VARIABLE GEOMETRY TERMINAL UNIT DESIGN

Having discussed overall design, we now consider the advantages of the RICKARD VARIABLE GEOMETRY air diffusion concept. Essentially, the concept utilises the principle of varying the shape of the diffusion device at the point of discharge into the air conditioned space. This ensures that constant air distribution patterns and room air induction rates are maintained from minimum to maximum flow conditions.

THE PRINCIPLE OF OPERATION

Cooling / Reheat Systems

1 Cooling

During the cooling cycle each terminal unit, or group of terminal units, varies the volume of supply air at a constant temperature to the airconditioned space, in response to the output from a room or zone temperature controller. Generally the range of volume control is 30% to 100%. The RICKARD range of VARIABLE GEOMETRY air diffusion equipment is so designed that room air movement remains essentially constant even at minimum terminal air volumes. This is because lower volumes are discharged at higher jet velocities.

2 Reheating

Should the temperature in the room or zone fall below setpoint, thereby requiring supplementary heating, the terminal reheaters, when fitted, are automatically energized. In this mode the heaters are only energized when the terminal unit is delivering minimum air so that energy consumption is kept as low as possible. In a well designed system, when heating is required, the cold air supplied to the terminal unit is in fact obtained "free" by economy cycle damper operation thereby avoiding unnecessary reheating of previously cooled air.

3 VAV Cooling / VAV Heating System

Through modern technology, at low cost, the action of the conventionally controlled RICKARD VARIABLE GEOMETRY VAV terminal equipment may be reversed such that during the heating mode, a decrease in room temperature will cause an increased volume of pre-heated air to enter the air conditioned space. This is brought about automatically by the integral controller detecting that the supply air temperature entering the VAV unit is above a predetermined set point. Therefore, through careful design and zoning considerations, the amount of reheat required could be substantially reduced without sacrificing flexibility or individual control.

THE PRINCIPLE OF AIR FLOW MODULATION OF VARIABLE GEOMETRY TERMINALS

The diagram below shows a supply air duct, a circular flexible connection and a RICKARD VARIABLE GEOMETRY VAV terminal. The terminal unit has a variable geometry air volume control device activated in response to room temperature conditions.

The static pressure in duct "B" is controlled at a constant level as the VARIABLE GEOMETRY control device varies the aperture of the terminal unit to modulate the air flow. As air volume flowing from "B" to "A" is decreased, the resultant static pressure in "A" is increased. Therefore, the increased static pressure at the lower flow rates is converted to velocity pressure resulting in increased air velocity entering the conditioned room.

Although mass flow at minimum supply air status is considerably reduced, the increased air velocity ensures a good induction rate and hence excellent room air movement.

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