When a single-component refrigerant is boiling in the evaporator at a steady pressure, what happens to its temperature?

Refrigeration · EPA 608 Core. The correct answer, and the reasoning behind it.

The refrigeration cycle, refrigerant states and pressures, gauges and leak detection.

The question

When a single-component refrigerant is boiling in the evaporator at a steady pressure, what happens to its temperature?

• AIt rises steadily as it boils
• BIt stays about the same while liquid remainsCorrect answer
• CIt falls steadily as it boils
• DIt swings up and down with each compressor stroke

Why “It stays about the same while liquid remains” is correct

While a pure refrigerant changes from liquid to vapor at a fixed pressure, it stays at its saturation temperature; the heat it absorbs goes into boiling, not into raising temperature. Only after all liquid has boiled does the vapor begin to warm (superheat).

Source: EPA: Test Topics

Why the other answers are wrong

• It rises steadily as it boils — Temperature rises only after all the liquid has boiled off.
• It falls steadily as it boils — At constant pressure the boiling temperature does not fall.
• It swings up and down with each compressor stroke — Saturation temperature follows pressure, not compressor strokes.

Practice more of this

More EPA 608 Core practice questions, or see every EPA 608 practice test.