How to Choose a HEPA Terminal Air Supply Unit

Start from the room, not the terminal

A HEPA terminal is the last point of the supply-air system: it holds the HEPA filter at the ceiling and discharges clean air into the room. It is tempting to start by picking a terminal model, but every later decision depends on what the room must achieve. The terminal type, the number of units, the filter class and the layout all follow from the required cleanliness and the process.

Cleanliness is defined by ISO 14644-1, which sets the maximum number of particles per cubic metre for each class [1]. For sterile medicinal products in Europe, EU GMP Annex 1 adds grades A to D with their own limits for the at-rest and in-operation states [2].

EU GMP Annex 1 also sets limits for particles ≥ 5 µm and for microbial contamination; check the full table for your grade.

Step 1: understand the unit types

There are three common forms of HEPA terminal, and the choice shapes everything else.

Step 2: decide the airflow

The total supply airflow comes from the room design: the particles and heat the process releases, the recovery time required after a disturbance, and the pressure cascade between rooms. ISO 14644-4 covers design and construction, and ISO 14644-16 encourages setting airflow from these needs rather than from a fixed rule of thumb, because supply airflow is the largest energy cost in most cleanrooms [3][4].

For unidirectional zones the airflow follows from the velocity. EU GMP Annex 1 gives 0.36–0.54 m/s as a guidance value for unidirectional airflow at the working position in Grade A, unless a different value is justified [2]. Over a 1.2 × 0.6 m filter face, 0.45 m/s is about 1,170 m³/h.

For non-unidirectional rooms, divide the total airflow by the number of terminals. As an illustration only: a room of 6 × 5 × 3 m (90 m³) designed for 30 air changes per hour needs 2,700 m³/h, which three terminals of about 900 m³/h each can supply. Your design must set the actual air change rate.

Step 3: size each terminal

• Check that the airflow per terminal is within the unit's rated range. Running a filter above its rated airflow raises pressure drop and noise; running far below wastes ceiling space and money.
• Check the air velocity at the inlet neck. Too high a neck velocity is a common source of whistling and duct noise.
• Compare the main technical data against the duty: rated airflow, initial pressure drop of the filter, filter class and sound level.
• Choose the filter class from the room class: H13 is the usual choice for ISO 7–8 rooms, and H14 for ISO 5 zones and where the specification names it (EN 1822 classes) [5].

Step 4: plan the layout

• Consider air distribution, noise, the architectural ceiling and maintenance access together, not one at a time.
• Place terminals over the critical work and spread the rest evenly. Keep return grilles low on the walls so air sweeps down through the work zone.
• Where there is a suspended ceiling, choose the discharge face (diffuser plate or perforated face, square or round pattern) from the room height and the airflow the process needs.
• Keep terminals away from doors and from heat sources that create rising plumes against the supply air.
• In rooms with high heat gain or tall spaces, low-level supply can give a more stable air pattern than ceiling supply.

Step 5: set the discharge velocity

Outlet velocity should be chosen from the supply method, the type of outlet, its mounting height, the air speed allowed in the occupied zone and the permitted noise level. As general design guidance for side-wall displacement outlets, the outlet is mounted below about 0.8 m with an outlet velocity of no more than about 0.2 m/s in civil buildings and 0.5 m/s in industrial buildings. Cleanroom terminals follow the design velocity for the cleanliness class, as in Step 2.

Step 6: plan for testing and maintenance

Every installed HEPA filter must be leak-tested in place, because most leaks occur at the seal and frame rather than in the media. ISO 14644-3 describes the scan test and the aerosol injection upstream of the filter [6]. A terminal design that cannot be tested is a problem that appears only at commissioning.

• Confirm the duct connection: top or side inlet, collar size and whether insulation is needed.
• Make sure there is an upstream test port for aerosol injection and a way to reach the full downstream filter face.
• Decide how filters will be changed later. Room-side replaceable designs avoid entering the ceiling void.
• Match the frame and seal (gasket or gel) to the ceiling grid or panel system.