Foundry Saves $10,000 per Year by Producing it's Own Nitrogen

Aluminum Foundries, Inc. saves $10,000 per year by using a nitrogen gas generator that eliminates the need to purchase nitrogen dewars. The company has also eliminated demurrage costs, labor required to change dewars and scrap caused by running out of compressed nitrogen. The foundry uses nitrogen to degas hydrogen from molten aluminum in order to reduce porosity of finished castings. The on-site nitrogen generation system uses a filtering process to produce nitrogen from air on demand with virtually no operating costs.

Aluminum Foundries provides one-stop shopping for producers of automobiles, oil field tools and electrical equipment by producing sand, permanent mold and die castings in a single foundry. Many of these companies have found they can reduce purchasing costs by using the 115-employee foundry to supply all their aluminum casting needs. Aluminum Foundries is capable of producing parts ranging from 8 ounces to 500 pounds.

Nitrogen plays a critical role in protecting the quality of most of the castings produced by Aluminum Foundries. Hydrogen is a particular problem in casting aluminum because it is soluble in the melt and can create voids that detract from the quality of the finished part. Nitrogen gas has an affinity for hydrogen so bubbling nitrogen through the melt eliminates these problems.

In the past, Aluminum Foundries purchased liquid nitrogen dewars on a regular basis. The cost of the gas itself when added to the demurrage charge, or dewar rental, amounted to about $10,000 per year. In addition, whenever a tank was emptied, which happened once or twice a week, the job would have to be interrupted while a worker was sent with a forklift to retrieve and install a new dewar.

The expense and inconvenience associated with the use of nitrogen dewars was high enough that management was motivated to investigate the recent development of inexpensive methods for on-site nitrogen generation. Joint ventures between industrial gas companies and chemical companies have resulted in successful R&D programs to improve gas generation technology. The most influential of these developments is the use of membranes and specialized adsorbents for the production of nitrogen gas.

Aluminum Foundries purchased a Balston 75-76 nitrogen generator. The system utilizes proprietary membrane separation technology. The generator separates air into its component gases by passing inexpensive, conventional compressed air through bundles of individual hollow fiber, semi-permeable membranes. Each fiber has a perfectly circular cross-section and a uniform bore through its center. Because the fibers are so small, a great many can be packed into a limited space, providing an extremely large membrane surface.

Air separation takes place in the membrane module. The inlet air enters the center bore of the membrane fibers and travels the length of the fibers. As the air passes through these hollow fibers, oxygen and water molecules pass through the membrane wall at a higher rate than nitrogen molecules. The membrane divides the air into two streams: one is up to 99.5% pure nitrogen and the other is oxygen-rich with carbon dioxide and other trace gases.

Compressed air is introduced to the center of the fibers at one end of the module and contacts the membrane as it flows through the fiber bores. Water vapor and other trace gases permeate the membrane fiber and are discharged through a permeate port. The nitrogen is contained within the hollow fiber membrane and flows through the outlet port of the module. Water vapor also permeates through the membrane, so the nitrogen product gas is very dry.

Two high efficiency coalescing filters, an activated carbon filter and a particulate filter, are incorporated into the nitrogen generator to protect the membrane module from contamination. The activated carbon filter removes hydrocarbons from the compressed air before it enters the separation module. The particulate filter protects the membrane from carbon dust. The coalescing filters remove liquids and particulate matter from the incoming air supply to 0.01 micron. These filters are equipped with automatic float drains to empty any liquids which accumulate inside the filter housing.

An optional oxygen monitor provides a continual check on the purity of the nitrogen process stream. It measures oxygen concentration with an accuracy of +1% of the calibrated set point. The unit also features audible alarms to signal high or low oxygen concentrations and alarm relays for process controls.

Installation consisted of simply connecting a line of standard compressed air to the inlet and connecting the outlet to a nitrogen line. All that is necessary is selecting the purity desired and setting the flow and pressure. The unit runs by itself without human monitoring or intervention.

Balston's nitrogen generators can produce nitrogen on demand at a purity of 95% to 99.5%, depending on the application. In this case, degassing aluminum requires a purity of 98% or better. The generator cost approximately $15,000, so it paid for itself in its first 18 months of operation and has provided at least $10,000 per year additional profit ever since. In addition, the generator has eliminated the need to handle nitrogen dewars and the danger of downtime caused by running out of nitrogen.

Operating costs of the unit are close to zero. The unit requires only two cubic feet of compressed air to produce one cubic foot of nitrogen. That means it uses less than 1% of the plant’s compressed air capacity, making the cost of supplying air almost too small to calculate. The only maintenance that is required is changing the filter which costs about $950 and is required approximately every six months. Besides this task, the unit has not required service since the time it was installed. An oxygen sensor located on the generator provides a continual reading of output purity and also determines when the filter needs to be changed.

The generator requires virtually no attention because it uses simple electromechanical components such as pressure vessels, and valves with a history of reliability in industrial applications. A key factor in the increased reliability provided by the generator is its elimination of the logistics of the gas supply chain.

Since the nitrogen generator simply separates air into its constituent parts, it has no adverse environmental effects. Both the nitrogen produced by the unit and the oxygen mixture generates as a byproduct can be released into the atmosphere.

Gas generators are much safer than high-pressure cylinders. The generator typically operates at a low pressure in the neighborhood of 100 psig and stores small volumes of compressed gas. The stored volume is much less than 1 cubic foot, compared to about 200 cubic feet stored in a typical high-pressure gas cylinder. Gas generators also eliminate the need to handle cylinders, which presents a risk of injury caused by dropping, lifting, asphyxiation or explosion.

The development of on-site gas generators is remarkably good news for foundries like this one that require a regular supply of nitrogen. Besides saving money, it provides long-term cost stability by eliminating the risk of gas shortages or uncontrollable vendor price increases. Aluminum Foundries was one of the first foundries to use this system and has experienced a steady $10,000 per year increase in profit since it was first installed. Since it was installed in May 1992, the unit has required no scheduled or unscheduled maintenance other than the six month filter changes. The product has been so successful that when Aluminum Foundries recently expanded its plant, the company installed a second nitrogen generator to provide its new casting lines with the same savings.

For more information, contact Parker Hannifin Corporation. by phone: 978-858-0505 or by fax: 978-858-0625.