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CRI
Catalog and Design Guide Section |
| For
Application and Design Information See: |
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| Product Name |
What is Adaptive Cooling?
Operating a fan or blower at full speed continuously,
or cycling it on and off, is a sub optimal solution
in most
cooling and ventilating applications. Ideally,
the fan or blower should be controlled to operate
at whatever speed
is necessary to satisfy the need of the moment,
no more and no less.
As
shown in Figure 1 and Figure 2, the SmartFan cooling system senses
environmental conditions such as temperature
(air, surface or liquid), humidity or pressure
and holds it nearly constant. A sensor connected
to
the
SmartFan cooling system continually monitors the environmental
condition. The SmartFan cooling product powers the
air movers, automatically adjusting their speed
over a predetermined range. A properly designed
adaptive
cooling or ventilation fan system will yield the following
benefits. |
- Noise
Reduction
- Energy
Savings
- Increased
Fan Life
- Automatic
Compensation for Changes in Cooling System Configuration
- Alarm
Byproduct
- Other
System Dependant Benefits
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Click on image for larger version
Figure 1 |
Click on image for larger version
Figure 2 |
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Noise
Reduction
A significant advantage of adaptive cooling is the potential
for considerable reduction in ventilation fan noise
under normal
environmental conditions. Most system designers select
fans for the worst case conditions
anticipated. This cooling product
creates excessive, unnecessary fan noise in a normal environment.
Because there is a 5th power relation between noise level
and fan speed, a small change in fan speed will
cause a large change in fan noise. The equation for determining
the noise level of a fan at less than full speed
is given below:
| LS
= L1 + 50 log S |
| S
= Fan speed as a fraction of full fan speed |
| LS
= A-weighted noise level at fan speed S |
| L1
= A-weighted noise level at full speed |
Example:
A 300 CFM fan has a full speed noise rating of 59 dB(A).
What would the noise rating be at ½ full speed?
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L1/2
= 59 + 50 log (½) |
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L1/2
= 59 -15 |
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L1/2
= 44 dB(A) |
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Energy
Savings
A well designed adaptive cooling or ventilation system
saves energy. By running the fans at reduced speed when
less air flow is required, energy is saved. There is often
an approximate square law relationship between fan speed
and power consumption.
| Ps
= P1(S)2 |
| S
= Fan speed as a fraction of full fan speed |
| PS
= Power consumption at a fan speed of S |
| P1
= Power consumption at full speed |
If
a fan is running at a reduced speed under normal environmental
conditions the potential for power saving
is very high.
Example: A 1200 CFM blower uses 200 Watts of power at
full speed. How much power would be required
to run the blower at 600 CFM (1/2 speed)?
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P1/2
= 200(1/2)2 |
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P1/2
= 50 Watts |
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Increased
Fan Life
One of the most intriguing benefits of adaptive
cooling and ventilation is its effect on system
reliability.
One might think that adding another feature to any
system could only decrease its mean time between
failure (MTBF). This is not the case with speed
control.
Bearing failure, caused by heat and wear, is the
most common cause of fan and blower failure. By
allowing
air movers to run at reduced speeds for much of
the time, speed control actually increases fan life.
Typical
MBTF for speed control circuitry is between 106
and 107 hours. Typical fan MBTF is 105hours
at full speed.
The increased fan MBTF at reduced speeds significantly
outweighs the negative impact of the addition of
the
control circuitry.
Automatic Compensation for Changes in Cooling
System Configuration
Because SmartFan turns a flow-regulating device
(a fan) into an environmental (temperature, humidity
or pressure) regulating device, it will automatically
adjust to changes in cooling system configuration
such as: |
- Changes
in altitude
- Clogged,
blocked or missing filters
- Single
fan failure in a multiple fan system
- Opening
and closing of windows and doors (HVAC application)
- Adding
or removing circuit cards (electronics application)
- Fluctuations
in voltage or frequency
- Variations
in fan performance (fan choice less critical)
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Alarm
Byproduct
Since temperature, humidity or pressure must be continually
monitored in adaptive cooling or ventilation
applications, the cost of adding an alarm output to indicate
abnormal environmental conditions is minimal.
Sensors placed in one or more locations can monitor temperature,
humidity or pressure and send a signal
to light an LED, sound an audible alarm, trip a relay
or send a logic signal.
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Other
System Dependent Benefits
In electronics applications, fluctuations of semiconductor
junction temperatures are a major cause of component
failure. By designing a closed loop cooling system around
temperature and adding the proper temperature slope
(sensor temperature at full speed - sensor temperature
at idle speed), semiconductor junction temperatures can
be held to near constant. As the air velocity over a typical
semiconductor device changes by two-to-one, as it
does when fan speed changes from full speed to half, the
difference between the air temperature and junction
temperature changes by a few degrees. Changing the air
temperature by an equal number of degrees (3°C to
6°C
for most off-the-shelf units) cancels this effect to hold
absolute junction temperature near constant, improving
product reliability.
For HVAC applications, controlling fans automatically
to regulate environmental conditions saves energy, adds
comfort and convenience, and reduces acoustical noise.
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Adaptive
Cooling System Design for Electronics
Balancing Benefits
The key to designing an effective adaptive cooling system
for electronics applications is striking the right
balance between reduced noise, energy savings, fan life,
constant semiconductor junction temperatures vs.
higher semiconductor junction temperatures at reduced
fan speeds.
Cooling
System Variables
Fan (air flow & pressure)
Enclosure Design (air flow path)
Heat Sources (power dissipation of devices)
Heatsinks (thermal resistance)
Using thermal analysis computer modeling early in the
product design is vital in determining your cooling
system variables. Contact CRI customer service for informantion
reguarding thermal analysis services.
"Courtesy of Harvard Thermal"
Click on image
above for larger version.
Once these
variables are established, the equipment's temperature
rise, Delta T can be determined.
Equipment
Temperature Rise, Delta T = TE - TA1
TE
= Exhaust temperature of equipment at full fan speed
under normal operating conditions, ie: 20°C, sea
level, clean filters, typical power usage.
TA1
= Ambient room temperature under normal operating conditions
ie: 20°C, sea level, clean filters, typical
power usage.
The temperature rise (or slope) of a particular fixed
fan speed system will remain constant reguardless of
the
inlet air temperature. In Figure 4 below, the equipment
temperature rise is 10°C
Figure 4
Click
on image for larger version
Adding
Variable Fan Speed to the Mix
To add variable fan speed to the mix, we need to determine
what the slowest fan speed (idle speed) will be
and what the Control Slope (sensitivity) TSwill
be.
Idle speed should be set so that under normal operating
conditions, semiconductor junction temperatures and fan
noise
levels are at a satisfactory level. Using computer modeling,
semiconductor junction temperatures can be
calculated at
various fan speeds (noise levels). Consult MTBF and
noise specifications as determined for your product.
If the goal is to maintain constant critical semiconductor
junction temperatures, the Control Slope Ts should be
equal to the temperature rise of semiconductor surface or
junction temperatures between fan idle speed (as
determined
above) and full fan speed.
Now
that these variables have been determined, one can calculate
a Control Temperature.
Control
Temperature Tc = The sensor (exhaust) air temperature
above which fans run at full speed.
TC
= TA2 + TS + (TE -
TA1) / S
TA2
= The ambinent room temperature below which fans will
run at the idle speed. To get maximum noise control
benefits under normal environmental conditions, this
temperature should be close to TA1 as determined above.
TS
= Control Slope (sensitivity)
S = Fan idle speed as a percent of full speed.
Using the 10°C equipment temperature rise example
above, selecting a 6°C control slope (TS),
a 53% idle speed
(S) and a 20°C ambient idle speed setting (TA2),
we calculate a control temperature of 45°C and a
control curve
like the one illustrated in Figure 5 below.
Figure
5
Click
on image for larger version
Alternate
Method (sensing surface temperature)
The
surface temperature of one or more critical semiconductors
can be sensed and the fan speed can be varied to
hold this temperature near constant.
To
do this properly, the control slope must be as small
as possible. We have found that a 3 - 4°C slope
is about
as low as one can go without introducing hystoresis
or "hunting". That is, fans would tend to
constantly cycle
up and down in speed "hunting" for an acceptable
speed.
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Adaptive
Cooling and Ventilation for HVAC
In HVAC cooling and ventilating applications such as;
- Clean
Room Pressurization (pressure control)
- Attic,
Whole House or Duct fans (temperature or pressure
control)
- Livestock
Ventilation (temperature control)
- Ceiling
Fans (temperature control)
- Indoor
Pools and Spas (humidity control)
- Locker
Rooms and Bathrooms (humidity control)
- Industrial
Equipment (temperature, humidity or pressure control)
SmartFan
saves energy, reduces noise, increases fan life, compensates
for changes in system configuration
and provides the end user with the comfort and convenience
of automatic control.
SmartFan Alarms
Should an air mover in a cooling or ventilating application
fail to operate properly, the results can be catastrophic.
In electronics equipment, failure of an air mover may
cause operational failure or even permanent damage.
In ventilating applications, contaminants may build
to dangerous levels, equipment may fail or livestock
may perish.
Here
are three effective methods of detecting air mover failure.
Temperature
or Humidity Alarms
Measuring temperature (air or surface) or humidity is
fundamental. In combination with speed control it adds
little
cost because the sensor used to control speed can also
provide a signal to the alarm circuitry. One disadvantage
is a possible delay after air mover failure, before
the alarm is triggered. A temperature or humidity alarm
will of
course also respond to failures other than the air mover.
For example, operating equipment at excessive ambient
temperature or with a clogged air filter could also
trigger the alarm.
Tach
Alarms
Air mover speed is sensed using pulses generated by
a Hall Effect device installed in the fan. Below a
predetermined speed, an alarm is triggered. This method
is very reliable, however it requires a special fan
with a Hall Effect output. Because the Hall Effect device
is part of any brushless DC fan, the output adds
little cost. This feature may however add significant
cost to an AC fan and may not be available on all models.
| Alarm
Pros & Cons |
| Tach
Alarm |
Temperature
Alarm |
| No
external sensor required |
Sensor
location critical |
| Quick
response time |
Slower
response time |
| Can
diagnose fan failure |
Cannot
diagnose fan failure |
| Requires
fan with tach output |
Uses
normal fan |
| Will
only signal fan failure |
Will
signal all types of thermal
failure scenarios |
| Can
be expensive in a
multiple fan system |
Economical,
(especially when used in conjunction
with fan speed control) |
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Alarm Outputs
Alarm outputs may be referenced to the negative terminal
of a fan or can be optically isolated. In applications
requiring higher switching (sinking) currents or voltages,
the standard optical isolator can be replaced with a
normally closed or normally open MOS relay. Alarms are
available as an:
- Electrical
signal to drive logic
- Visual
signal (LED)
- Audible
signal (piezo alarm)
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Control Resources Incorporated
11 Beaver Brook Road
Littleton MA, 01460 USA
Tel: 978-486-4160
Fax: 978-486-4772
email: sales@controlres.com
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