On a pressure-enthalpy (P-H) diagram, what is plotted along the horizontal axis?
- Absolute pressure, in psia
- Temperature, in degrees F
- Specific volume, in ft³ per pound
- Enthalpy, in BTU per pound
Show the answer
D. Enthalpy, in BTU per pound
A P-H diagram plots absolute pressure on the vertical axis and enthalpy (heat content per pound) on the horizontal axis. Temperature and specific volume appear as families of lines drawn across the chart, not as an axis.
Reference: Pressure-enthalpy diagram layout
On a pressure-enthalpy diagram, what does the area underneath the saturation dome represent?
- Subcooled liquid only
- A mixture of liquid and vapor
- Superheated vapor only
- Refrigerant above its critical point
Show the answer
B. A mixture of liquid and vapor
Inside the dome the refrigerant is saturated: part liquid and part vapor, changing state at constant temperature for a single-component refrigerant. Subcooled liquid lies to the left of the dome and superheated vapor to the right.
Reference: Pressure-enthalpy diagram; saturation dome
Which statement best describes the saturation temperature of a refrigerant?
- The temperature at which its oil separates from the liquid
- The highest temperature the refrigerant can safely reach
- The temperature of the vapor leaving the compressor
- The temperature at which it boils at a given pressure
Show the answer
D. The temperature at which it boils at a given pressure
Saturation temperature is the boiling or condensing temperature that goes with a particular pressure; it is what the pressure-temperature chart gives. Discharge vapor is well above saturation because it is superheated.
Reference: Pressure-temperature relationship; saturation