Relative humidity question (cooling the air in a room)

In summary, the individual is asking for help in calculating the relative humidity at the end of a cooling process where the surrounding temperature is 30 degrees Celsius and the relative humidity is 30%. They are using an air conditioner that cools the surrounding to 15 degrees Celsius and at the end of the process, the temperature is 25 degrees Celsius with no condensation. They are looking for a way to calculate the final relative humidity, which requires a psycrometric chart and may vary depending on elevation.
  • #1
Hello everyone,
I have a question about calculating Relative humidity.
If I have a surrounding with 30 degrees Celsius and RH of 30%.
I am cooling the surrounding with an air conditioner that works at 15 degrees Celcius.
At the end of the process, the room reaches 25 degrees Celcius.
In this situation, there is no condensation in the process.
How can I calculate what is the RH at the end of the process?
Thank you very much
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  • #2
Are you sure there is no condensation on the evaporator of the machine?
  • #3
The equilibrium vapor pressure of water at 30 C is 4250 Pa, and, for 30% RH, that means that the partial pressure is 1275 Pa. At 15 C, the equilibrium vapor pressure is 1700 Pa, so no water will condense. At 25 C, the equilibrium vapor pressure is 2340 Pa. So, with a partial pressure of 1275 Pa, what will the RH be at 25 C?
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  • #4
What you need is known as a "psycrometric chart." The answer will vary somewhat based on your elevation, but there are charts for many different elevations and some correction factors for stuff inbetween. Long story short, without removing any water from the air (looks about 8g H2O/kg dry air), at sea level your final humidity is around 40%.

Note: Although the cool air coming from the AC might be at 15 degrees, as Lnewqban points out, the temperature of your AC evaporator could be much less, and if the coils are less than about 10 degrees, you will be getting some condensation out of the air.
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1. What is relative humidity and how does it affect the air in a room?

Relative humidity is a measure of the amount of water vapor present in the air compared to the maximum amount of water vapor that can be held at a specific temperature. This can affect the air in a room by making it feel either dry or humid, which can impact comfort levels and even health.

2. How can I decrease relative humidity in a room?

One way to decrease relative humidity in a room is by using a dehumidifier. This machine sucks in the air and removes excess moisture, reducing the overall relative humidity in the room. Another way is by increasing ventilation, either by opening windows or using fans to circulate the air.

3. Will cooling the air in a room decrease relative humidity?

Yes, cooling the air in a room can decrease relative humidity. As the temperature drops, the air is less able to hold moisture, causing the relative humidity to decrease. This is why the air can feel drier in the winter when the temperature is lower.

4. Can high relative humidity in a room cause any health issues?

High relative humidity in a room can contribute to the growth of mold and bacteria, which can cause respiratory issues, allergies, and other health problems. It can also make the air feel sticky and uncomfortable, leading to difficulty breathing for some individuals.

5. How can I measure the relative humidity in a room?

There are several ways to measure relative humidity in a room. One option is to use a hygrometer, a device specifically designed to measure humidity levels. Another option is to use a psychrometer, which measures both temperature and humidity. You can also use a weather app or website that provides current humidity levels for your location.

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