Vacuum control in altitude simulation chamber

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7 December 2020

Aviation & Altitude Simulation

One of our German customers is a leading provider of aircraft maintenance, repair, overhaul, and modification services for civil aircraft, from commercial to VIP and special mission aircraft; activities include the development of components and electronics for use in aircraft.

Disclaimer: For the avoidance of doubt we state that Pressure Control Solutions does not typically sell into the aviation industry. Like many other companies, meeting the stringent requirements of this heavily regulated industry simply does not fit in our business model. Yet, in this case we did deliver a solution for an aircraft-oriented pressure control application: On-the-ground-altitude-simulation.

Although in 2020, flying seems somewhat far away for many of us, we also include some good advice for anyone boarding a flight again in the future … Read the blog below!

Altitude Simulation Chamber for Aircraft Component Testing

To continuously exceed user expectations and develop components and systems that are robust and durable enough for use in airplanes, aircraft companies obviously need to undertake lots of testing and validating. To do this effectively and with minimal impact on company resources and the environment, aircraft engineers often use altitude simulation chambers. Altitude simulation chambers allow engineers to create a mock-up of conditions that their test product could potentially be exposed to during aircraft use. Outside the aircraft industry, small and medium and large altitude simulation chambers are also used in the automotive industry for example; another example of altitude simulation chambers is in sports, for athletes to improve oxygen saturation of their blood.

The Role of Pressure Control in Altitude Simulation

To simulate altitude, pressure conditions are of vital importance: During take-off and landing of an airplane, air pressure can change rapidly. Who hasn’t felt that increasing pressure on their ears during take-off or landing, while frantically trying to pop or clear the ears to decrease pressure?

Off-topic, but perhaps a useful free tip from your pressure control expert for your next flight: Swallowing is less dangerous for your ears than pushing your finger onto your eardrum!

Process Conditions for Altitude Simulation

Just as the human ear, aircraft components also struggle with pressure changes: Low air pressure can create a stress factor for components in the aviation industry, as they can be in a zone without pressure equalization (higher altitudes). Components and devices are normally designed for an ambient pressure of 1013 hPa. However, as the flight altitude increases, the air becomes “thinner”, which means that the air density decreases significantly with decreasing pressure. This can affect the quality of components or devices and thus cause a wide variety of issues and risks that should be prevented or managed before allowing them to be used in aircraft.

Additional complicating factors are very low temperatures in the sky, swift temperature changes, and condensation. Therefore, every single component for aircraft use must be tested thoroughly for compatibility with the conditions mentioned. That includes subjecting test components to the same changes in pressure as our ears!

How to simulate the required (changes in) subatmospheric air pressure conditions?

At higher altitudes, air pressure is always subatmospheric. An altitude simulation chamber on the ground thus requires that pressure can be controlled below the pressure at sea level. On top of that, it must be possible to simulate rapid pressure changes, similar to the rapid pressure changes in an airplane. To facilitate this, a pressure reduction in quite a large volume that follows a steady curve of the measured pressure is necessary. Usually, the evacuation curve looks logarithmic, as with lower pressures the volume of the mass that has to be pumped out of the chamber expands exponentially.

Traditional pressure regulators in the market are often not sufficiently capable of rapid responses to sudden pressure changes. Traditional pressure control valves have a limited flow range and are often inadequate for this kind of pressure reduction. To achieve a pressure reduction that is steady enough, multiple valves and a very complex control algorithm are required. This was the pressure control issue that led the aircraft engineers to contact Pressure Control Solutions: Could we help them select a suitable pressure regulator to control the required vacuum and that has fewer limitations than traditional regulators? Obviously, the regulator should be resistant to harsh temperatures and condensation.

Precision Process Control Products suitable for Altitude Simulation

Due to its large rangeability capacity, a pilot-operated Equilibar vacuum regulator is the best-in-class solution to address the given pressure control challenges in altitude simulation with just one valve in a simple PID-loop. The flow capacity of the valve used for this application has a Cv range of 1e-2 up to 8.1, resulting in airflow of approximately 0,07 up to 56 m3N/h (@ 20 °C and a differential pressure (dp) of 500 mbar). On top of that, Equilibar vacuum regulators can handle harsh temperatures – thanks to the used materials – and condensation, as they can be used in two-phase flow as well.

PCS recommended combining the Equilibar vacuum regulator with the unique own-design electronic pilot regulator, the ERC. Controllers in the ERC Series work with a PID controller that controls the set pressure with a proportional valve. The valve works against a capillary to achieve excellent stability and precision. The pressure feedback comes from an absolute pressure sensor that can either be used in closed-loop when placed directly in the process or in open-loop when placed in the reference pressure system.

P&ID Schematic showing the use of an Equilibar Vacuum Regulator and an ERC Series electronic pressure controller

P&ID pressure control in altitude simulation chamber

In this schematic, the sub-atmospheric chamber pressure is controlled by an Equilibar Vacuum Regulator that throttles the vacuum provided by a vacuum pump. A vacuum pressure transmitter gives constant feedback from the altitude simulation chamber to a PID controller, which adjusts the proportional valve on the Electronic Reference Controller (ERC), controlling the Equilibar Vacuum Regulator.

With this setup, rapid and also steady increases in altitude can be simulated. The Equilibar vacuum regulator creates tight and constant pressure control. This patented back pressure regulator technology is unsurpassed in translating its precision into the vacuum flow stream that is required for this application. Because of the carefully selected diaphragm that is both flexible and strong enough for this application, the process pressure can be controlled accurately without any hysteresis, keeping the sensitivity to changes of pressure in the system well below 0,001 bar!

Want to know more?

Visit the product page on powerful vacuum control products by Pressure Control Solutions for more details and application examples of the ERC Series. Contact us if you would like to discuss the possibilities for your application.