Vapor Pressure
Vapor Pressure
"It does not matter if you choose to use a refrigerant dehumidifier, a desiccant dehumidifier, a furnace, an air mover or for that matter just opening a window, every effective drying strategy is a manipulation of vapor pressure - the force that drives all moisture phase changes. When there is a difference between the pressures exerted by these vapors of humidity, nature seeks to equalize in a specific fashion: from high pressure to low pressures." (Leadership In Restorative Drying, page 80)
How many of us have watched the weather on TV and the weather man talks about a high pressure area moving into a low pressure area? The same elements that cause this effect in a larger scale also happens within the micro environment of a building.
As we start to compile the information about water molecules and the force exerted on the surrounding environment by their characteristics, we see that if we have a wet structure we must alter the environment in such a way that the water molecules in the structure are excited (increased in their movement) until the bonds which hold them together as a liquid are not sufficient to keep them joined together in a liquid state. This process increases the vapor pressure in the hygroscopic material.
At the same time, as part of the process, if we create a lower vapor pressure beside the material we are drying as we are increasing the vapor pressure in the hygroscopic material, the water vapor will move out of the material into the low pressure environment. The greater the difference between the two pressure areas (delta vapor pressure), the more rapidly moisture movement will occur. (For delta vapor pressure discussion see Leadership In Restorative Drying, pages 86 - 91, esp scenario on page 85)
"Important Note: The psychrometric chart solely defines the metrics pertaining to water in its vapor state - not its wood EMC condition. When water is in a hygroscopic material, there are chemical bonds that form between the material and the water molecule. These bonds are not accounted for on the psychrometric chart." (Leadership In Restorative Drying, page 84)
"If the wood is heated, the water within the wood will also heat, and the water molecules shall move faster and more agressively. This additional motion of water molecules will increase to the point where the pressure within the wood will be substantially greater than the pressure outside the wood and the water shall seek to escape with a greater force. This force is actually quantifiable." (Leadership In Restorative Drying, page 86)
If we can get the molecules moving in a controlled fashion so they are released from the hygroscopic material and then cause the released molecules (water vapor) to be condensed and captured as water (liquid) or to be exhausted from the immediate environment, we are developing a controled drying strategy. We can use the psychrometric readings to analyze the present factors in the environment so that we can determine what can be influenced but we must understand what readings are relevant when assessing hygroscopic materials.
"Learning point: vapor pressure (along with dry bulb temperature/sensible BTU) is the only psychrometric value that can be responsibly interchanged between an environmental condition and a hygroscopic material. This is why scientists consistently use the vapor pressure metric when discussing evaporative forces. No other metric on the psychrometric chart can be interchanged in this manner." (Leadership In Restorative Drying, page 86)
Another important concept to consider in formulating a drying strategy is that of Dynamic Equilibrium Vapor Pressure. As we influence the environment to cause the different forces to work from high to low pressure we have to be aware that the forces can reach a point of balance or equilibrium where the evaporation and condensation equalize. This will stop the drying process in its tracks.
It is important to remember that at any given time there are water molecules leaving the surface of the water (becoming a vapor) and molecules returning to the water phase through the hydrogen bonding properties. One of the roles of the restorer is to prevent the vapor from condensing or returning to the water phase.
"(Accurate Science: There is no physical foundation for the statement that 'wet goes to dry' beyond the physical characteristics of liquid moisture movement: gravity, static pressure, seepage, capillary forces, wicking etc. In a gaseous state, water is constantly bonding and releasing from each other as they change phase. Thus, at any given moment, dry actually does go to wet'. When there are more molecules leaving the surface of water than are bonding, then there is net evaporation.)"
"For restorers, evaporation that is meaningful to our drying strategy is the difference between the rate of evaporation and the rate of condensation. Therefore:
- The drier we make the air, we slow the rate of condensation, and we increase the net evaporation.
- The warmer we make the liquid, we increase the rate of evaporation, and we increase the net evaporation." (Leadership In Restorative Drying, page 83)
"Given a scenario with no changes in temperature (i.e., the liquid and air is maintained at a constant temperature):
- The rate of evaporation from a liquid surface will transpire at a constant, unchanging rate.
- The rate of condensation from the air back into a liquid will transpire at a rate dictated by the quantity and temperature of the water molecules in the environment.
- There comes a point where the rate of evaporation and the rate of condensation occur at exactly the same rate. This is something called dynamic equilibrium vapor pressure." (Leadership In Restorative Drying, page 83)
"Equilibrium Vapor Pressure is such a good term to describe a very important psychrometric condition because it accurately describes a distinct point on a psychrometric chart. It also defines the quantity of energy in the system (this energy in the system is called enthalpy... " (Leadership In Restorative Drying, page 83)
