Why A Pale of Water Doesn’t Dry Easily?
Build yourself a drying cabinet and put a dehumidifier inside. Then fill a bucket or tray of water, switch on the dehumidifier and observe. What probably you think should happen in fact does not.
The water should disappear quickly right?
For sure the pale of water test is logical and simple way to show the performance of a dehumidifier in a tight space but it doesn’t work as expected.
The same example was demonstrated by an experienced restoration guy in the industry for over 30 years who said, “I’ve put a refrigerant dehumidifier in a bathroom of a flooded house. In the bathroom was a bathtub of water filled. I managed to dry the whole house in time but the bath tub was still full of water”. When I asked him why this was he said simply, “No idea.”.
Less Surface Area
The first obvious reason is the surface area of the pale of water compared to the surface area of say a room or house to the surface of the pale in the comparative test is far different. Moisture in the walls, ceilings and floor of a home provide a far bigger surface area than the surface area of a bucket.
In theory, the more humid the air immediately above the water is, the longer it will take to evaporate because if the air above the surface is already filled with water in the form of vapor & it cannot accommodate as much vapor. This is the vapor pressure.
The rate of drying should be inversely proportional to the relative humidity of the air above the water. If you can reach a very low relative humidity with one dehumidifier it should dry faster than a dehumidifier that can only reach relatively higher level of humidity taking temperature out of the equation. Simply, at the same temperature,
if air has a relative humidity (RH) of only 10% in the air above the water in the pale it should dry much faster if the air above the pale of water is at say 50% humidity.
In normal circumstances far lower humidity levels are possible with Ecor Pro desiccant dehumidifiers than refrigerant dehumidifiers so should always dry faster if that were the only factor.
What you see in the test
In the scenario of a drying cabinet with an Ecor Pro desiccant unit inside you may see a few litres of water disappearing in a 24 hour period. However, with a refrigerant unit you may see very little water disappear at all. That is true even if that refrigerant dehumidifier that is being used is quoted at a much higher extraction capacity than even the Ecor Pro desiccant dehumidifier.
Even though the Ecor Pro desiccant dehumidifier can achieve a lower humidity level than large refrigerant dehumidifiers and would still dry faster, there is still the question why doesn’t the water in the pale disappear?
Heat Energy Is Needed
Evaporation of the water molecules from the pale also is impacted by temperature also. Water evaporates more quickly at higher temperatures to give a higher flow rate between the gaseous and liquid phase. Molecules of water in the pale of water are in constant motion in all random directions in the liquid. Since only a small proportion of the molecules are located near the surface of the pale of water and are moving in the proper direction to escape the surface at any given instant, the rate of evaporation is limited.
As the faster-moving molecules escape and break away from the cohesive bonds of their neighboring around then and indeed form the majority of the liquid itself, those remaining molecules have lower average energy. The temperature of the liquid thus decreases. That is once energetic molecules of water that have energy to leave the liquid the liquid and go to the gaseous phase, there is an overall loss of energy in the bucket of water. In fact, this is known as the phenomenon called evaporative cooling which slows the process of evaporation. It is simply understood by human sweat. When sweat evaporates we cool down.
Relating this Theory to Drying a Wall for Flood Restoration
It is worth noting also that even when a wall or timber is drying, indeed any substance, it’s the heat energy at the surface that is taken away as the water evaporates. Just the same as the human sweat example.
The heat therefore needs to transverse the dry area at the very surface of the wall in order to reach the moisture that is deeper into the wall. However, heat travels faster through the damp wall than the dry wall. There can be an instances where using too much heat or drying too rapidly can occur. If the surface is too dry, then heat will find it difficult to penetrate the wall to the moisture deeper in the wall. The heat in a way finds it difficult to transverse the thickness of the wall closest to the surface. This can in effect seal in moisture completely and with certain substances can ruin them forever. Woods are particularly affected. In addition, the denser dry surface area can form a barrier that moisture finds more difficult to cross.
Care must be take to dry any substance at the correct rate. Rapid drying can damage the building materials otherwise.
Letting a Wall Relax as Too Much Heat or Fast Drying Can Damage Substances
In some instances it may be necessary to stop the drying process to allow the wall or other substance to relax. This means moisture can travel from the deeper more moist areas of the wall into the surface dry area and then drying can start again. Once moisture has reached the surface again, the drying process can resume.
The correct way to dry any medium is to apply enough heat or drying energy at a rate that moisture can travel through the substance being dried naturally.
If we think about dense objects they need to be dried slower than less dense items.
In a pale of water the body of water it is much more difficult to get the water to have enough energy to facilitate vaporization as it is in moving state of constant convection currents.
So How Strong is the Effect of Surface Tension on the Pale of Water Preventing Vaporization?
Besides mercury, water has the highest surface tension of all liquids. Water’s high surface tension is due to the hydrogen bonding in water molecules and they are strong giving cohesive forces with other water molecules.
Cohesion is an inter-molecular force between like molecules and is why water molecules are able to hold themselves together in a drop. Water molecules are very cohesive because of the molecule’s polarity. It is also the reason why you can fill a glass of water just barely above the rim without it spilling.
Water also has an exceptionally high heat of vaporization. Vaporization occurs when a liquid changes to a gas, which makes it an endothermic reaction described above. Vapor pressure is inversely related to inter-molecular forces, so those with stronger inter-molecular forces have a lower vapor pressure. Water has very strong inter-molecular forces, hence the low vapor pressure.
Keep Air Moving
Air movement will aid drying. Heat or dehumidification will dry in the immediate area but that will mean the room will not have air to the same level of dryness. evenly distributed. This can be achieved by circulating he air with additional fans if needed. A car windscreen we will see clearing even when the engine is cold and there is no heat. That is because the air closest to the cold wind shield is more saturated until fully saturated actually on the windshield as condensation. Putting even a cold fan on distributes the saturated air more evenily in the car so the car wind shield begins to clear.
To evaporate a pale of water, you need to break the surface tension aspect for the water molecules to escape with heat. To quicken the evaporation process circulate the air if needed and reduce the vapor pressure in the chamber by using an Ecor Pro desiccant dehumidifier which create a low relative humidity.
In the flood drying & restoration industry, it may not be possible to add heat to the environment or there may be concern of heat damage to items inside the property, In that instance the other variables still hold. Reduce the relative humidity in the property with an Ecor Pro desiccant dehumidifiers and circulate air if needed.
Changing the Pale Drying Test to Reduce the Heat Energy Needed & Look at Dehumidifier Performance
Breaking surface tension is difficult without heat. The easier way is we perform the same drying test but empty the pale of water onto a medium where the inter-molecular forces come less into play. A sponge like material, a brick, plaster or even drywall itself is enough to break the surface tension.
In a practical example for drying a building with pools of water standing is to break those surface tension bonds with a soak medium. For instance a cloth laid over a pool of standing water reducing surface tension will allow greater vapor pressure and thus much faster drying whether heat is added or not.
Vapor Pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system.
Compressor or Refrigerant dehumidifiers can not really reach below 45% Relative humidity. A Desiccant or Absorption dehumidifier can reach to around 25% giving a lower vapor pressure to gradually dry. There is always a degree of heat energy but more can be added.
Adding heat will excite the water molecules to the surface faster. Some traditional lumber dryers for instance use compressor dehumidifiers and heating elements in order to get a lower relative humidity and therefore lower vapor pressure.
Using too much heat or drying too quickly, may mean the timber or a wall becomes “case-hardened”. The very outer skin becomes impervious to the flow of moisture through it. In woods sometimes this can not be reversed. In walls time to allow the plaster to “relax” may be needed simply by removing heat or dehumidification.