A cleanroom that dries by its damper, not its heater

Meridian is a 300 m² ISO 7 room held at 22 °C and 50 % RH, twenty air changes an hour through a recirculating handler with 5 % makeup air, three ebm-papst RadiPac modules on the vendor's own pressure and power curves against 800 Pa of HEPA and duct static, and a chilled-water coil fed by a modular chiller holding 7 °C. On Taipei's year the first reading was 686 MWh at the coil for 549 MWh of electricity — about 300 of them an electric reheat, because holding a 50 % ceiling meant over-cooling all 5.6 m³/s to a 10.5 °C dew point and warming it back to a 16 °C supply.

The textbook answer is a makeup-air unit: dry the 5 % where it enters, run the recirculating coil dry. It was built — a leading outside-air unit with a plate core and a wrap-around heat pipe, the handler recirculating behind it — and measured at three sizes and two loop temperatures. It read worse every time (reheat 305 → 101 MWh at best, electricity 549 → 436, the chiller short three thousand hours): a recirculating handler cannot dry a little — any moisture it is asked for means bringing its whole 6.6 kg/s to dew point — and a makeup unit on chilled water never dries the room past its ceiling.

What a real cleanroom handler has is face-and-bypass: a damper that sends part of the air across the coil and mixes the rest back, so the coil dries the share it sees and the bypassed room air is the reheat, for nothing. The model already had it. On Taipei the handler now reads 386.5 MWh at the coil for 162.2 MWh of electricity, no reheat, 0 unmet and 0 humid hours, the room at 22.0 °C all year.

What it reads on Taipei

What it taught

• A humidity ceiling on a recirculating handler is a reheat bill unless the handler can bypass; size the damper, not the heater.
• A 45 % ceiling is below what 7 °C water reaches (a 10.5 °C leaving dew point); a ceiling the coil cannot reach is one the controller cannot hold.
• A makeup-air unit carries the latent only if it dries the room past its bound — on chilled water at 5–7 °C it does not.

The machines

• Example cleanroom AHU, 5.6 m³/s, RadiPac bank — open in the builder
• Example air-cooled chiller, 90 kW class — open in the builder

Sources

• ISO 14644-1 (ISO 7 air-change convention)
• ebm-papst RadiPac K3G450-AQ24-90 product data (curves)
• PVGIS TMY, Taipei

Readings of the shipped model on the building's own PVGIS typical meteorological year; engineering estimates from catalog data and published correlations. The building runs on every plan, including Free, a few times a day.