5th Class Power Engineering Practice Exam 2026 - Free Power Engineer Practice Questions and Study Guide

In a compression refrigeration system, during the compression phase, the vapor is compressed by the compressor, which significantly increases its pressure and temperature. This occurs due to the ideal gas law, where compressing a gas results in a decrease in its volume and an increase in temperature, provided that the process occurs quickly enough to prevent heat dissipation.

As the vapor's volume is reduced by the piston or rotary mechanism within the compressor, the energy imparted to the gas leads to molecular interactions that heighten the kinetic energy of the molecules, manifesting as an increase in temperature. Thus, the correct understanding of what happens to the vapor during this phase is that its temperature increases as it is converted from a low-pressure gas to a high-pressure gas ready for the next stage in the refrigeration cycle—the condensation phase.