HVAC, Refrigeration, and Building Systems
What the exam tests
This domain covers refrigeration cycles, HVAC air and water systems, and building heating and ventilation. Expect the four main refrigeration components, superheat and subcooling, psychrometric terms, airflow, and safe refrigerant handling.
The refrigeration cycle
- The four components are the compressor, condenser, metering device, and evaporator. The compressor raises refrigerant pressure and temperature; the condenser rejects heat and condenses liquid; the metering device drops pressure; the evaporator absorbs heat and boils refrigerant.
- Superheat is the temperature of vapor above its saturation temperature. It protects the compressor from liquid slugging and is measured at the evaporator outlet.
- Subcooling is liquid cooled below its condensing temperature. It indicates how much liquid is stacked in the condenser and helps ensure a solid liquid seal at the metering device.
- Refrigerant safety: never mix refrigerants, never add liquid to the low side of a running compressor, and follow recovery and labelling rules.
HVAC and building systems
- Psychrometrics describes moist air using dry-bulb temperature, wet-bulb temperature, relative humidity, dew point, and humidity ratio. Cooling below dew point removes moisture.
- Air-handling units move return and outdoor air through filters, coils, and fans. Airflow problems show as high temperature difference, frozen coils, or low static pressure.
- Heating systems use steam, hot water, or electric heat. Cooling towers reject condenser heat; they require water treatment and freeze protection.
Worked example
A refrigeration system has a suction line temperature of 12°C and an evaporating saturation temperature of 4°C. What is the superheat?
Superheat = 12 − 4 = 8 K (or 8°C). The reasoning is that superheat is the difference between actual vapor temperature and saturation temperature at the same pressure. A normal value depends on the system; too little risks liquid return, too much means a starved evaporator.
Traps that cost marks
- Confusing superheat with subcooling.
- Measuring superheat at the compressor discharge rather than the evaporator outlet.
- Charging refrigerant by guesswork without weighing or using superheat/subcooling.
- Ignoring dew point when diagnosing humidity problems.
- Treating a cooling tower as a closed-loop system; it is open to atmosphere.