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Evaporator Sizing Calculator

Work out evaporator sizing instantly with clear inputs, formula shown and shareable results.

About Evaporator Sizing Calculator

Coil area follows the basic heat exchanger relation Q = U x A x ΔTm, so the surface needed is the load divided by the product of the coefficient and the mean temperature difference. Face area is a separate constraint set by airflow and the velocity above which condensate is blown off the fins, usually about 500 fpm.

Formulas

Surface area

A = Q / (U x ΔTm)

Face area

Face ft² = airflow CFM / face velocity fpm, airflow = tons x 400

Worked example

With the default values below, Evaporator Sizing Calculator returns Coil surface area required of 72.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Cooling loadminimum 1 BTU/h
36,000 BTU/h
Coil overall heat transfer coefficientminimum 1 BTU/h·ft²·°F
25 BTU/h·ft²·°F
Mean air-to-refrigerant differenceminimum 1 °F
20 °F
Design coil face velocityminimum 100 fpm
500 fpm

Results

Coil surface area required
72.0 ft²
Load in tons
3.00 tons
Airflow at 400 CFM per ton
1,200 CFM
Coil face area
2.40 ft²

Step by step

  1. 1Values used
  2. 2Surface area
  3. 3Face area
  4. 4Coil surface area required= 72.0 ft²
  5. 5Load in tons= 3.00 tons
  6. 6Airflow at 400 CFM per ton= 1,200 CFM
  7. 7Coil face area= 2.40 ft²

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Evaporator Sizing figure worked out?

A = Q / (U x ΔTm). Coil area follows the basic heat exchanger relation Q = U x A x ΔTm, so the surface needed is the load divided by the product of the coefficient and the mean temperature difference.

Why cap face velocity at 500 fpm?

Above roughly 550 fpm on a wet coil, condensate is carried off the fins into the duct, causing water damage and microbial growth downstream.

What values do I need to enter?

This calculator takes 4 inputs: Cooling load, Coil overall heat transfer coefficient, Mean air-to-refrigerant difference, Design coil face velocity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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