Worked problem

Total Cooling Load from Air Enthalpy

Find total cooling capacity from dry-air mass flow and enthalpy change, then the latent capacity and sensible heat ratio against a 30 kW sensible load.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

Given

A cooling coil handles 2.5 kg/s of dry air. Read from a psychrometric chart, the entering air has an enthalpy of 55.0 kJ/kg of dry air (roughly 27 °C and 50% relative humidity) and the leaving air 38.0 kJ/kg (about 13.5 °C, close to saturated). The sensible part of the load is 30.0 kW.

Find

The total cooling capacity, the latent capacity, the sensible heat ratio and the capacity in tons of refrigeration.

Solution

Q(total) = m(dry air) × (h(in) − h(out))

Q(total) = 2.5 × (55.0 − 38.0) = 42.5 kW.
Latent capacity = total − sensible = 42.5 − 30.0 = 12.5 kW. That is the energy removed by condensing moisture.
Sensible heat ratio SHR = sensible ÷ total = 30.0 ÷ 42.5 = 0.71.
One ton of refrigeration is 3.517 kW, so 42.5 ÷ 3.517 = 12.1 tons.

Answer

42.5 kW total (12.1 tons)  |  12.5 kW latent  |  SHR 0.71

Why enthalpy and dry-air flow

The enthalpy of moist air includes the energy in the water vapor, so the enthalpy difference captures sensible and latent cooling together. The mass flow is the mass of dry air, because that is the quantity that stays constant while moisture is added or removed. The small energy of the condensate leaving the coil is neglected here.

Common mistakes

Using the sensible formula (1.08 × CFM × ΔT) as if it were the total load. It leaves out the moisture removed.
Using the moist-air mass flow instead of the dry-air mass flow.
Reading both enthalpies at the same humidity, which hides the latent load.
Mixing kJ/kg with Btu/lb without converting (1 Btu/lb = 2.326 kJ/kg).

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

Questions

What is a normal sensible heat ratio?

Many comfort-cooling applications run between about 0.7 and 0.9. A low value means a large latent load, such as high outdoor-air ventilation or a crowd of people.

Where do the enthalpies come from?

From a psychrometric chart or equations, using the dry-bulb temperature and humidity of the entering and leaving air.

What if the coil does not dehumidify?

If the coil surface stays above the dew point of the air, there is no condensation, the latent capacity is zero and the SHR is 1.0.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

← All worked problems