Enthalpy

Detailed Explanation

Enthalpy (h) is, in thermodynamics, the sum of a system's internal energy and the product of its pressure and volume. In HVAC context, it represents the total heat energy carried by moist air per unit mass of dry air, given in kJ/kg dry air. The reference point is 0°C dry air with 0 g/kg moisture content, so h = 0 at this point.

The importance of enthalpy in HVAC design is that it expresses the TOTAL energy load of a process in a single number. The work a cooling coil must perform is not how much air to cool, but how much enthalpy to drop. This difference is given by Δh = h₁ − h₂ and directly reflects in compressor electrical consumption, condenser heat rejection, and evaporator capacity selection.

Typical reference values: at 25°C, 50% RH, h ≈ 50.5 kJ/kg; at 30°C, 70% RH, h ≈ 78.4 kJ/kg; at 18°C, 50% RH, h ≈ 34.9 kJ/kg. The enthalpy difference between two points is a direct measure of system load.

Calculation

Moist air enthalpy (ASHRAE formula):

h = 1.006 × T + W × (2501 + 1.86 × T)

h: moist air enthalpy (kJ/kg dry air) T: dry bulb temperature (°C) W: specific humidity (kg/kg dry air)

Components: • 1.006 × T → sensible heat (specific heat of dry air) • W × 2501 → latent heat of vaporization of water (at 0°C) • W × 1.86 × T → sensible heat of water vapor

2501 kJ/kg is the latent heat absorbed by water at 0°C to transition into the vapor phase.

Practical Example

In a shopping mall ventilation system, 20,000 m³/h of outdoor air is processed. Summer design conditions:

Inlet (outdoor): 33°C, 65% RH → h₁ = 80.2 kJ/kg, W₁ = 18.3 g/kg Outlet (supply): 14°C, 95% RH → h₂ = 37.2 kJ/kg, W₂ = 9.4 g/kg

Dry air mass flow: 20,000 × 1.2 / 3600 ≈ 6.67 kg/s Total cooling load: Q = 6.67 × (80.2 − 37.2) = 286.7 kW

Sensible load (33→14°C reduction with W constant): Δhsen = 1.006 × (33 − 14) ≈ 19.1 kJ/kg → Qsen = 127.4 kW

Latent load (condensation portion): Qlat = 286.7 − 127.4 = 159.3 kW

Latent load is 55% of total. This ratio determines the economic rationale for dehumidifier or rotor integration: at high latent loads, desiccant technology can save 20–40% energy compared to conventional chillers.

Engineering Note

Critical points in enthalpy-based design:

• ERV (Energy Recovery Ventilator) selection is based on enthalpy recovery efficiency, not temperature efficiency. An ERV may offer 75% temperature efficiency but only 35% enthalpy efficiency. • Mixed air analysis: the enthalpy of an outdoor and return air mixture is calculated as the mass-weighted average: hmix = (mout × hout + mret × hret) / (mout + mret) • High altitude correction: above 1500 m, the pressure drop causes the enthalpy at the same temperature-humidity point to differ; ASHRAE altitude correction tables must be used. • Low-enthalpy designs (dry rooms, freeze-dryers): enthalpy drops to negative values; at −40°C, 5% RH, h ≈ −39 kJ/kg. Standard charts may not cover this region.

The NKT energy simulation tool calculates hourly enthalpy differences from annual outdoor weather data and reports total cooling + dehumidification energy consumption using the bin-hour method.