HVAC Systems
Comprehensive guide to Heating, Ventilation, and Air Conditioning (HVAC) principles, design, and applications.
Heat Load Calculation
Heat load calculations determine the amount of heating or cooling required for a space. The total heat load includes:
Q_total = Q_transmission + Q_infiltration + Q_ventilation + Q_internal + Q_solar
Where:
- Q_transmission = heat transfer through building envelope (W)
- Q_infiltration = heat gain/loss due to air leakage (W)
- Q_ventilation = heat gain/loss due to mechanical ventilation (W)
- Q_internal = heat gain from occupants, lighting, and equipment (W)
- Q_solar = heat gain from solar radiation (W)
Transmission heat transfer is calculated as:
Q_transmission = U × A × ΔT
Where:
- U = overall heat transfer coefficient (W/m²·K)
- A = surface area (m²)
- ΔT = temperature difference (K)
Psychrometrics
Psychrometrics is the study of air-water vapor mixtures. Key parameters include:
1. Relative Humidity (RH):
RH = (p_w / p_ws) × 100%
2. Humidity Ratio (W):
W = 0.622 × (p_w / (p - p_w))
3. Enthalpy (h):
h = c_p × t + W × (h_fg + c_pw × t)
Where:
- p_w = partial pressure of water vapor (Pa)
- p_ws = saturation pressure of water vapor (Pa)
- p = total atmospheric pressure (Pa)
- W = humidity ratio (kg water vapor/kg dry air)
- c_p = specific heat of dry air (J/kg·K)
- c_pw = specific heat of water vapor (J/kg·K)
- t = dry bulb temperature (°C)
- h_fg = latent heat of vaporization (J/kg)
Air Distribution
Air distribution systems deliver conditioned air to spaces. The required airflow rate is calculated as:
Q = q / (ρ × c_p × ΔT)
Where:
- Q = volumetric flow rate (m³/s)
- q = heat load (W)
- ρ = air density (kg/m³)
- c_p = specific heat of air (J/kg·K)
- ΔT = temperature difference between supply and room air (K)
Duct sizing is often done using the equal friction method, where the pressure drop per unit length is kept constant:
ΔP/L = f × (ρ × v²) / (2 × D_h)
Where:
- ΔP/L = pressure drop per unit length (Pa/m)
- f = friction factor
- ρ = air density (kg/m³)
- v = air velocity (m/s)
- D_h = hydraulic diameter (m)