Formula for Evaporative Cooloer (Swamp cooler)

In summary, the individual is trying to determine if they can grow cool weather vegetables in a hot and dry climate by using a swamp cooler to lower the temperature in a shade house. They have downloaded daily temperature and humidity data from Weather Underground and are looking for the formula behind a chart showing potential temperature drops. They have also obtained TMY3 data from NREL and are trying to analyze three years' worth of data, but are unsure if this is the most efficient approach. The person suggests using wet bulb temperature as a measure and sorting the data to find the hottest day of the year. They also mention that evaporative coolers are not perfectly efficient and may only provide air at 75% of the wet bulb temperature. The formula behind the
  • #1
Rose Gardener
I am trying to figure out if I could grow cool weather vegetable (say lettuce) in hot but dry climate by using swamp cooler to lower the temp of shade house. I have downloaded daily temp and humidity from and a chart on the potential temp drop:

The thing is that I have 3 years of data from weatherunderground on a spreadsheet, doing manual matching of data is laborious.

Does anyone know the formula behind the above chart? I think that chart assumed the airflow is perfectly matched to the room size.

Thanks for helping.
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  • #2
So, what data does Weather Underground give you...and by the way, where exactly did you get it, because I've only seen month-to-month spreadsheets on their website?

I use TMY3 data from NREL:

TMY stands for "typical meteorological year" - it's really what you want; what is typical, not just what the last 3 years were like.

Now, as it turns out, the calculation you are asking about is pretty involved and I haven't even found a good calculation for it, even though I would use it every day if I had one (I use a chart). But there are other ways to do this...

First, why are you trying to analyze 3 years' worth of data. Don't you really want a worst-case?

Evaporative coolers work by exchanging energy between sensible (what a thermometer reads) and latent (energy from evaporating water) heat. There is a measure that combines the two to find the total energy of the air. It's called "wet bulb" temperature and as you might expect, it is very similar to what an evaporative cooler does: it puts a wet "sock" on the bulb of a thermometer and finds the resulting temperature due to evaporation.

A good, professional weather site lists the wet bulb temperature. TMY3 data includes it -- not sure if weatherunderground does. But all you need to do - with either - is put the data into a spreadsheet and sort by wet bulb temperature. The highest wet bulb temperature you see is, thermodynamically, the hottest day of the year.

Now, an evaporative cooler is probably rated to come close to providing air at the wet bulb temperature, but not exactly, since it isn't perfectly efficient/effective. It looks to me like they assume 75% effectiveness, which means you get 75% of the way from the dry bulb to the wet bulb temperature. Meaning, if the dry bulb is 90 and the wet bulb is 50, you'd get 60F off your evaporative cooler (3/4 of the 40F difference).

I think the chart assumed the airflow is perfectly matched to the room size.
No, all it says is what the temperature of the air out of the evaporative cooler is: it doesn't say what that can do for a room.

Related to Formula for Evaporative Cooloer (Swamp cooler)

1. What is a swamp cooler?

A swamp cooler, also known as an evaporative cooler, is a cooling device that cools air by using the evaporation of water. It is commonly used in dry and hot climates to provide relief from the heat.

2. How does a swamp cooler work?

A swamp cooler works by pulling hot and dry air from the outside into the device through a series of wet pads. As the air passes through the wet pads, the water evaporates, causing the air to cool down. The cooled air is then blown back into the room, providing a refreshing and humidified atmosphere.

3. What is the formula for calculating the cooling capacity of a swamp cooler?

The formula for calculating the cooling capacity of a swamp cooler is: CFM x (Initial Dry Bulb Temperature - Final Dry Bulb Temperature) = BTU/h. CFM refers to the cubic feet per minute of air flow, and BTU/h stands for British Thermal Units per hour.

4. How do I determine the appropriate size of a swamp cooler for my space?

The appropriate size of a swamp cooler for a space is determined by calculating the cooling capacity needed based on the square footage of the space. As a general rule, you will need 20-25 CFM per square foot of space. You should also consider the climate and humidity level of your area, as well as the number of people and heat-generating appliances in the space.

5. Can I use a swamp cooler in any climate?

Swamp coolers are most effective in dry and hot climates, where the air is able to absorb more water and the humidity level is low. In humid climates, the cooling effect will be reduced, and the device may not be as effective. Additionally, swamp coolers require proper ventilation, so they may not be suitable for closed or tightly sealed spaces.

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