THE MAIN PLANT IN MORE DETAIL

The Air handling Unit(s) are normally installed on the roof and/or in the basement and sized to handle 100% fresh air for maximum free cooling.

Plenum Fans (scroll-less backward curved fans installed in a pressurised plenum) are used which are suitable for high volume, low-pressure applications.

The VAV fan is controlled by an inverter linked to a duct static pressure sensor, traditionally two thirds of the way down the main index run. Because the system is a “ductless system” and the pressure drop in the raised floor plenum is very low, the sensor is normally located in the main riser.

Cooling is achieved by using Chilled Water or Direct Expansion. The choice is normally dictated by the location and layout of the central plant. Not less than 6 circuits are used for DX Cooling.

Unless there is a requirement for LPHW on the site, Natural Gas fired indirect heaters are used.

If there is sufficient space available, a sidestream two-stage reheat is provided using a combination of filtered return air at 27°C, which is free, for the first stage and the gas burner for the second stage.

Return air is handled by either a full sized return air fan or an exhaust (dump) fan.