Refrigerated warehouses, walk-in coolers and freezers are among the most energy-intensive buildings on an industrial site. Two forces drive that load. First, the building envelope continuously leaks heat inward, so compressors run to pump it back out. Second, defrost cycles periodically dump large, concentrated heat into the airstream to melt frost off the evaporator coil. Much of that energy, plus heat from condenser relief and personnel areas, is simply vented. Air-to-air recovery captures part of it instead of letting it escape.
Where the recoverable heat actually sits
Three streams are worth engineering attention. Defrost relief air leaves the evaporator warm and very moist during an electric or hot-gas defrost; a -20°C freezer may briefly see this stream at -5 to +10°C. Condenser heat is rejected at 25 to 40°C and can pre-heat make-up air for dock offices. Buffer and personnel rooms at +12 to +18°C are often heated electrically and can instead be fed by recovered warmth. Identifying which stream is clean, which is moist, and which is remote decides the technology. In practice the defrost and buffer-room pair is the easiest win because both streams are cool and close together.
Which recovery technology fits a cold store
- Run-around coils when the cold and warm airstreams are in separate rooms or buildings and any cross-contamination path is unacceptable. A glycol loop carries the energy between them with no air mixing.
- Plate recuperators for same-room exhaust-to-supply pairs where both streams are clean and physically close.
- Heat-pipe sections for passive, maintenance-light duty at remote sites; they need no pump or controls and tolerate freezing well.
Frost risk on the recovery coil itself
The real danger is that the cold-side face of your recovery coil frosts up. If you pre-heat incoming freezer make-up air with warmer exhaust, keep the cold exhaust on the side that stays above its dewpoint, or add a pre-heat coil ahead of the recuperator. A common design rule is to hold the cold-face temperature above roughly 2°C to avoid ice build-up that would block airflow. Our frost-control guide covers this in detail, including bypass and recirculation strategies during deep cold snaps.
| Stream | Approx. temp | Typical effectiveness |
|---|---|---|
| Defrost relief air | -5 to +10°C | 65% to 75% |
| Buffer-room make-up | -5 to +15°C | 70% to 80% |
| Condenser pre-heat | +25 to +40°C | 60% to 70% |
Worked example and payback
Take 8,000 m³/h of defrost relief at -5°C being used to warm buffer-room make-up air toward +15°C. Air mass flow is 8,000 ÷ 3,600 × 1.2 ≈ 2.67 kg/s. With a 70% effective plate unit the recovered load is 2.67 × 1.0 × 20 × 0.70 ≈ 37 kW. Over 2,000 operating hours that avoids about 74,000 kWh/year of electric heating. At $0.10/kWh that is $7,400/year; a modest packaged unit often pays back in under three years, faster in cold climates where the heating season is long.
Specification checklist
- Map every exhaust and relief stream and tag it clean, moist or remote before choosing a device.
- Keep the coldest airstream on the side that stays above 2°C to prevent coil icing.
- Choose run-around coils wherever streams are in separate rooms to avoid any air cross-path.
- Add an automated bypass for extreme cold so recovery never fights the refrigeration load.
- Meter recovered kW and compare against the defrost schedule to confirm savings.
- Size the make-up air heater as a trim, not the primary, heat source.
For the broader picture see the industrial recovery overview, the efficiency deep-dive and the sizing guide.
Specifying a unit for your project?
EN159 builds plate, rotary, heat-pipe, run-around coil and counterflow air-to-air exchangers. Send airflow (m3/h), inlet temperatures and available space and we will size it.
Request a quote