Wet bulb is the ceiling, and it moves
Evaporation cools by taking energy out of the air to turn liquid water into vapour. The process runs until the air can hold no more, and the temperature it lands on at that point is the wet bulb temperature. No pad, no fan and no amount of water gets below it. The gap between the dry bulb and the wet bulb is the entire budget, and everything else is a question of what fraction of that budget the hardware captures.
At 95 degrees and 30 percent relative humidity the wet bulb is 71.9 degrees, so there is 23.1 degrees on the table. An 85 percent efficient pad delivers 19.7 of them and the air leaves at 75.3. Hold the temperature at 95 and raise the humidity to 70 percent and the wet bulb climbs to 86.5; now there are 8.5 degrees available and the same pad delivers 7.3. Same equipment, same electricity, a third of the result.
The calculator gets the wet bulb from an empirical fit rather than by iterating a psychrometric chart. It is accurate to a few tenths of a degree over the ordinary range of greenhouse conditions and it will not be the largest error in your calculation. The humidity reading will be, especially if it was taken at the wrong time of day.
Where the water goes
Sensible heat and latent heat trade one for one across a pad. The heat that leaves the air stream is exactly the heat that goes into evaporating water, so the water use follows directly from the temperature drop: 1.08 times the airflow times the drop gives BTU per hour, and dividing by about 1,061 BTU per pound gives pounds of water an hour.
On 10,000 CFM and a 19.7 degree drop that is 212,432 BTU an hour, 200.2 pounds an hour, 24.0 gallons an hour. Eight hours a day for ninety days is over 17,000 gallons before any bleed. On a well, that is a number worth checking against the recovery rate before the pad wall is built rather than after.
| Outside air | Wet bulb | Drop at 85% pad | Air leaving | Water at 10,000 CFM |
|---|---|---|---|---|
| 95°F, 20% RH | 66.7°F | 24.0°F | 71.0°F | 29.3 gal/hr |
| 95°F, 30% RH | 71.9°F | 19.7°F | 75.3°F | 24.0 gal/hr |
| 95°F, 50% RH | 79.9°F | 12.9°F | 82.1°F | 15.7 gal/hr |
| 95°F, 70% RH | 86.5°F | 7.3°F | 87.7°F | 8.9 gal/hr |
Notice that the water use falls with humidity at exactly the same rate as the cooling does. Evaporative cooling in a humid climate does not waste water; it simply does very little, and the water bill reports that honestly.
The bleed, and why skipping it ruins the pad
Only pure water leaves as vapour. Everything dissolved in the supply — calcium, magnesium, whatever the well brings up — stays in the sump and concentrates. Cycles of concentration is the ratio of what is in the sump to what is in the supply, and holding it at a chosen value requires bleeding off a fraction of the flow continuously. The relation is simple: bleed equals evaporation divided by cycles minus one. Four cycles means bleeding a third of what evaporates. Two cycles means bleeding as much as evaporates, doubling the water bill.
How many cycles your water tolerates is a question for your water report and the pad supplier, not for a web page. Hard water forces a low number. What is not negotiable is that the bleed exists: a sump run without one will scale the pad face, block the flutes unevenly, and turn an 85 percent pad into a 50 percent pad over a season without any visible moment where it happened.
Fitting it to the rest of the house
The airflow number this page starts from should come from the exhaust fan calculator, because pad and fan is one system: the fans pull the air, the pad conditions it on the way in, and the pad adds static pressure the fans have to overcome. Shade cloth reduces the airflow, which reduces the pad area and the water at the same time, so the shading decision comes first.
The other thing a pad changes is humidity, and a house running at 80 percent relative humidity all afternoon behaves differently from a dry one. The vapour pressure deficit calculator turns that into the number that actually describes what a leaf experiences, and it is a more useful reading than relative humidity once a pad wall is running.
Questions people ask
Why can a pad not cool below the wet bulb?
Because the process is adiabatic — the energy to evaporate water comes from the air itself, so as the air cools it also gets wetter, and it reaches saturation at a temperature that is fixed by its starting condition. The wet bulb temperature is the definition of that endpoint. Anything colder requires energy from somewhere other than the air stream, which is what a refrigeration cycle provides and an evaporative pad does not. A pad advertised as reaching some fixed number of degrees below ambient is quoting one particular weather condition.
What relative humidity should I enter?
The one measured at the same instant as the dry bulb, on the sort of afternoon you are sizing for. This trips people up because humidity is highest before dawn and lowest in mid-afternoon, and a daily average or a morning reading makes evaporative cooling look far worse than it will be. If you have an inexpensive logger, a week of readings through the hottest part of a few days is worth more than any published figure, because the afternoon depression at your site is what the pad lives on.
Is a pad wall worth it in a humid climate?
Usually not as the main cooling strategy, and the wet bulb depression tells you that before any money is spent. Where afternoon depressions run under about eight degrees, a pad delivers a few degrees at the cost of raising the humidity to near saturation, which is its own problem for the crop and for disease. The order of operations in a humid climate is shade first, then maximum ventilation, then think about whether the remaining gap justifies the water and the maintenance. The calculator flags a small depression for exactly this reason.
How often does the pad need replacing?
That depends on water chemistry, on whether the bleed was actually running, and on whether the pad was allowed to dry out with minerals sitting in it, so it is a question for the manufacturer of the material you have rather than for arithmetic. What the arithmetic can tell you is the load: at four cycles of concentration the sump is running at four times the supply mineral content continuously, and every gallon of the season total passed through that pad face. Getting the bleed right is the cheapest maintenance decision available.
Does the calculator account for the pad adding resistance to the fans?
No, and this matters. A wetted pad is a significant static pressure the fans have to work against, and a fan chosen from its free-air rating will move considerably less through a pad wall than the label suggests. The pad manufacturer publishes a pressure drop for each thickness at each face velocity, and the fan manufacturer publishes a performance curve; the two have to be read together. That is also the reason face velocity is a field here rather than a constant — running a pad faster to save on area costs efficiency and fan power at the same time.