Two conventional filter systems for metalworking fluids are vacuum filters and gravity bed filters.  Both types of filters use non-woven fabric filter media to remove solids from the liquid.  However, there is a significant difference between them.  Not only in principal of operation but also in the usage, and cost.  Gravity bed systems are better suited to low flow requirements and filtering larger contaminants. They are good entry level filters and their pricing reflects this lower flow, lighter loading capacity.  Vacuum filters edge ahead for finer filtration, higher flows and heavy solids loading,. They are more robust and costly, but minimize floor space requirement and provide low media use.  Mr. Chris Ashley, General Manager, Filtration Control Systems explains it in detail.  - Worth reading.