INTRODUCTION

The Rickard Variable Volume Pressure Control Dampers have been designed to enable supply air duct pressures to be accurately controlled. This facility is required in Variable Air Volume systems as VAV diffusers are sized and set to operate at a constant pressure, independent of air flow. Important factors in PCD design are linear operation, reliability, and unlike opposed blade types, low noise regeneration from maximum to minimum flow rates. The Rickard PCD has excellent linearity as is shown by extensive tests. Many types of dampers are produced by various manufacturers; opposed blade dampers are particularly common and there are several variations on the "butterfly-valve" type. While these dampers are well suited to many applications none of them are ideal for duct pressure control as their transfer characteristics are not linear. In any damper, noise generation is largely a function of pressure drop and air velocity, which may be countered by fitting a downstream sound actuator.

DAMPER SELECTION

Usually dampers are sized simply to match the duct into which they will be fitted. Generally, this is an adequate selection method, but under certain circumstances it can lead to difficulties. Normally the air velocities in air conditioning ductwork at full volume should be between 1200 and 1800 ft/min. If velocities outside these limits are used or if there are any substantial restrictions such as silencers, filters or multiple bends and transformation pieces in the ducting, then a closer examination of the situation may be warranted.

DIMENSIONS & CONFIGURATIONS

The PCD may be manufactured to any size required, both in terms of the width and the height of the unit. Although the standard depth is 6 in., this too may be varied to suit any requirements but may not be less than 6 inches. When damper widths exceed 40 in., the aluminum blades are split and a mullion is fitted down the center of the casing. In this case two actuators would be fitted, one for each section. A damper of say 72 x 72 in. would consist of four sections, each 36 x 36 in. and each having its own actuator. The standard PCD would be manufactured with sheet metal flanges to be drilled for installation purposes by the contractor. Pre-manufactured flanges are available as an option. The PCD must always be installed with the actuator on the downstream side of the vanes.

TIGHT SHUT-OFF PCD UNITS

In some system designs (usually of the smaller type) only very low leakage levels may be tolerated from the bypass pressure control dampers. If this facility is required, a tight shut-off PCD, which will ensure that the maximum possible volume of supply air is available, may be specified.

ACTUATION

The Rickard PCD is supplied suitable for electric actuation. The PCD is supplied with one or more Lindrive actuators. The Lindrive actuator requires a 24 volt A.C. signal for operation and has a linear, or push-pull, type of action.

PRESSURE CONTROL

The PCD may be supplied complete with electronic pressure controls. The controller would normally be mounted in a panel fitted to the side of the damper casing, while the pressure sensor would be supplied loose, to be fitted to the duct in a suitable position by the contractor. The interconnecting cable between the controller and sensor forms part of the supply. Although the standard length supplied is 60 ft, additional cable may be supplied if required. As an alternative, the pressure controller may be supplied loose suitable for mounting in a remote switchboard.

CONTROL OPERATION

The controller requires 24V to be brought to the unit.

PRESSURE SENSOR

The pressure sensor is supplied loose and is easily fitted to the duct on site. Drill a 3/8 in. diameter hole in the duct to accept the sensing probe and fix the sensor by means of two self tapping screws. Ensure that the sensing probe protrudes into the air stream and is not obstructed by internal duct lining or any other such obstruction.

PRESSURE INDICATION

For ease of adjustment during commissioning, the Rickard pressure controller has the facility to accept a plug-in pressure indicating meter.

FACE-AND-BYPASS DAMPER CONTROL

The Rickard static pressure controller may be used to control more than one damper, as in the case of a face-and-bypass damper. In this case the two actuators will simply be wired in the opposite sense so that the one damper drives open while the other drives shut.

COMMISSIONING PROCEDURE

The function of the PCD is to maintain a constant pressure in the systems regardless of airflow. This constant pressure is necessary to allow the VAV Variable Geometry diffusers to work correctly as they are sized and set to deliver design volumes at a specific pressure. For the commissioning technique to be useful, it should first be established that the thermostat/controllers are set to control at a realizable temperature. If the temperature set point requires more of the air conditioning system than the design conditions allow, the system may never perform satisfactorily.

METHOD

For each pressure control damper and associated branch duct, select the diffuser which needs the highest neck total pressure to enable it to supply maximum design air volume and drive this diffuser to fully open. Also drive all pressure control dampers to fully open, and then drive the other diffusers until design maximum air is leaving the Air Handling Unit and is being distributed uniformly throughout the conditioned space. At this stage diffuser neck total pressures at fully open diffusers will be on or slightly above their respective maximum air requirements. Adjust the duct static pressure controllers until the diffusers which need the highest neck total pressures have sufficient pressure available. At this stage the system is fully commissioned, but it may be tested by monitoring duct pressures while changing the air flows around the system. In a satisfactory system one would expect duct pressures to vary by 7 - 10 % as air flows vary from 30 % to 100 %. Please note that these commissioning recommendations do not check a system for all faults. It is intended only to set up and test the pressure controls and, to a lesser extent, the diffusers. Gross system faults, such as disconnected or ruptured flexible ducting, will not be traced using this method. Rickard Air Diffusion Pty. Ltd. reserves the right to change specifications and data without prior notice.