A dehumidifier is a machine that removes water vapor from indoor air to make a room feel drier and more comfortable. It matters because high humidity can encourage mold growth, damage materials, and make cooling systems work harder. Most household dehumidifiers use refrigeration, so they are closely related to air conditioners.
The key engineering idea is to move moist air across a cold surface until water vapor condenses into liquid water.
Key Facts
- Relative humidity = actual water vapor in air / maximum possible water vapor at that temperature x 100%
- Condensation happens when air is cooled below its dew point.
- Heat removed from air at the cold coil is partly released back at the warm coil.
- Latent heat is the energy released when water vapor changes into liquid water.
- Water collected per day depends on humidity, air temperature, airflow rate, and coil temperature.
- Electrical power use can be estimated with E = Pt, where E is energy, P is power, and t is time.
Vocabulary
- Relative humidity
- Relative humidity is the percentage of water vapor in air compared with the maximum amount the air could hold at the same temperature.
- Dew point
- The dew point is the temperature at which air becomes saturated and water vapor begins to condense.
- Evaporator coil
- The evaporator coil is the cold coil that cools incoming moist air and causes water vapor to condense.
- Condenser coil
- The condenser coil is the warm coil that releases heat back into the air after moisture has been removed.
- Compressor
- The compressor is the pump that raises the pressure and temperature of the refrigerant so heat can be moved through the system.
Common Mistakes to Avoid
- Thinking a dehumidifier destroys water vapor, which is wrong because it changes water vapor into liquid water and collects it in a tank or drain.
- Ignoring air temperature, which is wrong because warm air can hold more water vapor and cold coils must be below the dew point for condensation to occur.
- Assuming lower relative humidity always means less water in the air, which is wrong because relative humidity depends on temperature as well as water vapor content.
- Blocking the intake or exhaust vents, which is wrong because reduced airflow lowers the amount of moist air reaching the coils and can make the unit less efficient.
Practice Questions
- 1 A dehumidifier is rated at 500 W and runs for 6.0 h. How much electrical energy does it use in kWh?
- 2 A unit collects 9.0 L of water in 12 h. If it keeps collecting at the same rate, how many liters will it collect in 24 h?
- 3 Explain why water droplets form on the cold coil of a dehumidifier but not usually on the warm coil.