Psychrometrics for Sales Engineers — The Five State Points Every AHU Quote Implies

Every AHU quote, whether or not anyone draws a chart, implies five psychrometric state points: outdoor air, return air, mixed air, coil-leaving air, and delivered supply air. A sales engineer who can name the humidity ratio and enthalpy at each of those five points — not just the airflow and the kW — can defend a quote against a consultant's review instead of re-running the selection after the fact.

Why this matters even if you never open a psychrometric chart

The AHU software runs the psychrometrics for you. But every number on the technical data sheet — coil duty in kW, condensate rate in kg/h, supply dry-bulb and RH — is the output of a chain of state points, and a consultant reviewing the quote will check that chain, not just the final row. If the mixed-air state doesn't sit between outdoor and return in proportion to the fresh-air ratio, or the coil-leaving state implies an apparatus dew point (ADP) below what the coil's rows and fin pitch can actually reach, the quote gets sent back. Understanding the five points is what lets you answer that email in one line instead of a re-selection.

The five state points

Outdoor air (OA) — the design condition from the project's location and season, e.g. EN 16798-1 summer/winter design data for the city in question. Defined by dry-bulb temperature and either RH or humidity ratio.
Return air (RA) — the room condition the AHU is extracting from, typically the comfort setpoint (21–24°C, 40–60% RH per EN 16798-1 Category II) unless the project brief states otherwise.
Mixed air (MA) — outdoor and return air combined in proportion to the fresh-air fraction. A mass-weighted average of both dry-bulb and humidity ratio, not a simple average of percentages.
Coil-leaving air (post-treatment) — the state after heating, cooling, or dehumidification. For cooling coils this is bounded by the apparatus dew point (ADP) and the coil's bypass factor; for heating coils it is a dry-bulb shift at constant humidity ratio.
Supply air (SA) — the coil-leaving state plus fan heat gain (and duct heat gain/loss, usually neglected for short runs), i.e. what actually leaves the AHU casing and enters the ductwork.

Each transition between points is either a straight line of constant humidity ratio (sensible-only heating or fan heat) or a line toward the coil's ADP (cooling with latent load) — that shape is exactly what a psychrometric chart makes visible and a spec sheet does not.

A worked example

Office AHU, 30% fresh air, summer design, EN 16798-1 Category II room setpoint.

State point	Dry-bulb (°C)	RH (%)	Humidity ratio (g/kg)	Enthalpy (kJ/kg)
Outdoor air (OA)	32.0	45	14.8	70.0
Return air (RA)	24.0	50	9.3	47.7
Mixed air (MA)	26.4	—	10.9	54.3
Coil-leaving (cooling)	13.5	92	9.2	36.6
Supply air (SA, +0.8°C fan heat)	14.3	86	9.2	37.5

The mixed-air point is the 30/70 mass-weighted blend of OA and RA (not a straight average of the two dry-bulbs, which would overstate it slightly given the humidity-ratio weighting). The coil-leaving point sits close to the coil's ADP because most of the load here is latent; the gap between coil-leaving and mixed-air humidity ratio (1.7 g/kg removed) is the number that sets the condensate rate on the data sheet. Fan heat gain — a function of the fan's static pressure and efficiency — nudges the final two points up in dry-bulb without changing humidity ratio at all.

What a consultant actually checks

Mixed-air sanity: does MA fall on the straight line between OA and RA at the stated fresh-air ratio? A wrong fresh-air percentage on the AHU schedule shows up here first.
ADP feasibility: is the implied apparatus dew point achievable by the coil's row count and fin pitch, or does the selection imply an unrealistically "dry" coil? This is the same row/fin-pitch trade-off covered in heating coil selection — the cooling-coil case follows the same physics in reverse.
Fan heat gain accounted for: SA dry-bulb should be measurably above coil-leaving dry-bulb once fan and duct gains are added — a data sheet that shows them equal usually means the fan heat step was skipped.
Condensate rate consistency: (humidity ratio in − humidity ratio out) × mass flow should match the condensate figure quoted elsewhere on the sheet.

Where the tooling matters

AirSelect3D carries all five state points through the same calculation chain that sizes the coil and picks the fan — mixed air, ADP, condensate, and fan heat gain are computed from the same catalog data that drives face velocity and casing sizing, not re-derived separately for the data sheet. When a consultant asks for the psychrometric chart behind a quote, the chain is already there to hand over.

See the full psychrometric chain behind every AHU selection →