We frequently encounter return-line filters in hydraulics. On the one hand, this is due to their inexpensive housings, which have to withstand comparatively low pressures. On the other hand, they offer a variety of designs and variants that can be drawn from them. Thus, they are suitable for all filtration tasks in the return flow and leak oil area. Return flow filters are available in the classic version as tank-top or tank-mounted filters and as in-tank filters as well as filters for in-line installation.

Tank-top or tank-mounted filter
This design stands for maintenance and is environmentally friendly; the tank-mounted filters are usually easily accessible in order to carry out the filter service in just a few steps and with practically no loss of oil. Furthermore, many additional functions can be integrated in the filter head such as tank ventilation, tank level control, a clogging indicator or magnets. This means that users can implement separate interfaces for ventilating filters, for example. At the same time, the risk of leakage and costs in the manufacturing of the hydraulic tank are minimized.

In-tank filter
In-tank-mounted filters are found especially in machines with very high return flow rates. These are tank-mounted filters without a head piece, which can be suspended in a customized tank chamber. The chamber then serves as a head piece and as a collector. All the return flows that have to be filtered are directed into this tank chamber and flow into the tank via the built-in filter. Since very large return filters can be disadvantageous in terms of handling and costs, the in-tank filters that are often installed are designed for only half or one-third of the return volume. Therefore, two or three compact filters are installed, which means a larger purchasing volume for series and maintenance parts – which in turn minimizes costs. The ease of maintenance and environmental friendliness are comparable to tank-mounted filters. In terms of functional integration, however, minor compromises have to be made.

Air separation – 2 designs to choose from
ARGO-HYTOS has developed a new type of air separation system for return flow filters. Taking into account all working conditions, the AirEX air separation reduces the air content in the hydraulic medium by around 40 % , independent of volume flow and viscosity.
By using AirEX, it is in many cases possible to reduce the tank volume by around 30% and still improve air separation. This avoids malfunctions caused by air bubbles, damage and thus unplanned repair costs and downtimes.

Efficiency of air separation
In the past, the outgassing of air was favored by large-volume tanks and long dwell times since it is usually not possible to completely avoid the introduction of air into the hydraulic medium. Increasingly compact solutions are now reducing the installation space in the machine. Continuously increasing environmental and cost pressures also mean that large quantities of oil in large-volume tanks are no longer appropriate. However, the logical reduction in tank volume and dwell times can lead to problems as air contained in the oil is sucked back in faster than it is outgassed.

ARGO-HYTOS has simulated this in a comparative test on its air separation test rig. 4 return flow filter designs were compared:

• Standard return filter (tank-top mounted)
• Standard return filter with AirEX air separation module
• In-tank return filter AirEX-Light (without guiding tube)
• In-tank return filter AirEX (with guiding tube)

The first two filters are designated TE (Top Entry) since the oil first flows into the filter head. The flow direction of the filter element inside the filters is from the outside to the inside, as is common in most cases.

In the two tank-mounted filter designs, the oil first flows into the filter from below. For this reason, they are referred to as BE (bottom entry). The flow direction is from the inside to the outside, which is advantageous in terms of air separation and further flow guidance in the tank.

In the comparative test, the TE type “Standard return filter with AirEX air separation module” performed best, reducing the air content by 40 % compared to the identical filter without AirEX. The two BE types achieved a 15 % reduction (see graph below).

At the end of the test, the filters with AirEX continued to outperform the standard filter with the advantage of the “Standard return filter with AirEX air separation module” becoming even clearer. It was thus possible to clearly demonstrate how quickly and efficiently the different air separation technologies work.

When are the BE variants useful?
Since all types of designs are used successfully in practice and have their justification, the following recommendations can nevertheless be made:

The in-tank return flow filters with or without guiding tube (BE AirEX or BE AirEX-Light) should be used only if air enters the system sporadically and there is no acute air problem (e.g. permanent noise or cavitation). In addition, they are particularly suitable for machines in which the hydraulic tank is positioned at the highest point (i.e. harvesters or beet harvesters). The return line does not have to be routed upward to the tank surface, but can be routed directly through the tank bottom or laterally through a tank wall to the filter. This design is also recommended for machines where the operators have to walk on the tank (excavators) since only a flat cover of the filter is visible on the tank.

This filter design consists of only a few components (cover, filter element and eventually a guiding tube), which makes it the most cost-effective return flow filter design, especially for large volume flows.

When are the TE variants useful?
The standard tank-top return filter or tank-mounted filter should be supplemented with an AirEX air separator module for machines with regularly occurring air issues. The AirEX air separator module can also be retrofitted for this purpose, provided there is sufficient space to the tank bottom. In this way, the advantages of the proven return filter concept are retained and supplemented by the most effective air separation technology currently available on the market. Depending on the tank design, further potential savings can be achieved by significantly reducing the tank volume. 

Your Advantages

• Reduction of the air content in the hydraulic system by up to 40%
• Avoidance of cavitation damage
• Possible reduction of tank volume up to 30%
• Cost and installation space savings
• Reduction of oil change quantity and extension of oil change interval