13.3 Thermal Comfort, ASHRAE Standard 55 & Psychrometrics

Thermal Comfort & Building Thermodynamics

Heating, ventilating, and air-conditioning (HVAC) systems are engineered to control indoor atmospheric conditions and maintain human thermal comfort while satisfying indoor air quality mandates. For the ARE 5.0 Project Planning & Design (PPD) division, architects must understand the thermodynamic principles governing human comfort, evaluate mechanical system typologies, and coordinate extensive spatial requirements for central equipment plants, vertical distribution chases, and horizontal ceiling plenums.

Human Thermal Comfort & ASHRAE Standard 55

Thermal comfort is defined by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) as "that condition of mind that expresses satisfaction with the thermal environment." Under ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy), human thermal comfort is governed by six primary variables:

1. Dry-Bulb Temperature ($T_{db}$): The ambient air temperature measured by a standard thermometer shielded from direct solar radiation. Standard indoor comfort ranges from 68°F to 74°F in winter heating conditions and 73°F to 78°F in summer cooling conditions.
2. Relative Humidity (RH): The ratio of the actual partial pressure of water vapor in air to the saturation vapor pressure at the same temperature. The acceptable indoor comfort envelope spans 30% to 60% RH. Relative humidity below 30% dries human mucous membranes and induces electrostatic discharges; humidity above 60% inhibits natural evaporative cooling through perspiration and accelerates toxic fungal/mold proliferation.
3. Indoor Air Velocity: The speed of air movement across human skin (typically maintained between 30 and 50 feet per minute [fpm]). Air velocities exceeding 50 fpm cause unwanted draft sensations in winter, but elevated air velocity (up to 120–160 fpm) can be deliberately introduced via ceiling fans in summer to extend the acceptable cooling comfort range upward by 3°F to 5°F through enhanced skin evaporation.
4. Mean Radiant Temperature (MRT): The uniform surface temperature of an imaginary enclosing blackbody that emits the same radiant heat toward a person as the actual non-uniform surroundings. MRT represents the weighted average temperature of all bounding surfaces (walls, windows, floors, ceilings, lighting fixtures). Because radiation accounts for approximately 40% to 50% of human body heat dissipation, cold interior glass surfaces or uninsulated roofs cause occupants to feel uncomfortably chilled even when the dry-bulb room thermostat reads 72°F. The Operative Temperature ($T_o$) balances these effects: $T_o \approx 0.5 T_{db} + 0.5 MRT$.
5. Clothing Insulation ($I_{cl}$): The thermal resistance of clothing worn by occupants, quantified in clo units ($1 \text{ clo} = 0.155 \text{ m}^2\cdot\text{K/W} = 0.88 \text{ }^\circ\text{F}\cdot\text{ft}^2\cdot\text{hr/Btu}$). A heavy three-piece business suit with long underwear equals approximately 1.0 to 1.5 clo, whereas light summer shorts and a t-shirt equal 0.3 to 0.5 clo.
6. Metabolic Rate ($M$): The rate of human internal chemical energy transformation into heat, measured in met units ($1 \text{ met} = 58.2 \text{ W/m}^2 = 18.4 \text{ Btu/hr}\cdot\text{sq ft}$ of body surface area, roughly 360 to 400 Btu/hr for a seated adult at rest). Sedentary office computer work generates approximately 1.1 to 1.2 met; vigorous athletic exercise in gymnasiums exceeds 3.0 to 4.0+ met.

Psychrometric Chart Fundamentals

The psychrometric chart graphically depicts the thermodynamic properties of moist air at standard atmospheric pressure:

• Dry-Bulb Temperature: Plotted along the horizontal x-axis (increasing from left to right).
• Humidity Ratio (Specific Humidity): Plotted along the vertical y-axis on the right (grains of moisture per pound of dry air, or pounds of moisture per pound of dry air).
• Relative Humidity Curves: Curved lines radiating from bottom-left to top-right. The uppermost curve represents 100% Relative Humidity (Saturation Curve / Dew Point).
• Wet-Bulb Temperature & Enthalpy: Diagonal lines sloping downward from upper-left (on the saturation curve) toward lower-right. Enthalpy represents the total heat content of moist air (Btu/lb of dry air), combining sensible and latent energy.
• Thermodynamic Processes:
  • Sensible Heating / Cooling: Moves purely horizontally (left/right) along lines of constant moisture content without adding or removing water vapor.
  • Latent Humidification / Dehumidification: Moves purely vertically (up/down) along lines of constant dry-bulb temperature.
  • Evaporative Cooling: Moves upward and leftward along lines of constant wet-bulb temperature / constant enthalpy, exchanging sensible heat for latent heat (cooling the air while adding humidity).
  • Cooling & Dehumidification: The standard summer air conditioning cycle; air moves leftward until reaching saturation (dew point), then slides down the saturation curve, condensing out liquid moisture while chilling the air.