Why is there no flash or tax gas across a metering valve?

The reason there is no flash or gas across a metering valve is that there is no temperature drop associated with the process. In a metering valve scenario, the refrigerant experiences a pressure drop as it passes through the valve, which typically leads to a reduction in temperature. However, if the temperature remains constant during this process, the refrigerant will stay in the liquid state without transitioning into a vapor or "flash" situation.

In this context, the other options provide conditions that might normally cause issues like flashing or vaporization if they were applicable. For example, a maximum pressure drop could lead to flashing if the refrigerant was already close to its boiling point. Similarly, a speed reduction can contribute to changes in phase states, but it is not the primary reason for the absence of flash gas in this scenario. Lastly, subcooling would imply that the refrigerant is in a chilled state that should prevent vaporization, but it is not the direct reason for the absence of gas across the metering valve when no temperature drop occurs. Therefore, the fundamental reason lies in the absence of temperature change, which ensures that the refrigerant remains in the liquid state as it passes through the valve.