Adaptive Cooling using Fan Speed Control

What is Adaptive Cooling using Fan Speed Control?

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, variable fan speed control should be used to maintain 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 fan speed control system senses environmental conditions such as temperature (air, surface or liquid), humidity or pressure and holds it nearly constant. A sensor continually monitors the environmental condition. The fan speed control powers the air movers, automatically adjusting speed over a predetermined range. A properly designed adaptive cooling or ventilating fan speed control 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
fan speed control closed loop
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Figure 1
Closed loop Fan Speed Control
fan speed control pressure sensing
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Figure 2
Pressure sensing Fan Speed Control

Noise Reduction using Fan Speed Control
A significant advantage of adaptive cooling using fan speed control 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 which 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?

L1/2 = 59 + 50 log (½)
L1/2 = 59 -15
L1/2 = 44 dB(A)

Energy Savings using Fan Speed Control
A well designed adaptive cooling or ventilating fan speed control 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)?

P1/2 = 200(1/2)2
P1/2 = 50 Watts

Increased Fan Life using Fan Speed Control
One of the most intriguing benefits of an adaptive cooling and ventilating fan speed control system 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 fan 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, fan speed control actually increases fan life. Typical MBTF for fan 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 fan speed control circuitry.

Automatic Compensation for Changes in Cooling System Configuration using Fan Speed Control Because fan speed control 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)

Alarm Byproduct of Fan Speed Control
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.

Other System Dependent Benefits using Fan Speed Control In electronics applications, fluctuations of semiconductor junction temperatures are a major cause of component failure. By designing a closed loop fan speed control 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, using fan speed control to regulate environmental conditions saves energy, adds comfort and convenience, and reduces acoustical noise.

Click on the following link for Fan Speed Control Adaptive Cooling Examples.

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