Example Data Table
| Case | Dry Bulb | Humidity | Airflow | Effectiveness | Expected Use |
|---|---|---|---|---|---|
| Dry desert room | 102 °F | 15% | 4200 CFM | 80% | High cooling output |
| Mixed climate shop | 94 °F | 35% | 3000 CFM | 72% | Moderate cooling output |
| Humid evening patio | 88 °F | 65% | 2500 CFM | 70% | Limited cooling output |
Formula Used
The calculator first estimates wet bulb temperature from dry bulb temperature and relative humidity. It then applies cooler effectiveness.
Supply Temperature = Dry Bulb - Effectiveness × (Dry Bulb - Wet Bulb)
Adjusted CFM = Rated CFM × (1 - Duct Loss) × Pad Condition Factor
Gross BTU/hr = 1.08 × Adjusted CFM × Temperature Drop × Density Factor
Net BTU/hr = Gross BTU/hr - Fan Watts × 3.412
Moisture gain is estimated from humidity ratio and constant enthalpy movement. Water use is converted from pounds per hour to gallons per hour.
How To Use This Calculator
- Enter outdoor dry bulb temperature in degrees Fahrenheit.
- Enter the relative humidity near the cooler inlet.
- Add rated or measured airflow in CFM.
- Set cooler effectiveness based on media condition.
- Add altitude, fan watts, duct losses, and room size.
- Press calculate and read the result above the form.
- Use CSV or PDF buttons to save the result.
Understanding Evaporative Cooling BTU
Evaporative cooling works by moving air through wet media. Water absorbs heat as it changes into vapor. The leaving air becomes cooler, but its moisture content rises. This calculator estimates the cooling effect in BTU per hour. It uses dry bulb temperature, relative humidity, airflow, and cooler effectiveness. These inputs show how close the unit can move toward the wet bulb limit.
Why Temperature And Humidity Matter
Dry air can accept more water vapor. That gives strong cooling. Humid air is already closer to saturation. That gives weak cooling. A hot and dry afternoon may deliver a large temperature drop. A warm and sticky evening may deliver a small drop. This is why the same cooler can feel powerful one day and poor the next. The wet bulb temperature explains that change clearly.
Reading The Capacity Result
The BTU result is based on sensible air cooling. It uses airflow and the predicted temperature drop. The altitude correction reduces capacity when air is thinner. The fan heat option subtracts motor heat from useful cooling. The calculator also estimates tons of cooling, water use, air changes, and daily BTU output. These values help compare coolers, rooms, ducts, and schedules.
Practical Design Notes
Evaporative coolers need air relief. Open windows, vents, or exhaust paths are important. Without relief, humidity rises and comfort drops. Airflow also falls when ducts, pads, or grilles are restrictive. Use measured airflow when possible. Rated airflow can be higher than real installed airflow. A clean pad, correct pump flow, and balanced openings improve performance.
Comfort And Safety Limits
A low supply temperature does not always mean good comfort. High indoor humidity can feel heavy. It can also affect materials, stored goods, and electronics. In homes, many users prefer moderate indoor humidity. In workshops or greenhouses, higher moisture may be acceptable. Always compare the supply humidity estimate with the space use. The tool gives guidance, not a certified design.
Better Inputs Give Better Answers
Use local weather readings near the cooler inlet. Avoid sun heated wall readings. Enter actual fan speed airflow when known. Set effectiveness based on pad type and condition. Rigid media often performs better than thin fiber pads. Very old pads may perform much worse. Update the values after maintenance, seasonal changes, or duct adjustments.
Common Mistakes To Avoid
Do not size a cooler from floor area alone. Weather and ventilation control the result. Do not ignore pressure losses through long duct runs. Do not assume colder supply air means lower room humidity. Direct evaporative systems add water to the space. Also avoid using average monthly humidity for peak design. Use the worst expected hour instead. That choice gives a safer BTU estimate for hot days and crowded rooms. Record each assumption beside the result. This makes later checks easier. It helps explain field readings during testing. Small notes prevent wrong upgrades later.
FAQs
What does this evaporative cooling calculator measure?
It estimates cooling capacity in BTU per hour. It also predicts wet bulb temperature, supply air temperature, water use, fan heat, cooling tons, and air changes per hour.
Why does humidity reduce evaporative cooling?
Humid air already contains more water vapor. It cannot absorb as much extra moisture. That reduces evaporation and limits the temperature drop across the wet media.
What is wet bulb temperature?
Wet bulb temperature is the lowest air temperature possible through evaporation under current air conditions. Direct evaporative cooling cannot cool below this limit.
What is cooler effectiveness?
Effectiveness shows how much of the dry bulb to wet bulb difference the cooler can achieve. Clean media and good water distribution raise effectiveness.
Why is altitude included?
Air becomes thinner at higher elevations. Thin air carries less heat for the same airflow volume. The altitude correction lowers the estimated BTU capacity.
Does the calculator include fan heat?
Yes. Fan and pump watts are converted to BTU per hour. That heat is subtracted from gross cooling to show a more practical net output.
Why can water use differ from the estimate?
Real water use changes with bleed rate, pump cycling, pad wetting, leaks, and mineral control. The estimate focuses on evaporation needed for cooling.
Can this replace a professional load calculation?
No. It is a planning tool. Final designs should include building heat gain, local weather data, ventilation strategy, equipment ratings, and safety margins.
What airflow value should I enter?
Measured installed airflow is best. If that is unavailable, enter rated CFM and add a realistic duct and grille loss percentage.
Why is supply humidity shown?
Direct evaporative cooling adds moisture to air. Supply humidity helps judge comfort, material risk, and whether extra ventilation is needed.
When does evaporative cooling work best?
It works best in hot, dry weather. It performs poorly when outdoor humidity is high or when the space lacks enough exhaust relief.