A Helpful Guide for Technicians and Administrative Staff to Understand Moisture Transfer and Dehumidification.
Water: The extraordinary versatility of water allows it to exist as a solid, liquid, or gas at ordinary temperatures. It can transform effortlessly between these forms as it gains or loses heat. One of the most significant transformations is the evaporation of liquid water into water vapour. We rely on evapouration to dry anything that is wet or moist. However, when water vapour condenses back into liquid water, it can cause problems such as slippery surfaces and hidden condensation within building materials, leading to leaks, corrosion, decay, and microbial hazards.
Water Vapour: Although invisible, water vapour has a powerful impact on our comfort and interactions with materials. Clouds, fog, mist, and steam are not water vapour, but rather accumulations of water droplets or ice crystals. Water vapour is simply a collection of individual molecules that have escaped the surface of water. These molecules interact with the gases in the air and travel with them within the moist air mass that forms our atmosphere. The amount of water vapour in the atmosphere varies depending on location and temperature, with even the driest places containing a small trace. Surprisingly, adding water vapour to air makes it less dense and more buoyant due to the lighter weight of water vapour molecules compared to nitrogen and oxygen molecules. The small size of these molecules also allows them to travel through materials that would block liquid water and air.
Water Vapour Diffusion: Unlike liquid water, individual water vapour molecules are small enough to penetrate materials that would typically prevent the passage of liquids and air. If there is a concentration gradient, with more water vapour on one side of a material than the other, the water vapour will attempt to diffuse through the material towards the lower concentration. Some building materials are permeable to water vapour, and the chart compares the rates of water vapour diffusion through these materials. Impermeable materials like glass and most metals effectively block diffusion. The driving force for diffusion is the difference in vapour pressure across a material or between spaces. Each gas in a mixture, such as air and water vapour, acts independently and exerts its own vapour pressure based on temperature. Water vapour responds to differences in its own partial vapour pressure, regardless of the overall atmospheric pressure. However, most water vapour travels through accidental or intentional gaps and holes, rather than through diffusion in the building fabric.
In humans: When it’s hot, our bodies cool down by sweating. But as humidity rises, this cooling process becomes less effective. This puts stress on our bodies, making our heart work harder to recirculate blood.
4 Effective Methods to Reduce Relative Humidity:
Lowering the relative humidity of moist air can be achieved through four straightforward approaches, visually depicted on the psychrometric chart.
Ref: Handbook of Dehumidification | By G.W. Brundrett.
Increase the temperature: By raising the dry-bulb temperature, the saturation vapor pressure elevates, consequently reducing the relative humidity. However, the overall amount of water present remains unchanged.
Harness heat pump dehumidification: The heat pump dehumidifier amplifies its efficiency along constant enthalpy lines. This means that the latent heat of the condensate is utilised to warm up the dry air. As cool moist air primarily stems from the latent heat of water vapour, the potential energy that can be recovered is substantial. Practically, the dehumidifier does add sensible heat to the air passing through it, causing the slope of this line on the psychrometric chart to tilt more towards the “heating” side.
Implement moisture removal: Sorbent systems effectively eliminate water vapour, thus effectively reducing the relative humidity. It should be noted that the desiccant heating up may result in a slight increase in dry-bulb temperatures. However, the energy penalty incurred by such a system is noteworthy since the sorbent needs to be regenerated at a high temperature.
Dilution with dry air: The conventional approach to lowering relative humidity involves blending dry air with moist air. If the dry air is colder than the moist air, the temperature of the mixture will drop. However, it’s important to acknowledge that such methods come with a high energy cost.
Refrigerant Dehumidification: Efficient Moisture Removal
Refrigerant dehumidification is a unique method that converts the enthalpy of moist air into sensible heat. By refrigerating a heat-exchanger surface below the air’s dew point, moisture condenses, releasing its latent heat of condensation. The resulting cold, dry air is then passed over the hot condenser of the refrigerant circuit. This cycle acts as a heat pump, converting the latent heat of condensation into sensible heat, providing more energy output than the electricity consumed.
The efficiency of the dehumidifier depends on the relative humidity of the incoming air. Cooling even slightly can cause water to condense in 100% relative humidity air. However, at low relative humidities, the air needs to be cooled to its dew point before any water is released. In this case, most of the refrigerant cooling is used to lower the air temperature, leaving less capacity to remove moisture. This makes the cycle less effective at low relative humidities. Additionally, the water extraction rate decreases as the room temperature declines, due to a reduction in refrigerant recycling within the dehumidifier and a corresponding decrease in electricity consumption.
The basic components of a refrigerated dehumidifier include the compressor, condenser, expansion valve, evaporator, refrigerant fluid, and the fan for air recirculation.
The compressor takes the refrigerant from the low-pressure evapourator and increases its pressure, pushing it into the condenser. The process consists of two constant temperature and pressure operations.
The evaporator, a low-pressure heat exchanger, requires heat from the air to evaporate the refrigerant. This causes the evaporator to operate at a very cold temperature, colder than the airstream passing through the heat exchanger. If the evaporator temperature is lower than the dewpoint of the air, some dehumidification occurs.
After compression, the refrigerant flows into the condenser. Since it is hotter than the air passing through the condenser, it condenses and transfers its latent heat to the airstream, thereby heating it up.
Refrigerant Authority: Discover the Key to Cooling Success
When it comes to refrigerants, fluorinated hydrocarbons reign supreme. Recognisable by the prefix R and a series of numbers derived from their chemical formulae, they are the absolute essentials for effective cooling.
Relative humidity (RH) is often mentioned as a measure of water vapour in the atmosphere, but it is frequently misunderstood. RH tells us the amount of water vapour present in relation to the maximum amount that the current temperature can hold.
The Impact of Relative Humidity on Comfort and Health
Relative humidity (RH) can pose problems when it’s too low or too high. Low RH levels can cause dryness of the eyes, skin, and mucosal membranes, leading to discomfort and fatigue. This is often experienced by travelers in passenger aircraft cabins, where RH is typically around 10%. Experts recommend a minimum RH of 30% and a maximum of 60% for optimal human comfort, without considering temperature. On the other hand, high RH can impede the skin’s natural cooling process through perspiration evaporation, resulting in that bothersome “sticky” sensation. Even within the recommended 30–60% RH range, other organisms may also thrive, creating additional challenges.
The continuous airflow from the air conditioning unit to the thermostat. This image represents localised variation in surface temperature with artificial cooling.
Thanks for reading — we hope this has been useful!
The CARSI Team.