How Do You Size an Industrial Power Supply Cooling Fan?

A smaller fan can replace the original only if it moves enough air through the actual power-supply restriction to hold internal components within their permitted temperatures. Diameter affects the fan’s possible operating range, but it does not directly state cooling capacity. Follow the airflow from inlet to outlet, calculate the required heat removal, then compare both fans at the required static pressure.

Where Does the Cooling Air Travel?

Layer one first: map the physical air path before comparing fan specifications. The fan draws air through an inlet, filter, guard, grille, wiring space, heat sinks, internal components, and an outlet. Every restriction consumes pressure. Recirculation around an incomplete seal can produce an acceptable fan flow while bypassing the components that need cooling.

Air-path point	Recurring failure	Commissioning check
Inlet and filter	Blocked area or excessive filter resistance	Inspect the full inlet area and record pressure drop if instrumentation is available.
Fan mounting	Wrong direction, leakage, or grille too close to the blades	Confirm the airflow arrow and seal the fan frame to the intended opening.
Heat sinks and components	Air bypasses hot surfaces	Use smoke or another safe airflow indicator with power isolated.
Outlet	Backpressure or discharged air returns to the inlet	Check outlet clearance and separation from the inlet.

A smaller footprint may reduce the effective inlet or outlet area even when the catalog airflow looks adequate. Establish that the replacement can use the same controlled air path, and verify direction, sealing, and clearances before calculating capacity.

What Heat Load Must the Fan Remove?

Use the power supply’s dissipated heat, not its electrical output rating, as the thermal load. Obtain dissipation from manufacturer data or measure electrical input power and delivered output power at the target load; their difference is the heat that must leave the assembly, apart from any separately measured energy path.

For dry-air sensible cooling in English units, the stated relationship is:

Q = 1.08 * cfm * DT

Q is heat transfer in BTU/hr, cfm is volumetric airflow in cubic feet per minute, and DT is the air-temperature rise in degrees Fahrenheit. Rearranging gives the required airflow:

cfm = Q / (1.08 * DT)

The coefficient 1.08 bundles air-density, heat-capacity, and time conversions. Actual density changes with inlet conditions and elevation, so use the appropriate air properties when those effects matter. The equivalent general relationship is sensible heat rate equals mass flow multiplied by specific heat and temperature rise.

Calculation	Expression	Use
Sensible heat	Q = 1.08 * cfm * DT	Normal electronics cooling without a phase change
Sensible plus latent heat	Q = 19 * cfm * Dh	Processes where an enthalpy change must be included

In the second expression, Dh is the enthalpy difference in BTU/lbm. Latent capacity normally does not govern a ventilated power supply unless moisture treatment or condensation is part of the design. Confirm the heat load and allowable inlet-to-outlet temperature rise before selecting airflow.

What Fan Operating Point Must the Replacement Meet?

Catalog airflow and diameter do not define installed airflow. A fan curve relates flow to static pressure; the enclosure has a system-resistance curve. Their intersection is the operating point. A free-air rating represents little external resistance and can overstate flow through filters, guards, narrow passages, and heat sinks.

Measure static pressure across the relevant air path at the original fan’s operating condition, or derive the requirement from an enclosure resistance curve. Then read the original and proposed fan curves at that pressure. Compare like-for-like air conditions and use the manufacturer’s stated measurement basis.

Setting or quantity	Original fan	Proposed fan	Acceptance basis
Airflow at required static pressure	Read from curve or measure	Read from curve or measure	Meets calculated heat-removal airflow
Mounting opening and seal	Record installed arrangement	Document adapter or revised opening	No bypass path
Electrical supply	Read fan data	Read fan data	Compatible voltage, current, and control method
Airflow direction	Record	Match intended path	Air crosses the required components

The proof at this stage is a proposed operating point that supplies at least the calculated airflow at the measured or specified system pressure.

How Can a Smaller Fan Produce the Required Flow?

A smaller fan generally needs greater speed to recover flow. For geometrically similar fans operating in comparable air density, the fan-law relationships supplied for comparison are:

Flow is proportional to Diameter^3 * RPM
Pressure is proportional to Diameter^2 * RPM^2

These proportional relationships are scaling tools, not substitutes for the proposed fan’s curve. Changing blade geometry, hub ratio, motor construction, housing, or mounting invalidates a direct diameter-and-speed comparison. Higher rotational speed can also change electrical load, acoustic output, vibration, and service life; obtain those limits from the proposed fan’s data.

1. Calculate the airflow required from the heat load and permitted DT.
2. Establish the static pressure imposed by the real air path.
3. Select a fan whose curve crosses that duty point.
4. Check supply compatibility, starting behavior, control input, alarm output, mounting, and environmental ratings against the existing design.
5. Review the fan curve near the duty point so normal filter loading or production variation does not immediately move airflow below the requirement.

Before purchase approval, verify the duty point on the actual proposed fan curve rather than extrapolating from diameter alone.

How Should the Replacement Be Installed and Commissioned?

The installation must preserve both the designed flow path and the fan’s electrical interface. An adapter plate can reduce the opening, create a leakage slot, or place a grille close enough to disturb blade inlet flow. Route wiring outside the inlet area and keep loose conductors away from the rotor.

1. Record the original fan orientation, connector functions, mounting depth, opening area, and clearances.
2. Fit the proposed fan and seal unused portions of the former opening.
3. Confirm the supply and any speed-control or alarm connections before energizing.
4. Start the fan and confirm rotation, airflow direction, abnormal vibration, and contact-free operation.
5. Measure airflow or static pressure using the same method and locations used for the baseline.
6. Run the power supply at the defined test load and inlet-air condition until temperatures stop trending upward.

Do not infer successful cooling from fan rotation or outlet velocity at one point. Proceed only after the measured operating condition matches the selected duty point and air reaches the intended heat-transfer surfaces.

How Is End-to-End Cooling Performance Verified?

Verify the complete path under the most demanding defined operating condition: inlet air enters without recirculation, the fan overcomes system resistance, flow crosses the critical components, and heated air exits without returning to the inlet. Measure inlet temperature, outlet temperature, and the temperatures of the components that set the thermal limit.

Compare the measured air-temperature rise with the DT used in the airflow calculation. If temperature rises beyond the design value, inspect restrictions, bypass leakage, recirculation, fan speed, and the actual load before increasing the nominal CFM rating. Repeat the test with the production guard, filter, cover, wiring, and mounting hardware installed. Approve the change only when temperatures stabilize below the applicable component limits and the fan remains at the required operating point.

Frequently Asked Questions