This is a type of pressure cap which seals the radiator by blocking the overflow pipe is shown in Figure. The cap is held tightly on the filler neck, with a gasket placed between it and the neck to ensure an air-tight seal. A vacuum valve is also provided in the cap to admit air when the pressure in the system falls below that of the outside atmosphere due to the condensation of steam vapors on cooling.
opened to atmospheric pressure. The boiling point of the water or the coolant is raised by keeping it under pressure in the closed cooling system. A pressure relief valve is provided in the system to prevent pressure from becoming excessive and causing leaks in the system. Whenever the pressure exceeds a predetermined value, the relief valve opens to the atmosphere and the excess of pressure is released. The relief valve is generally incorporated in a tight-fitting radiator filler cap.
Radiator Cap Pressure Limit
The relief valve is generally set to open at the pressure of 5.5 to 11N/cm. A valve set to open at 3 N/cm2 would raise the boiling point of such a 100% water cooling system to about 105°C at sea level. An 11 kg/cm2 valve would issue a boiling point of about 125°C.
In the closed system, by raising the boiling point of the coolant, the cooling capacity of the system is raised. Also, there is less likelihood of boiling over; and fuel vaporization and distribution are improved.