Normally the main factor for sizing a filter is the allowable pressure drop over the filter housing and clean element.
The pressure drop over the housing depends mainly on the size of the housing, the flow path through the housing and the density of the fluid.
The pressure drop over the element depends mainly on the size of the element, the filter material and fineness, and the viscosity of the fluid.
Viscosity is strongly related to the temperature of the oil.
For every filter type the Filter Selector mentions a nominal flow. Usually this is the flow that gives a reasonable pressure drop over the filter with standard bypass setting, using a 10µ micron Glassfibre element with oil having a density of 0,87 kg/l and 30 cSt. If different parameters apply, calculations need to be done.
Calculating the pressure drop is often done with oil specifications under normal operation circumstances (temperature/viscosity). However also during cold start the filter should function well and not run in bypass.
For applications subject to frequent cold start with high oil viscosity, we kindly advice to review and apply the actual pump flow output and fluid viscosity to calculate the pressure drop across the filter.
Within the Pressure drop calculator of the Advanced Selector the oil viscosity at different temperatures can easily be determined by entering working temperatures in the “Oil type selector” or in the “Viscosity calculator”.
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