Cooling System Design Tips
Main ConsiderationsAir 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. 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. 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. 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 and Nimbus-HP 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%.
Temperature Control & Product Operating PrinciplesRecommended 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.
|For 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|
- Larger equipment
- Machine room equipment
- Equipment designed for use in both normal and high temperature environments, e.g., office or storage room
- Good choice for many purposes
- Equipment of moderate size intended for installation in open offices, hospitals
- Smaller equipment
- Equipment to be installed in private offices and homes
- Equipment that will be used in proximity to an operator (e.g., desk-mounted)