For a fab manager designing a gas system at a research facility, this is a fairly common question. As with most complex issues, the answer to the question is “it depends”. Here is a list of the key considerations to address when making that decision:
- Purity Considerations: Many of the small (under 1 liter per minute) Hydrogen Generation systems are designed primarily for gas chromatography service. The stainless tubing inside is passivated, not electropolished, and many of these systems typically use compression fittings, which are not as leak tight as VCR® type fittings. Some of the bigger systems have VCR bulkhead out, but there are still compression fittings on the inside. Gas Cabinets, on the other hand, are provided with electropolished tubing and both VCR® type, as well as welded fittings, which severely reduces the change for atmospheric inboard leakage which might affect your process. While Hydrogen is an inert gas, and passivated tubing is fine, a downstream purifier should be used if your process is sensitive to atmospheric contamination should a leaky compression fitting exist.
- Code Requirements: Code requires a way to shut down the gas system if your hazardous gas monitor trips, indicating a leak. Some Hydrogen Generators have the ability to receive this signal from the monitor while others do not. Should the generator you’re considering not have this ability, you can run the signal from the gas monitor to a breaker which will kill power to the unit, effectively shutting down the generator. You will want to check with your Safety group to determine if this kind of a configuration is acceptable. Properly designed Gas Cabinets, on the other hand, come standard with the ability to receive a signal from a gas monitor to shut down the cylinder in the event of a leak.
- Moisture Extraction: Typical smaller Hydrogen Generators utilize a palladium purifier to remove moisture down to < 1 ppm, while the larger ones usually utilize a dryer to remove the moisture, still keeping moisture at <1 ppm. If your tool’s moisture spec is tighter than this, external purifier are needed. Gas Cabinets rely on gas from cylinders which, with electronics grade gases, are sub-ppb moisture rated. Typical gas cabinet specs are <100 ppb, so purifiers are not typically needed.
- MFC Output Requirements: If your Hydrogen requirements are small (under 1 liter), and isolated to a specific tool, the thought may be to hook the gas line from the Generator directly to the gas box on the tool. A word of caution here. Small Hydrogen Generation units are typically designed for gas chromatography, so they aren’t designed to deliver continual stable flow if connected directly to an MFC. Non-stable flows into an MFC can cause fluctuations in your MFC’s output depending on what type of MFC you’re using (Some are more stable than others). Always use a line regulator in front of the gas box to even out any fluctuations in flow rate from the Generator. Gas Cabinets, due to the nature of design, deliver much higher flow rates so fluctuation typically isn’t an issue.
- Gas Cylinders: Hydrogen Generators are excellent for high flow requirements, such as with Thin Film Solar production. Having to change our multiple bottles on a daily basis can be a drain on manpower, high in cylinder cost, and hazardous when it comes to changing out bottles. Should your demand for Hydrogen require daily cylinder changes, you should compare the cost of a Generation unit to a Bulk System to see which makes the most sense. There is a definite place for Generators in a facility and this is, in our opinion, where the need exists.
- Price: Hydrogen Generators are not inexpensive. Granted, you do save on buying gas and storing cylinders, as well as in the manpower required to service the gas cabinets, but a Hydrogen Generator capable of providing around 600cc/min of continual flow will cost approx. $12K, which is less than a Gas Cabinet, but most applications require more than a liter of gas. Jump up to 5 liters a minute and you’ll pay $50K – $60K. It’s easy to spend several hundred thousand dollars on a system that produces 30 to 50 slpm of gas continually. Compare this to a gas cabinet which can flow up to 100 slpm of gas and will run you $20 to $30K, or better yet, choose a reconditioned gas cabinet that will run you from $14 to $18K, and you can see that you can buy a lot of gas bottles for the difference.
Ultimately, the decision comes to your needs and available budget. Always run the numbers, and keep in mind potential increases in demand as your facility grows.