Diesel cooling methods and cooling tower

Diesel engines are cooled to remove the heat (about 25–35 % of fuel energy) that flows into cylinder walls, head and pistons, so that metal and lubricating oil stay within safe temperatures.

Methods of cooling diesel engines

1. Air cooling: fins on the cylinder and head increase surface area; air from a fan or vehicle motion carries heat away. Used only for small engines (a few kW) because air has low heat capacity.
2. Liquid (water) cooling: water circulates through jackets; used for all power-plant engines. Types:
   • Open (once-through) system – raw water used once and discharged.
   • Natural (thermosiphon) circulation – circulation by density difference, no pump; small engines.
   • Forced circulation – pump drives water through jackets and radiator/cooling tower.
   • Closed (double-circuit) system – soft water in a closed loop through jackets and a heat exchanger; raw water in the secondary loop goes to a cooling tower or spray pond.
   • Evaporative cooling – water allowed to boil in jackets, steam condensed and returned.
3. Oil cooling of pistons in large engines (oil sprayed under the piston crown).

Function of a cooling tower

A cooling tower cools the hot water coming from the engine jackets or heat exchanger so the same water can be reused, which saves water where a river or lake is not available.

      Air + vapour out
          ^  ^  ^
   +----------------+
   |  Spray nozzles | <-- hot water in
   |  ~~~~~~~~~~~~  |
   |  Fill/packing  |
   |  ~~~~~~~~~~~~  |
 ->| air in   air in|<-
   +----------------+
   |  Cold water    | --> pump -> engine/HX
   +----------------+

• Hot water (about 50–60 °C) is sprayed at the top and trickles down over fill/packing that breaks it into thin films and drops.
• Air flows upward (by natural draught or fans – induced or forced draught) through the falling water.
• A small part of the water evaporates; the latent heat for evaporation is taken from the rest of the water, which cools by 8–12 °C. Some sensible heat also passes to the air.
• Cold water collects in the basin and is pumped back. Make-up water replaces evaporation, drift and blow-down losses (about 1–3 %).

Thus the cooling tower rejects engine heat to the atmosphere with very small water consumption.