Dec 17, 2015

Temperature Effects on Pre-Charge Pressure


In previous newsletter articles we explained the importance of a correct set pre-charge.
Category: News
Posted by: admin

In previous newsletter articles we explained the importance of a correct set pre-charge. One important thing we did not mention yet is the effect of the temperature on the pre-charge.

As the air used to charge a tank can be considered an ideal gas, we can use the ideal gas law to calculate the temperature effect on the pre-charge:

For a fixed mass of gas, at a constant volume, the pressure (p) is directly proportional to the absolute temperature (T). 

The effect of this is that the pre-charge of a tank decreases when the air in the air chamber cools down and the pre-charge increases when the air is heated up.

If we want to calculate the change in pre-charge pressure based on the temperature change, we can use the following formula: p2 = p1*(T2/T1)

It is necessary to use absolute temperatures in the calculation. To get the absolute temperature we add 273.16 to the °C value.

The below diagram shows the effect of a temperature variation for a tank that was charged to 1.9 bar at 20°C as an example.

 

 

 

Real world examples:

1)    A tank that was pre-charged to 1.9 bar at 20°C then stored in a cold warehouse where the winter temperature was 5°C:

 

T1 = 20°C = 293.16 K

T2 = 5°C = 278.16 K

P1 = 1.9 bar

P2 = pre-charge pressure when the pre-charge temperature is lowered to 5°C

 

 

 

2)    A tank was pre-charged to 1.9 bar at 20°C and then installed on a roof in direct sunlight where the temperature increased to 40°C at noon:

 

T1 = 20°C = 293.16 K

T2 = 40°C = 313.16 K

P1 = 1.9 bar

P2 = pre-charge pressure when the pre-charge temperature is increased to 40°C