to fan dia. |
0.925 |
| 0.30%
to fan dia. |
0.975 |
0.70%
to fan dia. |
0.900 |
| 0.40%
to fan dia. |
0.965 |
0.80%
to fan dia. |
0.875 |
(7) Fan Performance Tolerance:
The fan performance also has a tolerance which must
be considered. The AMCA allowable certified ratings
tolerance is ?2.5% in flow and 5% on pressure.
It is quite common practice
to anticipate the system tolerance by applying a safety
factor to the design. This is most often done by adding
some (10%) additional static pressure requirement to
the calculated static and some (5%) additional airflow
requirement to the calculated air flow. So, the actual
point of fan operation will be different from the design
fan performance, since the estimated system resistance
and airflow are usually result in excess of actual system.
Assuming the fan is rated
correctly, the three most common causes of deficient
performance of the fan are "improper air discharge
through fan stack", "non-uniform inlet airflow",
and "swirl at the fan inlet". These conditions
alter the aerodynamic characteristics of the fan so
that its full flow potential is not realized. They will
occur if the fan inlet and outlet are not properly designed
or installed. Other major causes of deficient performance
are:
- ● The air performance characteristics
of the installed system are significantly different
from the system design.
- ● Dirty fills will increase
the system resistance and consequently reduce the
air flow rate.
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