A coil does not care about temperature; it cares about ENERGY, and the energy in moist air is its enthalpy. $h = 1.006\,t + W(2501 + 1.86\,t)$ — read aloud h equals one-point-oh-oh-six t, plus W times two-five-oh-one plus one-point-eight-six t. $h$ is the enthalpy in kJ per kilogram of dry air; $t$ is the dry-bulb temperature in °C; $W$ is the humidity ratio as a bare kg/kg, so 9 g/kg enters as 0.009. The 1.006 is dry air's specific heat, the 2501 is water's latent heat of vaporisation at 0 °C, and the 1.86·t corrects that latent heat for vapour that is not sitting at 0 °C.

The basis is one kilogram of DRY air, not one kilogram of the mixture, and this is the single thing people get wrong. The reason is bookkeeping, not pedantry: run the stream through a coil, condense water out of it, and the dry air is STILL one kilogram. So enthalpies before and after can be subtracted directly. Divide by the mixture mass instead and the denominator moves under you between the two readings.

The same basis carries specific volume: $v = \dfrac{0.287042\,T\,(1 + 1.6078\,W)}{p}$, cubic metres of mixture per kilogram of dry air, with $T$ the absolute temperature in kelvin — it is a gas law, so Celsius will not do — and $p$ the barometric pressure in kPa. Around 0.85 m³/kg at room conditions. This is the number that turns a fan's cubic metres per second into the kilograms per second everything else is priced in. The 1.6078 is $1/0.62198$, and it is why moist air is LIGHTER than dry air at the same temperature and pressure: a cubic metre holds a fixed number of molecules, and every water molecule that joined displaced a heavier nitrogen or oxygen one. Damp air feels heavy and is not.

And the altitude trap, which costs real money. $\rho = \dfrac{p_z(1 + W)}{R_{da}T(1 + 1.6078\,W)}$, where $p_z$ is the standard-day pressure at your altitude and $R_{da} = 287.042\ \mathrm{J/(kg\cdot K)}$ is dry air's gas constant. The trade's 1.08, 0.68 and 4.5 rules are sea-level, standard-air bookkeeping with a density baked in — $1.2014\ \mathrm{kg/m^3}$. At 1 000 m the air is roughly 11 % thinner, so a unit sized by the sea-level rule comes up short by about that much. The fan moves the cubic metres it was rated for and every one of them contains less air.