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Using moist air enthalpy to size cooling coils in Canada
Enthalpy is the total heat content of moist air per unit mass of dry air, combining the sensible heat stored in the air's temperature with the latent heat stored in the water vapour. The 1061 BTU/lb constant in the latent term is the approximate enthalpy of vaporization of water near typical HVAC conditions, the same reference property tabulated in NIST's water thermophysical data. When a cooling coil processes air, it removes total heat equal to the airflow times the enthalpy difference between entering and leaving conditions. This single number captures both the temperature drop (sensible) and the moisture removal (latent) in one calculation.
The formula for total coil capacity is: Q (BTU/hr) = CFM x 4.5 x (h_entering - h_leaving), where h is in BTU/lb and 4.5 is the standard air mass flow factor at sea level (60.45 minutes/hr x 0.075 lb/ft³). At Canadian cities with significant elevation, the density factor drops. Calgary at 1045 m has a standard air density of about 0.066 lb/ft³, giving a factor of about 3.95 instead of 4.5, roughly a 12% reduction. This calculator applies the elevation-corrected factor automatically.
Sensible vs. latent enthalpy components
Total enthalpy splits into sensible and latent components. Sensible enthalpy (BTU/lb) = 0.240 x T_db (°F). Latent enthalpy (BTU/lb) = W (lb/lb) x (1061 + 0.444 x T_db). For typical summer comfort cooling from 80°F/50% RH entering to 55°F/95% RH leaving air, sensible enthalpy drops from about 19.2 to 13.2 BTU/lb (6.0 BTU/lb sensible change) while latent enthalpy drops from about 12.0 to 9.5 BTU/lb (2.5 BTU/lb latent change). Total enthalpy change is about 8.5 BTU/lb, with an SHR of roughly 0.70, indicating a predominantly sensible-cooling coil at this condition.
For dry climates like Calgary and interior BC, summer outdoor air has low humidity ratio. Entering air enthalpy is dominated by sensible heat, and the latent component is small. Coils in these climates have high SHR (0.85-0.95), since most of the cooling work is sensible, which also makes them good candidates for the free cooling an economizer control calculator can size around low-enthalpy outdoor air. Use the cooling coil calculator to compute SHR and apparatus dew point from entering and leaving conditions.
Ventilation enthalpy loads in Canadian climates
Ventilation energy recovery is sized from the enthalpy difference between outdoor and exhaust air. In Toronto summers, outdoor air at 35°C and 60% RH has an enthalpy of about 90 kJ/kg (38.8 BTU/lb), while exhaust air at 24°C and 50% RH has about 48 kJ/kg (20.6 BTU/lb). The enthalpy recovery opportunity is about 42 kJ/kg (18.2 BTU/lb) per unit mass of outdoor air, substantial enough to justify energy recovery ventilators (ERVs) in most Canadian commercial buildings. See the HRV/ERV calculator for recovery efficiency analysis.
Frequently Asked Questions
Moist air enthalpy (BTU/lb dry air) = 0.240 x T_db (°F) + W (lb/lb) x (1061 + 0.444 x T_db). For coil sizing, calculate enthalpy at entering conditions (return plus outdoor air mix) and at leaving conditions (design supply air). Total coil capacity (BTU/hr) = CFM x 4.5 x (h_entering - h_leaving). At higher Canadian cities, replace 4.5 with an elevation-corrected density factor, which this calculator applies automatically. Use the cooling coil calculator for full SHR and bypass factor analysis.
Total enthalpy has 2 components. Sensible enthalpy (= 0.240 x T_db) changes with temperature at constant humidity. Latent enthalpy (= W x 1061) changes with moisture content at constant temperature. A cooling coil removes both: sensible cooling reduces dry-bulb temperature, latent removes moisture as condensate. The Sensible Heat Ratio (SHR = sensible / total) is typically 0.75-0.85 for comfort cooling, and this same enthalpy split is what energy recovery ventilators are rated on under NRCan's ENERGY STAR technical specification for H/ERVs. Use the cooling coil calculator to compute SHR from entering and leaving conditions, and the humidity ratio calculator for latent-only analysis.