목차
- Drying and baking exhaust typically leaves the process at 50–90°C and 15–40 g/kg absolute humidity. Recovering that energy cuts fuel or electricity by 15–40%, but the wrong recovery device can re-wet the product, contaminate the supply air, or freeze in winter. Plate and rotary-wheel exchangers solve the same problem in fundamentally different ways, so the selection rule is not "which is more efficient" but "which failure modes the process can tolerate".
- Plate (fixed-plate) exchanger Counter-flow or cross-flow stacks of thin aluminium or polymer plates. Sensible-only plates transfer heat with zero moisture carry-over. Polymer (enthalpy) plates add selective water-vapour transfer while still blocking aerosols and odours. No moving parts, no rotating seal, no cross-stream leakage path. Rotary wheel A slowly rotating desiccant-coated or all-aluminium matrix absorbs heat and moisture on the exhaust side and releases both to the incoming air as it turns. Typical effectiveness is 70–85% for heat and 60–80% for moisture, but a fraction of exhaust always leaks into the supply through the purge-sector gap.
- Aspect Plate (sensible) Rotary wheel Moisture recovered No (sensible only) Yes (latent + sensible) Cross-contamination risk None (separated streams) Low but non-zero (purge sector) Re-wetting product air Impossible Possible if wheel saturated Frost / condensation Easy to bypass or pre-heat Needs frost-control sector Moving parts / maintenance None Belt, bearing, seal Pressure drop Moderate Moderate, flow-dependent Best fit High-humidity, no-return-air duty Need both sensible + latent recovery
- Plate sensible is the default when exhaust is very humid and the product must not be re-wetted — grain, food, and most industrial drying fall here. Avoid the rotary wheel where return air is forbidden by code or food-safety practice (pharmaceutical exhaust, anywhere "no re-circulation" is specified). Avoid the wheel in mine ventilation and data-centre cooling — rotation, seal leakage and servicing downtime outweigh the extra latent recovery. Choose the wheel only when you genuinely need both sensible and latent recovery and can tolerate a small exhaust leak plus the maintenance of a rotating assembly.
- For the large majority of industrial drying exhaust-heat recovery, the fixed plate sensible exchanger is the safer, lower-maintenance choice: it recovers heat without moving moisture or contaminants. Reach for a rotary wheel only when latent recovery is a hard requirement and a purge sector is acceptable.
- Worked Example — Sensible Recovery on a Grain Dryer
디rying and baking exhaust typically leaves the process at 50–90°C and 15–40 g/kg absolute humidity. Recovering that energy cuts fuel or electricity by 15–40%, but the wrong recovery device can re-wet the product, contaminate the supply air, or freeze in winter. Plate and rotary-wheel exchangers solve the same problem in fundamentally different ways, so the selection rule is not "which is more efficient" but "which failure modes the process can tolerate".
Plate (fixed-plate) exchanger
Counter-flow or cross-flow stacks of thin aluminium or polymer plates. Sensible-only plates transfer heat with zero moisture carry-over. Polymer (enthalpy) plates add selective water-vapour transfer while still blocking aerosols and odours. No moving parts, no rotating seal, no cross-stream leakage path.
Rotary wheel
A slowly rotating desiccant-coated or all-aluminium matrix absorbs heat and moisture on the exhaust side and releases both to the incoming air as it turns. Typical effectiveness is 70–85% for heat and 60–80% for moisture, but a fraction of exhaust always leaks into the supply through the purge-sector gap.
| Aspect | Plate (sensible) | Rotary wheel |
|---|---|---|
| Moisture recovered | No (sensible only) | Yes (latent + sensible) |
| Cross-contamination risk | None (separated streams) | Low but non-zero (purge sector) |
| Re-wetting product air | Impossible | Possible if wheel saturated |
| Frost / condensation | Easy to bypass or pre-heat | Needs frost-control sector |
| Moving parts / maintenance | None | Belt, bearing, seal |
| Pressure drop | Moderate | Moderate, flow-dependent |
| Best fit | High-humidity, no-return-air duty | Need both sensible + latent recovery |
- Plate sensible is the default when exhaust is very humid and the product must not be re-wetted — grain, food, and most industrial drying fall here.
- Avoid the rotary wheel where return air is forbidden by code or food-safety practice (pharmaceutical exhaust, anywhere "no re-circulation" is specified).
- Avoid the wheel in mine ventilation and data-centre cooling — rotation, seal leakage and servicing downtime outweigh the extra latent recovery.
- Choose the wheel only when you genuinely need both sensible and latent recovery and can tolerate a small exhaust leak plus the maintenance of a rotating assembly.
For the large majority of industrial drying exhaust-heat recovery, the fixed plate sensible exchanger is the safer, lower-maintenance choice: it recovers heat without moving moisture or contaminants. Reach for a rotary wheel only when latent recovery is a hard requirement and a purge sector is acceptable.
Worked Example — Sensible Recovery on a Grain Dryer
To make the selection rule concrete, size a plate sensible exchanger for a typical grain-drying exhaust and compare it with a rotary wheel on the same duty.
Given duty
| Parameter | Value | Note |
|---|---|---|
| Exhaust flow | 20,000 m³/h | ≈ 6.67 kg/s at 1.2 kg/m³ |
| Exhaust temperature | 75 °C | Leaving the dryer |
| Inlet (make-up) air | 15 °C | Ambient |
| Mean ΔT | 60 K | Driving potential |
| Specific heat cp | 1.005 kJ/kg·K | Air |
| Plate sensible effectiveness η | 75 % | Counter-flow aluminium |
| Rotary wheel effectiveness | 80 % sensible / 70 % latent | Desiccant-coated |
| Annual run time | 4,000 h | Seasonal drying |
Calculation
Recovered sensible power:
Q = ṁ · cp · ΔT · η = 6.67 · 1.005 · 60 · 0.75 ≈ 301 kW
- Annual energy recovered ≈ 301 kW × 4,000 h ≈ 1.20 GWh.
- At an electricity-equivalent value of $0.04/kWh, that is about $48,000/year of avoided heating cost.
- A typical 20,000 m³/h plate unit costs roughly $40,000 installed, giving a payback under 12 months.
Plate vs Wheel on the Same Duty
| Metric | Plate (sensible) | Rotary wheel |
|---|---|---|
| Recovered heat | 301 kW (sensible) | ~321 kW sensible + latent |
| Moisture to supply | None | Yes (re-wets make-up air) |
| Fit for grain drying | Recommended | Not advised (product must stay dry) |
| Maintenance | None | Belt + seal service |
The wheel recovers a few percent more energy, but on a drying duty the re-wetted make-up air fights the dryer — so the plate is the correct economic and technical choice despite the slightly lower number.