Energy Systems and HVAC
Energy Systems and HVAC tests psychrometrics, cooling and heating loads, refrigeration and heat pumps, duct and fan systems, combustion, and energy efficiency.
Core rules and concepts
- Psychrometrics: humidity ratio W=0.622 p_v/(P-p_v); dew point is saturation at p_v; moist-air enthalpy combines dry-air and water-vapor terms.
- Sensible heat q_s is approximately 1.2 Q delta T in kW when Q is in m^3/s and delta T is in K at standard air; latent heat depends on moisture removal.
- Refrigeration COP=Q_L/W_in; heat-pump COP=Q_H/W_in=refrigeration COP+1 for the same cycle.
- Duct losses scale with velocity squared; fan laws give Q proportional to N, delta p proportional to N^2, and P proportional to N^3.
- Cooling load includes conduction, solar, internal gains, infiltration, and ventilation latent load; equipment sensible heat ratio must match the load.
Worked example
A cooling coil handles 1.0 m^3/s of air with a 10 K sensible temperature drop. Using q_s approximately 1.2 Q delta T=1.2 x 1.0 x 10=12 kW. If the coil also removes 0.002 kg/s of water vapor, latent heat is about m_w h_fg=0.002 x 2450=4.9 kW. Total coil load is about 16.9 kW. The sensible heat ratio is 12/16.9=0.71, which must be considered when selecting the coil and apparatus dew point.
Traps
Using total pressure instead of vapor pressure in humidity ratio, mixing sensible and latent loads, ignoring altitude corrections for air density, and applying fan laws across different system curves are common.