HVAC and Refrigeration Systems
Core concepts
Study the vapor-compression cycle: compression, condensation, expansion, and evaporation, and the pressure-enthalpy diagram that ties them together. Know the roles of compressor, condenser, evaporator, expansion valve, and refrigerant, plus superheat and subcooling as charge and performance indicators. Understand psychrometrics: dry-bulb, wet-bulb, dew point, humidity ratio, and sensible versus latent heat. Air-side topics include airflow, duct static pressure, filters, and coils. Know refrigerant safety classifications, phase-out rules, and leak detection.
Traps to watch
A classic trap is confusing superheat with subcooling when diagnosing charge. High superheat at the evaporator outlet suggests undercharge; high subcooling suggests overcharge or a restriction. Questions may hide the difference between sensible and latent cooling, or between wet-bulb and dew point. Watch for refrigerant handling items where recovery, not venting, is required. Another trap is assuming a lower suction pressure always means better capacity when it actually reduces density and capacity.
Worked example
On a P-h diagram, the refrigeration effect is the enthalpy difference across the evaporator, and compressor work is the enthalpy rise across the compressor. If evaporator inlet enthalpy is 250 kJ/kg and outlet is 400 kJ/kg, the refrigeration effect is 150 kJ/kg. Rule of thumb: target superheat of roughly 5–8 K at the evaporator outlet for a properly charged system, and confirm with subcooling at the condenser.