Latent Sensible Cooling Load

Overview

Engineering reference data for Latent Sensible Cooling Load in mechanics.

Key Formulas

Newton's Second Law

$F = ma$

Force = mass × acceleration.

Work

$W = Fd\cos\theta$

Work = force × displacement × cos(angle).

Kinetic Energy

$E_k = \frac{1}{2}mv^2$

Energy of motion.

Potential Energy

$E_p = mgh$

Gravitational potential energy.

Variables

Sensible vs. Latent Cooling Load

The total design cooling load required for an HVAC system is the sum of the sensible cooling load and the latent cooling load. These two components address different aspects of thermal comfort.

• Sensible Cooling Load: This is the heat gain that results in a temperature increase of the air. It is associated with the dry bulb temperature and must be removed to maintain the desired room air temperature.
• Latent Cooling Load: This is the heat gain associated with moisture (water vapor) entering the conditioned space. It is tied to the wet bulb temperature and must be removed to maintain the desired humidity level.

For summer design conditions, both loads must be calculated to ensure the HVAC equipment can handle the combined total.

Factors Influencing Sensible Cooling Load

Key heat sources that contribute to the sensible cooling load include:

• Fenestration: Sunlight striking glass windows, skylights, or glass doors.
• Building Envelope: Heat transfer through exterior walls and roofs.
• Interior Surfaces: Partitions separating spaces of different temperatures, and ceilings under an attic.
• Infiltration: Air leaking through cracks in the building envelope, doors, and windows.
• Internal Gains: Heat emitted from people, equipment, appliances, and lighting.

Note: Below-grade walls, below-grade floors, and floors on concrete slabs do not contribute to the cooling load and are ignored in the calculation.

Factors Influencing Latent Cooling Load

Moisture is introduced into the conditioned space primarily through:

• Occupants: People through respiration and perspiration.
• Internal Processes: Equipment and appliances that generate moisture.
• Infiltration: Air leakage carrying outdoor humidity into the building.

Important Distinction: System Gains vs. Room Gains

The text highlights a critical design consideration:

• Room Gains: The sensible and latent loads listed above occur within the conditioned space.
• System Gains: Additional sensible and latent heat gains can occur upstream in the HVAC system, such as from ductwork located in an unconditioned attic or from the energy required to condition ventilation air. These system gains must be added to the room loads to determine the total required capacity of the HVAC equipment.

Key Heat Transfer Formulas

While the original text references charts, the fundamental equations for calculating sensible and latent heat loads are essential:

Sensible Heat Load (W or Btu/h): $Q_{sensible} = \dot{m} \cdot c_p \cdot (T_{in} - T_{out})$ where $\dot{m}$ is the mass flow rate of air, $c_p$ is the specific heat of air, and $(T_{in} - T_{out})$ is the dry-bulb temperature difference.

Latent Heat Load (W or Btu/h): $Q_{latent} = \dot{m} \cdot h_{fg} \cdot (W_{in} - W_{out})$ where $h_{fg}$ is the latent heat of vaporization of water and $(W_{in} - W_{out})$ is the humidity ratio difference.

Total Cooling Load: $Q_{total} = Q_{sensible} + Q_{latent}$

References

• Original Source