1. AHU
Air handling unit. Draws return air from the room and some fresh air to control supply temperature and airflow and hold the room temperature.
2. PAU
Primary air unit. Pre-treats outdoor fresh air before sending it to the fan coil unit (FCU).
3. RCU
Recycled air handling unit. Circulates and conditions the room air.
4. MAU
Make-up air unit. Fresh-air only. By environment it can give constant temperature and humidity or just fresh air. It draws outdoor air, then filters, dehumidifies (or humidifies) and cools (or heats) it, and a fan sends it indoors, replacing the stale air. The functions above depend on the site; more functions cost more.
5. DDC
Dry cooling coil (dry coil). Removes the room's sensible and latent heat.
Direct digital control (DDC).
6. HEPA
High-efficiency particulate air filter. A HEPA-grade filter removes 99.998% of particles at 0.1 µm and 0.3 µm. Air passes through but fine particles do not. For particles of 0.3 µm (1/200 of a hair diameter) and larger, removal reaches over 99.7%; it is the most effective filter for smoke, dust and bacteria. Smoke particles are about 0.5 µm. HEPA is the recognised standard for high-efficiency filtration and is used widely in operating theatres, animal labs, crystal work and aviation.
7. FCU
Fan coil unit. The terminal of the air-conditioning system. It recirculates the room air through a cold (or hot) water coil to cool (or heat) the room and hold a steady temperature. The fan forces the air across the coil, strengthening heat exchange with the room.
8. VAV
Variable air volume system. A VAV controller and a room thermostat form a cascade: the room temperature is the main control and airflow the secondary. The controller reads the room sensor, compares it with the setpoint, and sends a signal to adjust the VAV box and change supply airflow, holding the room in range. A duct pressure sensor reads duct pressure; with PI or PID control and a variable-frequency drive on the supply fan, it removes pressure swings and keeps airflow steady.
9. FFU
Fan filter unit. A fan and a filter (HEPA or ULPA) in one self-powered terminal clean-air device. The fan draws air from the top, filters it through HEPA, and the clean air leaves the whole face at 0.45 m/s ±20%.