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Cooling System Design Tips
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Air Flow, Distribution and System Pressure
The greater the airflow and pressure of a given system, the greater the noise. Total airflow can often be
minimized by carefully distributing flow. This is done by placing heat sources with highest power levels
where the air stream has highest velocity. Baffles can be used to concentrate airflow on higher power
devices and heat sinks to improve transfer of heat to the air stream. Pressure can be minimized by
avoiding changes in airflow direction. Air inlet and exhaust ports must not be constricted.
Any close obstruction upstream or downstream to the fan will increase noise, especially obstructions
with sharp edges. Choose rounded finger guards when possible. |
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How Many Fans
In multiple fan applications, lower noise will be achieved by using a small number of large fans, rather
than a larger number of small fans. |
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Fan/Blower
Size
Most air movers operate more efficiently
at less than their nominal rated voltage. Size the air
mover for
worst-case thermal conditions. Don't compromise available
airflow. The controller will power the air mover
only as necessary to maintain the selected control temperature.
Consider allowing maximum flow somewhat
greater than for a fixed speed design.
For least
noise, an air mover should be selected and its speed
set such that it will provide the required airflow
and pressure at about 70% of free airflow on its performance
curve. Choosing a high-pressure air mover in a low
pressure application will result in significantly greater
noise. Conversely, a low-pressure air mover heavily
loaded
in a high-pressure application will also increase noise. |
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AC
or DC?
Choosing an AC or DC fan is generally
a matter of power availability. In HVAC applications and
in some
electronic cooling applications at power levels above
100 watts, AC power is usually preferred. At lower
power levels, DC power is usually preferred because it
avoids high voltage AC wiring, and provides airflow
independent of power line voltage and frequency. All things
being equal, nearly all DC fans are speed
controllable, not all AC fans are controllable.
SmartFan Nimbus, Nimbus-HP and
AC-VX fan controls use a triac phase control principle
to vary the voltage applied to the fan.
As the figure below illustrates, the voltage applied
to the fan is switched off for a period of time, called
TOFF
and switched on for a period of time TON during each half of one line cycle. As a result, the
fan RMS voltage
changes proportionally to increases and decreases in TON Since fan speed is proportional to RMS voltage
applied, it will also vary in proportion to changes
in TON.
Click on image for larger version.
SmartFan Stratus II is a Variable Frequency Drive that controls motor speed by varying the output frequency from 0 to 400 Hz for small single phase or three phase fractional HP motors.
Many permanent split capacitor (PSC), shaded pole, and universal single-phase motors are compatible with
SmartFan AC controls. Three phase motors can only be used with the SmartFan Stratus II VFD. Use of SmartFan controls with capacitor start motors, where a capacitor is switched in and out of the motor windings, should be avoided. Confirm controllability with the fan manufacturer before installation or contact CRI customer service for testing recommendations. Attempting to control a fan that is not compatible could cause excessive heating and could permanently damage the fan motor. Click here for AC Motor Compatibility checklist.
The speed of a DC fan is nearly proportional to the DC voltage applied. Lower powered controls use a linear
dissipative principle to vary applied voltage. At higher power levels, CRI uses a high frequency switching
principle to achieve power efficiency of more than 90%. | |
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Temperature Control & Product Operating Principles |
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Recommended
Control Temperatures based on equipment
temperature rise are given in the following table. Temperature
rise is measured from equipment air inlet to the
point where the sensor is located, often near the exhaust,
with air movers running at full speed. These Control
Temperatures will result in minimum noise under normal
conditions of room temperature, altitude, and system
resistance while maintaining lowest operating temperatures
under all conditions.
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this design temperature rise from air inlet to air
sensor with air movers running at full speed |
3-6°C |
6-8°C |
8-12°C |
| Choose
this Control Temperature |
35°C |
40°C |
45°C |
Recommended
Control Temperatures
Recommended Control Temperatures based on equipment
application are given in the following table.
35°C
Control Temperature
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Larger
equipment
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Machine
room equipment
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Equipment
designed for use in both normal and high temperature
environments, e.g., office or storage room
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40°C
Control Temperature
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Good
choice for many purposes
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Equipment
of moderate size intended for installation
in
open offices, hospitals
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45°C
Control Temperature
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Smaller
equipment
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Equipment
to be installed in private offices and homes
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Equipment
that will be used in proximity to an
operator (e.g., desk-mounted)
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How
SmartFan® Regulates Component Temperatures
in Electronic Equipment
A
3° to 4°C temperature change causes a closed-loop SmartFan
controller to change speed from idle
to maximum. A unit with a 40°C Control Temperature regulates
temperature within a range from about
36° to 40°C. In principle, a controller could be made
to regulate air temperature more precisely. It would
have very high sensitivity such that at a fraction of
one degree below 40°C, fans would idle and at 40°C
they would reach full speed. There are two reasons why
SmartFan sensitivity is controlled at a moderate
level. First, very high sensitivity could result in
instability (sometimes called hunting) with fans having
difficulty finding a stable speed. Secondly, the temperature
that we really want to control is that of
semiconductor junctions, not air. As the air velocity
over a typical semiconductor device (e.g., DIP
or similar) changes by two-to-one, as it does when fan
speed changes from full speed to idle, the
difference between the air temperature and junction
temperature changes by a few degrees. Changing
the air temperature by an equal number of degrees cancels
this effect to hold absolute junction temperature
more nearly constant.
Temperature
Compensation -The Open-Loop Alternative
An alternative to regulating temperature inside equipment is to sense and compensate for changes in inlet air
(room) temperature as shown in the figure below. A disadvantage of such an open-loop system is that it is
responsive only to inlet air temperature and cannot respond to changes in power dissipation, altitude, or
system resistance. As the curve shows, a change in inlet air temperature from 23°C (73°F) to 35°C (95°F)
causes fan speed to change from 50 to 100%. Open-loop versions of SmartFan controllers are available
and, like closed-loop units, can reduce noise levels by 15 dB(A) or more.
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