The Optimal Operating Range of VAV Supply Units

Good indoor air quality requires sufficient amount of air to be supplied and well distributed within a space. Air diffusers equipped with variable air volume control function (VAV) regulate the air flow rate to meet the rising and falling heat gains or losses within the thermal zone served. The VAV unit is basically a calibrated air damper with an automatic actuator. When the air is supplied parralel to the ceiling surface negative pressure occurs between the air jet and the ceiling, causing the jet to attract to the ceiling (Coanda effect). This effect is of great importance, particularly when supplying cooling air. The velocity of the supplied air should be kept at a level which ensures that the mixing is effective, but at the same time ensures that the air velocity has fallen to the required level by the time it reaches the occupied zone.

Several types of diffusers were tested to determine the influence of the shape and constructional features of diffusers on the air distribution pattern within a premises. The tests were performed at Lindab Comfort aerodynamics laboratory in specially designed test chamber which represented a single office room. 24 probes were installed at 3 levels: feet, waist and head area of a standing occupant in accordance with ASHRAE:55-2004. At each point air temperature and velocity were measured. The test chamber was isolated so that isothermal conditions were maintained and all measurements were carried out at 8oK temperature difference between supply air and room air. Measurements were recorded every 20 seconds over the period of 2 minutes so that any fluctuations or deviations were excluded.

Two air flow values are of great importance to establish the operating range: the lowest threshold at which Coanda effect appears and the highest threshold above which allowed noise level is exceeded (noise level data were gained from technical specifications). Thus we traced the path of air distribution pattern within the operating range and built the model to demonstrate the air distribution pattern on each tested diffuser. Acquired data are applicable in VAV systems where airflow varies depending on a specific room conditions.

Citation: Cold Climate Conference



Product Code(s): D-CCC12-33