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.
