In process technology, pressure and flow rate are two relevant quantities - analogous to voltage and current in electrical technology. The pressure of a liquid, gas or vapour is the force that this medium exerts onto its surroundings, and generally this pressure is a driving force for the medium to flow. For example, the pressure in the municipal water supply allows water to flow from the tap in your home when you open it.
Although Bronkhorst is renowned for the measurement and control of liquid, gas and vapour flows, pressure regulation is another of the company’s pillars. Let’s dive into this world of pressure control, with a versatile Bronkhorst pressure regulator as an example.
Bronkhorst EL-PRESS pressure regulator as an example
A pressure regulator is used to make sure that the process pressure does not exceed a predefined setpoint. In this respect, the Bronkhorst EL-PRESS digital electronic pressure controller can control the pressure of liquids or gases. It is a device that consists of a pressure sensor, an adjacent valve and control software.
The pressure sensor continuously measures the actual pressure of the flowing medium. Smart PID control software compares this actual value with a desired setpoint value and subsequently controls the valve. By (further) opening or closing this valve, the gas/liquid will flow faster or slower, and as a result the medium’s actual pressure will reach the setpoint pressure and stay to this value as close as possible. This is a continuous process.
In the pressure sensor, the liquid or gas flow exerts its pressure onto a flexible stainless-steel diaphragm. An intermediate volume of oil transfers this pressure to the surface of a piezo-based chip – the ‘real sensor’ – that converts the pressure into an electrical voltage signal for further processing. The thickness of the chip surface determines the pressure range - several millibars to several hundreds of bars.
The valve of the EL-PRESS is a proportional, electromagnetic control valve with extremely fast and smooth control characteristics. A pressure controller minus the control valve is ‘simply’ a pressure meter.
Cross sectional drawing of a pressure sensor
Forward pressure control, back pressure control and differential pressure (ΔP) control, schematically
Types of pressure regulation
The position of the pressure regulation components largely determines the type of pressure regulation. In forward pressure control to regulate the downstream pressure, the sensor is positioned downstream of the valve, for example to gain a constant pressure of a gas flow that will enter a burner jet. In back pressure control to regulate the upstream pressure, the sensor is positioned upstream of the valve, for example to relief the pressure of a chemical reaction vessel in a controlled way. In these configurations, the pressure sensor is always closest to the chamber or vessel of which the pressure needs to be controlled.
In differential pressure control, the pressure difference between two (gas) lines is regulated. A pressure meter is placed between both lines, and a control valve is positioned inline into one of the two lines. This can be done, for example, in membrane testing applications to obtain a constant pressure difference over the membrane.
In process pressure control, the pressure inside closed vessels with one fluid connection is regulated, combining one pressure sensor with dual valve pressure control for inlet and relief.
Bronkhorst pressure regulation devices
The following Bronkhorst devices contain a pressure sensor, and are therefore able to take part of a pressure regulation setup:
- The versatile EL-PRESS digital electronic pressure controller/meter for gases and liquids can measure in the range from 2 mbar up to 400 bar, for forward as well as back pressure control. EL-PRESS Metal Sealed devices are for high-purity gas applications up to 64 bars, such as for the semiconductor market.
- The robust IN-PRESS digital electronic pressure controller/meter for gases and liquids also operates in the 2 mbar - 400 bar range, and is equipped with a rugged, 'industrial style' (IP65) pc-board housing for use in industrial environments.
- The compact IQ+FLOW digital pressure controller/meter for gases (‘the yellow one’) is able to measure in the range between 0.01 and 10 bar on a small (0.75”) footprint.
- The highly compact chip-based FLEXI-FLOW Compact mass flow controller/meter for gases contains two pressure sensors, one on the inlet and one on the outlet, to measure the upstream and downstream pressure.
- The EL-FLOW Prestige mass flow controller/meter for gases with ‘Pressure Insensitive’ option contains an onboard pressure sensor to compensate inlet pressure variations.
An EL-FLOW mass flow meter in series combined with an EL-PRESS forward pressure controller in a burner test application
Combined mass flow - pressure control (‘a Bronkhorst specialty’)
For an optimum control of process conditions such as flow rate and pressure, a pressure meter or controller is often combined with a flow rate controller or meter. This combination allows a user to control the flow rate and measure the pressure, or to control the pressure and measure the flow rate.
For example, with these configurations the desired process pressure is kept constant within a chemical reaction vessel, whereas the incoming required reaction gas flow is maintained at a constant flow rate. In the examples on the left, these combinations are often applied.
Pressure regulation applications
Typical applications for pressure regulators are in leak test benches, in high pressure or vacuum test benches for chemical reactions, in permeability tests for rock or sediment, and in membrane permeability/separation tests. Concrete applications are:
- Bronkhorst EL-PRESS digital electronic back pressure controllers are applied to ensure the required constant value of the low pressure inside the plasma chamber for the manufacturing of high-quality lab-grown diamonds by CVD.
- In a lithium-ion battery manufacturing process, to fill the pouch-type battery cells, the pressure of the electrolyte supply vessel is kept on a constant value by a Bronkhorst EL-PRESS pressure controller.
- In a test bench for the humidification of fuel cells for automotive applications, the absolute pressure and pressure difference are controlled by Bronkhorst IN-PRESS instruments.
What is a flow meter
Theory of CEM vapor control
Thermal mass flow measurement