October 10, 2013
Have you ever wondered how moisture is removed from the air? Have you ever had problems with moisture in the air? These are the basics of how and why it happens.
Cooling air is the basic method for removing moisture from air. When warm air passes through a cooling coil, the air temperature decreases and moisture is removed from the air. Cooling systems are equipped with drainage systems to collect and take away the water condensation.
When air is too humid, it is circulated through cooling systems so that moisture is removed from the air. This process is called dehumidification. For most spaces, dehumidification is only required during the cooling season. Dehumidification and cooling can be achieved in one process.
Water can be vaporized in any temperature. For example, if an ice block is kept in a minus temperature freezer for a long time, the size of the ice gets smaller. It means the ice has vaporized and the air has the capacity to hold moisture even at minus temperatures. Warm air can hold more moisture than cold air. Therefore, by cooling the air, the capacity of air to hold the moisture reduces.
At what temperature is moisture removed from air?
When the air cools down below its dew point temperature, moisture starts to be removed (condense) from the air.
What is dew point temperature?
Dew point is the temperature that the moisture in the air starts to condense and forms liquid water. The dew point temperature and saturation temperature are the same. Saturation is a condition at which the air is not able to hold any additional moisture and its relative humidity is 100%.
How to determine dew point temperature?
Dew point temperature is determined by measuring two common parameters, dry-bulb temperature and relative humidity, and using the psychometric chart (A psychometric chart can be found at this link.
http://www.coolerado.com/wp-content/uploads/2010/04/0000Psych11x17US_SI.pdf
Dry-bulb temperature: The temperature that is measured by usual thermometers.
Relative Humidity (RH): The ratio of the measured amount of moisture in the air to the maximum amount of moisture that air can hold at the same temperature and pressure. Relative humidity is shown in percentage (%) of saturation. “30% RH” means air has 30% of the moisture that it is capable to hold at that temperature and pressure.
As air cools down, its capability to hold moisture reduces and relative humidity of air increases. When relative humidity of air is 100% the capacity of air to hold moisture is full. If air cools down more, it causes moisture to condense.
There is a challenge to control indoor humidity in humid climates. Air- conditioning system designers usually choose 50%-60% RH for indoor relative humidity during cooling season. ASHRAE Standard 62.1-2007 requires that relative humidity in indoor spaces to be kept less than 65% during cooling season.
Psychometric Chart: The ASHRAE Psychometric chart is a graphical form of the thermodynamic data of air. It helps to understand thermodynamic properties of air and the air-conditioning system process better. The following data is shown on the psychometric chart.
–Dry bulb temperature
–Wet bulb temperature
–Relative humidity
–Saturation temperature or dew point
–Enthalpy or total heat
–Humidity ratio (moisture amount)
–Specific volume
An example of the Psychometric chart application in the cooling process:
When the outdoor temperature increases during the day, the capacity of air to hold moisture increases and it absorbs more moisture. If the outdoor air is 80°F with 60% relative humidity enters a coiling coil with 50°F chilled water.
As per the Psychrometric chart, the dew point of outdoor air is 70°F. If the air temperature decreases below 70°F, the air cannot hold more moisture and the moisture starts to be removed from the air. Moisture condenses on the cooling coil as the cooling coil temperature (50°F) is less than outdoor air dew point temperature (70°F).
If the cooling coil is 100 percent efficient, all entering air contacts the coil surfaces and the air temperature after the coil is the same as the cooling coil temperature of 50°F with the relative humidity of 100%. However, coils are not 100 percent efficient, therefore the air temperature after the coil is between the coil temperature 50°F and entering air temperature 80°F. Temperature and relative humidity after the cooling coil should be measured in order to be determined on the chart. In this example, the air temperature and relative humidity after the cooling coil are 60°F and 100% respectively. Also, the air temperature and relative humidity after the supply fan are 62°F and 93% respectively. The fan motor usually increases the air temperature by 0.5°F to 2°F. As the air temperature after the fan is increased, air can hold more moisture and its relative humidity is decreased.
Moisture amount (humidity ratio) is shown on the vertical axis of the Psychrometric chart. As no moisture is added or removed from the air after the fan, the moisture amount is constant. Even when the air gets to the room temperature of 70°F, its moisture amount is still constant and its relative humidity is 71%. The same amount of moisture results in a lower percentage relative humidity at 70°F than at 62°F.
71% relative humidity is high for the room. The air temperature after the cooling coil should be cooled down to lower than 60°F till more moisture is removed from air. For example, if 40% relative humidity at 70°F is required for room conditions, the air should be cooled down below its dew point 45°F.
I hope this blog has been informative and helpful.
