Face Velocity Defined

Face velocity is the airflow divided by the core face area. It sets both effectiveness and pressure drop.

The Trade-off

• Low velocity - high effectiveness, low pressure drop, but a larger (costlier) core
• High velocity - compact core, but lower effectiveness and higher fan energy

Typical Bands

Plate cores often run 1.5 to 3.0 m/s; rotary wheels similar; heat pipes a bit lower. The optimum is where net energy saving peaks.

How to Pick

Start from the manufacturer's recommended band, then check effectiveness and pressure drop at your actual flow. Avoid the top of the band unless space forces it.

Recovery in the Whole Building

A building's energy use is an energy balance: gains (people, equipment, sun) minus losses (envelope, ventilation). Recovery attacks the ventilation loss directly.

The Ventilation Load

Ventilation brings in untreated outside air, so it adds both a heating load in winter and a cooling load in summer. Recovery moves that energy back before the coil sees it.

Quantify the Impact

If recovery removes 70 percent of the ventilation load, the coil, plant, and distribution are all sized smaller. That cuts both capital and running cost.

Standards Context

EN 16798 and ASHRAE 90.1 set recovery expectations because this lever is so effective at the whole-building scale.

Pressure Drop Is Energy

Every core resists airflow. The fan must overcome that resistance, and fan power rises with pressure. Ignored, it can erase part of the saving.

What Drives It

• Face velocity - higher velocity, much higher drop
• Channel size - narrower channels raise drop
• Core depth and configuration

Typical Values

Plate cores: 50 to 150 Pa per side; rotary wheels: 100 to 250 Pa; heat pipes: 40 to 120 Pa. These are design-point figures.

Balance

There is a sweet spot: lower velocity lifts effectiveness but needs a bigger core. Size for the point where recovered energy minus extra fan energy is maximal.

What to Measure

On site you need supply and exhaust airflow, and dry-bulb (and wet-bulb for enthalpy) temperatures on both sides.

Procedure

1. Balance and confirm airflow with a hood or pitot
2. Log inlet and outlet temperatures on both streams for a stable period
3. Record pressure drop across the core

Compute Effectiveness

Use the measured temperatures in the effectiveness formula. For enthalpy wheels include humidity.

Judge the Result

Compare the measured value with the rated curve at the same conditions. A large gap means fouling, leakage, or a wrong selection. Keep the sheet as the commissioning baseline.

The Definition

Effectiveness epsilon = actual heat transferred / maximum possible heat transferred. Maximum possible = C_min x (T_hot_in - T_cold_in), where C_min is the smaller heat-capacity rate.

Worked Example

Exhaust 20 C, supply inlet -2 C, 70 percent effectiveness. Recovered temperature rise = 0.70 x (20 - (-2)) = 15.4 K, so supply leaves at about 13.4 C.

Check the Power

Q = m_dot x cp x 15.4. For 10,000 m3/h (3.33 kg/s): Q = 3.33 x 1.006 x 15.4 is about 51.6 kW.

Compare to Rating

Always compare your hand calc to the manufacturer's EN 308 or AHRI 1060 rating at the same face velocity.

Find a State Point

Locate the intersection of dry-bulb (x-axis) and the diagonal humidity lines. That point gives wet-bulb, humidity ratio, and enthalpy at once.

Trace a Recovery Process

• Mark the exhaust state and the supply state
• For a sensible device, move horizontally (temperature changes, moisture stays)
• For an enthalpy device, move along a line that also drops humidity ratio

Read the Result

The enthalpy difference between states is the recovered energy per kg of air. Multiply by mass flow to get power.

Use It For Selection

The chart shows whether a sensible or enthalpy device meets your supply-air target, which settles the technology choice.