Flow Controllers; an introduction

Flow control is indispensable in biological, chemical, food and manufacturing processes or for analytical purposes. In these process applications, it is obviously necessary to supply fluids (gases, liquids or vapours) in a controlled way - which is essentially the definition of flow control. Accurate flow control will improve final product quality, improve process understanding by investigating varying process flows and may speed up process steps. Ultimately, the flow control will improve process economics. To this end, flow control devices or ‘flow controllers’ are used.

  1. What is a flow controller?
  2. How does a flow controller work?
  3. What kinds of flow controllers exist?
  4. Examples of flow controllers used in customer applications

Flow Controller

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.

How does a Flow Controller work?

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 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.

Flow Controller

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, LIQUI-FLOW and MASS-VIEW 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.

Examples of Flow Controllers used in customer applications

Flow Controller for dosing liquids in pharmaceutical process

For accurate control of liquid flows for dosing liquid additives in a continuous pharmaceutical manufacturing application for a leading company in this field, Bronkhorst supplied a complete customised liquid dosing skid solution. This solution combined a mini CORI-FLOW mass flow meter with a gear pump and a pressure sensor from the IN-PRESS range, resulting in a faster, more reliable and flexible process. Read more...

Flow Controller in liquid dosing skid
Liquid Dosing skid with CORI-FLOW (copyright: ISPE)

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...

Flow Controllers for aeration

For ice cream production, the aeration process is essential. Here, air is blended into a liquid mixture of various food ingredients. For the right final product consistency and structure, the ice cream must contain the correct amount of air bubbles. To this end, aeration mixer manufacturers use EL-FLOW Select mass flow controllers to dose an exact amount of air into the cooled mixer. Read more...

Flow controllers in ice cream production

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