At an open industrial door, two air streams (cold outdoor inward, warm indoor outward) flow simultaneously through the same opening. Without obstruction they exchange up to 0.8 m³/s per m² of door area. For a freight door 4 × 4 m that means 12.8 m³/s of air exchange, an energy hole no heater can keep up with.
Operating principle
An air curtain creates a high-velocity air jet across the opening. With correct setup (flow angle 15° outward from vertical, exit velocity 8–12 m/s) it prevents turbulent mixing between the two sides. Effective seal: 65 to 90 % of the energy loss through the door is eliminated.
Three types by design
1. Vertical ambient (unheated). Airflow from the top of the door downward, no active heating. Cheapest type (€1 200–3 500), suitable for doors up to 3 m height and moderate ΔT (< 15 K). Solaronics water- and electric-heated SW/SE range covers this segment. Annual energy-loss reduction: 40–55 %.
2. Vertical heated (gas or electric). Airflow heated to 25–35 °C before exit. Efficiency: 65–80 % loss reduction. Cost €3 500–8 500 plus gas (G20/G31) connection or electricity (>15 kW supply).
3. Side-mounted lateral curtain (industrial heavy-duty for large doors). For doors above 4 m height or > 5 m width. Two units on either side of the door create a horizontal screen. Efficiency: 80–90 %, cost €8 000–18 000 per pair. Solaronics partners on this type: specified case by case after project audit.
Selection by door height
| Door height | Curtain type | Solaronics recommendation |
|---|---|---|
| up to 2.5 m | Vertical ambient | Solaronics SW/SE range (in-catalog) |
| 2.5–4 m | Vertical heated (gas or electric) | Solaronics MRA gas range |
| 4–6 m | Vertical heated, exposed turbine | Solaronics MRX condensing range |
| above 6 m | Side-mounted lateral, two units | Per project specification |
Flow calculation
Curtain flow must cover the minimum air exchange through the door at the operating ΔT. Formula:
Q (m³/s) = V × 0.7, where V is the door-opening volume (m³) = width × height × equivalent depth (1 m).
Example: door 4 × 4 m → V = 16 m³ → Q = 11.2 m³/s = 40 320 m³/h of flow.
Commercial curtains state their flow in m³/h. Pick a unit with at least 1.1× the calculated value, for margin on windy days.
Correct installation
- Door-open sensor: the curtain activates automatically on door open, deactivates on close. Without a sensor the curtain runs all day, wasting electricity (5–15 kW supply).
- Flow direction: 15° outward from vertical, never inward. Inward tilt causes turbulence and degrades performance.
- Gap between ceiling and the curtain outlet slot: 30 cm max. Larger gaps cause the curtain to break up before reaching the floor.
- No obstruction under the outlet: a forklift, conveyor, or rack under the door means the curtain never reaches the floor and cold air flows around the gap.
TCO threshold
Door 4 × 4 m, 12 minutes of open time per day (warehouse average), 250 working days. Annual air exchange: 12 × 60 × 12.8 = 9 216 m³ per day × 250 = 2 304 000 m³.
Heating this air from −2 °C (winter average) to 14 °C (warehouse target) = 2 304 000 × 0.34 × 16 / 1000 = 12 530 kWh per year = €5 010 at €0.40/kWh of gas.
Solaronics MRA curtain (≈ €5 000) at 75 % efficiency: €3 760 annual saving. Payback: 16 months.