How a greenhouse cools with wet pads

Big exhaust fans at one end of the greenhouse pull outside air through a wall of wet, corrugated pad material at the other end. As the air squeezes through, water evaporates into it and soaks up heat, so the air comes out cooler, though never cooler than the outdoor wet bulb temperature.

Controls

Labels

Animation speed

Outside air temperature (dry bulb)

Outside wet bulb temperature

Lower wet bulb means drier air and more cooling.

Pad efficiency

Step through it

The pad wallCorrugated cellulose pads fill one end wall. Their folded channels give a huge wet surface for air to pass over.
Keeping it wetA pipe along the top trickles water down the pads. A gutter and sump catch the excess and a pump sends it back up.
Fans do the pullingExhaust fans at the far end create a slight vacuum, so all incoming air must come through the pads. A common target is about one air change per minute.
Humid dayWhen the wet bulb climbs close to the air temperature, there is little room for evaporation and the pads can only cool a few degrees.
Across the houseThe cooled air picks up heat from sun and plants on its way to the fans, so the fan end runs several degrees warmer than the pad end.

The pad formula

Evaporative cooling can at best bring air down to its wet bulb temperature. Pad efficiency says how much of that gap a pad closes: leaving temperature = dry bulb minus efficiency x (dry bulb minus wet bulb).

A well kept system can reach about 85 percent efficiency. Clogged, dry or poorly sized pads fall well below that.

Try it: keep the air at 35 °C and move the wet bulb from 18 to 29 °C. On the dry day the pad cools air by more than 13 degrees; on the humid day, by less than 5. Then drop the efficiency to 50 percent to see what a neglected pad costs.

Why the fan end is warmer

Sunlight and the crop keep adding heat as air crosses the house. The third readout adds a typical 4 °C rise between pads and fans, a rule of thumb rather than a calculation for your house. Keeping the pad to fan distance short keeps that rise small.

Where Avid uses this

Avid's founder began his career as an HVAC apprentice at 16, and the same psychrometrics behind building cooling drive greenhouse pad and fan design.

Sources

Fan and Pad Greenhouse Evaporative Cooling Systems (CIR1135)University of Florida IFAS Extension
Summer Cooling SystemsUniversity of Arkansas Greenhouse Management
Energy Efficiency in Greenhouse Evaporative Cooling Techniques: Cooling Boxes versus Cellulose PadsEnergies, MDPI (open access)

How to cite this model

Avid Solutions International. (2026, November 11). How a greenhouse cools with wet pads [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/greenhouse-pad-fan

Last updated November 11, 2026

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