HVAC Systems
Comprehensive guide to Heating, Ventilation, and Air Conditioning (HVAC) principles, design methodologies, and system optimization for building comfort and energy efficiency.
Psychrometrics and Air Properties
Psychrometrics is the study of air-water vapor mixtures and their thermodynamic properties. Understanding these relationships is fundamental to HVAC system design and analysis.
Humidity Relationships
RH = (p_w / p_ws) × 100%
W = 0.622 × (p_w / (p - p_w))
Where:
RH = relative humidity (%)
p_w = partial pressure of water vapor (Pa)
p_ws = saturation pressure of water vapor (Pa)
W = humidity ratio (kg water/kg dry air)
p = total atmospheric pressure (Pa)
Enthalpy and Specific Volume
h = c_p × t + W × (h_fg + c_pw × t)
v = (R_a × T × (1 + 1.608 × W)) / p
Where:
h = specific enthalpy (kJ/kg dry air)
c_p = specific heat of dry air (1.006 kJ/kg·K)
c_pw = specific heat of water vapor (1.86 kJ/kg·K)
t = dry bulb temperature (°C)
h_fg = latent heat of vaporization (2501 kJ/kg at 0°C)
v = specific volume (m³/kg dry air)
R_a = gas constant for dry air (287 J/kg·K)
T = absolute temperature (K)
Wet Bulb and Dew Point
t_wb = t - (t - t_dp) × (0.00066 × p × (1 + 0.00115 × t_wb))
t_dp = temperature at which p_w = p_ws
Where:
t_wb = wet bulb temperature (°C)
t_dp = dew point temperature (°C)
t = dry bulb temperature (°C)
Comfort Conditions
Summer comfort: 24-27°C, 40-60% RH
Winter comfort: 20-24°C, 30-50% RH
Air velocity: 0.1-0.2 m/s (occupied zone)
Operative temperature: (t_air + t_radiant)/2
Heat Load Calculations
Heat load calculations determine the amount of heating or cooling required to maintain desired indoor conditions. Accurate load calculations are essential for proper equipment sizing.
Total Cooling Load
Q_total = Q_sensible + Q_latent
Q_sensible = Q_transmission + Q_solar + Q_internal + Q_infiltration
Where:
Q_total = total cooling load (W)
Q_sensible = sensible heat gain (W)
Q_latent = latent heat gain (W)
Transmission Heat Gain
Q_transmission = U × A × CLTD
CLTD = (t_o - t_i) + LM + (t_r - 25.5) - (t_i - 25.5)
Where:
U = overall heat transfer coefficient (W/m²·K)
A = surface area (m²)
CLTD = cooling load temperature difference (K)
t_o = outdoor temperature (°C)
t_i = indoor temperature (°C)
LM = latitude and month correction
t_r = room temperature (°C)
Solar Heat Gain
Q_solar = A × SHGC × SCL × CLF
SHGC = SC × 0.87 (for standard glass)
Where:
A = window area (m²)
SHGC = solar heat gain coefficient
SCL = solar cooling load factor
CLF = cooling load factor
SC = shading coefficient
Internal Heat Gains
Q_people = N × q_sensible × CLF + N × q_latent
Q_lighting = W × FUF × FBF × CLF
Q_equipment = W × FLF × FRF × CLF
Where:
N = number of people
q = heat gain per person (W)
W = installed wattage
FUF = use factor, FBF = ballast factor
FLF = load factor, FRF = radiation factor
Typical Heat Gain Values
| Source | Sensible (W) | Latent (W) |
|---|---|---|
| Adult, seated, light work | 75 | 55 |
| Adult, seated, office work | 70 | 45 |
| Adult, standing, light work | 75 | 75 |
| Fluorescent lighting | 10-20 W/m² | 0 |
| Office equipment | 10-25 W/m² | 0 |
Air Distribution Systems
Air distribution systems deliver conditioned air to occupied spaces and return air to the central equipment. Proper design ensures adequate air quality, comfort, and energy efficiency.
Airflow Rate Calculations
Q_sensible = ṁ × c_p × ΔT = ρ × V̇ × c_p × ΔT
Q_latent = ṁ × h_fg × ΔW = ρ × V̇ × h_fg × ΔW
Where:
ṁ = mass flow rate (kg/s)
V̇ = volumetric flow rate (m³/s)
ρ = air density (kg/m³)
c_p = specific heat of air (1006 J/kg·K)
ΔT = temperature difference (K)
ΔW = humidity ratio difference (kg/kg)
Duct Sizing Methods
Equal Friction: ΔP/L = constant
Static Regain: ΔP_static = ΔP_velocity
Velocity Reduction: v₂ = v₁ × √(A₁/A₂)
Where:
ΔP/L = pressure drop per unit length (Pa/m)
v = air velocity (m/s)
A = duct cross-sectional area (m²)
Pressure Drop in Ducts
ΔP_friction = f × (L/D_h) × (ρv²/2)
ΔP_fitting = K × (ρv²/2)
Where:
f = friction factor
L = duct length (m)
D_h = hydraulic diameter (m)
K = loss coefficient for fittings
ρ = air density (kg/m³)
v = air velocity (m/s)
Air Distribution Patterns
Mixing ventilation: Supply air mixes with room air
Displacement ventilation: Cool air supplied at floor level
Underfloor air distribution: Supply through raised floor
Chilled beam systems: Convective and radiant cooling
Recommended Air Velocities
| Application | Velocity (m/s) | Noise Level |
|---|---|---|
| Main ducts | 6-10 | Low |
| Branch ducts | 4-6 | Medium |
| Supply outlets | 2-5 | Quiet |
| Return inlets | 2-4 | Very quiet |
HVAC Equipment and Systems
HVAC equipment selection and system design must consider capacity, efficiency, control, and maintenance requirements to achieve optimal performance and energy consumption.
Cooling Equipment Performance
COP = Q_cooling / W_input
EER = Q_cooling(Btu/h) / W_input(W)
SEER = Seasonal cooling output / Seasonal energy input
Where:
COP = coefficient of performance
EER = energy efficiency ratio
SEER = seasonal energy efficiency ratio
Q_cooling = cooling capacity (W or Btu/h)
W_input = electrical power input (W)
Heating Equipment Performance
η_combustion = (Q_output / Q_input) × 100%
AFUE = Annual fuel utilization efficiency
HSPF = Heating seasonal performance factor
Where:
η = efficiency (%)
Q_output = useful heat output
Q_input = fuel energy input
Fan Performance
P_fan = (V̇ × ΔP) / η_fan
η_fan = (V̇ × ΔP) / (P_input × 1000)
Where:
P_fan = fan power (W)
V̇ = airflow rate (m³/s)
ΔP = total pressure rise (Pa)
η_fan = fan efficiency
P_input = electrical power input (kW)
System Types and Applications
| System Type | Capacity Range | Applications |
|---|---|---|
| Split systems | 1.5-20 kW | Residential, small commercial |
| Package units | 5-100 kW | Commercial, industrial |
| Chillers | 50-5000 kW | Large commercial, industrial |
| VRF systems | 10-200 kW | Multi-zone commercial |
Energy Efficiency and Controls
Energy-efficient HVAC design and advanced control strategies are essential for reducing operating costs and environmental impact while maintaining occupant comfort.
Energy Recovery Systems
η_sensible = (t_supply - t_outdoor) / (t_exhaust - t_outdoor)
η_latent = (W_supply - W_outdoor) / (W_exhaust - W_outdoor)
η_total = (h_supply - h_outdoor) / (h_exhaust - h_outdoor)
Where:
η = effectiveness (%)
t = temperature (°C)
W = humidity ratio (kg/kg)
h = enthalpy (kJ/kg)
Variable Air Volume (VAV)
P_fan ∝ V̇³ (for constant pressure systems)
Energy savings = 1 - (V̇_reduced/V̇_design)³
VAV systems reduce energy consumption by varying airflow based on load requirements.
Economizer Operation
If h_outdoor < h_return, use 100% outdoor air
If t_outdoor < t_return (dry bulb economizer)
Economizers use favorable outdoor conditions to reduce mechanical cooling loads.
Control Strategies
Demand-controlled ventilation: CO₂-based outdoor air control
Optimal start/stop: Pre-conditioning based on thermal mass
Reset strategies: Supply air temperature and pressure reset
Occupancy-based control: Presence detection for zone control
Energy Performance Metrics
| Metric | Units | Typical Values |
|---|---|---|
| EUI (Energy Use Intensity) | kWh/m²·year | 100-300 |
| Chiller efficiency | kW/ton | 0.5-0.8 |
| Fan power | W/(L/s) | 1.0-2.5 |
| Pump power | W/gpm | 15-30 |