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Understand the full role of the air-compressor system to optimize and protect your press investment.
If the performance of your automatic isn't up to par, the problem may not be with the press itself but with the air going into it. Most automatic printing machines use compressed air to operate some portion of their movements, and over time, poor-qualityair can impact performance to the point of failure.
Incoming air needs to be handled in such a way that contaminants such as water, dirt, and oil are removed. If this isn't done, the machine functions controlled by air-operated parts will become erratic or fail completely due to the infiltration of contaminants into the airstream. Many times, parts failure is put down to normal wear when, in actuality, the root cause lies in the design or equipping of the air-delivery system, and the life expectancy of the parts should have been much longer.
Some machines rely exclusively on air for their operation, while others use it only in certain areas—in the squeegee/floodbar chop system, for example. The amount, or volume, of air required for operation of individual printing machines varies, as do the designs. However, the quality requirements for the air being delivered to the press remain the same across the board.
Since the package price of a machine includes a compressed-air system, and the cost of a compressor is the greatest single-item cost within an air system, the focus is normally on the size of compressor needed to run a machine.
Frequently, little or no attention is given to the quality of air. This may be because most machine manufacturers realize that many customers choose to purchase their air systems locally for service reasons. Therefore, they tend to leave responsibility for the system to the local vendor. Specifications are furnished in terms of SCFM (standard cubic feet per minute), but they rarely detail air-quality requirements. In a sense, this leaves air quality to chance.The local air system vendor may or may not sell the print machine owner the correct hardware for proper air treatment. The vendor may not emphasize the importance of air quality, or the customer may ignore his suggestions.
Although the cost of the compressor system may be minimal compared to the total dollars spent on the rest of the printing setup, it may seem monumental if not planned for in advance.
This is typically a pay-now-or-pay-later situation. What tips the balance is that paying later causes more problems and associated costs when parts fail "prematurely" and, more importantly, "unexpectedly."
Which parts diminish in performance or fail when incoming air is not of the desired "quality," and why do they do so? In short, almost all components that have air running through them. This includes cylinders, valves, solenoids, and much more.
What causes air-quality problems?
Water, dust, rust, and oil are the main causes of air-quality problems, but sometimes ink and spray glue are causes as well. More severe problems arise when any of these items combine and form a sludge that typically clogs valves and causes intermittent sticking problems until the part fails totally.
How do these contaminants enter the air system?
Solids—dust, dirt, pollen, spray glue, etc.—can enter the compressor intake. Some components such as rodless air cylinders can allow contamination such as ink into the cylinder if it's not equipped with sealed bands.
Typical city air can contain four million dust particles per cubic foot of air. After compression, the particle concentration is likely to be nine times as much, or 36 million particles per cubic foot.
Oil and lubricants can enter the system in the form of liquid, aerosols, and/or vapors. It is necessary to lubricate the compression chamber of a compressor to protect the moving parts against wear. However, the presence of oil within the compression chamber results in the oil being vaporized and made part of the air being expelled.
Moisture (water) enters the compressor in the form of humidity or water vapor. The compression of the humid air generates heat. This heated air cools as it condenses, turning the vapor to liquid (water).
Try looking at it this way: It takes about 8 cubic feet of free air to make 1 cubic foot of compressed air at 100 PSIG. While the volume of air is compressed or decreased, the moisture remains the same or eight times greater than before.
A 25-horsepower compressor can generate 18 gallons of water per day when running at 90° F and 50% relative humidity, delivering 100 CFM at 100 PSIG.
Rusty excess water held within the air distribution lines and components can cause the pipe to corrode and rust to form on some parts. This damages seals and/or gaskets where loose particles can clog components by blocking the air pathways.
What's the solution?
When an air system is designed and installed, more should be considered than the delivery of volume. The system should also take into account the critical components needed to clean the air of contaminants.
These include:
Air Intake: Locate your air intake in the coolest, cleanest, driest area possible (outside, on the roof is generally best). Pulling air from your shop where contaminants such as spray glue or water vapor from a washout/reclaiming booth are present is counterproductive. Also, the cooling demands on the system are decreased when the compressor is not stored in a small room that promotes overheating.
After Cooler: An after cooler is a must on any air compressor, but it is not considered standard equipment. In other words, if you don't ask for it, your compressor may not come with it. The after cooler works like a car radiator in the way it dissipates heat through a finlike structure. An after cooler is very inexpensive and provides a tremendous amount of cooling for the money. Most average range compressors have inner coolers, but only better compressors have inner and after coolers built in.
If moisture removal is necessary— which it is—an after cooler is the first necessary component in the water-removal process. Other hardware items, such as refrigerated dryers, cannot perform their necessary functions without the help of the after cooler.
The amount of water removed from the system by the use of the after cooler makes it a must for any quality air system. An after cooler helps remove approximately 66% of moisture so the 25-horsepower compressor generating 18 gallons of water per day in the earlier example will produce only 6.2 gallons. However, that 6.2 gallons will still make its way into the print machine components if not addressed by other water-extraction methods/devices.
Water Separators: At best, these devices remove 98% of any free water running through the system, but they still permit 100% of the saturated air to pass through. Thus, the only way to trap the saturated air is to condense it. Complete moisture removal happens when the dew point (temperature at which vapors condense) can be reduced physically by refrigeration or chemically by a drying medium that attracts moisture.
Storage Tanks/Air Receivers: Last, but not least, is the role of the air receiver. Air receivers or storage tanks normally serve the primary function of storing air to meet sudden demands, thus reducing the compressor load cycle. The typical rule of thumb in a 100 PSIG compressed-air system is 1 gallon of storage for every 1 CFM of compressor capacity.
In addition to its primary functions, the receiver also acts as a cool-down device and water separator if the air is left in the receiver long enough before it is used. Because of this, it's important to size your receiver properly or use more than one.
It is generally also a good idea to use a vertical tank when possible. This allows incoming air to enter at the bottom and exit through the top. Pulling air out of the top ensures the driest air is used first. The use of two receiver tanks is ideal but not necessary. A second tank, if used, is placed after the air dryer and coalescing filter to store the treated clean, dry air before use.
Refrigerated Dryers: Because of the relatively low price of these devices compared to other drying methods/ hardware, refrigerated dryers are the most popular in our industry. Most of these devices use the outgoing cold air to precool the incoming air. The cooler air coming in puts less demand on the chiller, and a smaller refrigerant compressor can be used. The basic approach involves mechanically refrigerating the water vapor to condense it. Once the water condenses, a water separator removes the condensation (water).
Filters: Solid contaminate particles such as dust, dirt, rust, and oil can be removed more effectively by filters than by any other means. Devices designed to remove moisture, such as after coolers and refrigerator dryers, have little or no effect on these contaminants.
Most filters are considered to be particulate, coalescing, or a combination; particulate filters remove solid particles by mechanically separating them. This is often accomplished by trapping them in a material that allows air to pass but catches the particles within the passageways that are smaller than the particles. Mechanical separation also occurs when the contaminant drops out of the airstream due to an acceleration of the air velocity. Coalescence occurs when very small droplets of oil and or water collide with small-diameter fibers causing them to merge into larger droplets and drain from the filter by gravity.
Particulate Filters. Particle size ranges from 5 to 40 microns and filtration can be 100% at any size. Pressure drop due to the use of this filter is usually only 0.5 PSI to 1 PSI when the filter is clean. As particles accumulate on the replaceable element, the air restriction continues to rise until the air cannot pass or the seal is broken, allowing large sizes and quantities of contaminants to enter the air system/press components.
Coalescing Filter. Glass fibers, with diameters about the size of the oil droplets they are designed to remove intercept the oil droplets when they collide with the fibers. After this collision, the spacing between the fibers is designed to be three to four times greater than the fiber diameter, allowing air to pass but promoting the flow of oil down the fiber to the filter bottom where it is expelled. Solid particles are trapped in the fibers and not the drain, eventually clogging the filter and requiring replacement. Pressure drop on this unit can be as much as 10 PSI or greater when new. This puts some additional requirements on the compressor to deliver the added pressure to the machine. Differential pressure gauges in the air line may be mounted in order to detect when these filters need changing.
Lubricators: Finally, remember that after the air is delivered to the press, it must be lubricated before it enters the machine. Machine lubricators are provided by the equipment manufacturer and mounted on the press. The addition of replacement oil to the system is your responsibility, as is checking to see whether the machine is being fed the proper amount of oil by the oiler. Too much clogs the system and causes problems; too little allows parts to run dry, generating friction that, in time, causes premature wear on seals/gaskets and results in parts failures.
Theideal system incorporating the components needed to deliver clean dry air to your printing machine. Changing the filters when necessary and draining the units on schedule will greatly improve the life expectancy of your press's pneumatic components.
If maintenance scheduling is a problem, consider using automatic drains. These electrically operated devices purge themselves, relieving you of the task. The addition of pressure gauges before and after the filters (not pictured) also will help in determining when filters need replacing.
Real Performance Machinery L.L.C.
Need an answer to your question right away? Phone 317-876-9230 Mon-Fri 8a.m. - 5.p.m.
Indianapolis, IN |
