Designing the valve manifold for a lightweight, portable oxygen concentrator required an engineering balancing act to achieve the compact size, low power consumption and flow rates required to make the product a success.
When a manufacturer of respiratory and rehabilitative care equipment recognized a need for a portable O2 concentrator to replace the relatively heavy and cumbersome portable O2 tanks, they began developing a prototype using a lapped spool valve. Since the unit would be battery-powered, low current consumption was critical with a goal of producing a small, compact unit with up to four hours of battery life.
But when the flow rate specification for the product changed, and the size and leak rate of the resulting spool valve proved unacceptable, the customer turned to a custom valve manifold design and worked with the engineering group at Humphrey Products. A new design met the target flow rate, spatial requirements, low current consumption, life cycle performance and enabled the customer to create the portable oxygen concentrator.
The engineered solution for the application included a custom manifold with three HK5 direct-acting solenoid valves having virtually zero leakage. The standard leak rate for these valves is 2 cc/min and the noise specification results in virtually silent operation. This is a big advantage because the patient can't hear the valves making any clicking sound when the unit is operating. Two of the HK5 valves were modified for 0.8W current consumption, for an overall total current consumption of 3.2W.
"The valves were customized by reducing the air gap in the solenoid which caused the valve to shift to lower power," says Joshua Haug, a territorial manager for Humphrey Products. "We reduced the stroke on one valve to reduce power consumption and increased the stroke on one valve to increase the flow. The HK5 valve was versatile enough to accommodate both situations."
Modified Humphrey quick exhaust valves utilizing a unique shuttle design were used as check valves to achieve a target cracking pressure of 1 psig or less. Special low profile fittings reduced the overall package size and the valve assembly was tested to 6 million cycles without failure.
Each of the three HK5 valves provided different functions. One controlled patient O2 delivery with a sensor circuit, providing oxygen only when the patient was inhaling. Pressure equalization valves and a bleed valve controlled the production of O2 in the sieve bed, where nitrogen atoms are removed from the atmospheric air.
Within the custom manifold, Humphrey also integrated customer circuits and additional inline components that previously had been plugged into the system externally to eliminate fittings, tubing and possible leak points. The components were integrated into a small manifold to reduce the size, the overall number of components and time required to assemble the product.
As the design progressed from concept to prototype and the customer modified the key flow rate specifications, one requirement was being able to adjust the internal orifice sizes in the manifold quickly to enable the customer to fine-tune the design.
"We would much rather have the flow dictated by the orifice in the manifold because it is more consistent than attempting to specify a certain stroke for the valve," says Haug. "The valve is built with an overflow and restricting the flow at the orifice makes the rate more consistent from product to product."
The biggest challenge for the application, apart from the tight delivery schedule, was the overall envelope size and packaging required to achieve the unit's portable design objectives. "We were designing to make sure the package would achieve the goals set by the original conceptual design," Haug says.