FAHU cooling: sensible load, latent load and airflow
A fresh air handling unit conditions outdoor air before delivering it to a building. In hot, humid conditions, cooling the dry-bulb temperature is only part of the duty. Removing moisture can account for a substantial share of the coil load, and the control sequence must be consistent with the required supply conditions.
Start with airflow and air conditions
Record the design outdoor airflow, entering dry-bulb temperature and moisture condition, leaving condition, heat recovery arrangement and fan location. Airflow must be converted consistently: 3,600 m³/h equals 1 m³/s. For these examples, assume air density 1.20 kg/m³ and specific heat 1.005 kJ/(kg·K). These are simplified teaching assumptions.
Worked example: sensible duty
At 3,600 m³/h, mass flow is 1.20 × 1 = 1.20 kg/s. If air cools from 35°C to 15°C, sensible cooling is Qs = ṁ × cp × ΔT = 1.20 × 1.005 × 20 = 24.12 kW. This calculation uses temperature change alone.
Now include moisture through enthalpy
For a separate illustrative condition, suppose the entering and leaving moist-air enthalpies are 80 and 42 kJ/kg of dry air, and dry-air mass flow is 1.20 kg/s. Total cooling is Qt = ṁdry × (hin − hout) = 1.20 × 38 = 45.60 kW. Using 1 refrigeration ton = 3.517 kW gives approximately 12.97 TR.
Enthalpy values must correspond to the actual dry-bulb temperature, humidity and pressure. They were supplied here to demonstrate the calculation; they are not asserted as the psychrometric states of the earlier sensible example. Do not subtract the two examples to claim a verified latent load.
Heat recovery and coil selection
A recovery device changes the condition entering the cooling coil. Evaluate the resulting air state, leakage implications and available controls rather than assuming that all recovery equipment performs the same way. Reheat can raise supply temperature after dehumidification, while adding a separate energy duty.
- Use manufacturer coil selections at the actual entering air and chilled-water conditions.
- Check coil pressure drop, condensate drainage, filter losses and fan duty.
- Distinguish installed equipment capacity from the approved district-cooling load and design flow.
- Commission airflow, supply conditions, valves, sensors and the humidity-control sequence together.
Use the FAHU lab to explore cause and effect. These simplified examples do not replace a psychrometric calculation or an equipment selection.
Further reference
EnergyPlus engineering reference: cooling coils. This reference explains coil energy modelling; the numerical conditions in this guide are illustrative.
Verify assumptions and results before project use. These worked examples are educational and do not provide authority approval. Report a correction.