In Figure 7-7, a schematic drawing of a hydraulic circuit shows filters in the standard locations, with typical filtration ratings listed next to them. Note that most circuits would not have all of these filters, but every circuit should have adequate filtration to protect the pump, valves, and actuators from contamination.

7-7. Relative size of contamination particles at 500X magnification.

There are several sources of contamination in and around hydraulic units. Normal component wear, contamination in new oil, sloppy filling practices, poor plumbing installation, and dirt carried in on piston rods are the main ones. Some of these areas are simple to address ahead of time, while others can only be handled by filtration.

New oil from the supplier is not as clean as most hydraulic circuits require. At best about 25-µ cleanliness is all most suppliers will offer. One reason: the drums that new oil comes in probably were used before. Although they were cleaned, they are not contaminant-free. In addition, a standard drum pump draws fluid from the bottom of the drum where all the residue has settled.

Figure 7-8 shows a manually operated 3-way valve installed in the suction line of an offline filter pump. When this valve is shifted, all new fluid has to go through the offline filter before it enters the tank.

7-8. Schematic drawing of hydraulic circuit showing typical locations for filters.

Another item in Figure 7-8 is a tank-fill filter with no bypass, which filters all new oil entering the tank. Another option is to use a purchased filter cart to fill the tank through its normal fill port.

Whatever the method, it is important to keep filling practices from introducing contamination. While the fill port offers one of the simplest places to address contamination problems, it often is overlooked.

Another way that contamination enters a hydraulic circuit – even before startup -- is through poor plumbing practices. All pipe, tube, and hoses should be inspected for contamination before installation and cleaned if necessary. It’s good practice to seal clean conductors with caps until installation time. Use care when cutting and preparing pipe or tube ends to make sure no metal chips or filings stay in the conduits. If the system includes servovalves, flush it with filtered oil through flushing covers for the time recommended by the valve manufacturer before startup. Use every possible precaution on a new or replacement plumbing system to make startup go smoothly.

In a running circuit, one of the most common sources of ingressed contamination is the cylinder piston rods. Every time a piston rod extends, it is damp with system oil. In a dusty atmosphere this oil-dampened piston rod attracts and holds fine particles. Many of these are dragged back into the cylinder and washed off. Most cylinders have a rod wiper to help keep out contamination but this wiper only catches large pieces. Everything smaller passes by. This type contamination must be filtered out continuously to protect system components. Some suggest flexible boots or bellows over the rod end of the cylinder to eliminate rod contamination ingression. A flexible bellows does a good job unless it gets a tear or other type hole. At this point it actually sucks in ambient air with its contamination and holds it closely to the rod.

A similar situation takes place at the tank breather. During each cycle, the fluid level in the reservoir changes -- either drawing dirty atmospheric air in or discharging air through the breather. The filler breather should be capable of trapping the inbound contaminants in this air flow.

The other main cause of contamination is normal component wear. Dynamic pumps, motors, and cylinders have constantly rubbing metal-to-metal contact areas. Even with good lubrication, small eroded particles get into the fluid. This contamination must be constantly captured by filters to eliminate a damaging buildup of residue.

Condensed water is another form of contaminant that should be addressed. Water in hydraulic fluid can cause corrosion, break down fluid additives, and make viscosity vary. Water vapor often enters the system through the breather and condenses to liquid form when the tank cools. Using a breather with a hygroscopic media can eliminate most water contamination. Most of these hygroscopic breathers must be changed when they become saturated. They often use silica gel that is blue tinted when dry and turns a pink tint as it gets wet, signaling that a change is needed.

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Why are air dryers necessary? All air compressors take in atmospheric and compress it eight or ten times to a pressure between 110 and 125 psig. All the moisture vapor and heat in the atmospheric air is also compressed and concentrated, so air at the compressor outlet is hot and wet. The temperature of the air at the compressor outlet can be as high as 350ºF. The air also can be saturated with water vapor. As this air cools in the receiver and plant piping, the water vapor in it condenses into water droplets.

A 25-hp compressor running at 75% capacity pumps 18 gallons of water into a plant air system on a day with average ambient humidity. An aftercooler condenses and removes about 11 gallons of this liquid, but that still leaves 7 gallons to collect in low spots, retard valve movement, damage production parts, and cause problems in general.

Types of air dryers

Figure 7-4 shows a cutaway view of (and the symbol for) a deliquescent air dryer. Wet air enters the dryer (which is a pressure vessel), passes up through a bed of hygroscopic chemicals, and flows on to the outlet. The chemicals (often a form of sodium) absorb moisture from the air as it passes through the bed. As they dry the air, the chemicals break down into a slurry of water and chemical drops that falls to the bottom of the tank. A manual or automatic drain keeps the water mixture from rising too high and mixing with inlet air flow.

7-4. Absorbent-type deliquescent air dryer.

A typical deliquescent dryer removes moisture to a dew point of about 40ºF. Air that has passed through this dryer must be cooled below 40ºF before any more moisture will condense. This dew point is satisfactory for most plants -- even during winter cold. However, the hotter the air passing through the chemicals, the less the amount of moisture the chemicals will collect. Thus it is important to keep the incoming air at or below 100ºF. This usually requires an upstream aftercooler to lower the temperature of the air being delivered from the compressor. Air at higher temperatures at the inlet results in higher dew points at the outlet.

Deliquescent dryers are the least expensive of the three types mentioned in this section, but they might cause problems in some installations. There is always a chance of the chemicals or their vapors being picked up by the air stream and sent into the pneumatic system. These chemicals are corrosive and can damage internal parts. Also chemicals must be replenished on a regular basis. This means shutting down the compressor or bypassing the dryer when chemicals get low. Finally, slurry must be removed.

7-5. Condensing-type refrigeration dryer.

Figure 7-5 shows a cutaway view of a refrigeration-type dryer. This unit condenses water vapor by cooling compressed air to lower its dew point. There are no chemicals to replace or break down, so the system can run as long as required. A refrigeration dryer is more complex and more expensive to purchase than chemical dryers, but has a lower operating cost. The main operating cost is electricity. Maintenance cost is minimal and life expectancy is high, so this type dryer can be cost effective over the long haul.

To keep operating cost down these systems usually have an upstream water-cooled aftercooler to cool compressor air and take out the bulk of the moisture. Normally a maximum incoming temperature of 100ºF is recommended. When a higher inbound temperature is present, oversize the refrigerated dryer to handle the extra energy removal. (Of course, this adds extra cost to the purchase of the unit, as well as increasing operating cost over the life of the system.)

Wet air enters the unit through an internal air-to-air heat exchanger. This unit is piped to pre-cool the hot inbound air and re-warm the cold dry air before it exits the dryer to enter the plant piping. This arrangement saves energy by removing some of the heat in the incoming air. It also keeps the plant piping from condensing water vapor from ambient air on its cold exterior surfaces and dripping water on the production floor.

The pre-cooled air then passes through a Freon-to-air heat exchanger that reduces its temperature to approximately 35ºF. This procedure condenses more water vapor to achieve a 35ºF dew. If plant temperature stays above 35ºF (as it does in most plants), there will be no more condensation inside the air piping. The condensed water drains from the dryer through a water separator.

The main potential problem with refrigerated dryers is they will freeze up if temperature is set too low or if the amount of air passing through them is low and intermittent.

Another approach to refrigerated drying is drying the air before it enters the compressor. One company offers chillers that take atmospheric air down to –40ºF. and feed it to the compressor. After it is compressed to 100 psi, the air has a pressure dew point of approximately 35ºF. Because the compressor takes in air that is denser and at such a low temperature, the heat of compression is negligible. Also, most of the airborne contaminants are removed during the cooling, condensing, and freezing process. These inlet air dryers use dual refrigeration units. While one is drying input air, the wasted heat from the drying unit defrosts the other.

7-6. Adsorbing-type desiccant air dryer.

Figure 7-6 pictures a twin-tank desiccant dryer that uses a hygroscopic material (such as silica gel or activated alumina) that collects water vapor but is not broken down by it. This type dryer is called an adsorber because it collects water vapor but once the moisture is removed by heat or other methods, the chemical is ready to work again. This desiccant dryer may achieve dew points of –40ºF or lower, so the air can be used in most outdoor circuits without fear of freeze ups.

Electric heaters and purge air from the opposite tank handle the drying process. Other methods are steam heat, dried purge air only, and desiccant replacement.

As wet compressed air enters the control valve, it is channeled to one of the desiccant tanks. (The control valves are usually set up to shift automatically, triggered by a signal from a device that monitors the output air’s dew point.) Wet air is forced through the desiccant material to take out water vapor, then sent on to the plant. Some of the dried air is diverted through an orifice to the spent tank, where it is heated and then passed through the wet desiccant in the other tank. Water again vaporizes and exhausts to atmosphere. This drying process continues at the rate necessary to maintain the required dew point.

Desiccant dryers are the most expensive type to operate and maintain. In particular, they are subject to failure if the incoming air contains carry-over compressor oil. The oil can coat the desiccant and make it incapable of collecting moisture. The main expense of operating these dryers is the energy used to dry out the idle tank of desiccant.

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Fluid power filters

Moving parts in fluid power systems are subject to wear from contamination. Neither atmospheric air nor hydraulic fluids are clean enough as supplied to avoid this and they both become more contaminated with use. Therefore all fluid power systems require filters to remove contamination and thereby increase component life.

Pneumatic filters and lubricators

A pneumatic filter should be the first component at the inlet of most air circuits. This unit usually is one part of a combination of components that filters the air, regulates its pressure, and adds lubricants for moving parts in the circuit. The air filter and lubricator are covered in this section. (An air line regulator performs the same function as a hydraulic reducing valve and is covered in that section.)

Air from the compressor contains dust from the ambient atmosphere, condensed water, and rust and oil sludge that bypass the compressor rings. These by-products of compressing and transmitting air must be removed to keep moving parts of the machine working properly. Most filters clean the air and separate condensed water from it before the air enters the circuit.

7-1. Cross-section of typical air filter, with ISO symbols.

Figure 7-1 shows a simple air filter (and the ISO symbols that represent it). These units usually require little attention if the compressor has an air dryer at its outlet. Air enters at the left and is channeled into the bowl with a downward circular motion. The centrifugal force of this swirling action slings water droplets outward. They collect and fall to the bottom of the bowl below the baffle into a quiet zone for draining either manually or automatically. The air then flows through a porous filter element to the outlet. These units typically remove particles of 40-micron (40-µ) size or larger but they also are available for particles as small as 5 µ if required.

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Cooling hydraulic systems is necessary more often than heating them due to wasted energy from inefficiency and/or poor circuit design. A well-designed circuit eliminates most heat generation and may not need a heat exchanger. Use the same method to estimate how much heat a system generates as was used for the previous tank-cooling example.

When figuring wasted horsepower, see if there is any way to reduce or eliminate it so it does not have to be paid for twice. It costs money to produce the unused heat and it is expensive to get rid of it after it enters the system. Heat exchangers are expensive, the water that through them is not free, and maintenance of this cooling system can run high.

Items such as flow controls, sequence valves, reducing valves, and undersized directional control valves can add heat to any circuit. Are these items absolutely necessary? Can they be replaced with another valve or part that does the same thing with less pressure drop? Anytime these questions can be answered with a yes, the circuit is not ready to build.

Air-cooled heat exchangers

After calculating wasted horsepower, review heat exchanger manufacturers’ catalog to pick out a unit that will dissipate that amount of energy. Most catalogs include charts for given size heat exchangers that show the amount of horsepower and/or BTU they can remove at different flows, oil temperatures, and ambient air temperatures.

Figure 6-9. Typical radiator-and-fan oil cooler.

Figure 6-9 shows a typical air-cooled heat exchanger that may be used in place of a water-cooled unit in some applications. Air-cooled heat exchangers are not as efficient as water-cooled heat exchangers, but they require only an electrical outside hookup. They work well in cool atmospheres or when the amount of heat to be removed is low. Note that airborne contaminants such as heavy dust or water and coolant vapors can quickly reduce an air-cooled heat exchangers low efficiency to almost nothing. Some manufacturers offer a filter pack to take out airborne contamination before it clogs the heat exchanger’s radiator fins and tubes.

On circuits with pressure-compensated pumps, a small air-cooled heat exchanger often is used to cool case drain flow. On this type system, most of the heat generation is from internal leakage and control oil that flows to tank through the pump case drain. One type heat exchanger -- called a coupling cooler -- is a finned tube formed into a circle and wrapped around a blower that is driven by the motor turning the pump. A similar arrangement uses a small flat radiator attached to the intake end of the fan-cooled electric motor that drives the pump. Both units are low-flow, low-backpressure devices and dissipate only a small amount of heat.

Some systems use a water-cooled heat exchanger in the summer and an air-cooled one in the winter. This arrangement eliminates plant heating in summer weather and saves heating expense in the winter.

Water-cooled heat exchangers

Two popular water-cooled heat exchanger designs. As before, manufacturers’ catalogs will assist you in picking out a unit to dissipate the amount of wasted heat energy. For water-cooled heat exchangers, catalogs ask for information such as how much and what temperature water is available, how many horsepower or BTU of energy must be dissipated, what is the fluid flow in gpm, and how many passes will the water makes to get through the body. The more passes -- up to four maximum usually -- the greater the heat dissipation per gallon of water flow. Charts that use this information make it easy to pick the correct size heat exchanger.

Figure 6-10. Two popular water-cooled heat exchanger designs.

Figure 6-10shows two types of water-cooled heat exchangers commonly used for hydraulic systems. The shell-and-tube design is the most common one at present, but the plate-and-frame or brazed-plate type are coming along because they are much smaller and easier to maintain. It is important to use clean water in either type unit to keep from building up insulating deposits or corroding the tubes until they leak water into the oil. Treated water from a cooling tower works best.

Figure 6-11. Temperature-controlled water valve.

Either type heat exchanger should have a thermostatic control to turn on the water or fan only when the fluid temperature rises above its normal operating range. Without a thermostatic control, fluid could be too cold and thick, while wasting the energy to operate the heat exchanger. Figure 6-11shows one type of water-control valve that requires no electrical hookup. Heat-sensitive liquid in a thermometer-type probe in the tank expands and opens a water valve upon reaching a preset temperature. The temperature is fully adjustable to meet any requirement and it operates in all types of fluid. Another option is an electrically operated temperature sensor that controls a solenoid-operated water valve. This installation requires an electrical hookup but is able to maintain a fluid-temperature range at any desired setting.

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The inlet line to the pump should be at the same end of the reservoir as the return line, with the baffle between them forcing returning fluid to travel to the opposite end of the tank and back. The inlet must be below the fluid level and may include a strainer. If the inlet line is just a straight piece of pipe installed vertically, it is best to cut the line’s open end at 45° so it is impossible to butt it against the floor of the tank, which could block flow. Outside the tank, this line should lead as directly to the pump as possible, with no unnecessary bends or connections. Never use a pipe union in the inlet line; unions are almost impossible to seal against air leaks. Even a minute leak in the inlet line can cause pump cavitation and all of its problems.

The return line should be located in the same end of the tank as the inlet line and on the opposite side of the baffle (as shown in Figure 6-5). The return line must terminate below fluid level to reduce turbulence and aeration. The open end of the return line also should be cut at 45° to eliminate the chance of stopping flow if it gets pushed to the bottom. Another good practice is to point the opening toward the side wall to get the most heat-transfer surface contact possible.

When a hydraulic reservoir is part of the machine base or body, it may not be possible to incorporate some of the features discussed in this chapter. However, keep in mind the different functions mentioned to try to eliminate ongoing problems.

Non-pressurized and pressurized reservoirs

Reservoirs are seldom pressurized because that feature is not required under most circumstances. One reason for using a pressurized reservoir is to provide the positive inlet pressure required by some pumps -- usually in line piston types. Another reason is to force fluid into a cylinder through an undersized prefill valve. Both of these reasons may require pressures between 5 and 25 psi and could not use a conventional rectangular reservoir design.

Figure 6-6. Formula for estimating how much heat a tank of a given size can dissipate.

Another reason for pressurizing a tank is to keep out contaminates. If the reservoir always has a positive pressure in it there is no way for atmospheric air with its contaminants to enter. Pressure for this application is very low – 0.1 to 1.0 psi -- and may be all right even in a rectangular design tank.

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Fluid power reservoirs

Fluid power systems require air or a liquid fluid to transmit energy. Pneumatic systems use the atmosphere -- the air we breathe -- as the source or reservoir for their fluid. A compressor takes in atmospheric air at 14.7 psia, compresses it to between 90 and 125 psig, and then stores it in a receiver tank. A receiver tank is similar to a hydraulic system’s accumulator. A receiver tank, Figure 6-1, stores energy for future use similar to a hydraulic accumulator. This is possible because air is a gas and thus is compressible. A receiver tank is a pressure vessel and is constructed to pressure vessel standards. At the end of the work cycle the air is simply returned to the atmosphere.

Hydraulic reservoirs

Figure 6-1. Simple pneumatic power unit.

Hydraulic systems, on the other hand, need a finite amount of liquid fluid that must be stored and reused continually as the circuit works. Therefore, part of any hydraulic circuit is a storage reservoir or tank. This tank may be part of the machine framework or a separate stand-alone unit. In either case, reservoir design and implementation is very important. The efficiency of a well-designed hydraulic circuit can be greatly reduced by poor tank design. A hydraulic reservoir does much more than just provide a place to put fluid. A well-designed reservoir also dissipates heat, allows time for contamination to drop out of the fluid, and allows air bubbles to come to the surface and dissipate. It may give a positive pressure to the pump inlet and makes a convenient mounting place for the pump and its motor, and valves.

Some standard reservoir layouts

Pump on top. Figure 6-2 shows this common reservoir/pump layout -- used by many suppliers. The flat top surface of a standard reservoir makes a perfect place to mount the pump and motor.

Figure 6-2. Pump and motor mounted on top of tank.

The main disadvantage to this configuration is that the pump must create enough vacuum to raise and accelerate the fluid into the pump inlet. For most pumps, this is not a big problem, but it is not the best situation for any of them. Axial or in-line piston pump life can be adversely affected by medium to high vacuum at its inlet when using this layout. The piping in this configuration must be sealed, should be as short as possible, and have few or no bends.

Pump alongside tank. Figure 6-3 shows another design that is satisfactory for any type pump. (Many suppliers prefer this layout.) This arrangement is sometime called a flooded suction, because the pump inlet always is filled with fluid.

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Pressurized fluids act in a certain manner in most situations. However, there are instances where a gas-type fluid does not perform as its liquid counterpart does. As mentioned earlier in this chapter, a pneumatic actuator is incapable of holding a position against increasing external forces because the air can be compressed more. Other situations such as flow-control circuits, return-line backpressure, energy-transfer considerations, and more are covered and explained in the text.

Conventions used in this manual

All schematic symbols and drawings are in accordance with the International Standards Organization (ISO) format. These symbols and representative parts are laid out in Chapter 4 either in whole or in part. Some symbols are made up of several standard parts and are not shown in their entirety in Chapter 4.

When a symbol is not shown it is good practice to use the symbol shown in the suppliers catalog. If no symbol is given there then use standard symbol parts to make a representation of the new item.

As in all cases of drawings using schematic symbols, the circuit designer may use his or her experience or opinion to interpret some parts. This usually does not make the schematic harder to read, just different. If a part representation is not clear, refer to the material list and check the supplier's catalog for an explanation of the valve's function.

Color coding

To better understand how a part or circuit works, consider using color coding for the lines and components. Color coding is instituted by the instructor, designer, or engineer and is according to his or her interpretation, so it might not be consistent in each case. Most training manuals and manufacturers use the following color code.

• Red: Working fluid flow lines, usually from the pump to a device. This line is always solid. It can represent plastic tubing as small as 5/32-in. OD for air or any size pipe or tubing for hydraulics.
• Blue: Return lines from valves and other devices for hydraulic circuits. This line always is solid, and can represent any size pipe or tubing.
• Yellow: Metered or flow-controlled fluid that is at a reduced speed in relation to the same line without a restriction. This line could be solid or a series of long dashes if pilot flow must be metered.
• Orange: A reduced-pressure line, such as a pilot-pressure line or one carrying accumulator precharge gas. This line could be a solid after a reducing valve or a long-dashed line for pilot flow.
• Green: Pump inlet lines (suction lines) or drain lines. These lines would be solid for the pump inlet and a series of short dashes for drains. Two types of lines with the same color are not confusing -- even when in close proximity to each other.
• Purple or indigo: These colors usually indicate working fluid that has been pressure-intensified by area differences or load-induced conditions. These pressures are usually greater than the setting of the main relief valve or reducing valve that feeds the circuit.
• Lines without color are considered non-working or to have no flow at present.

This color-coding technique is used with this manual and can be seen in Chapter 4.

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