Psychrometric Air Properties & Process

Air is never just a temperature

Most heating and cooling sums treat air as if it were dry. It isn't. Every airstream in a factory carries water, and that water carries heat — often more heat than the temperature change does. Get the moisture wrong and you undersize the coil, oversize the burner, or condense acid onto the inside of a flue you have just paid to reline.

This calculator resolves the complete state of moist air from two measurements, models what happens to it through a real process step, and puts a pound figure on the difference between two designs.

Where the money actually is

Coil sizing. Cooling 2.5 kg/s of air from 35 °C / 60 % RH to a 10 °C coil is a 151 kW duty. A dry-bulb-only calculation gives 65 kW. The other 86 kW is latent — the heat released as 124 kg/h of water condenses out. Buy the 65 kW coil and it will never hit setpoint on a humid day.

Make-up air. Heating 2 kg/s of winter air (5 °C, 70 % RH) to 45 °C takes 81 kW. Landing it at 30 °C instead takes 51 kW. Over 6,000 operating hours that is 182 MWh, £13,676 and 33 tonnes of CO₂e a year — a £40,000 recovery scheme pays back in 2.9 years. That is the whole argument for a heat-recovery coil on a make-up air unit, in one line.

Evaporative pre-cooling. Spraying water into 5 kg/s of 32 °C / 35 % RH air until it reaches 80 % RH drops it to 22.9 °C — a 9.1 K temperature drop for 66 kg/h of water and a pump, with no refrigeration at all.

Combustion air. Air at 30 °C / 80 % RH carries 0.0216 kg of water per kg of dry air. At 5 °C / 60 % RH it carries 0.0032 — nearly seven times less. A burner trimmed in February is not trimmed in August.

The three tools

State lookup

Fix the air with dry-bulb temperature plus any one of relative humidity, wet-bulb, dew point, humidity ratio or enthalpy, and read the complete state: wet-bulb, dew point, RH, humidity ratio $W$, enthalpy $h$, specific volume, vapour pressure, degree of saturation and density. Pressure can be entered directly or derived from site altitude.

Moist-air enthalpy uses the ASHRAE simplified datum:

with $h$ in kJ per kg of dry air — the basis every duty below is calculated on, because dry-air mass is conserved while moisture is added or removed.

Process

Six transformations, each returning the outlet state, the duty and the condensate, drawn as a vector on the psychrometric chart:

• Sensible heating / cooling — constant humidity ratio, specified by outlet temperature or by duty. Cooling below the inlet dew point is flagged rather than silently mis-reported.
• Cooling with condensation — an apparatus-dew-point coil model with a bypass factor. Real coils bypass 5–20 % of the air untreated; at a bypass factor of 0 the air leaves saturated at the coil temperature.
• Mixing — any number of streams, blended on dry-air-mass-weighted humidity ratio and enthalpy. A supersaturated mix (fog) is detected and reported at the saturated state.
• Adiabatic humidification — evaporative spray along a constant-enthalpy line: moisture up, temperature down.
• Steam humidification — direct injection at near-constant dry-bulb, with an editable steam enthalpy (about 2,676 kJ/kg for saturated steam near 100 °C).
• Dew-point / condensation risk — whether a duct, flue or heat-exchanger surface sits close enough to the stream dew point to condense, within a safety margin you set.

Each model closes its own mass and energy balance rather than calling a library and hoping. Condensing cooling, for instance, accounts for the enthalpy leaving with the liquid water:

Savings comparison

Build a baseline chain and a proposed chain — several steps each — and the calculator solves both, sums the duty by fuel, and returns annual energy, running cost, CO₂e and simple payback. Heating defaults to gas and everything else to electricity; utility prices and carbon factors are editable, because yours are not ours.

Method, range and what it will not do

Two property engines run behind the same interface:

• Model 1 — the standard ASHRAE 2017 moist-air formulation. Fast, ideal-gas, and accurate from about −60 °C to 80 °C near atmospheric pressure. This is what runs by default.
• Model 2 — a real-gas humid-air formulation that stays valid well above that range, for high-temperature and foul-gas cases such as dryer exhaust and combustion air. It loads in the background and the calculator switches to it automatically when a case falls outside Model 1's range.

The roughly 0.5 % difference between the two at everyday conditions is the expected ideal-gas versus real-gas gap, not an error. Both use the same enthalpy datum, so duties, mixing and scenario comparisons stay consistent whichever engine answers.

Limits worth stating plainly. This is a moist-air model: it assumes a well-mixed stream at a single pressure, no heat loss through duct walls, and complete condensate removal at the coil. It does not size equipment, model coil rows or fin geometry, predict fouling, or correct for the composition of a genuine combustion product stream. Utility prices and the DESNZ carbon factors are defaults with a vintage attached — confirm them against your own tariff and the current published table before quoting a saving.

For a coil selection, a dryer mass balance or a heat-recovery scheme you intend to build, these numbers are the opening argument, not the design. That part is what we do.