Coil Leaving Air Calculator

Cooling coil leaving-air conditions from entering air, capacity, and SHR, with apparatus dew point.

Coil Inputs

Typical DX: 0.70–0.80; CHW: 0.65–0.85

Leaving Air Conditions

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Leaving Dry Bulb (°F)
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Leaving W (gr/lb)
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Sensible Cap (tons)
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Latent Cap (tons)
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Temp Drop ΔT (°F)
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Leaving Enthalpy (BTU/lb)
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Leaving RH (%)
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Leaving Wet Bulb (°F)
Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This coil leaving air calculator predicts the dry-bulb temperature and humidity of air leaving a cooling coil from the entering condition, airflow, total capacity in tons, and sensible heat ratio. It also splits the load into sensible and latent tons, reports the temperature drop, leaving enthalpy, RH, and wet bulb, finds the apparatus dew point and bypass factor, and is altitude-corrected for the lower air density at elevation.

Leaving air conditions set the supply temperature your diffusers deliver and the moisture the coil removes, so they drive both comfort and dehumidification. The tool flags leaving temperatures low enough to risk coil frost and high enough to suggest the coil or airflow is mismatched, helping you sanity-check a selection before specifying equipment.

Formula & Method

Load splitQsens = tons × 12000 × SHR,   Qlat = tons × 12000 × (1 − SHR)
Air mass flowṁ = 60 · CFM ÷ vin,   vin = 0.370486 · (EAT + 459.67) · (1 + 1.607858 · Win) ÷ P
Sensible ΔTΔT = Qsens ÷ [ṁ · (0.240 + 0.444 · Win)],   LAT = EAT − ΔT
Latent ΔWΔW = Qlat ÷ [ṁ · (1061 + 0.444 · LAT)]  [lb/lb]
PressureP = 14.696 · (1 − 6.8754×10−6 · Z)5.2559 psia, Z = elevation (ft)

Total capacity is split by SHR. The airflow is treated as actual CFM at the entering condition and converted to a dry-air mass flow; the sensible part sets the temperature drop and the latent part the humidity-ratio drop, so the leaving enthalpy is exactly hin − Qtotal ÷ ṁ. At standard air (v = 13.33 ft³/lb, W ≈ 0) the sensible relation reduces to the familiar Qsens = 1.08 · CFM · ΔT; using the real entering density keeps the result consistent with the AIM Works cooling coil calculator and correct at altitude. Humidity ratio, enthalpy, RH, and wet bulb use the ASHRAE Handbook—Fundamentals (2021) Chapter 1 moist-air equations at the barometric pressure for your elevation. The apparatus dew point (ADP) is where the entering-to-leaving process line, extended, meets the saturation curve, and the bypass factor is (LAT − ADP) ÷ (EAT − ADP). A leaving state above 100 % RH, or a line with no ADP, cannot be produced by a cooling coil and is flagged rather than reported as normal. This checks a selection; it is not a coil rating, so confirm with the manufacturer’s selection software.

Frequently Asked Questions

How is leaving air temperature calculated?
The tool splits total coil capacity into sensible and latent parts using the sensible heat ratio. It converts the airflow to a dry-air mass flow using the specific volume of the entering air, then divides the sensible BTU per hour by mass flow times the moist-air specific heat (0.240 + 0.444 W) to get the temperature drop, and the latent BTU per hour by mass flow times the latent heat (1061 + 0.444 T) to get the humidity-ratio drop. At standard air this is the familiar 1.08 × CFM × ΔT; at 80 °F entering air the effective factor is about 1.06, so the drop reads slightly larger than the 1.08 rule.
What is sensible heat ratio (SHR)?
Sensible heat ratio is the fraction of total coil capacity that goes to lowering air temperature rather than removing moisture. SHR of 1.0 is pure sensible cooling, and lower values mean more dehumidification. Typical direct-expansion coils run about 0.70 to 0.80, and chilled-water coils about 0.65 to 0.85. A lower SHR drives a colder, drier supply air condition.
Does altitude affect the result?
Yes. The tool computes barometric pressure from elevation with the ASHRAE standard atmosphere, then uses it for the entering humidity ratio and the air specific volume. Thinner air at altitude means the same CFM carries less air mass, so the same coil capacity produces a larger temperature and humidity drop, and the entering air holds more moisture per pound at the same RH.
Why does the tool warn about temperatures below 45 degrees?
A leaving dry bulb below about 45 degrees Fahrenheit suggests the coil surface may drop near or below freezing, risking frost or ice on a direct-expansion coil. The tool flags that case, and also warns when leaving temperature is above 65 degrees, which usually means the coil capacity or airflow is mismatched. Normal cooling-coil leaving air is roughly 50 to 60 degrees Fahrenheit. It also rejects impossible results: a leaving state above 100 % RH, or a process line that never reaches the saturation curve (no apparatus dew point), means no real cooling coil can deliver that capacity and SHR at that airflow.

Related Tools

Psychrometric Calculator Enthalpy Calculator Humidity Ratio Dew Point Calculator Wet Bulb / RH
AIM Works · launches Oct 15, 2026

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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.