Definition
Heat energy spent on phase change (evaporation or condensation) without changing temperature. Evaporation of water in air absorbs latent heat, while condensation releases it. Dehumidifiers are the primary equipment controlling latent heat load.
Detailed Explanation
Latent heat is the heat energy spent on a phase change without altering the substance's temperature. The name "latent" (hidden) comes from being undetectable by a thermometer — the temperature reading remains constant while large amounts of energy are absorbed or released. The latent heat of water vaporization is 2,501 kJ/kg at 0°C and 2,257 kJ/kg at 100°C.
In HVAC context, latent heat is identical to moisture change in air. When air is dehumidified, the latent heat load decreases (via condensation); when air is humidified, the latent heat load increases (via evaporation). Latent load is generated when people perspire, water evaporates from product surfaces, or moisture diffuses from open baths in a space.
The importance of latent load: evaporating 1 kg of water absorbs approximately 0.7 kWh of energy. This is roughly 6 times the energy needed to heat the same water from 0°C to 100°C (~0.12 kWh). For this reason, in high-moisture-load environments (swimming pools, food processing, lithium battery production), latent load constitutes 50–95% of the total energy load.
Calculation
Latent heat load for an air stream:
Qlat = mda × hfg × ΔW ≈ V̇ × ρ × 2,501 × ΔW (0°C reference) ≈ V̇ × 3,001 × ΔW (standard conditions, quick rule)
Qlat: latent heat load (kW) V̇: volumetric air flow (m³/s) ρ: air density (≈ 1.2 kg/m³) hfg: latent heat of vaporization of water (≈ 2,501 kJ/kg, at 0°C) ΔW: specific humidity difference (kg/kg dry air)
Quick rule: a 1 g/kg moisture change in 1 m³/s of air flow equals approximately 3 kW of latent load.
Note: ΔW is in kg/kg; remember to divide by 1000 when working with g/kg.
Practical Example
Let us examine a lithium battery dry room: 800 m³ volume, target dew point of −40°Cdp (approximately 1% RH at 25°C). Due to personnel and airlock infiltration, 2 kg/h of moisture enters the room.
Latent load: m_water = 2 kg/h = 0.556 g/s Qlat = 0.556 × 2,501 / 1,000 = 1.39 kW
This 1.39 kW seems like a small number, but to drop this moisture to −40°Cdp, a silica gel rotor system must be installed; its total electrical consumption is approximately 30–50 kW (including reactivation heating). That is, ~25–35 kW of electricity is consumed for 1 kW of actual moisture removal load. COP ≈ 0.03–0.04.
A conventional chiller cannot reach this dew point (for −40°Cdp the cooling coil must be cooled to −45°C, where the evaporator freezes). Therefore, in lithium battery and low dew point applications, desiccant technology is the only practical solution, and reactivation heat recovery, multi-stage rotors, and low flow design are critical for energy optimization.
Engineering Note
Common mistakes in latent load analysis:
• Temperature-based control setpoint — systems controlled solely by thermostat cannot capture latent load; especially in low-SHR applications, ambient humidity remains far above the setpoint. • Ignoring reheat energy — in latent-load-dominated environments, chillers first cool air below the dew point, then reheat. This double-payment wastes 20–35% of annual energy bills. • Seasonal load profile — latent load can vary 5–10 times between summer and winter. Fixed capacity design is continuous energy waste; modulating (VSD inverter, multi-stage) equipment must be selected. • Recovery of condensation heat — condensation-type dehumidifiers convert latent load into sensible heat. This is the "free" side of reheat; in pool applications, this feature contributes to room heating.
The NKT psychrometric calculator and energy simulation tools report latent load separately; design decisions are made with the correct baseline.
