Environmental Protection Agency (EPA) MVAC Certification

What is the internal pressure of a refrigerant cylinder containing one ounce of liquid refrigerant compared to a full cylinder?

1. Higher than a full cylinder

2. Lower than a full cylinder

3. The same as a full cylinder

4. It varies with temperature

The correct answer is: The same as a full cylinder

The internal pressure of a refrigerant cylinder is largely determined by the vapor pressure of the refrigerant at a specific temperature. In general, if a cylinder holds some liquid refrigerant, the pressure inside corresponds to the vapor equilibrium between the liquid and the vapor above it, regardless of the quantity of liquid present. In a full cylinder, the pressure will be at a certain level depending on the refrigerant's vapor pressure at the prevailing temperature. In the case of a partially filled cylinder with liquid refrigerant, as long as there is some liquid present, the pressure will remain the same as that in a full cylinder at the same temperature, because the vapor pressure of the refrigerant does not change with the amount of liquid present, provided there is excess liquid to maintain equilibrium. Therefore, the pressure inside both a full cylinder and a partially filled cylinder with liquid is essentially the same, given that the temperature remains constant. Understanding how vapor pressure works is key to grasping why the internal pressure remains unchanged in such scenarios. This principle underscores the importance of vapor-liquid equilibrium in refrigerant systems.