The evaporator is the heat exchanger where the refrigerant absorbs heat and evaporates.
In heating mode
In an air-to-water heat pump, the outdoor heat exchanger usually functions as the evaporator.
What happens to the refrigerant
The refrigerant enters at low pressure and, by absorbing heat, gradually turns into vapor.
Role of airflow
A dirty heat exchanger or insufficient airflow reduces heat absorption and affects the evaporation pressure.