Dedicated Outdoor Air Systems (DOAS), ASHRAE 84 Psychrometrics & Energy Recovery
HVAC Rotary Enthalpy Wheel Sensible & Latent Recovery Formula
Calculate ASHRAE Standard 84 Desiccant Rotary Enthalpy Wheel Leaving Supply Air Temperature (°C), Total Recovered Enthalpy Power (kW), Sensible Heat Recovered (kW), and Latent Dehumidification/Humidification Power Recovered (kW).
See why desiccant enthalpy wheels recover 3x more summer cooling than sensible plate exchangers!
In Dedicated Outdoor Air Systems (DOAS) serving hospitals, labs, and schools, humid summer ventilation air carries far more LATENT moisture enthalpy than sensible dry-bulb heat! Under ASHRAE Standard 84 with balanced supply and exhaust dry-air mass flow m_da (kg/s) and wheel effectiveness ε, Leaving Supply Temperature is T_sup = T_oa - ε * (T_oa - T_ret), Sensible Power Recovered is Q_sens = m_da * 1.006 * ε * ABS(T_oa - T_ret) (kW), and Total Enthalpy Power Recovered is Q_tot = m_da * ε * ABS(h_oa - h_ret) (kW)—isolating Latent Moisture Power Recovered as Q_lat = Q_tot - Q_sens!
Verify on a Humid Summer DOAS Design Day—Balanced_Dry_Air_Flow = 2.40 kg/s (B2), Outdoor_Air_Temp = 35.0°C (B3), Return_Exhaust_Temp = 23.0°C (B4), Outdoor_Enthalpy = 92.0 kJ/kg (B5), Return_Enthalpy = 48.0 kJ/kg (B6), and Desiccant_Wheel_Effectiveness = 0.75 (75.0%, B7): Leaving Pre-Conditioned Supply Temp drops to 35 - 0.75*(35 - 23) = 26.00°C (E2), Total Enthalpy Power Recovered reaches 2.40 * 0.75 * 44 = 79.20 kW (E3), Sensible Cooling Recovered is 21.73 kW (E4), and Latent Dehumidification Recovered is 57.47 kW (E5—72.6% of total savings)!
| Rotary Enthalpy Wheel Metric (2.4 kg/s, 35°C/92 kJ Outdoor -> 23°C/48 kJ Return, ε=0.75) | Formula | Verified Output |
|---|---|---|
| Leaving Supply Temp (°C) | T_oa - ε * (T_oa - T_ret) | 26.00°C (+14.50°C @ -5°C Winter) |
| Total Enthalpy Recovered (kW) | m_da * ε * ABS(h_oa - h_ret) | 79.20 kW (Same @ Δh=44 Winter) |
| Sensible Heat Recovered (kW) | m_da * 1.006 * ε * ABS(T_oa - T_ret) | 21.73 kW (47.08 kW @ -5°C Winter) |
| Latent Moisture Recovered (kW) | Total_Recovered_kW - Sensible_kW | 57.47 kW (32.12 kW @ -5°C Winter) |
Desiccant Wheel Latent Moisture Power Recovered Formula
Validate m_da > 0, effectiveness ε in (0, 1), and physically consistent psychrometric enthalpy swing ABS(h_oa - h_ret) >= 1.006 * ABS(T_oa - T_ret)!
=LET(md_a, B2, to_c, B3, tr_c, B4, ho_k, B5, hr_k, B6, ep_w, B7, dh_t, ABS(ho_k - hr_k), ds_t, 1.006 * ABS(to_c - tr_c), IF(OR(md_a <= 0, ep_w <= 0, ep_w >= 1, dh_t < ds_t), "Invalid Psychrometric Wheel State", LET(qt_k, md_a * ep_w * dh_t, qs_k, md_a * ep_w * ds_t, ROUND(qt_k - qs_k, 2))))Sub-Freezing Winter Heating & Humidification Check (Set Outdoor = -5°C, -2 kJ/kg; Return = 21°C, 42 kJ/kg)
Flip to winter design (B3 = -5, B4 = 21, B5 = -2, B6 = 42): Leaving Supply Air E2 preheats automatically from -5°C up to +14.50°C, recovering 47.08 kW sensible heat (E4) plus 32.12 kW latent indoor moisture (E5)!
=IF(ISTEXT(E5), E5, ROUND(B3 - B7 * (B3 - B4), 2))Try the example
Use the original values or change them to test your understanding.