Importance of Pressure Cap and Expansion Tank in the Radiator

  Modern Radiators have their filler necks covered with a Pressure Cap. It maintains an air tight joint which helps to keep the coolant at a pressure higher than atmospheric pressure.

This helps in the following ways:

1.       The engine can operate at higher temperature without boiling the coolant. This ensures higher rate of heat transfer.  (Rate of heat transfer depends on the difference in temperature between the Coolant and Atmosphere. Higher difference in temperature ensures higher rate of heat transfer.)

2.       At higher altitude, atmospheric pressure is low. This causes the coolant to boil at comparatively low temperature.  With the Pressure Cap, a higher pressure is maintained inside, irrespective of any change in atmospheric pressure. Thus maintaining cooling system efficiency.

The Pressure Cap contains a Pressure Valve and a Vacuum Valve. If, due to severe working condition, the coolant starts boiling or vaporise, the pressure in the system exceeds a certain value, the Pressure Valve opens and releases the excess pressure to the atmosphere through an over flow pipe.

On the other hand if sudden cooling happens, a vacuum is created inside the Radiator, then the vacuum valve opens to avoid collapse of the Radiator.

It is important to remember that the Pressure Cap should never be opened when the Radiator is still hot. Because, on removing the cap, the pressure inside the Radiator will suddenly drop, causing the boiling point of the coolant to decrease all of a sudden. This causes the coolant to start boiling immediately. This may cause spill over and burn anyone standing nearby. In case of loss of a Pressure Cap, the replacement cap should be of the same pressure rating as the original one.

In most of the modern engines, instead of overflow pipe an Expansion Tank is provided. This tank is so connected with the Radiator that it receives the excess coolant when the engine is heated. When temperature decreases, the coolant volume reduces and the coolant in the Expansion Tank returns to the Radiator. This ensures that the Radiator is always full of coolant. The reservoir is usually made of semi transparent plastic so that it can indicate the level of coolant.

Such a system is also called as Coolant Recovery System, it has the following advantages:

1.       There is no loss of coolant due to overflow.

2.       Air does not enter into the system. Corrosion of the cooling jackets is reduced considerably.