Calculator Inputs
Formula Used
W = 0.62198 × Pv / (P − Pv)
h = 0.24 × T + W × (1061 + 0.444 × T)
Qs = 1.08 × CFM × (Tentering − Tleaving)
Ql = 4840 × CFM × (Wentering − Wleaving)
Qt = 4.5 × CFM × (hentering − hleaving)
Tons = Qt / 12000
Moisture Removal = 270 × CFM × (Wentering − Wleaving)
These formulas estimate air-side evaporator performance for planning and comparison. Final equipment selection should also consider refrigerant conditions, coil rows, fin spacing, face velocity, and manufacturer data.
How to Use This Calculator
- Enter the supply airflow in CFM.
- Provide entering and leaving dry-bulb temperatures.
- Enter the entering and leaving relative humidity values.
- Adjust barometric pressure if the project elevation differs from standard conditions.
- Input coil width and height to estimate face area and face velocity.
- Set a design margin for recommended installed capacity.
- Click Calculate Capacity to see total, sensible, and latent load results.
- Use the CSV button for spreadsheets and the PDF button for a printable report.
Example Data Table
| Example Item | Value |
|---|---|
| Airflow | 1,800 CFM |
| Entering Dry-Bulb | 78 °F |
| Leaving Dry-Bulb | 58 °F |
| Entering Relative Humidity | 55 % |
| Leaving Relative Humidity | 92 % |
| Barometric Pressure | 101.325 kPa |
| Coil Size | 3.0 ft × 4.0 ft |
| Design Margin | 10 % |
| Estimated Total Capacity | 55,696.23 Btu/hr |
| Estimated Capacity | 4.641 tons |
| Recommended Design Capacity | 61,265.85 Btu/hr |
| Face Velocity | 150.00 fpm |
Frequently Asked Questions
1) What does this evaporator capacity calculator estimate?
It estimates sensible, latent, and total air-side cooling capacity using airflow, dry-bulb temperatures, humidity, and coil dimensions. It also reports tons, moisture removal, and face velocity for quick construction planning decisions.
2) Why are entering and leaving humidity values important?
Humidity changes drive latent load. When moisture is removed from the air, the evaporator does additional work beyond simple temperature reduction. Ignoring humidity can understate actual cooling capacity requirements.
3) What is the difference between sensible and latent capacity?
Sensible capacity reduces air temperature. Latent capacity removes moisture from the airstream. Total capacity combines both effects and better reflects real evaporator duty in occupied or humid environments.
4) Why does the calculator ask for barometric pressure?
Pressure affects psychrometric properties. At higher elevations, air behaves differently, so humidity ratio and enthalpy shift. Including pressure improves estimate quality for nonstandard project locations.
5) What does face velocity tell me?
Face velocity is airflow divided by coil face area. It helps you review whether air moves too quickly across the coil, which can affect pressure drop, moisture carryover, and coil performance.
6) Why add a design margin?
A design margin provides a practical allowance for field variation, fouling, measurement uncertainty, and future drift. It helps convert calculated load into a more realistic target for selection.
7) Can I use this for final equipment selection?
Use it for early sizing, budgeting, and comparison. Final selection should still be checked against manufacturer coil data, refrigerant conditions, fan performance, and full psychrometric design criteria.
8) Why might my latent load calculate near zero?
That usually means entering and leaving humidity ratios are nearly equal. In that case, the coil is cooling mainly sensibly, with little dehumidification occurring under the entered conditions.