Water-free, synthetically produced fire-resistant fluids. They rely on the following compounds:
- Phosphoric ester (HFD R), currently the most widely used HFD fluid; performs worst in terms of fire resistance, however.
- Chlorinated hydrocarbons (HFD S). Only used in closed systems within the coal industry for reasons of environmental protection.
- Mixtures of HFD R and HFD S.
- Other compositions (HFD U).
Common to All HFD fluids is the lack of compatibility with most materials for seals and a high density of 1.15 to 1.4 g/cm3 (intake characteristic!), which complicates any subsequent adjustment.