Moist Air Enthalpy (per kg DRY air)

Enthalpy is the number that decides coil selections, because it is the only one that captures both halves of the job at once. Cooling air involves dropping its temperature (sensible) and condensing water out of it (latent), and a coil has to do both from the same finite capacity. Enthalpy adds them into one figure, so the total load is simply mass flow times the enthalpy change across the coil.

The basis is one kilogram of DRY AIR, not one kilogram of the mixture, and this is the single thing people get wrong. It is worth stating why the convention exists rather than treating it as an oddity. Run air through a cooling coil and condense water out of it: the mixture mass has changed, so enthalpies before and after are on different bases and cannot be subtracted. The dry-air mass has not changed, and will not, whatever the coil does. Anchor everything to the part that stays constant and before-and-after values subtract directly, which is precisely what a load calculation needs. The cost is that the numbers look strange until you accept the basis. At 24 °C and 50 % RH the enthalpy is 47.76 kJ per kg of dry air; per kilogram of the actual mixture it would be 47.32, and that figure appears on no chart anywhere.

Read the terms. $1.006t$ is the sensible heat of the dry air itself. $2501W$ is the latent heat of the water it carries, using the heat of vaporisation at 0 °C. $1.86Wt$ corrects that latent term for vapour that is not sitting at 0 °C. The proportions are startling: at that same 24 °C / 50 % RH point the air contributes 24.1 kJ and the 9.3 grams of water contribute 23.6 kJ. Half the heat content of ordinary room air is in a quantity of water you could hold in a tablespoon. That is why dehumidification is expensive and why a coil sized on temperature alone comes up short in a humid climate.

The zero point is arbitrary. This scale sets $h = 0$ at 0 °C and $W = 0$, so below freezing the enthalpy is negative — and that is not an error. Only DIFFERENCES in enthalpy have physical meaning, and a difference is all a coil load ever asks for. Imperial charts zero at 0 °F and give different absolute numbers for identical air, which is harmless as long as you never mix the two scales in one subtraction.

This is what the 4.5 rule approximates: BTU/hr = 4.5 × CFM × Δh, where the 4.5 is 60 min/hr × 0.075 lb/ft³. Like 1.08 and 0.68 it carries a sea-level standard density and derates at altitude.