Flow Controllers

What is a Flow Controller?

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.

Have a look at our blog 'Which control valve to use for your flow meter' to see how control valves can be an integral part of the flow controller device, or how they can be a separate component in combination with the flow meter. Directly coupled electromagnetically operated control valves are fast, low cost and use very little power to control the flow. To handle higher pressures and flows, redesigned (somewhat larger) direct valves or indirect control valve configurations are available.

What kinds of Flow Controllers exist?

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 a bypass sensor (as in the EL-FLOW Select and IN-FLOW series), which is ideal for clean and dry gas applications.
• By using the Constant Temperature Anemometry (CTA) principle with the sensor inline in the main flow (as in the MASS-STREAM, MASS-VIEW and LIQUI-FLOW series), for example for robust applications with slightly moist gases or for liquids.
• By using a MEMS chip-based sensor for gas flow control. FLEXI-FLOW devices control gas flow rates and measure temperature as well as pressure. Sensors from the highly compact IQ+FLOW series are ideal for use in confined spaces.

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.
 

Flow Controllers using positive displacement principle

A gear pump can be considered a volumetric flow controller according to the positive displacement principle. A motor causes gears to rotate, and the space between the teeth of two interlocking rotating gears determines the stroke volume. Successive strokes generate a continuous fluid flow. 

Some flow controllers are particularly suitable for gases, and some specifically for liquids. Moreover, there are flow controllers available which are independent of fluid properties, and which can therefore control the flow rate of gases as well as liquids.
 

Flow Controllers in hydrogen application

To investigate the storage of hydrogen in metal hydride containers, the German Aerospace Center wanted a solution for the controlled introduction of hydrogen gas into the container. Devices from the IN-FLOW flow meter series in combination with Vary-P valves did the trick. To control the pressure in the metal hydride container, pressure controllers from the IN-PRESS series were used, connected to Vary-P valves. Read more...