Coolers are essential parts of rotary kiln systems. Clinker leaves the kiln at 1200°C or more. Clinker at this temperature has a heat content of 1.22 MJ/kg and must be cooled in order to:
- avoid wasting this heat energy
- obtain clinker at a temperature suitable for grinding - less than 100°C
- rapidly cool ("quench") - the clinker, thus preserving reactive metastable high-temperature minerals in the clinker and maximising its strength potential
The cooler transfers the heat from the clinker to the combustion air, and this is also an essential function, recognised since the earliest rotary kiln designs of Ransome - it is difficult to produce a sufficiently hot flame in the kiln unless the combustion air is pre-heated. The hot air thus passing into the kiln is referred to as secondary air as distinct from the primary air which conveys the fuel into the kiln through the firing pipe. In more efficient kiln systems (in which fuel usage, and therefore combustion air usage, is reduced) there may be additional hot air available for other purposes, and this is referred to as tertiary air. There have been a variety of designs for coolers, broadly categorised as follows:
| Rotary coolers |
| Planetary (or Satellite) coolers |
| Grate coolers |
