HYDAC Filter Sizing: Flow, Viscosity and Pressure Drop
Once pressure rating and filtration grade are established, size a HYDAC filter by the clean pressure drop at actual oil conditions. Passing 100 L/min does not establish that resistance at 100 L/min is acceptable. A new element that already consumes most of the alarm differential leaves little margin for contamination loading.
At the same flow, media, filtration area and viscosity change pressure drop. Calculate both warm operation and cold start to determine whether more element area or a larger housing connection is needed.
Add housing and element losses
HYDAC separates the calculation into two components.
Total filter pressure drop = housing pressure drop + clean-element pressure drop.
Read housing loss from the curve for the selected connection and flow direction. Calculate element loss using flow, viscosity and gradient coefficient SK. SK is expressed in mbar/(L/min), referenced to a kinematic viscosity of 30 mm²/s.
Element pressure drop (bar) = Q × SK ÷ 1000 × ν ÷ 30. Q is in L/min and actual kinematic viscosity ν is in mm²/s.
The viscosity correction in this formula applies to the element. Do not multiply the combined housing-and-element loss by the same viscosity factor. Use the housing curve and its stated correction method for the housing contribution.
Comparing sizes 330 and 660 at 100 L/min
The following calculation uses the standard ON, 10 μm column in DF catalog E7.555.11. Sizes 330, 500 and 660 have SK values of 2.46, 1.23 and 0.93 respectively. These are separate from the ON/PS column.
| DF element size, ON 10 μm | SK, mbar/(L/min) | Element drop at 100 L/min and 30 mm²/s | Element drop at 100 L/min and 150 mm²/s |
|---|---|---|---|
| 330 | 2.46 | 0.246 bar | 1.230 bar |
| 500 | 1.23 | 0.123 bar | 0.615 bar |
| 660 | 0.93 | 0.093 bar | 0.465 bar |
For size 330, the warm calculation is 100 × 2.46 ÷ 1000 = 0.246 bar. At 150 mm²/s, it rises to 1.230 bar. Flow and filtration grade are unchanged; viscosity causes the increase.
These values include only the element. Size 660 has about 62% less clean-element pressure drop than size 330, but the reduction across the complete filter also depends on its housing and connections.
The example uses 150 mm²/s as cold-start viscosity. Obtain the value at the machine's minimum starting temperature from the oil's viscosity-temperature curve. VG46 describes the viscosity grade around 40 °C; 46 is not the viscosity at every temperature.
Include the housing before choosing a size
Suppose an industrial power unit has an average flow and, for this example, a maximum flow of 100 L/min. It uses a 2 bar differential indicator. Assume the selected housing connection has a pressure drop of 0.20 bar at that flow under the relevant oil conditions. Adding it gives:
| Size | Warm total drop | Cold-start total drop | Cold-start share of the 2 bar trip setting |
|---|---|---|---|
| 330 | 0.446 bar | 1.430 bar | 71.5% |
| 500 | 0.323 bar | 0.815 bar | 40.8% |
| 660 | 0.293 bar | 0.665 bar | 33.3% |
HYDAC's general guidance for industrial applications is to keep clean pressure drop within 20%–25% of indicator setting at average flow. If cold-start pressure drop at maximum flow exceeds 50% of the setting, consider a larger size.
Under this screen, size 330 is acceptable warm but reaches 71.5% at cold start, so it is unsuitable for this example. Size 500 passes the pressure-drop screen; 660 reduces resistance further. Where space and cost are constrained, 500 can proceed to the remaining selection checks. Choose 660 when more differential-pressure margin is desired. Housing pressure rating, element collapse rating and seals must also suit the circuit.
More element area and larger ports solve different losses
If the element dominates total pressure drop, more filtration area has a clear effect. Moving from 330 to 500 in this example reduces cold element loss by 0.615 bar. Moving from 500 to 660 reduces it by 0.150 bar. The gains are not equal.
If housing, connections and piping dominate, more element area has limited benefit. For example, with 0.80 bar housing loss and 0.10 bar element loss, halving the element loss reduces the total only from 0.90 to 0.85 bar. Address port size and the flow path instead.
A larger element generally offers more filtration area, but this does not establish twice the service life. Contaminant ingress, particle distribution and actual dirt capacity affect life. Record clean pressure drop at the same flow and comparable oil temperature after replacement to track its subsequent rise.
Replacing an element in an existing housing
An existing housing limits the element dimensions it can accept. An installed 0660 D 010 ON cannot simply be replaced with a longer element to reduce pressure drop. Increasing area requires an approved housing configuration or a newly selected complete filter.
Do not hide excessive differential pressure by increasing the alarm setting. Preserve warning margin below bypass opening. The bypass flow path changes how much oil receives filtration.
Calculate total pressure drop warm and cold. Increase filtration area when the element dominates; improve connections and flow passages when the housing dominates. This connects filter size to the machine's flow, oil temperature and alarm setting.