General Description:
Rabie VAV terminal boxes type RAV were developed for
the control of air flow, especially in variable volume systems.
They come in different models:
- RAVBS Single Duct Terminal Box - (VAV
or CAV) (Pressure independent)
- RAVBP By-Pass Terminal Box (Pressure Dependent)
Factory fitted control package, ensures the most
accurate setup of required flow rates. The flow rate control is a closed circuit system with
external power supply. RAV boxes can be supplied with control components from any reputable
manufacturer to suit the project specification
RAV contains a differential pressure sensor for air flow
measurement (not for Bypass type), a control damper and an integral sound attenuator for
reducing the air regenerated noise.
The control damper blade with seal insures best air tightness
when closed.
RAV has a circular spigot connection up to 200 (8”) size and
Oval connection for larger sizes on the fan side (high pressure end) & a rectangular
flange connection on the room side (low pressure end)
Room temperature controller
In VAV systems, the room temperature control takes the form of a
cascade control. The primary controlled variable is the room temperature. The
output signal of the room temperature controller is not fed directly to the
supply air control damper but alters the setpoint value of the supply air flow
rate control circuit. The flow rate control also generates minimum and maximum limits for the
air flow which has benefits in keeping the room temperature constant and for
the functioning of the overall room air conditioning
system.
Flow rate measurement
A sensor is necessary for the positive measurement of air flows,
this is achieved by pressure measurement at several points distributed over the
cross-section and provision of the resulting average value. The differential
pressure sensor is an optimum solution in terms of economics and product
technology. This sensor delivers exact results
for most room air conditioning applications based on normal upstream flow
conditions.
Volume flow control
The control of the flow rate takes place in a closed control
circuit, i.e. measurement – comparison – control. A pressure transducer
converts the pressure differential into an electrical signal, which is
interpreted as the actual value by the controller. In most applications, the
set point value comes from a room temperature controller. The controller compares the actual value with the
set point value and from the difference between these two alters the control
signal to the damper actuator.
Supply/extract air tracking control
In individual rooms and closed-off office areas, where the balance
between supply and extract air flow rate has to be maintained. Otherwise,
annoying whistling noises can occur at door gaps, and the doors can be
difficult to open. For this reason, the extract air should also have variable
control in a VAV system. The actual value of the supply air is fed as an input
signal to the extract air controller (slave controller). In this way, the
extract air flow rate automatically follows the supply air flow rate, even in
the case where this has not reached its set point value.
Bypass VAV - Pressure Dependent
Bypass VAV is usually installed in a pressure dependent system
(application) where there is no variable speed control of the supply fan and so
back pressure should be avoided. For this reason a bypass VAV requires a
pressure relief damper (or bypass damper) to avoid exceeding the maximum
pressure of the system. Flow measurement is not possible for Bypass type and so
the Bypass VAV comes without flow sensor. The control of Bypass VAV shall be
directly from the set point of the room Temperature Controller OR BMS.
Pressure Independent CAV Operation
CAV operation is principally the same as VAV Terminal Boxes but
the concept of function is different. The CAV is intended to keep preset
constant air volume inter the duct branch afterword the box. To do so, the
compact controller will receive a signal of the preset volume flow required and
the actuator will act to keep the flow constant depending on differential pressure measurement
taken (corresponding to air speed).
For more details and further reading, refer to the Attachments.
