This was a Senior Design I & II  (product ideation) project to design a manual (hand-pump) nebulizer for patients with COPD. 

The main constraint was a low budget, using a manual power system for off-grid use. We used an off-the-shelf mouthpiece to take advantage of the Venturi effect, so the primary target for the flow and control system was generating the pressure + flow rate required for this mouthpiece. 

For implementation, we changed the design to use a pressure vessel and bellows system which allowed for fluctuations in pressure during hand-pumping. 

Scope of Work

As part of the pump controls team, I was responsible for designing the pressure vessel used to pressurize and maintain a constant flow for the user once the threshold pressure was achieved. I worked alongside the flow, testing, and HMI team to ensure all requirements  were met. 

The pressure vessel was designed to support a continuous 18 psig internal pressure combined with an average 5-pound input force on the pump’s lever arm. This input force is redirected to an oscillatory ± 40 lbf. at the lever fulcrum point. The vessel should also drain for a minimum of 10 seconds from maximum to minimum internal pressure to allow for potential breaks to be taken during medicine administration. For this to occur, a volumetric capacity of 2.5 L was required.

Mechanical Analysis

Due to compressible flow conditions of this project, the internal volume found for the desired drain time was an iterative, empirical procedure. However, an estimated internal volume could be found by using incompressible assumptions. Combining Boyle’s Law and flow rate, the required internal volume was found (6). This provided a goal volume of 2.2L. Erring on the side of caution and leaving room for lower pressures to be used to achieve the same drain time, a larger volume of 2.5L was chosen.

V_internal  = P_external ∗ Q_target ∗ (time_target/ΔPinternal) 

= 101.3kPa ∗ 7LPM ∗ (10s/55kPa) =2.2L

FEA

The pressure vessel’s use-case loading was simulated using a fixed contact at the bottom bolt flanges as well as on the posterior side of the bracket overhang, for a total of 6 fixed points of contact. The clevis slots have a simulated applied load of 140 lbf (safety factor = 3.5) each. The simulated internal pressure is 126 psi, and the maximum combined internal pressure and external loading concentrated stress is approximately 29.1 MPa. With a yield strength of 30-50 MPa for ABS, this design is well within the intended use specifications with a factor of safety greater than 7.0. This complies with the standard of critical component safety factors in many medical devices today.