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Complete air compressor installation
Figure 8-2 combines the schematic diagram and picture representation of a typical air compressor installation. (The compressor could be a reciprocating or rotary type.) The aftercooler may not be required on installations under 50 hp, and it could be air-cooled instead of water-cooled. An air dryer is necessary in certain applications, but is often left out due to added cost. As noted earlier, a receiver tank might be eliminated with a rotary compressor is there never is a demand for short bursts of high-volume air. Water traps with drains are required on all systems because a compressor takes in a lot of water with the ambient air. (Even with an air dryer there is always the time when the dryer needs service but the system cannot be shut down. A trap will help during these times.) Other components, such as isolation or bypass valves for the aftercooler and air dryer, often are part of the circuit.
Most hydraulic pumps are positive-displacement devices. Pumps with positive sealing parts -- whether rotary or reciprocating -- move fluid every time they operate. This means that if the pump is turning, it produces flow. (Conversely; blocking flow stops the pump’s rotation mechanically.) Positive-displacement pumps have higher efficiencies than their non-positive-displacement counterparts, such as impeller or centrifugal designs. Figure 8-3 illustrates some non-positive-displacement designs that could be used to run hydraulic circuits. Because these pumps only run at 50 to 75% efficiency, they are not used in high-pressure circuits. They are frequently found in systems with high-water-content fluids (HWCF), such as 95% water and 5% soluble oil, because these pumps require little or no lubrication. Also, these systems usually operate at or below 400 psi.
Some positive-displacement pumps are paired with centrifugal pumps to pressurize their inlets to keep them from cavitating. Or, when a positive-displacement pump is run at higher rpm than specified, the inlet may not be large enough to let in enough fluid at atmospheric pressure. In this case a non-positive-displacement pump can force fluid into the undersized inlet and eliminate cavitation.
A non-positive-displacement pump does not require a relief valve in many installations. There is enough slippage in most designs to allow for stopping flow while not over pressuring the circuit. However, if the pump operates at no flow for more than two or three minutes, simple bypass circuit to move fluid for cooling purposes should be added. The bypass circuit could be a small relief valve, a manual petcock, or a normally closed solenoid valve operated by a timer or pressure switch.
The propeller design is the least efficient of these pumps because there is a direct path from inlet to outlet through the blades. The minimum rpm of this type pump is high due to this open path. The centrifugal-impeller design operates at much closer tolerance so it slips less fluid while operating.
Fixed-displacement pumps are found most commonly in circuits with a single actuator. This allows the pump to be unloaded at little or no pressure when not performing work. A multiple-actuator circuit, where only one device moves at a time, can also be practical for fixed-displacement pumps if the actuators use about the same volume of fluid. This means total pump flow is either doing work at load pressure or is being sent to tank at very low pressure.
Avoid using meter-in or meter-out flow controls with fixed-volume pumps because a flow restriction increases pressure and the increase sends fluid to tank at the relief valve setting. This produces excess heat and all the problems associated with it. One way to use fixed-displacement pumps with multiple-actuator circuits is to include an accumulator with an unloading and dump valve. With this circuit, the pump is only on pressure when fluid is required. The accumulator accepts excess pump flow and provides working flow when the pump is unloaded. Figure 8-12 shows a fixed-volume pump with an accumulator.
Fixed-displacement pumps are usually less expensive and more contamination tolerant than pressure-compensated pump. Note: this does not mean they should be run with dirty fluid or that cheaper is really less expensive. It only means they fill the bill in many applications where cost is a factor.