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.
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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
Welcome!
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?

Relative humidity is a measure of the amount of water vapor present in the air compared to the maximum amount of water vapor the air can hold at a given temperature. It is expressed as a percentage, with higher percentages indicating higher levels of water vapor in the air.

2. How does cooling the air affect relative humidity?

Cooling the air decreases its ability to hold water vapor, causing the relative humidity to increase. This is because cooler air can hold less water vapor than warmer air. For example, if the relative humidity in a room is 50% at a temperature of 80°F, cooling the air to 70°F would cause the relative humidity to increase to approximately 65%.

3. Why is it important to maintain a comfortable relative humidity level in a room?

A comfortable relative humidity level is important for both health and comfort. High humidity can lead to mold growth, while low humidity can cause dryness and irritation of the skin, eyes, and respiratory system. It is recommended to maintain a relative humidity level between 30-60% for optimal comfort and health.

4. How can I lower the relative humidity in a room?

To lower the relative humidity in a room, you can use a dehumidifier or air conditioner. These devices remove excess moisture from the air, reducing the relative humidity. You can also improve ventilation in the room by opening windows or using exhaust fans to allow moisture to escape.

5. Can I control the relative humidity in a room using a humidifier?

Yes, a humidifier can be used to increase the relative humidity in a room. However, it is important to monitor the humidity level and not let it get too high, as this can lead to mold growth and other issues. It is recommended to use a hygrometer to measure the relative humidity and adjust the humidifier accordingly.

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