Questions that come up when selecting products.

The average air velocity across a section is the volumetric airflow rate divided by the area of that same section. With airflow in m³/h and area in m², velocity = airflow ÷ (3600 × area). For example, 720 m³/h across 0.10 m² gives an average velocity of 2 m/s. A grille’s neck area, free area and effective area are different, so catalogue comparisons must use the same area definition.

ACH is the volume of air supplied to or extracted from a room in one hour divided by the room volume. With airflow in m³/h and room volume in m³, ACH = airflow ÷ room volume. For example, a 150 m³ room with an airflow of 900 m³/h has 6 ACH. State whether the calculation uses total supply, extract or outdoor airflow; total ACH alone does not indicate the amount of outdoor air.

Grille outlet velocity describes the air speed at the section where air leaves the product. Occupied-zone velocity describes air movement where people are located. In mixed-air distribution, the supply jet entrains surrounding room air as it travels. The aim is for its velocity and temperature difference to reach suitable levels before it enters the occupied zone.

Throw is the distance to the point where the air jet slows to a specified terminal velocity. T0.25 indicates that the distance is stated for a terminal velocity of 0.25 m/s. Selection should account for the catalogue airflow rate, discharge direction and temperature conditions, as well as collisions between adjacent jets and cold air dropping into the occupied zone too soon.

First check the design airflow, pressure drop, damper opening and duct connection approaching the grille. Select an appropriate size and position while maintaining the required airflow, and assess catalogue sound data alongside the room acoustics and number of units. Also investigate whether noise is being transmitted from the fan, ductwork or another component.

Selection considers the active discharge length, number of slots, airflow, discharge direction and plenum arrangement together. Concealed installations require verification of throw, pressure drop and acoustic performance alongside the architectural appearance. Check the active length and installation conditions to which the catalogue data apply.

A CAV controller helps maintain a set volumetric airflow within its specified operating pressure and airflow range. Grille selection must still account for that airflow, throw, temperature difference and acoustic target. The system must provide the pressure needed for the controller to operate, and the operating range must be checked against the product’s own technical data.

In displacement systems, cool air supplied at low velocity spreads across the floor and rises around heat sources. Selection considers diffuser face velocity, the near zone in front of the diffuser, air movement at ankle level and the vertical temperature difference together. Regularly occupied seating areas should be positioned using the product’s near-zone data.

The velocity target depends on the position of beds and seating, supply-air temperature and operating conditions. Direct cold airflow around beds and system noise during night-time use should be controlled. Grille selection is assessed together with the room layout; a single velocity value is not a sufficient design criterion for every hotel room.

ASHRAE’s room-air-distribution guidance gives an occupied-zone velocity below 0.25 m/s as a general design approach for typical mixed-air office layouts. Comfort assessment considers air and radiant temperatures, humidity, activity and clothing together. This value must not be used as a grille outlet velocity or a stand-alone compliance limit under all conditions.

A classroom’s outdoor-air requirement is determined by student numbers, floor area and the applicable ventilation criteria. Grille selection should distribute this airflow appropriately to the occupied zone while controlling draughts and system noise. Total supply airflow and outdoor airflow are assessed separately.

An unsuitable discharge direction, throw or airflow setting can cause the air stream to reach seated guests directly. Air distribution should be adjusted by considering the table layout together with kitchen extract, make-up air and the air balance between the dining area and kitchen.

Air-terminal placement, throw, temperature difference, and heating and cooling operating modes are assessed together. Long-throw or adjustable diffusers and displacement solutions close to the occupied zone may be considered according to the project geometry. In complex spaces, computational fluid dynamics (CFD) analysis or full-scale testing can help validate the design.

There is no single velocity that applies to every operating theatre. First establish the system type, applicable standard and measurement plane. Average outlet velocity at the diffuser face, local velocity in the protected zone and duct velocity are different quantities. Acceptance criteria for unidirectional ultraclean ventilation (UCV) canopies must be assessed separately from conventional operating-theatre air distribution.

For vertical-flow UCV canopies, NHS England HTM 03-01 Part A (2021) assesses the average of test points 2 m above the floor: for a canopy without side walls or with side walls ending 2 m above the floor, at least 0.38 m/s; where the side walls end 1 m above the floor, at least 0.30 m/s. At 1 m above the floor, every test point in the inner clean zone must reach at least 0.20 m/s. These defined acceptance tests in the English guidance must not be treated as general room-velocity requirements for every operating theatre.

The requirement depends on the room’s function and the edition of the applicable standard. For example, the operating-theatre table in the 2023 ASHRAE Handbook, reproduced from Standard 170-2021, gives a minimum of 20 total ACH, including a minimum outdoor-air component of 4 ACH. These values must not be generalised to all hospital rooms. For a new project, verify the adopted edition and its addenda; publication of 170-2025 does not automatically make that edition applicable to every facility.

The pressure difference controls the direction of airflow between adjacent spaces. Positive pressure in an operating theatre limits the entry of contaminated air from outside the room. Negative pressure in an airborne infection isolation room draws air inward to help limit the spread of contaminants to surrounding areas. The pressure direction must not be changed without a clinical risk assessment and a decision by the responsible technical team.

A HEPA filter alone does not satisfy all of a room’s acceptance criteria. Airflow, air distribution, pressure relationships, filter installation and leak testing are assessed together. A portable HEPA device does not automatically replace the required outdoor-air supply. A filter efficiency certificate does not demonstrate that the installed system meets every applicable requirement.

Under the EN 1822-1 classification, H13 has a minimum overall efficiency of 99.95%, and H14 has a minimum of 99.995%. These values are assessed at the particle size at which the filter has its lowest efficiency, known as the most penetrating particle size (MPPS). Selection also considers design airflow, pressure drop, leak-tight installation and project requirements.

There is no single universal replacement interval. Manufacturer instructions, operating airflow, pressure drop, contamination loading, physical condition and performance checks are considered together. Correct installation and application-appropriate verification are required after replacement. In clinical areas, competent personnel should carry out maintenance using a defined safe procedure.

Pressure drop monitors the filter’s resistance to airflow at a given flow rate. Leak testing investigates the passage of unfiltered air through the filter element or around seals and the housing. A normal pressure drop alone does not prove leak-tightness. Tests are planned according to the applicable application standard and the manufacturer’s method.

ISO 14644-1:2015 classifies air cleanliness by airborne particle concentration. The class designation alone does not specify an air velocity, ACH or number of HEPA filters. Airflow patterns and operating conditions are assessed through project design and performance testing. An ISO class alone does not establish whether particles are viable or demonstrate clinical safety.