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Real Performance Machinery Screen Printing Article :
Select Compressor With Needs In Mind

Select Compressor With Needs In Mind
Make sure your compressor can supply the amount of air and pressure your equipment requires.
By Rick Fuqua

Just as poor-quality air can impede press performance and harm parts (see IMPRESSIONS, January 1998), "starving" your automatic of air can keep it from doing its best and possibly damage components or shorten their life. So, it pays to ensure the air-compressor system you buy can supply the proper amount of air and pressure for your machine.

Insufficient air can hamper press functioning in a number of ways. For example, when running all print heads at the same time with full strokes:

  • Some print heads do not have the same power (their cycle speed is slow or they stall at certain points) as they do when fewer heads are used.
  • The same lack of power is evident in certain heads when the press cycle speed is faster or dwell time is closer to zero.
  • After hesitating, when certain heads begin to move, they move too quickly.
  • The indexer table raises more slowly than normal or does not raise all the way before printing.
  • The indexer table appears to lower when print pressure is applied.
  • The indexer bounces at the end of its cycle or stops so abruptly it makes a banging noise.

Less-than-ideal machine functioning may impact print quality and/or cause the machine to need service. For instance, when print heads lack the power to move smoothly, machine output is diminished, or the print may appear to have squeegee start and stop lines. A good, smooth print stroke at any speed requires adequate air pressure in addition to volume.

Air pressure is required to overcome frictions that are present in or result from the air cylinders' internal seals, mechanical squeegee/flood bar carriage guidance mechanism, squeegee print pressure, and screen/squeegee friction.

Unless these are largely overcome, the stroke may not start, appear slow,"stutter" or pause then race over the screen too quickly.
Parts are not damaged in these instances, but the print can be adversely affected. When the squeegee moves with a stutter or refuses to move under extra squeegee pressure, one solution is to lessen pressure. However; this may result in a light print where more ink is needed. A second stroke or new screen with coarser mesh may be needed for proper ink coverage. Also, if the stroke pauses at its start until enough pressure builds to overcome the friction, the squeegee may move too quickly for the remainder of the stroke. When the squeegee cannot stroke slowly under heavier print pressure, the ink doesn't have a chance to flow, and textured surfaces may print well on the hills but not in the valleys.

A coarser mesh or less pressure is not always the answer. The problem may not be that the design requires more ink, but that the ink needs to go down into the valleys of the material. This is best achieved with more pressure and a slower print speed.

Lift cylinders for raising the indexer tables are the largest cylinders on most pneumatic textile machines. A loss of air volume/pressure can be evident in the motion they provide for. If the rate at which they lift is slower than normal, it may be indicate the cylinders are starving for air. If they appear to lower during the print sequence, pressure may be dropping in the system. In this case, there isn't adequate air pressure to hold the table up against the combined squeegee pressures of all the heads. The stroke of the print head cylinders may have robbed the lift cylinders of volume and/or the pressure may be insufficient.

In indexing systems that incorporate shock absorbers/linear decelerators, the shocks are set to decelerate and stop the indexer at predictable ranges of force (speed and weight of the indexer table). When pressure and/or volume varies during indexing, the shock absorbers' ability to compensate for these variables may result in a rough motion causing excessive parts wear. If, for example, the shock is set to take a high level of energy from a heavy, fast-moving indexer table and, instead, the table is moving slower without much force, the settings may be too strong for the application. The result may be a banging or bouncing of the indexer once the shock is impacted. Banging indicates the shock did not collapse quickly enough to reduce impact.

Bouncing normally results when the shock collapses too quickly. But it also can occur after a hard impact if the force carrying the indexer forward is less than the energy built into the shock, allowing the throwing back of the weight. The abrupt impact and/or bouncing of the drive unit can directly or indirectly cause premature wear. These forces directly affect the wear to the drive fork or nest, bearings, slides, and the shock. Indirectly, these excessive vibrations can cause fasteners, such as bolts, to work loose. Once these parts are able to move freely, wear can be accelerated.

To avoid these problems, the air system for your press should be designed to handle the highest demands required without experiencing capacity/pressure drop. This means the system should operate comfortably with the press running at full speed with all print heads at full stroke.

The first step toward meeting this capacity/pressure goal is making sure you have the correct compressor type and size. A large compressor does not always guarantee the press will be fed correctly. A number of other factors also influence proper delivery of air to the machine.

Compressor Types
Most of the compressors on the market are one of three basic types: reciprocal, rotary, or centrifugal.
These types can be further divided into subcategories: single or multistage and air- or water-cooled. The proper size of any of these types of units can handle a single press. But issues other than cost—long-term performance/ maintenance, multiple-machine capacity, and compressor noise—may make a certain type preferable.

Reciprocating compressors cost the least and are the most popular for small to medium-size textile shops. This unit is a positive-displacement compressor. It takes in successive volumes of air, which are confined in a close space, and raises them to a higher pressure. The reciprocating element is a piston within a cylinder. The piston within the cylinder is the compressing and displacing unit.
A compressor of this type can be single- or double-acting. A compressor is considered single-acting when the air is compressed using one side of the piston and double-acting when both sides are used.

When the total air pressure is accomplished using only one piston, the compressor is considered "single stage." "Multistage" means the air is brought up to its required pressure in stages using more than one cylinder. The need for multistage compression comes from the design problems related to excessive discharge temperatures when the ratio between absolute intake pressure and absolute discharge temperature is too great. To avoid these problems, air is cooled between stages.

The reciprocating air compressor uses spring-loaded valves in each cylinder that open when the proper differential pressure exists across the valve.

Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure of the cylinder is slightly above the discharge pressure.

Reciprocating compressors come in lubricated and non lubricated configurations. Lubrication can be accomplished using a simple splash or dip arrangement wherein the parts are lubricated by being cycled into the sump of oil or coated automatically, in more expensive pump systems.

The advantage of the pump is parts can be lubricated at startup rather than after cycling. It is argued that, with splash systems, parts lose their lubrication during long waiting periods and experience excessive wear when forced to operate at startup without adequate lubrication.

Because of the reciprocating nature of these compressors, they tend to generate a sometimes objectionable amount of noise. But, caution should be used in attempting to reduce noise by placing the compressor in a small room or insulating around it. This can prevent sufficient ventilation to keep the operating temperature of the unit at a reasonable level, causing it to overheat.

Single-acting, air-cooled reciprocating air compressors should be sized to operate 30% to 40% of the time unloaded, based on a constant speed control system. Water-cooled reciprocating air compressors can operate continuously; but, it's a good practice to size for a 20% to 25% unloaded period.

Rotary screw compressors are not typically manufactured in sizes smaller than 30 cubic feet per minute, but they range as high as 3,000 cfm. The price of rotary units is higher than comparably sized piston types. However, the added life expectancy and lack of noise generated by units of this design may make up for the higher cost.

Rotary compressors are positive-displacement compressors. The most common is the single-stage helical or spiral-lobe, oil-flooded screw compressor. These two compressors consist of two rotors within a casing that compress the air internally. There are no valves. These units are basically oil-cooled where the oil seals the internal clearances. Since the cooling takes place inside the compressor, the working parts never see extreme operating temperatures. This makes the rotary compressor a continuous-duty, air- or water-cooled unit.

Because of their elementary design and few operating parts, these compressors are easy to maintain, operate, and install. Unlike reciprocating piston compressors, screw compressors can be installed on any floor or shelf capable of supporting only the weight of the unit.

Rotary screw compressors are considered two-stage when a pair of rotors are combined in the assembly. Compression is accomplished by sharing the duty in series between the two rotors. Up to 15% energy savings can be realized due to the design of the two-stage unit.
Two-stage rotary screw compressors are available in air- and water-cooled versions as well as oil-free models. Oil-free types eliminate the oil in the compression chamber for applications where oil is not desired.

Centrifugal compressors are designed primarily for large-volume (400 cfm to 15,000 cfm) applications requiring oil-free air. The air is compressed using a rotating impeller. Price and application differences make this type outside our discussion.
Compressor Size

The size compressor chosen is based on cubic feet of air your press requires at a certain pressure (normally 90 to 100 pounds per square inch). The pressure that is needed remains a constant. Only the amount of air or cfm of consumption varies depending on the number of heads being used at the same time and the cycle rate. Some manufacturers base the amount of cfm the press requires on the speed of the machine in dozens per hour. For example, according to M&R Equipment, a six-color Gauntlet with air drive requires 16.68 cfm at 50 dozen per hour at 100 psi and 23.35 cfm at 70 dozen per hour. A 5 horsepower compressor is adequate for 16 cfm, but a 7.5 horse is needed for 23 cfm.

Calculating the needs of a machine involves figuring the capacity of each cylinder and the sequence in which cylinders need to be fed. Given the dynamics of press operation, it is simpler to follow the manufacturer's suggestions. Of course, if you use the compressor for other purposes at the same time, those needs must be accounted for as well if those items are to be run at the same time as your press.

Systems for delivering air to machines range from simple to complex in design, depending on the number of devices/machines operating from the system. Consultation with an expert is recommended when feeding multiple machines. Specific plant variables, such as the floor plan, machine types, and the location of feeding devices from the air source make it impossible to provide exact installation information here. Rules of thumb can help, but each shop's plumbing and system design is unique. The following points provide food for thought in evaluating a system.

Other System Considerations
Leaks. A 1/4-inch orifice allows more than 100 cfm of compressed air to pass at 100 psi. Translation: Even small leaks collectively add up to significant energy losses. If you know some leaks are going to be present, choose the size of the compressor to allow for some air loss.

Air receiver. Storage tanks or air receivers store excess compressed air to deal with sudden demands, dampen discharge pressure pulsation, and reduce compressor load cycling.

In a typical 100 psi compressed-air system, a rule of thumb is 1 gallon of storage for 1 cfm of compressor capacity. Due to the high intermittent demand of most pneumatic textile presses and the relatively low cost of storage tanks, most systems sold for this application should provide 80-100 gallons of storage for even the smallest textile presses using 5-7.5 HP compressors.

Size, length, and type of pipe. The longer the hose, the less water pressure is available at the discharge end. Pressure loss is due to friction from the inside walls of the hose or pipe that is increased by the length. With air, as with water, the larger the pipe, the greater volume of fluid that can move in the center, escaping the wall friction and reducing pressure drop. The pressure drop is a function of the volume of air flow; initial air pressure; the size and length of the pipe; and the number of valves, couplings, and bends in the system.

  • For a given pipe or hose size and length, pressure loss increases as the volume of air flow increases.
  • Under the same conditions, pressure loss increases with a lower initial pressure and decreases with a higher initial pressure.
  • A smooth inner lining of the pipe or hose causes less pressure drop.
  • Fittings, couplings, and valves increase the pressure drop.

Try to:

  • Cut the effective distance of flow for the air, looping the system, if necessary.
  • Reduce friction losses caused by restrictions to flow. (A 1 1/2-inch pipe may cost 50% more than a 1-inch pipe, but deliver three times as much air with the same pressure drop.)
  • Use a large enough pipe with an inside diameter so as not to exceed 3 psi of pressure loss through the entire line. A goal of 10% pressure drop to the furthest point in the system is ideal.
  • Reduce the velocity, or flow rate, of air through the system. (Add receivers at the point of use.)
    Many issues other than the hardware items (compressor and chiller) figure into installing a top-notch air system. Your facility needs and the particular layout and machine locations, as well as the specific type and number of machines, must be considered. The best result will be obtained by working with qualified people on-site.

Real Performance Machinery L.L.C.

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