What Is A Fan and How To Choose The Right One For Your Application?
In Thermal management, fans are classified as objects
that forces air at a volumetric rate to cool certain devices,
like a CPU microprocessor. There are numerous types of
air moving products including axial, propeller, and tubeaxial
fans. Other air movers consist of impellers and blowers
(centrifugal & crossflow). Fans, impellers, and blowers
could be distinguished by their size, shape, but more
importantly, the flow of air (measured in CFM) given the
static air pressure. A fan can basically have a high output
of CFM, but at a lower level of air pressure, whereas
blowers and impellers move smaller amounts of air at a
greater level of pressure.
Basic Description of Fans, Impellers, and Blowers
Fans: Tubeaxial fans could vary in many sizes, from 20x20mm
(0.79" inches) to 172x51mm (6.77 inches). They are
relatively inexpensive and readily available due to the
high volume usage in the automotive, computer, and power
supply markets. For its low cost, using tubeaxial fans
is an efficient approach to dissipate heat. Fans: Axial
fans have high performance airflows, relatively good efficiency,
and has an axial airflow pattern. Also known as "box
fans", the optimal operating performance of axial
fans is at low-pressure or low system impedance conditions.
One special characteristic is that acoustics could be
reduced or increased by simply modifying the speed of
the fan.
Impellers: Impellers have the best performance when it
comes to noise. Typically, they are less powerful when
it comes to airflow, but impellers do have various airflow
alternatives, including tangential air flow. Extremely
efficient, there is greater pressurization in water with
impellers than fans.
Blowers: Blowers are usually installed in high system
impedance that requires a backpressure, like in network
servers and telecommunication systems. A drawback for
blowers is their high level of noise compared to fans.
Some benefits include more concentrated airflow (largely
due to its ability to in take air from one side and release
air directly onto the unit that requires cooling on the
other); a circular impeller that provides direct airflow
given a substantially high-pressure environment; and blowers
have option for single or dual inlets.
COOLTRON Fan Composition
COOLTRON sells both inductive AC and brushless DC fans.
The major raw materials that are used in the production
of our fans include the following: housing, impellers,
rotor, bearing, PC board, lead wires, and terminal leads
(AC only).
Housing:
- The housing material on COOLTRON DC fans is made of
a plastic material which is UL 94V-O, has a P.B.T. +30%
GF black, has various dimensions, and usually 4 poles
for convenient installation.
- Most AC fan housings is made of an aluminum painted
/ plastic material.
Impellers:
- The blades on the DC fans are of a plastic material
with UL 94V-O P.B.T., whereas the AC line is made of thermoplastic
material.
Rotor:
- The rotor runs in a counter-clockwise direction. COOLTRON
fans have a locked rotor protection where the fans were
tested for a continuous period locked at the rated voltage
and no damage was made.
Bearing:
- Bearings can be classified as sleeve, dual ball, and
ball/sleeve. COOLTRON bearings have a life of 30,000,
45,000, and 60,000 for sleeve, ball/sleeve, and dual ball,
respectively.
PC Board:
- A PC board is built into the fan and acts as the central
control that keeps the fan operating. Special functions,
like alarm and speed sensor function can be added to the
PC board to detect abnormal operating conditions.
Lead Wire:
- Both DC and AC fans can have lead wires to connect to
a power source. The standard lengths are 12 inches or
7 inches. Customers may specify lead wires to any length
necessary for an application.
Terminal Leads:
- Sometimes, in the design process, customers would like
to do away with dangling wires. Terminal leads offer an
alternative for those customers who have done so. Unfortunately,
COOLTRON only provides this alternative for our AC line
at the current moment.
Selecting the Right Fan (Part I)
Measuring Heat
The main purpose of a fan is to cool other components;
therefore, the most important factor to consider before
choosing a fan is exactly how much heat dissipation is
necessary. The amount of heat transferred could be derived
using the following equation:
Q = m Cp DT
Where,
Q = the amount of heat transferred to system, Watts Cp
= the specific heat of air, J/kg x K
m = the mass flow rate of air, kg/s
DT = the desired air temperature differential (cabinet
to ambient outside air), K
Yet, there still a relationship between mass flow rate
and volumetric flow rate:
m = rG
Where,
G = the volumetric flow rate, m3/s
r = the air density, kg/m3
Therefore, the required volumetric flow rate is then calculated
as:
G = Q / (rCp DT)
By utilizing this formula, a rough estimate of the airflow
to obtain the desired overall air temperature rise in
DT, but it does not specify actual airflow. Actual operating
airflow is determined by the intersection of the fan curve
and the system resistance curve. This point of intersection
can be calculated using one of three approaches.
1) Airflow network methods:
For this approach to be effective requires certain criteria
to be true. First, the flow path to the cabinet must be
known or roughly estimated. Secondly, the geometry must
remain simple, meaning the three-dimensional flow path
cannot be complex.
2) Computational commercial software:
When the flow path is more complicated, the use of computational
fluid dynamics (CFD) may simplify things greatly. A fan's
performance curve is used as reference input to the CFD
software where the operating point and system resistance
is determined. While even taking into account the effects
of turbulence and gravity, CFD assesses the flow of air
and heat transfer in a three dimensional view, as in a
real life application. Even more complex calculations,
like fans in series or in parallel formation, can be easier
analyzed using CFD software.
3) Experimental evaluation mockup of the system:
The total airflow or the system resistance curve can be
measured using the experimental method. In this method,
the operating airflow is obtained when the engineer superimpose
the airflow and system resistance curve.
Regardless of how the operating airflow within a system
is derived, all systems are depicted by a system resistance
curve as the one shown below.
This non-linear expression of airflow versus static pressure
can be stated as:
DP = KrGN
Where,
DP = system pressure loss
K = the load factor specific to the system
r = density of air
G = rate of airflow
N = a constant which varies between 1 and 2. If N=1, then
airflow is
completely laminar. If N=2, then completely turbulent.
Once actual airflow is determined, a comparison between
actual and required airflow need to be evaluated. If the
actual value is considerably less than the required value,
the packaging system must be reexamined so the diminishment
of airflow resistance can be asserted and renovated.
In summary, the first and foremost important factor to
choosing your fan is determining the airflow and the required
pressure to move the volumetric rate flow to dissipate
heat within your system. Before first making this analysis,
to determine other factors like fan dimension, bearing,
and the need for special functions might be meaningless
or irrelevant. Part I serves as an introduction to understand
the main purpose of a fan and equations necessary to determine
basis of fan selection. In Part II, we will continue to
explore other important variables such as noise, voltage,
and bearing, which are more specific to the selection
of a fan. Furthermore, in the proceeding section, other
alternatives like fans in series and in parallel will
be offered to the reader as a consideration in the selection
process.
Selecting the Right Fan (Part II)
In Part I, we established the basis of selecting a cooling
unit by determining the most important attribute, which
is the airflow given a static amount of pressure. In the
section,"What is a fan," we described the composition
within a fan but since there are different variables within
each attribute, the major issue is to determine which
combination of variables is best suited for the application.
Furthermore, the reader will find a brief analysis of
using multiple fans and a small portion devoted to selecting
a fan from an economical standpoint in this section.
What is the best combination of variables to choose?
There is no universal answer to the above question because
each application is unique due to the originality of the
engineer. For example, ten years ago, computers used a
92x92x25mm to cool the internal system. Today, we have
engineers designing their computers using both a 60x60mm
and an 80x80mm. And hypothetically speaking, engineers
might create desktop computer systems using just a 40x20mm
fan ten years from now.
Though impelling air is the most important concept when
choosing a fan, one cannot ignore size. An illustration
of how important size is can be observed in laptop computers.
Due to portability, the evolution of laptops has changed
greatly in terms of compactness. Today, one can walk into
an electronic store and find laptops that are less than
one inch thick. This makes it absolutely impossible and
impractical to install a fan with 25mm thickness because
there would be no room for operation. And if a fan becomes
non-functional, the amount of air exerted by it is irrelevant;
therefore, size is a mandatory decision based on the design
by the engineer.
Next, using the correct voltage is important. Voltage
is very product specific and in some cases, industry specific.
In the telecommunications industry, it is very common
for engineers to use 48-volt DC fans. The components on
the PCB require a great deal of power. To employ a fan
with low voltage might cause the fan to burn out. Eventually,
without a fan to dissipate heat, product failure becomes
inevitable.
Other attributes of a fan like noise and bearing are also
important because it enhances the concept of quality in
the end product. For bearing, COOLTRON manufacture fans
with dual ball, sleeve, or a hybrid, ball/sleeve. These
three types of bearings can be distinguished by price
and the quality each one adds to the fan. Dual ball fans
last longer in terms of operation hours; therefore, are
most expensive. These fans last up to 60,000 hours. Next
in line would be fans that are assembled using ball/sleeve
bearings. These fans typically operate for 50,000 hours.
As for sleeve bearings, they cost the least and function
for around 30,000 hours.
Noise is an attribute that varies based on fan speed and
bearing. One approach to reduce acoustical noise is by
assembling a fan using a sleeve bearing. Overtime, however,
when the sleeve bearing loses lubrication (which it does
so faster than ball bearing), noise level will increase
beyond fans with ball bearing. Another alternative to
modifying noise level is to control fan speed. As the
velocity of a fan rise, so does noise level. In effect,
a direct correlation can be drawn between noise and airflow.
Finally, there exist value-added features for fan performance
monitoring and failure detection. Lets take COOLTRON's
120x120 DC models for example. Large fans have a higher
likelihood of failure because of heavier composition material.
COOLTRON's 120x120 come standard with an auto-restart
function to prevent fan failure. Once the fan stops or
locks up, it will stop running and restarts itself after
2 - 6 seconds.
Some engineers may like to design fans with speed control
to conserve power. COOLTRON offers these engineers a thermister
speed control option. While in operation, a temperature
sensor is in place to automatically regulate speed. If
a system's internal temperature rises, the sensor will
detect the change and speed up the rotation of the fan's
rotor.
The consideration of multiple fans
Previously, the idea of using two or more fans was mentioned
as an option. Utilizing more than one fan has its benefits.
First, using two smaller fans rather than one large fan
may save additional space inside the system. Also, by
applying multiple fans during design can increase the
airflow and pressure to reduce heat transfer. Two techniques
being applied by engineers are 1) having fans in series
and 2) having fans in parallel.
Fans in Series - Fans in series is simply having two fans
operating one above the other. This technique achieves
the most desired results in systems where resistance is
high. Since static pressure nearly doubles when fans are
in series, this technique might welcomed where there is
a great amount of distance between the fans and the components
that require cooling in the system.
Fans in Parallel - This technique is when multiple fans
are set side by side. Different from fans in series, having
fans parallel operates best in systems with low resistance.
Likewise, parallel fans increases not static pressure
but airflow.
Regardless of which method is used, a caution of using
multiple fans is that too many will cause instability
to the performance curve.
Price as the determining factor
Buyers who see fans as common commodities usually use
price to form their final decision as to which manufacturer
to choose. However, the importance of quality can never
be over-emphasized. Some people have used the analogy
of comparing systems to humans. If that is the case, the
fan within the system is equivalent to the lungs in a
human. Without healthy lungs, humans have a hard time
breathing, sometimes leading to fatality. Likewise, if
a fan stops operating, the system will be disrupted and
in the long run malfunction due to over heating. COOLTRON
fans are most definitely price competitive, but unlike
ordinary lungs, fans sold by COOLTRON are of excellent
quality and have great features to make sure the fans
live up to their life expectancies.
Basically, the choice of fan is pretty much left up to
how engineers wish to design their products. The essential
concept to keep in mind is how much airflow is required
to cool the enclosure. Once realized, the designer can
then opt for the best method and characteristics.
