– Humidity has many different interpretations and its value changes continuously per time. The definition of “absolute humidity” is easier to understand, its calculation is small, but it is not used much in practice because it is not suitable for the change of temperature. Meanwhile, “relative humidity” is more difficult to understand, is a large value but uses a lot to calculate.

– Humidity varies with pressure and temperature. Therefore, this note should keep in mind to be ease of next discussion.

Abbreviation:

– mH2O: Mass of water vapor.

– VAir-mix: Mixed volume of air and water.

– mAir-mix: Air mass + water vapor mass = mAir + mH2O.

1.1 Absolute humidity

Expressed as the mass of water vapor per volume of air (g/m3).

AH = mH2O / VAir-mix (g / m3) or AH = mH2O / mAir-mix (%)

The disadvantage of this definition is that the absolute humidity does not change when the temperature or air pressure changes, and if the volume is fixed. This would not be suitable in chemical and physical engineering calculations that related to temperature, volume and humidity of the air.

Relating to temperature – water vapor (absolute humidity, grams of water vapor/m3 of air)

1.2 Relative humidity

The ratio of the partial pressure of water vapor to the saturated vapor pressure of water at the same temperature.

pH2O: The partial pressure of water vapor.

– p*H2O: Saturation pressure of water.

– RH = pH2O/p*H2O (%).

The more humid air, the higher relative humidity. Relative humidity depends on the temperature and pressure of the system. The same amount of water vapor results in the different relative humidity.

Relative humidity in some areas in Vietnam

100% humidity can be understood as when the relative humidity RH reaches 100%, it means that the water vapor in the air has reached the maximum level, so when adding water vapor (mass: m, unit: gram ), all this whole mass (m, gram) added will go from vapor to mist. If the temperature drops below the dew point, the clouds will turn into condensation and rain.

Relative humidity from 30 – 60% is suitable for living and health.

Health related to air temperature and humidity

Example: With clothing level = 1, metabolic rate = 1.1 and air speed 0.1m/s, the change in air temperature and average radiant temperature is from 20 – 24°C will reduce the maximum acceptable relative humidity from 100% to 65% to maintain thermal comfort conditions (https://en.wikipedia.org/wiki).

1.3 Dewpoint temperature

– Dew point, also known as the point at which water vapor becomes dew, the dew point of an air mass at a fixed atmospheric pressure, is the temperature at which the water vapor component in the air mass condenses into liquid water. . In other words, the dew point is the temperature at which the relative humidity of the air mass reaches 100%.

– When the dew point is lower than the freezing point it is called a ice freezing point, instead of creating dew, it will create frost or hoarfrost due to ice freezing

The dew point does not depend on the presence of oxygen, nitrogen or gases beyond water vapor in the air. The formation of dew can occur at the dew point even if the only gas that exists in the air mixture is water vapor.

When the temperature is higher than the dew point, the partial pressure of the water vapor increases and the water will be vapor into the air.

(Nguồn: Wiki Điểm sương)

The dew point determines the relative humidity. When the relative humidity is high, the dew point is close to the current temperature of the air. If the relative humidity is 100%, the dew point will be equal to or higher than the air temperature at that time. If the relative humidity decreases, the dew point will be lower than the temperature of that air volume.

– Principle to calculate the dew point temperature in a simple and quite accurate when the relative humidity RH > 50% by the formula of scientist Lawrence.

Td = T – [(100 – RH)/5]

Where:

+ Td is the dew point temperature (oC).

+ T is the observed temperature (oC).

+ RH is the relative humidity (%).

– The calculation of the dew point can be done with software temperature (described in the figure below).

Dew point temperature proportional to relative humidity RH
The higher relative humidity, the easier collecting water from air

2. Overview of water collection from the air and treatment

– The principle is to lower the air temperature to the dew point (15 – 20°C), then water will be separated and collected from the air.

– As a general rule, condensed – chilling atmospheric water generator does not operate effectively when the temperature drops below 18.3°C or the relative humidity drops below 30%. This means they are relatively inefficient when placed inside air-conditioned offices.

– Increase humidity by spraying brackish water, salt water into the air to collect more fresh water. However, in practice, this method of capturing water from the air is difficult to implement.

– A special case is water generation in a greenhouse because the air inside the greenhouse is much hotter and more humid than outside. In particular, in regions of water scarcity, a greenhouse can drastically enhance the conditions required for atmospheric water generation.

Please refer more information from RO water filtration systems at What is RO water filtration system? Industrial RO filtration process
.

3. Methods to collect and treat water from the air

3.1 Traditional method to collect water from the air

The net and fence are traditional and require no energy to operate

More water collected from the air with high humidity around the plant

3.2 Take advantage of modern technical products to collect water from the air

Solar panels generate electricity during the day – collect water from the air at night

3.3 Collecting water from the air for drinking by cold compression technique

– The two main techniques used to chill and dehumidification. The amount of water collected depends on the humidity in the air and the capacity of the chilling machine.

Flowchart of water collection technology from the air by reducing dew point temperature by refrigeration compressor

– Refrigeration compressor have an important role in reducing the air temperature (the air has been cleaned by electrostatic filtration) to condense water vapor. The water collected at the collector also still contains bacteria, so it must be strongly sterilized with the built-in Ozone unit. Then, the collected water is continued to be pumped through the activated carbon filter column to remove odors and residual ozone. The second filter column is to add minerals with mineral-containing materials.

– A running fan to collect the air, also collect water vapor in the air as well as to cool down the hot condenser

– Energy for refrigeration compressors to operate can be from available electricity source or using solar panels with integrated UPS to operate at night.

4. Collect water from the air for what purposes?

– Create the drinking water, sometimes including a part of domestic water

Provide drinking water for people in arid and desert areas.
Use for the military (DARPA, 4 people carry a machine enough to supply 150 people, source:
https://en.wikipedia.org/wiki/Atmospheric_water_generator.
These specialized machines are said to need 310Wh to produce 1 liter of water.

FMobile machine for water collected from the air for military – Use solar or direct electricity

Machine to collect water from the air for drinking located in the park

– Collecting water from the air used in offices and homes: Water receivers in the office are in the form of cold compressing, sterilizing and mineral supplement technology. However, currently, the price is still a bit high, so it is only used in the office, not at home.

The machine to collector water from the air placed in office is integrated with hot and cold water faucets

Please refer more: Collecting water from AHU, FCU, aircon for cooling tower at

5. How to collect and treat water from the air at home?

– From the air conditioner: Regarding thermal energy, at 50% relative humidity, the theory can be calculated as follows: For every 1 horse power air conditioner (1HP = 0.75kW) running continuously for 1 hour will collect 1.28 liters water. Taking an average of air conditioner: 1 HP in 1 hour will have 1 liter of water

– Collecting from the solar panels available on the roof (can gutter to collect water at night).

– Place the cone-shape collecting funnel: Place it near the base of the plant to collect more water.

At home, water collected from the air needs to be filtered with 2 layers of cloth, then boiling to disinfect safely before drinking. If used continuously, it is necessary to supplement minerals from mineral materials and boil to cool for drinking.

Thus, there are different ways to collect water in the air in each specific case to be flexible and autonomous.

Regarding this topic or any other water treatment, when needed, contact us, UCE Co., Ltd are willing to help at highest level.