Psychrometric Chart & Calculator

Psychrometric Chart & Interactive Calculator

1Location & Site Conditions

Location Elevation

—

Atm. Press.

—

APinHg

—

Atm. pressure derived from elevation: 14.696 × (1 − 0.0000068753 × Elev)^5.2559 psia

2Entering & Leaving Air Properties

HumRat · RelHum · DewPoint functions live

Entering

Tdb₁ Dry bulb temp

°F

Twb₁ Wet bulb temp

°F

Humidity ratio HRatio₁

—

lbw/lba

Grains of moisture

—

gr/lb

Relative humidity

—

Specific heat SpHt₁

—

Btu/lb·°F

Enthalpy Enal₁

—

Btu/lb

Specific volume SpVol₁

—

ft³/lb

Dew point DewPt₁

—

°F

Water per cu.ft. lbpCuFt

—

lbw/ft³

Leaving

Tdb₂ Dry bulb temp

°F

Twb₂ Wet bulb temp

°F

Humidity ratio HRatio₂

—

lbw/lba

Grains of moisture

—

gr/lb

Relative humidity

—

Specific heat SpHt₂

—

Btu/lb·°F

Enthalpy Enal₂

—

Btu/lb

Specific volume SpVol₂

—

ft³/lb

Dew point DewPt₂

—

°F

Water per cu.ft. lbpCuFt

—

lbw/ft³

3Capacity Calculation — Known Airflow

Total / Sensible / Latent

CFM Input Tons

—

Total Capacity

—BTU/h

cfm × ΔH × 4.5 (enthalpy method)

Sensible Capacity

—BTU/h

cfm × ΔTdb × 1.085

Latent Capacity

—BTU/h

Total − Sensible

Sensible Heat Ratio (SHR = Sensible ÷ Total): —

4Sensible Capacity & 5Coil Face Velocity

Coil sizing & sensible load

Sensible Capacity (Δ Dry Bulb)

Total CFM (from Sec. 3)

—

cfm

Inlet DB temp (Tdb₁)

—

°F

Outlet DB temp (Tdb₂)

—

°F

ΔT (Tdb₁ − Tdb₂)

—

°F

Sensible BTU/h

—BTU/h

BTU/h = 1.085 × cfm × ΔT

Coil Face Velocity

Coil height

Coil width

Face area (H × W ÷ 144)

—

ft²

CFM (from Sec. 3)

—

cfm

Face Velocity

—FPM

550 FPM maximum

Reference Chart

Linric Company psychrometric chart — click to enlarge

6Gas Furnace CFM

Temperature rise (ΔT)

°F

Furnace output

BTU/h

ΔT × 1.085 constant

—

Required CFM

—

cfm

cfm = BTU/h ÷ (1.085 × ΔT)

7Condensate — Grains to Gallons

Entering grains

—

gr/lb

Leaving grains

—

gr/lb

Grains difference

—

gr/lb

lbs water (÷ 7000)

—

Gallons (÷ 8.33)

—

gal

7000 gr/lb · 8.33 lb/gal

8Pulley Sizing

Motor pulley dia.

Blower pulley dia.

Motor RPM

rpm

Current airflow

cfm

Desired airflow

cfm

Blower pulley RPM

—

rpm

Needed RPM to reach CFM

—

rpm

Suggested motor pulley dia.

—

Suggested blower pulley dia.

—

Adjust motor dia.

Adjust blower dia.

Adjusted CFM (motor-side)

—

cfm

Adjusted CFM (blower-side)

—

cfm

Blower RPM = Motor dia × Motor RPM / Blower dia · Red flag if adjusted values exceed desired airflow

9Mixed Air — CFM Weighted

Airflow

Outside air

cfm

Return air

cfm

Total supply

cfm

Summer Design — Outdoor

Outdoor DB

°F

Outdoor WB

°F

Winter Design — Outdoor

Outdoor DB

°F

Outdoor WB

°F

Return Air

Return DB

°F

Return WB

°F

Summer Mixed Air

Mixed DB

—

°F

Mixed WB

—

°F

Winter Mixed Air

Mixed DB

—

°F

Mixed WB

—

°F

Enter any two of Outside Air / Return Air / Total Supply — the third fills in automatically (Supply = OA + RA). Mixed = (Outdoor × OA cfm + Return × RA cfm) ÷ Total cfm · DB and WB each weighted by airflow.

10DOAS — Sole-Source Heating & Cooling

Winter

Desired space DB temp

°F

DOAS leaving air temp

°F

CFM

cfm

Effective heat capacity

—

BTU/h

Summer

Desired space DB (Tdb₁)

—

°F

DOAS DB LAT

°F

CFM (sensible)

cfm

Effective sensible capacity

—

BTU/h

Space WB (Twb₁)

—

°F

WB LAT of DOAS (Twb₂)

—

°F

Space enthalpy (H₁)

—

Btu/lb

Leaving enthalpy (H₂)

—

Btu/lb

CFM (total)

cfm

Total effective capacity

—

BTU/h

Latent (Total − Sensible)

—

BTU/h

11Formula Reference & Notes

Key variables:

Q = Air volumetric flow (cfm)
H = Enthalpy (BTU/lb)
T = Dry bulb temperature (°F)
W = Humidity ratio (lb water / lb dry air)

Primary Capacity Formulas

Total BTU/h = cfm × (H₁ − H₂) × 4.5

Sensible BTU/h = cfm × (Tdb₁ − Tdb₂) × 1.085

Latent BTU/h = Total BTU/h − Sensible BTU/h

Back-Calculating Leaving Air

To find an unknown leaving-air dry bulb when the sensible capacity is known (from the equipment tech guide capacity table at a given entering condition):

Tdb₂ = Tdb₁ − (Sensible BTU/h ÷ (cfm × 1.085))

To find an unknown leaving-air enthalpy (then read wet bulb off the chart):

H₂ = H₁ − (Total BTU/h ÷ (cfm × 4.5))

Elevation & Atmospheric Pressure

Atm. Press. (psia) = 14.696 × (1 − 0.0000068753 × Elev)^5.2559

APinHg = AtmPress × 2.03602

Psychrometric Functions (from workbook VBA)

The HumRat, RelHum, and DewPoint macros in the original workbook are ASHRAE-based. Saturation vapor pressure uses the Hyland-Wexler polynomial over Rankine temperature, with constants c8…c13. This page runs an exact JavaScript port so the numbers match the spreadsheet cell-for-cell.

Rules of Thumb

Face velocity should not exceed 550 FPM through a wet cooling coil to avoid moisture carryover.
The 1.085 sensible constant = 0.24 (Cp of air) × 60 (min/hr) × 0.075 (lb/ft³ at sea level) ≈ 1.08. This tool uses 1.085 throughout for consistency with modern ASHRAE practice; older references use 1.08 or 1.1.
Water conversion: 7000 grains per pound, 8.33 pounds per gallon.