Off-line filtration systems -- sometimes called kidney loops or bypass filters -- consist of a separate pump, motor, and filter circuit that takes oil from the reservoir and re-circulates it. The system pumps oil from one end of the tank, passes it through a filter, and returns it to the opposite end of the tank. Figure 7-8 shows an off-line filtration system at the lower right. This arrangement is a good way to provide high-micron continuous filtration. The systems are easy to retrofit to existing hydraulic circuits and offer an excellent way for new installations to get high cleanliness levels.

Off-line filter circuits are usually rated in the 3- to 10-µ range and should sized to filter the volume of fluid in the reservoir every 1 to 3 hours minimum. This low, constant flow rate makes the filter very efficient, never opens the bypass, never causes media channeling, and never blows holes in the element.

When the filter indicator shows a clogged element in the off-line system, the main hydraulic circuit can continue to operate during filter change. Conversely, this off-line filter system can continue to run when the main hydraulic circuit is off overnight or weekends.

System-fill filters

New oil is not as clean as most hydraulic systems require so it should be filtered before use. To do this, introduce new oil to the tank through a pair of shut off valves, or a 3-way ball valve (as shown in Figure7-8 at the lower right) in the suction line of the off-line filter pump. Rotate the 3-way ball valve 180 degrees, hooking the off-line pump’s suction to a flexible hose from the oil drum or fluid container. This setup filters all oil from the fluid container as it fills the reservoir.

Another way to make sure all fluid is filtered before use is through the use of a tank-fill filter (as shown in Figure 7-8 at the lower middle). Here a low-micron pressure filter is installed in the tank wall and provides the only way to fill it. The filling process can only introduce clean fluid to the reservoir.

Additions to a filter loop

An off-line filter circuit also can provide heating or cooling functions. Figure 7-8 shows a bypass circuit with a normally open solenoid relief valve, a high-horsepower motor, a temperature switch, a heat exchanger, and a temperature-controlled water valve or switch. These additions can effectively control temperature while filtering the fluid. To only filter the oil, leave the water or fan turned off and the solenoid relief valve de-energized (or open).

If oil temperature drops, a temperature switch energizes the solenoid on the relief valve, and pressure rises. All electric motor horsepower converts to heat until the temperature switch indicates correct oil temperature. Unlike an immersion-type electric tank heater, the fluid is being circulated, so there are no hot spots.

For every electric horsepower, there will be 2544 Btu/hour heating capacity. After figuring the Btu/hour to heat or maintain minimum temperature, divide by 2544 to calculate the horsepower needed. (The formula for calculating tank heating appeared in Chapter 6.)

If the tank fluid temperature goes over a preset limit, a temperature-controlled water valve opens to send water through a heat exchanger or a temperature switch turns on the fan of an air-cooled heat exchanger. All filtered flow is cooled when the temperature-control device indicates elevated temperatures.

When installed in an off-line filtration loop, the heat exchanger receives constant flow, so it needs no bypass valve. Also, the heat exchanger sees flow even when the system uses a pressure-compensated pump.

Bidirectional pressure filters

The only difference between a bidirectional pressure filter, Figure 7-9, and a standard pressure filter is the four check valves in the housing. These check valves cause oil flow to pass through filter element in the same direction regardless of the direction fluid enters the housing. Another name for a bidirectional filter is last chance filter. It is installed in a working line to an actuator so it must withstand maximum system pressure. Bidirectional filters with 3- to 10-µ ratings are adequate for most circuits.

7-9. Schematic drawing of hydrostatic drive circuit with bi-directional filter.

Closed-loop hydrostatic transmission circuits are one place to use bidirectional filters. The oil can stay in the loop between the pump and motor for long periods. Any contamination in this closed loop continues to cause damage, even with ample filtering of oil in the tank.