Select a cold storage fan by matching three things: the evaporator airflow needed to hold the room temperature difference at 5–8 K, an air throw that reaches the far wall, and a motor type whose waste heat and low-temperature durability suit the duty. For chill rooms above 0 °C, EC or PSC axial motors are usually adequate; for freezer rooms at −18 to −30 °C, sealed EC or three-phase induction motors with low-temperature grease and IP54–IP55 protection are the practical choice because shaded-pole motors waste 70–75 % of their input as heat inside the cold envelope.
Step 1: Set the airflow from the coil duty and air-side temperature difference
Evaporator airflow follows from the sensible duty and the air temperature rise across the coil. For air at typical cold-room density, Q ≈ 3,000 × kW / ΔT in m³/h, where ΔT is the air-side temperature difference in kelvin. A 20 kW unit cooler running a 6 K air-side difference therefore needs roughly 10,000 m³/h. Chill rooms usually run 4–6 K, freezer rooms 5–8 K, and blast freezers 8–12 K with much higher face velocity.
Coil face velocity is normally kept to 2.0–3.5 m/s in storage rooms. Below 2.0 m/s the coil under-performs; above about 4.0 m/s water and frost carry-over begins and the fan static rises steeply. Static pressure for a bare unit cooler is only 50–150 Pa, but a ducted penthouse or a coil with an air sock can reach 250–450 Pa, which changes the fan family entirely.
Step 2: Check the air throw against room geometry
A cold store fails on temperature uniformity far more often than on total capacity. The rule of thumb is that the evaporator must throw air to the far wall so that the return sweeps the whole floor plate. Typical throw is 25–35 times the fan diameter for a well-designed axial unit cooler: a 500 mm fan throws roughly 12–18 m, and a 630 mm fan roughly 16–22 m. A 40 m long room served from one end therefore needs either two fan banks, a centre-mounted unit throwing both ways, or a textile air sock.
- Storage rooms: 0.3–1.0 m/s average air velocity in the racking aisles — enough to remove infiltration heat without drying product.
- Blast freezing: 2.0–5.0 m/s across the product face, which can cut freezing time by 40–60 % against still-air freezing.
- Fresh produce with high respiration heat: keep 0.5–1.5 m/s but hold 90–95 % relative humidity to limit weight loss.
- Leave at least 300–500 mm clear between the fan discharge and any stacked pallet, and never block the coil return face.
Step 3: Choose the motor type
| Motor type | Typical efficiency | Heat added to room | Speed control | Best duty |
|---|---|---|---|---|
| Shaded pole, single phase | 15–30 % | Very high | Voltage tap only | Legacy small chill rooms; avoid in new work |
| PSC (permanent split capacitor) | 30–50 % | High | Voltage or triac | Small chill rooms up to about 0.25 kW |
| EC / brushless DC | 70–90 % | Low | 0–10 V or Modbus, 10–100 % | Chill and freezer rooms, best energy case |
| Three-phase induction, IE3 | 75–88 % | Low | VFD | Large freezers, blast cells, ducted coils |
| Axial fan, T35-11 type with external motor | 65–75 % (fan) | None inside room | VFD | Ducted penthouses and jacketed cold stores |
The heat column matters more than it looks. Fan motors inside a freezer contribute their full electrical input as a cooling load, and that load must then be removed at a coefficient of performance of roughly 1.3–2.0 at −30 °C evaporating. In practice, every 1 kW of motor input inside the room costs 1.5–1.8 kW of total plant energy. Replacing 8 × 0.25 kW shaded-pole motors with EC equivalents typically cuts 1.2–1.5 kW of continuous load and pays back within 12–24 months on a 24/7 store.
Defrost strategy changes the fan specification
Frost is unavoidable below 0 °C surface temperature, and the defrost method dictates how the fan is controlled and built.
- Electric defrost: the simplest, but the fan must stop during heating and stay off for a 3–10 minute drip and re-freeze delay, otherwise melt water is blown into the room and forms floor ice.
- Hot gas defrost: faster and more efficient for large systems, typically 8–15 minutes against 25–40 minutes for electric; the same fan-off and drip-delay logic applies.
- Air or off-cycle defrost: only viable above about +2 °C room temperature, and here the fan runs continuously while the coil warms.
- Fan cycling on demand: reducing fan speed to 40–60 % during the long idle periods of a full store cuts fan energy by 75–90 % because power varies with the cube of speed, and it also reduces product dehydration.
Defrost frequency is usually 2–4 cycles per day for freezer rooms and 1–2 for chill rooms; terminating defrost on coil temperature rather than a fixed timer typically saves 5–10 % of plant energy.
Low-temperature construction requirements
Cold-store fans see thermal cycling, condensation and ice loading that ambient fans never experience. Specify class F or class H insulation with thermistor protection, low-temperature synthetic bearing grease rated to −40 °C, sealed bearings with an L10 life of at least 40,000 hours, and hot-dip galvanised or epoxy-coated steel guards and mounting rings. Aluminium or glass-reinforced polymer impellers are preferred over painted steel because they do not shed rust flakes into food areas. Cable glands must be rated for the low temperature too — standard PVC gland seals harden and crack below about −25 °C. Keruite produces low-temperature fan assemblies at its 80,000 m² Dezhou facility using an in-house AMCA-210-style test laboratory, with every rotor balanced to G2.5 before shipment.
When to choose which fan configuration
- Choose a standard axial unit cooler fan when the room is under about 20 m long, static pressure is below 150 Pa, and hygiene rules allow motors in the airstream.
- Choose EC motors whenever the store runs more than about 4,000 hours per year, or when the client wants demand-based speed control and Modbus monitoring.
- Choose three-phase induction motors with VFDs for blast freezers, ducted coils above 250 Pa, and any fan above roughly 3 kW.
- Choose an external-motor arrangement or a ducted penthouse when the room is below −25 °C, when washdown is frequent, or when motor heat inside the envelope must be eliminated.
- Choose a textile air sock instead of a longer throw when the product is humidity-sensitive and you need low, even velocity rather than a strong jet.
FAQ
How do you calculate evaporator fan airflow for a cold room?
Use airflow in cubic metres per hour of roughly 3,000 times the sensible kilowatts divided by the air-side temperature difference in kelvin. Chill rooms typically use 4 to 6 K, freezers 5 to 8 K and blast freezers 8 to 12 K, giving about 10,000 cubic metres per hour for a 20 kW coil at 6 K.
Are EC motors worth the extra cost in cold storage?
Usually yes. EC motors run at 70 to 90 percent efficiency against 15 to 30 percent for shaded pole units, and every kilowatt of motor input inside a freezer costs another 0.5 to 0.8 kilowatts of compressor energy. Payback on continuously running stores is typically 12 to 24 months.
Why must the fan stop during electric defrost?
A running fan blows melt water and warm moist air into the room, which forms floor ice and refreezes on the coil. Standard practice stops the fan through the heating phase and holds it off for a further three to ten minutes of drip and re-freeze delay before restart.
How far will a cold room evaporator fan throw air?
A well-designed axial unit cooler throws roughly 25 to 35 fan diameters, so a 500 mm fan reaches about 12 to 18 metres and a 630 mm fan about 16 to 22 metres. Rooms longer than that need two banks, a central twin-throw unit, or a textile air sock.
What air velocity should be used in a freezer storage room?
Storage rooms usually target 0.3 to 1.0 metres per second in the racking aisles, which removes infiltration heat without excessive product drying. Blast freezing is different and uses 2.0 to 5.0 metres per second across the product face to shorten freezing time substantially.