To prevent freezing the water in a water-cooled heat exchanger of an R-134a system during charging, what initial vapor pressure should be used?

The correct answer is based on the relationship between the pressure and temperature of R-134a. In a water-cooled heat exchanger system, maintaining a proper vapor pressure is crucial to avoid freezing the water in the system.

R-134a has specific pressure-temperature relationships which indicate that at a pressure of 30 psig, the corresponding saturation temperature is just above the freezing point of water. This ensures that the refrigerant remains in a vapor state and does not risk causing water freeze-up, which could lead to damage or inefficiencies in the system.

Choosing a lower pressure, such as 20 or 25 psig, may result in saturation temperatures low enough to approach or fall below freezing, thus increasing the risk of ice formation. Similarly, while 35 psig might maintain the refrigerant in a vapor state, it could introduce unnecessary pressure that may not be needed for optimal performance.

Therefore, the initial vapor pressure of 30 psig is the most appropriate choice to effectively manage the charging process while safeguarding against freezing conditions in the water-cooled heat exchanger.