Pressure Control System Setup

Introduction

Process control via a closed feedback loop requires the use of a sensor, controller, and actuator. In the case of BFS, controller and actuator (motor) are integrated in the fan, while the sensor selection and installation are up to the user’s choice. A few guidelines apply to improve the quality of the control.

Pressure Control Installation Guidelines

For an optimal pressure control, the setup should be installed as shown in Figure 1: the fan should be installed in a duct, with as little restrictions as possible in proximity of the fan (uniform streamline distribution is needed for stability).

Unavoidable restrictions (valves, elbows, etc) should be places as sufficient distance from the fan, so that the deviated flow has the chance to stabilize and gain uniformity.

The distance between sensors (pressure sensors, flow velocity sensors) and other components (both fans and system restrictions) should be maintained as described in the sensors installation guidelines.

The use of flow straightening devices (i.e. honeycomb structures, filter meshes..) helps to straighten the streamlines and reduce the amount of space needed between elbows and fans, for example.

Pressure Control Pitfalls

Levitronix doesn’t recommend a pressure sensor technology, therefore the type and the installation will depend on the customer process. Technology, type, accuracy, pressure range, installation, and transmission noise can affect the quality of the closed control loop.

For example, pressure waves propagates in air at relatively low speed (about 340 m/s at room conditions). As a result, if the sensor is placed far from the fan, there will be a delay between fan action and the sensor detecting the change in pressure.

The sensor location might also affect the performance of the pressure control. For example, in a duct the air flows in a main direction, but secondary flows might be present too (e.g. after a bend, swirling fluid and vortexes are usually present). Measuring the pressure in a duct where these secondary flows are present might result in inaccurate or oscillating measurements (Figure 2).

Pumps generate a pressure difference between suction and discharge side, but they are typically installed so that the suction generated is low, and most of the pressure is developed at the discharge side. This is needed for priming and cavitation avoidance reasons.

Fans don’t suffer from these issues, therefore they can be used either as a pressure generating devices, or as suction generating devices. This second case is often preferable, because if the air duct is under slightly negative pressure, the risk of external contamination will be reduced in case of a leak.