About Cooling Coil Load Calculator
The cooling coil load calculator determines how much cooling a coil must provide to move air from an entering (on-coil) condition to a leaving (off-coil) condition. It computes the total cooling load from the enthalpy drop across the coil and splits it into a sensible part, which lowers the dry-bulb temperature, and a latent part, which condenses moisture out of the air.
Enter the air flow rate, the entering and leaving dry-bulb temperatures and relative humidities, and the atmospheric pressure. The tool derives the humidity ratio and enthalpy at each face from the Magnus saturation-pressure relation, converts the volumetric flow to a dry-air mass flow using the moist-air specific volume, and reports the total, sensible, and latent loads, the sensible heat ratio, and the condensate rate.
How It Works
- Enter the air flow (m^3/s, L/s, or CFM) and the entering and leaving conditions as dry-bulb temperature plus relative humidity.
- The calculator finds the humidity ratio W and enthalpy h at each face using Pws = 610.94 exp(17.625 T/(T+243.04)), W = 0.62198 Pw/(P-Pw), and h = 1.006 T + W(2501 + 1.86 T).
- It converts the entering volumetric flow to a dry-air mass flow m_dot using the moist-air specific volume, then evaluates total cooling = m_dot (h_in - h_out) and sensible cooling = m_dot * cp_moist (T_in - T_out).
- The latent load is the remainder Q_total - Q_sensible, the SHR is Q_sensible / Q_total, and the condensate rate is m_dot (W_in - W_out).
Worked Example
A coil handles 1.0 m^3/s of air entering at 25 C / 50% RH and leaving at 13 C / 90% RH at 101.325 kPa. The entering humidity ratio is W_in = 0.00986 kg/kg with enthalpy h_in = 50.26 kJ/kg; the leaving values are W_out = 0.00837 kg/kg and h_out = 34.21 kJ/kg. The entering specific volume is 0.858 m^3/kg, so the dry-air mass flow is 1.0 / 0.858 = 1.166 kg/s. Total cooling = 1.166 (50.26 - 34.21) = 18.70 kW (5.32 tons). Sensible cooling = 1.166 * 1.024 * (25 - 13) = 14.33 kW, so latent = 18.70 - 14.33 = 4.38 kW and SHR = 14.33 / 18.70 = 0.766. The condensate rate is 1.166 (0.00986 - 0.00837) = 0.00173 kg/s = 6.24 L/h.
Formulas
- Total cooling load
Q_total = m_dot_da * (h_in - h_out)- Sensible cooling load
Q_sensible = m_dot_da * (1.006 + 1.86*W_in) * (T_in - T_out)- Latent cooling load and SHR
Q_latent = Q_total - Q_sensible ; SHR = Q_sensible / Q_total- Dry-air mass flow and condensate
m_dot_da = V_dot / v_in ; condensate = m_dot_da * (W_in - W_out)
Standards & References
- ASHRAE Handbook of Fundamentals - Psychrometrics
- Magnus-Tetens saturation-pressure relation (Alduchov & Eskridge)
- ASHRAE Systems and Equipment - air-cooling coils
Frequently Asked Questions
How is the total cooling load calculated?
The total cooling load equals the dry-air mass flow times the enthalpy drop across the coil, Q_total = m_dot (h_in - h_out). Using enthalpy captures both the temperature drop (sensible) and the moisture removal (latent) in a single number, expressed in kW or refrigeration tons.
What is the sensible heat ratio (SHR)?
The sensible heat ratio is the sensible cooling divided by the total cooling, SHR = Q_sensible / Q_total. An SHR near 1.0 means almost all cooling lowers temperature with little dehumidification, while a lower SHR (for example 0.7) indicates a significant latent load from condensing moisture.
Why is the mass flow based on dry air rather than total air?
Humidity ratio and enthalpy in psychrometrics are defined per kilogram of dry air, because the mass of dry air is conserved across the coil while water vapour is removed. Using the dry-air mass flow, found from the entering specific volume, keeps the energy and moisture balances consistent.
What happens if the leaving air is warmer than the entering air?
That describes a heating process, not cooling, so the calculator flags it as invalid. A cooling coil must lower the dry-bulb temperature; the leaving dry-bulb must be at or below the entering dry-bulb for the sensible and latent split to be physically meaningful.