The Frost Problem

When cold outside air meets warm, moist exhaust, the exhaust surface can drop below 0 C and ice forms. Left unchecked, frost blocks channels and collapses effectiveness.

Selection Choices That Help

• Wider channels - slow frost buildup and ease defrost
• Preheat coil - warm intake air above the frost point
• Defrost bypass or recirculation - periodic warm-air purge
• Frost-resistant media - hydrophobic surfaces

Technology Note

Rotary wheels handle frost well via purge sectors and modulation, but need controls. Plate cores need an explicit defrost strategy set from the coldest expected temperature, not the annual average.

Climate Thresholds

Below about -2 C exhaust dewpoint, plan active frost protection. The colder the design temperature, the more robust the strategy must be.

Verify

Model the worst week of the year, not the average, and confirm the controls actually trigger defrost at the set point.

Pressure Drop Is Part of the Cost

Every exchanger adds static pressure. The fan must deliver the design airflow at the new total pressure, and fan power rises with both flow and pressure. Ignore this and you can lose a chunk of the recovered energy to the fan.

Typical Ranges

• Plate core: 50-150 Pa per side at design velocity
• Rotary wheel: 100-250 Pa including the wheel and seals
• Heat pipe: 40-120 Pa, usually the lowest

Sizing the Fan

Take the clean-filter pressure drop from the datasheet, add duct and filter losses, then select a fan with adequate static capability and check its efficiency at the operating point.

Trade-off With Effectiveness

Lower face velocity raises effectiveness but enlarges the core and can change pressure drop non-linearly. The optimum is where the value of recovered energy minus extra fan energy is maximised.

Verification

During commissioning, measure actual pressure drop and compare with the design; a large gap signals fouling or a wrong selection.

The Core Formula

Recovered power Q = m_dot x cp x DeltaT x effectiveness, where m_dot is mass flow (kg/s), cp for air is about 1.006 kJ/kg.K, and DeltaT is the temperature difference between streams.

Worked Example

For a 10,000 m3/h stream (density 1.2 kg/m3 -> 3.33 kg/s) at a 20 K difference with 70 percent effectiveness: Q = 3.33 x 1.006 x 20 x 0.70 is about 46 kW removed from the fresh-air heating load.

From Power to Size

• Choose target effectiveness from the device type
• Pick face velocity from the manufacturer's band
• Face area = airflow / face velocity; core volume follows from depth

Don't Forget

• Use part-load average airflow, not peak, for realistic savings
• Add the extra fan power from pressure drop to the energy balance
• Include frost-protection energy in cold climates

Tools

Use our sizing spreadsheet or an online calculator to vary inputs and watch payback change before you commit to a size.

The Three Main Options

Decision Path

• Need peak effectiveness + humidity control? -> Rotary wheel
• Need isolation + simplicity? -> Plate core
• Need rugged, no-moving-parts, sensible only? -> Heat pipe
• Need humidity control but no moving parts? -> Membrane plate

Secondary Filters

Also weigh space, available fan pressure, climate (frost risk), and maintenance staff. The "best" device is the one that meets the duty at the lowest lifecycle cost.

Next Step

Once the technology is chosen, move to the specific selection guide for that device and size it against your duty.

What You Are Selecting

A heat pipe exchanger is a bundle of sealed tubes with a working fluid. It transfers sensible heat passively - no motor, no airstream mixing. Effectiveness is lower (50-65 percent) but reliability is very high.

Key Selection Parameters

• Tube bundle size - more tubes and surface area raise capacity
• Working fluid - chosen for the operating temperature range
• Orientation - evaporator end must sit in the hot stream, condenser in the cold
• Fin spacing - tighter fins improve transfer but foul faster

Sizing Approach

Estimate the sensible load to recover, then size the bundle so the temperature lift stays within the fluid's effective range. Heat pipes are often used where a failed moving part would be costly.

Best-Fit Applications

• Remote or harsh environments
• Industrial extracts where reliability beats peak efficiency
• Any space where zero cross-contamination is mandatory

Verify

Confirm the rated effectiveness at your airflow and the fluid's temperature limits; heat pipes do not recover moisture.

Why a Wheel

A rotary wheel is a slowly spinning matrix that can reach 80-85 percent sensible effectiveness and 70-80 percent latent (enthalpy) effectiveness - the highest of any air-to-air device. It also recovers moisture, which matters in humid climates.

Key Selection Parameters

• Wheel diameter and depth - deeper media holds more energy per rotation
• Media type - aluminium (sensible) or molecular-sieve coated (enthalpy)
• Rotation speed - set so the wheel spends enough time in each stream
• Seal quality - better seals mean less stream carryover

Sizing Approach

Define the required sensible and latent recovery, then select wheel size and speed to meet both at your design airflow. Because a wheel is compact for its capacity, it suits large air-handling units.

Watch-outs

• Slight carryover between airstreams - avoid in strict containment zones
• Motor, seals, and bearings need a maintenance plan
• Summer bypass is needed when no recovery is wanted

Verify

Check the certified effectiveness (Eurovent or AHRI 1060) at your operating point, not the headline number.

Start With the Duty

Plate exchangers are stationary, no-moving-parts cores. Selection begins with supply and exhaust airflow, indoor and outdoor design temperatures, and the allowable supply-air temperature.

Key Selection Parameters

• Face area - larger area at lower face velocity gives higher effectiveness
• Configuration - counterflow (70-75 percent) outperforms crossflow (60-65 percent)
• Material - aluminium for cost, polymer or epoxy-coated for corrosive or hygiene duty
• Separators - brazed, bonded, or welded depending on pressure and temperature

Sizing Rule of Thumb

Pick a target effectiveness (often 70 percent), then size the core so face velocity stays in the manufacturer's recommended band. Too high a velocity lowers effectiveness and raises fan power.

When Plate Is the Right Choice

• You want zero moving parts and minimal maintenance
• Air streams must stay fully separated (labs, hospitals)
• Sensible-only recovery is sufficient for your climate

Verify

Confirm the selection against an EN 308 or AHRI 1060 rating at your actual operating point, and check the added pressure drop against your fan's capability.