Introduction to HVACR
What this domain tests
The Introduction to HVACR domain checks whether you can explain what HVACR systems do and the physical principles behind them: heat moves from warmer to cooler, and comfort depends on controlling temperature, humidity, airflow, and cleanliness. Expect questions on sensible versus latent heat, the Btu and the refrigeration ton, pressure-temperature relationships, and the four basic refrigeration components.
Core rules and concepts
- One Btu raises 1 lb of water 1°F; one refrigeration ton removes 12,000 Btu/h.
- Sensible heat changes dry-bulb temperature; latent heat changes moisture content with no dry-bulb change.
- In a sealed refrigerant system, saturation pressure corresponds to saturation temperature; gauges read pressure, not temperature.
- The basic cycle: compressor raises pressure and temperature; condenser rejects heat and condenses vapor; metering device drops pressure; evaporator absorbs heat and boils liquid.
- Absolute pressure = gauge pressure + atmospheric pressure, about 14.7 psi at sea level.
- Airflow affects delivered capacity; a dirty filter or coil reduces heat transfer even when the equipment is energized.
Worked example
A 3-ton system is specified. Convert capacity to Btu/h: 3 × 12,000 = 36,000 Btu/h. If it runs at full capacity for 2 hours, it removes 72,000 Btu. For a heating coil with 1,200 CFM and a 20°F dry-bulb rise, sensible output is about 1.08 × 1,200 × 20 = 25,920 Btu/h. Use the ton definition for capacity; use the sensible heat equation only when airflow and temperature difference are measured.
Traps
A ton is a rate of heat removal, not 2,000 lb of refrigerant. Gauge pressure must be converted to absolute pressure before gas-law calculations. Low airflow can mimic a refrigerant charge problem by changing superheat, subcooling, and compressor load.