Latent Loads from Occupancy in High-Density Spaces

Occupant Latent Heat Generation

Human occupants generate latent heat through two primary physiological mechanisms: perspiration (evaporation from skin) and respiration (moisture in exhaled breath). The latent heat component represents 20-50% of total metabolic heat release depending on activity level and environmental conditions.

The total latent heat gain from occupants is calculated as:

$$Q_{lat} = N \cdot q_{lat} \cdot CLF$$

Where:

• $Q_{lat}$ = total latent heat gain (Btu/hr or W)
• $N$ = number of occupants
• $q_{lat}$ = latent heat gain per person (Btu/hr)
• $CLF$ = cooling load factor (typically 1.0 for latent loads)

Moisture Generation Mechanism

The moisture generation rate from occupants is expressed as:

$$\dot{m}{moisture} = N \cdot \frac{q{lat}}{h_{fg}}$$

Where:

• $\dot{m}_{moisture}$ = moisture generation rate (lb/hr or kg/hr)
• $h_{fg}$ = latent heat of vaporization (1050 Btu/lb or 2440 kJ/kg at standard conditions)

This moisture addition increases the space humidity ratio:

$$\Delta W = \frac{\dot{m}{moisture}}{\dot{m}{air}} = \frac{N \cdot q_{lat}}{h_{fg} \cdot \rho_{air} \cdot \dot{V}}$$

Where:

• $\Delta W$ = humidity ratio increase (lb moisture/lb dry air)
• $\dot{V}$ = ventilation airflow rate (CFM or m³/s)
• $\rho_{air}$ = air density (0.075 lb/ft³ or 1.2 kg/m³)

Latent Load by Activity Level

ASHRAE Fundamentals provides standardized latent heat generation rates based on activity intensity:

Note: Values assume 75°F space temperature. Latent fraction increases as space temperature rises.

Sensible Heat Ratio Impact

The sensible heat ratio (SHR) for occupied spaces is calculated as:

$$SHR = \frac{Q_{sensible}}{Q_{sensible} + Q_{latent}}$$

High-density occupancy drives SHR down to 0.60-0.75, significantly below the typical cooling coil SHR of 0.75-0.80. This mismatch creates humidity control challenges requiring specialized equipment selection.

graph TD
    A[Occupant Metabolism] --> B[Sensible Heat<br/>Convection + Radiation]
    A --> C[Latent Heat<br/>Perspiration + Respiration]

    B --> D[Temperature Rise]
    C --> E[Humidity Ratio Increase]

    D --> F{Space Conditions}
    E --> F

    F --> G[Required SHR = 0.60-0.75]
    G --> H[Cooling Coil Selection]
    H --> I[Adequate Latent Capacity]

    style C fill:#e6f3ff
    style E fill:#e6f3ff
    style G fill:#fff4e6
    style I fill:#e6ffe6

Dehumidification Requirements

graph LR
    A[Return Air<br/>78°F, 65% RH] --> B[Cooling Coil]
    B --> C[Supply Air<br/>55°F, 90% RH]
    C --> D[Duct Heat Gain]
    D --> E[Diffuser Discharge<br/>58°F, 85% RH]
    E --> F[Space Mixing]
    F --> A

    G[Occupant Moisture<br/>0.5 lb/hr per person] --> F
    H[Ventilation Air<br/>95°F, 55% RH] --> B

    style B fill:#cce6ff
    style G fill:#ffe6cc

Coil Performance Requirements

To handle high latent loads, the cooling coil must achieve sufficient moisture removal:

$$\dot{m}{removed} = \rho{air} \cdot \dot{V} \cdot (W_{entering} - W_{leaving})$$

Where:

• $W_{entering}$ = entering air humidity ratio
• $W_{leaving}$ = leaving air humidity ratio

The apparatus dew point (ADP) must be selected to provide adequate latent capacity:

$$ADP_{required} = T_{space} - \frac{Q_{total}}{1.08 \cdot \dot{V} \cdot BF}$$

Where $BF$ is the coil bypass factor (typically 0.05-0.15 for dehumidification applications).

Moisture Load Distribution

High-occupancy spaces experience time-varying latent loads:

Latent load diversity accounts for simultaneous occupancy patterns. Design diversity factors range from 0.75-0.95 depending on space function.

Design Considerations

Critical parameters for high-latent-load applications:

1. Supply air temperature: Lower supply air temperatures (52-55°F) improve dehumidification but require careful condensation control
2. Coil rows: 6-8 row coils with face velocities below 500 FPM maximize moisture removal
3. Reheat strategies: Subcool-and-reheat approaches maintain humidity control during partial load
4. Dedicated outdoor air systems (DOAS): Separately condition ventilation air to reduce space latent load
5. Desiccant dehumidification: Supplement mechanical cooling in extreme latent load conditions (SHR < 0.65)

Relative humidity control challenges:

• Occupant comfort range: 30-60% RH per ASHRAE Standard 55
• Mold prevention: maintain below 60% RH per ASHRAE Standard 62.1
• Condensation avoidance: surface temperatures must exceed dew point
• Part-load operation: reduced sensible loads increase space RH if latent capacity is insufficient

System Selection Criteria

Select HVAC equipment based on space SHR requirements:

$$\frac{Q_{lat,space}}{Q_{sen,space}} \leq \frac{Q_{lat,coil}}{Q_{sen,coil}}$$

When space latent load fraction exceeds coil latent capacity fraction, supplemental dehumidification is required. This commonly occurs in theaters, auditoriums, gymnasiums, and assembly spaces where occupant density exceeds 15 ft²/person and activity levels are moderate to high.

Reference: ASHRAE Handbook—Fundamentals, Chapter 18: Nonresidential Cooling and Heating Load Calculations (metabolic heat generation rates and occupancy load profiles).