CAPSTONE: HIGH PRESSURE NITROGEN GAS HEATER FOR COLD-SPRAY ADDITIVE MANUFACTURING
For my senior capstone design project, I've teamed up with three other mechanical engineering students to take on a challenging project. We are working to design a high-pressure nitrogen gas heater that will be used to study and improve upon cold-spray additive manufacturing. The project is in progress but we are learning every day along the way and are excited to cross the finish line by the end of this year!
COLD SPRAY FUNDAMENTALS
Cold Spray Additive Manufacturing (CSAM) is simple in principle but challenging in its application. A heated carrier gas transports high-temperature metallic particles through a supersonic nozzle and deposits the particles onto the workpiece. Mechanical interlocking and metallurgical bonding takes care of the rest as the particles deform on the substrate to form a solid surface. Our project focuses on the design of a high-temperature pressure vessel meant to heat and accelerate the nitrogen carrier gas as it reaches the nozzle. Safety is a main concern as we are dealing with temperatures up to 500°C and pressures up to 50 bar (725 psi).
FIRST PROTOTYPE
Our team is making steady progress in the design of our heater. The reason behind this project stems from our Professor's desire to construct a lower cost yet higher capability gas heater compared to those that are available on the market today. These devices can cost as much as $50,000 while we have managed to keep our budget below $10,000 without sacrificing any performance or safety characteristics. Below is a drawing of our initial prototype which features an off-the-shelf heating element encased in a pressure vessel which seals via flanges on each end. The flanges are not secured to the vessel itself, rather the tension in the bolts maintains a sealing surface between the ends of the pressure vessel and the surfaces of the flanges. We found that this design, while simple to assemble, makes maintenance and cleaning challenging as the bolts and flanges are extremely unwieldy. Instead, we are working to redesign the system to simplify operation and maintenance. Scroll down to see more!
SECOND PROTOTYPE
For our second iteration, we chose to explore the idea of welding the flanges to the pressure vessel itself. This drastically reduces the amount of parts required in the system and simplifies the design as a whole. Additionally, a certified pressure weld will give us confidence in the sealing capability of the device. The worst case scenario is that heated nitrogen gas escapes the chamber through a leak and harms the user or surrounding equipment. For this reason, we have designed a test protocol to ensure that the system is safe before it is pressurized to its operating conditions. Note the orange flange cap attached at the left-hand side of the device. This is known as a flanged immersion heater and was our best bet at streamlining the temperature control feedback loop to regulate temperature as accurately as possible. Unfortunately, this part alone would have exceeded our entire project budget so we have chosen to investigate alternative options for the heating element itself.
SIMULATION & ITERATION
Executing both thermal and mechanical simulations to verify our design has been one of my main contributions to the project. The results from my analyses are used to improve upon designs and verify that we can meet and maintain the required operating conditions. I very much enjoy learning about simulation softwares and have become proficient using COMSOL, Star-CCM and Ansys Fluent & Mechanical thanks to experiences in both capstone and on the job at co-op.
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While we are certainly not averse to applying the heat transfer principles we've learned in class to our design project, simulations have proven to be a powerful tool in optimizing the device. Above is a sample output which I generated using Siemens' Star-CCM+ CFD program. After creating numerous CAD prototypes and evaluating each and every one via simulations, we have been able to gain a thorough understanding of the thermofluidic principles which govern the performance of our device. Maximum heating efficiency is crucial to avoid wasted energy and higher manufacturing cost for CSAM parts. Similarly, simulations can be used in conjunction with ASME Boiler & Pressure Vessel codes to ensure structural integrity of the device, a key element in the success of our project. Electrical concerns aside, our heater is effectively a pipe-bomb meaning a high safety factor is required for every component.
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BAFFLES
As we continue to produce new simulation results, one main optimization factor has been the use of ceramic baffles placed within the vessel to tidy up the flow of nitrogen and ensure maximum contact with the heating elements. Above is an image of our first baffle prototype. Lots of effort went into choosing materials and optimizing its shape in terms of passthrough diameters, spacing, length, etc. We hope to maintain enough budget to implement this baffle into our final prototype, but are mainly focused on safety per dollar rather than performance.
CERAMIC HEATING ELEMENT
Our investigation into the heating elements available to use has lead us to select the device shown above. This is a 15KW, 3x 480V ceramic-braced heater which contains a number of coiled heating elements that are supported by the ceramic backbone. This device, used in conjunction with a solid state relay and PID temperature controller will comprise the electrical system which features a closed-loop temperature control cycle that reads data from an associated thermocouple. We have chosen a safe, cost-effective alternative to the flanged immersion heater which allows the user to control temperature to an accuracy of +/- 1°C, a feature which is extremely useful towards researching the effects of temperature on surface quality of CSAM parts.
ELECTRICAL SYSTEM
In addition to extensive mechanical design and verification both within our team and from outside contractors, it is crucial that the electrical system be designed safely. At maximum operating conditions, as much as 480V and 25A will be running through our system. As mechanical engineering students, approaching this problem has been a huge learning experience. A particularly important component of the electrical design is the electrical feed-throughs. These components are used to deliver power to the heating element through the vessel walls and must be capable of maintaining a perfect seal as we must not leak heated gas into the Cold Spray lab! Similarly, we must design a circuit to incorporate heater burnout detection, a hugely desirable feature meant to protect the heating element from overheating. Myself and my team have been in close contact with various electricians and electrical engineers to both assist in our design and verify our own work. I have found this to be an interesting challenge as an electrical failure may not only mean damage to components in our system, but potential harm to its user. I am happy to report great progress as the team has progressed to placing orders for the majority of our electrical components!
Please check in regularly for updates on our design progress!