Air dryer prevents damage to critical mold engraving and machining equipment.
King Machine is a producer of molds used to produce sidewall plates and tread segments for automobile tires. The molds are typically machined from mild steel and aluminum billets. King uses CNC machining centers to perform a wide range of operations required to produce molds to the approximate geometric requirements, and then uses special engraving machines to produce final details, such as lettering on the sidewalls.
Importance of compressed air service
Compressed air is one of the most critical inputs to the company's manufacturing operations. The process of producing molds generates large volumes of fine chips of metal that, if they are not promptly removed, will lodge in precision equipment and make it impossible to meet close tolerances required. The tools used in both the engravers and machining centers are changed on a regular basis. In fact, the tools used in the machining center are automatically moved into and out of spindles of the machine during each machining cycle, as required to perform specific operations. Each time a tool change occurs, both the old and new tools are exposed to chips and other particles from the cutting bed. If any of these particles were to stick to the shank, the tool would not run true when it was inserted into the spindle. This might make it impossible for the machine to meet the level of precision required in producing the molds.
To prevent this from happening, the spindle and tool are both blasted with compressed air each time a tool is changed. The machining center spindle and tool are cleaned automatically by the compressed air service installed in the machine, while the engraver spindle is cleaned manually by the operator. The problem, in the past, was that the air used to clean the spindles contained water that would collect in the compressed air lines whenever humidity was high. The result was that the water would be sprayed into the spindle along with the air, causing rust that creates the same accuracy problems as the chips that the air was designed to remove. In at least two cases, the company was forced to scrap $3,000 engraver spindles because rust had reduced their accuracy. In many other cases, expensive repairs were required.
The presence of water in the air lines causes other maintenance problems as well. The plant uses air motors in a wide range of tools such as pneumatic drills. These motors would rust to the point that they stopped functioning after just a few weeks of service. Napier would have to rebuild about ten of them per week, which took a total of about five hours of his time and perhaps $500 in parts. He spent perhaps an equal amount of time cleaning rust off machine spindles. Recognizing that the time he spent fighting rust was making it difficult for him to keep up with his other responsibilities, he sought and obtained authorization from his management to seek out a method of solving the problem at its source by removing water from the compressed air lines.
Napier considered a number of options in trying to solve this problem. He looked first at a refrigerated dryer that was relatively inexpensive to purchase. However, when he considered the costs of operating the refrigerated dryer, about $650 per year for electricity, plus regular maintenance required for the motors and other moving parts, it seemed as though it would be less expensive. Then he saw an article in a magazine about a new type of Balston compressed air dryer based on membrane separation from Parker Hannifin's Filtration and Separation Division in Tewksbury, MA. This dryer cost slightly more from an initial purchase standpoint, but has no moving parts and requires no maintenance except for changing the filter every year. Napier's calculations showed that this unit would be considerably less expensive over the long run than the refrigerated dryer, so he recommended it to his management, and the purchase was authorized.
Operation of the membrane filter
Prior to entering the membrane drying module on the Balston unit, the compressed air passes through high efficiency coalescing filters. These filters remove oil and water droplets and particulate contamination with an efficiency of 99.99% at 0.01 micron. Balston dehydration membranes consist of bundles of hollow membrane fibers, each permeable only to water vapor. As the compressed air passes through the center of these fibers, water vapor permeates the walls of the fiber and the dry air exits from the other end. A small portion of the dry air (regeneration flow) is redirected along the length of the membrane fiber to carry away the moisture laden air which surrounds it. The remainder of the dry air is piped to the application.
Liquids removed by the filter continuously drain from the filter cartridge into the bottom of the housing, where they are automatically emptied by an autodrain assembly. The air leaving the prefilters carries only water vapor, which is removed in the membrane module. The dryers deliver air with a dewpoint of 35[degrees]F. Selective permeation membranes remove water vapor from compressed air. The Balston membrane air dryer is designed to operate continuously, 24 hours per day, seven days per week. No electrical supply is required to use these dryers. The only maintenance required is changing the prefilter cartridge a maximum of once a year. The time required to change the prefilter cartridge is approximately five minutes.
Solving the problem
Napier selected the Model 76-200-35 dryer that produces a flow rate of 200 scfm. He also purchased a five-year supply of filters. The next time that he purged the compressed air lines he could verify the performance of the dryer. In the past, he would open up the drip leg and water would 2pour out. Since installing the dryer, he has never seen a single drop of water. The dryer has now been in service for about a year, and it has completely eliminated rusting problems on spindles and air motors. In addition, the dryer itself has worked without requiring any maintenance. When they originally purchased the dryer, Napier was informed by its manufacturer that the filter elements would last for at least one year. At the one year interval, a Parker service engineer visited their plant to check their condition. The filters were in nearly new condition, and the service engineer informed management that he estimated their life would be several years in this application.
According to Napier, this new type of membrane dryer was needed to eliminate problems with water building up in compressed air lines. Since the day it was installed, the company has not had a single problem with rust. The time and money saved by not having to repair spindles and air motors are said to pay for the cost of the filter every few months. The elimination of rust on spindles also helps to improve the dimensional accuracy of the products. The fact that the dryer operates with no electricity or maintenance is said to help to keep its operating cost low.
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|Title Annotation:||Process Machinery|
|Date:||Jun 1, 2004|
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