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Instead of buying a high-pressure compressor when only a small volume of compressed air is needed, consider using an air-to-air intensifier. Air-to-air intensifiers are small self-contained units that operate automatically as long as they have a supply of compressed air. Figure 13-30 shows a generic schematic of a simple air-to-air intensifier made from stock cylinders and valving. The arrangement has two cylinders connected at their rod ends and mounted on a beam, with limit switches or limit valves, a directional control valve, and four check valves. As long as compressed air is supplied to the intensifier, it takes in atmospheric air, compresses it, and sends it to a receiver and/or the system.
As the cylinders in Figure 13-30 stroke to the left, the intensifier takes in atmospheric air at the cap end of the 3.0-in. bore cylinder. Compressed air discharges from the rod end to a high-pressure receiver. After the cylinders fully stroke to the left, a limit valve pilot-shifts the directional control valve to stroke the cylinders to the right. When the cylinders stroke to the right, the opposite check valves take in and discharge air. Reciprocation continues until outlet pressure from the 3.0-in. cylinder reaches approximately twice the inlet pressure at the 4.0-in. bore cylinder.
The piping arrangement in Figure 13-30 produces less intensified air per compressor horsepower than the circuits in Figures 13-31 and 13-32. Taking in and compressing atmospheric air to a higher pressure gives a minimal high-pressure volume for each stroke. When compressing a gas, reducing volume by one half doubles absolute pressure. If the 3-in. bore cylinder has a 6-in. stroke and intake pressure is 14 psia, then as the cylinder moves through 3 in. of stroke, pressure climbs to 28 psia. As the 3-in. bore cylinder continues to stroke, pressure goes to 56 psia 1.5 in. from the end and to 112 psia 0.75 in. from the end. The cylinder finally starts discharging 160-psia air about 0.625 in. from the end of its stroke. Volume entering the high-pressure receiver is minimal for each stroke, and continues to decrease as the pressure level increases.
Using shop-air pressure in the intensifying cylinder, Figures 13-31 and -32, greatly reduces this high-pressure/low-flow problem. First, the high-pressure receiver starts with 80 psig and the air in the intensifying cylinder starts at 94 psia. This circuit discharges intensified air for more than half its stroke, making it a smaller, more-efficient package.
Notice also, the approximately 2:1 intensification from a 4-in. cylinder driving a 4-in. cylinder. This is possible because two areas, pressurized by shop air, push against one area of the intensifier cylinder. The actual intensification of the unit in Figure 13-31 is 2.06:1 when stroking to the left, and 1.93:1 when stroking to the right.
For higher pressure use a smaller-bore intensifier cylinder or a larger-bore driving cylinder. Figure 13-32 depicts a 4-in. bore driving cylinder and a 2-in. bore intensifier cylinder. This combination increases inlet air pressure about five times. The actual intensification ratios are 6.33:1 as the cylinders stroke left, and 4.74:1 as the cylinders stroke right.
When specifying cylinders to build an air-to-air intensifier, be careful not exceed their pressure rating. Pre-lubed cylinders are best for this type of operation because they keep excess lubricator oil out of the high-pressure circuit.
As an air-to-air intensifier pumps air to a maximum pressure, the volume decreases as the pressure increases. It is best to operate the intensifier to produce a pressure 15 to 20% higher than the system needs, with a regulator to set the maximum pressure at the work.