Flow controllers

A flow controller adds one or more chemical compounds in gaseous, vapour or liquid form to a process by controlling the compound’s flow rate. This can either be a mass flow rate, expressed in grams of compound per minute, or a volumetric flow rate expressed in litres per hour. 

Mass flow rate and volumetric flow rate are related to each other via the temperature and pressure dependent density of the compound and can thus be converted into each other. In many research and production processes, mass flow is preferred over volumetric flow control because mass and not volume is the important variable. However, many users think and work in units of volume, which is where volumetric flow controllers come in.

A flow controller is a flow meter combined with a control valve and control firmware. The flow meter measures the flow rate of the compound that is supplied to the process. If this measured value deviates from the setpoint value – the desired flow rate of the compound to be added – then the control valve is opened or closed to such an extent that the actual (measured) flow rate will eventually reach the setpoint value.

To explain how a flow controller operates, the main components of a Bronkhorst EL-FLOW mass flow controller are shown in the upper figure. The laminar flow element inside the body guides part of the fluid flow to the sensor which measures the flow rate. The control valve, positioned in series with this flow measurement part of the device, will vary the size of the flow passage by opening or closing, matching actual flow to setpoint. The pc-board inside the enclosure houses the control and feedback software between flow meter and valve.

The sensor type largely determines the kinds of flow controllers that are available. 

• Flow Controllers based on the thermal principle
  Flow controllers that operate according to the thermal principle employ the heat capacity of gases or liquids to control their mass flow rate. Bronkhorst has several of these thermal mass flow controllers available, each with their own sensor technique.
• Flow Controllers using the Coriolis principle
  Flow controllers working via the Coriolis principle (such as the Bronkhorst mini CORI-FLOW series) use a direct relationship between fluid mass flow rate and mass inertia. They operate fluid independently, which means that they can be used for varying and unknown mixtures. Moreover, with these Coriolis-based flow controllers there is no need for conversion when the device is used for other liquids. 
• Flow Controllers with ultrasound technique
  Bronkhorst ES-FLOW flow controllers control volumetric liquid flow rates through ultrasound. Their sensors measure the flow velocity and multiplied by the tube cross-section inside the device, this results in volumetric liquid flow rates.