Drying Difficult Materials
2005 Article
Since the early days of restorative drying, restorers have been challenged with providing assurance that wet materials have returned to their normal dry state, or what is commonly referred to as 'dry standard.' Some materials prove to be more difficult than others to dry. Additionally, client(s) may pressure the restorer to cut the drying process short in an effort to eliminate some of the equipment rental expense. "It looks dry, so let's not worry about it." Or, "I think you inflated the expenses on the project through extra drying efforts."
Both of these scenarios can aggravate the client to the point where they may threaten legal action or demand compensation.
The Physics of Drying Have Not Changed
Of all the materials in a structure that may become wet from a water intrusion, the client frequently considers the carpet and pad to be the only issue at hand. But most carpet and pad is highly porous and can release its moisture quite readily. With today's equipment, it is not uncommon for restorers to have the carpet returned to a dry standard in less than a day. Issues of greater concern should be the other materials found in the home that do not release their moisture quite as readily. Think of sheetrock, lathe and plaster, concrete, cinder-block walls, natural wood like hardwood and timber, engineered wood like plywood and pressboards, soil in crawlspaces. Each of these materials is able to take on moisture from the humidity present in the air (we call this quality hygroscopic).
These materials take on moisture through adsorption or absorption, and depending on the nature of the material, may forcefully retain this excess moisture. For instance, bound water is held within the cell walls of wood fiber through an incredibly strong chemical bond. It is part of the nature of the wood to resist the release of its moisture. Concrete, on the other hand, is denser than most wood and possesses the unique quality of extraordinarily strong capillary forces that pull the water deeper within the material. This can be a difficult force to overcome.
The three elements of the drying pie — humidity control, airflow and temperature ( or "H.A.T.") — are the same forces we use to overcome these challenges. But we may use a different strategy in how they are administered. It is important to understand the process of making water change state from a liquid to a gas, so that our dehumidifiers may then capture this excess moisture. It is a very simple formula:
To make water change state from a liquid to a gas, you need to add energy… to the water.
It is how we add this energy to these dense materials that becomes a challenge. Let's review the forces used in HAT that drive moisture out of wet materials.
Humidity Control
Energy form: vapor pressure
Pressure is a form of stored energy. When you lower the specific humidity (grains per pound/GPP) in an environment through dehumidification, vapor pressure is also lowered.
The greater the difference between the vapor pressures within the wet materials and the surrounding environment… the greater the influence for the wet material to release its moisture to the area of less vapor pressure.
It is the correct use of these three powerful forces that will drive moisture to release its bonds within wood, concrete or any hygroscopic material.
Airflow
Energy form: kinetic or momentum
Have you ever seen a Newton's Cradle? It is a small desktop device with 5 steel balls, each suspended on a string. If you swing one of those balls so that it impacts the other four, there is an equal and opposite reaction with the ball on the other side. The role of airmovers in our drying strategy implements the use of this energy phenomenon.
When you place an airmover on a wet surface, where does it dry first? Directly in front of the air mover! This is because the airflow that impacts the water molecule transfers its momentum to the water (think of Newton's Cradle here…) and causes the molecule to release its bond to other water molecules and become a vapor.
There is one other benefit of airflow worth mentioning here, but it isn't an energy form. A higher relative humidity is found at the surface of wet materials. There is a benefit to be able to 'sweep' away this high humidity so that freer evaporation may occur. This is of value. But in the case of wood or concrete, do you need highly compressed air to sweep away a thin layer of gas found at the surface of a material? Or do you only need a mild draft to move this gas vapor? It's worth thinking about. (Don't waste your amps on static pressure if you don't need it!)
Temperature
Energy form: heat
Perhaps the most obvious energy form used in our drying strategy — but frequently misused — is heat. Recently, there have been some aggressive claims that if you have enough temperature, this is the only force needed to dry a structure effectively. I suppose you could claim to build a beautiful 3-ingredient cake with only one of the ingredients. But I doubt many would call it a 'cake.' The greatest error in the use of temperature is the failure to realize that to make water change state - you need to add the energy — to the water… not the air. If the air becomes hot, but the materials remain cool or cold, then the water will not be gaining this energy, will it? To use heat effectively, you must heat the wet materials.
A New Way of Looking at HAT: MPH
In focusing on the effort to add energy to water trapped deep within hygroscopic materials, there is value to step back from looking at 'HAT — a mechanical view of drying — and focus on 'MPH.' With MPH, we focus on the physics of making water change state: Momentum, vapor Pressure and Heat.
Imagine that you are trying to dry plywood that has moisture trapped deep within it, and ask yourself:
- "Are my high velocity air movers adding energy to the water trapped at mid-depth inside the plywood?"
- "If I use a conventional dehumidifier in a room that will only reduce the vapor pressure to approximately .30 Hg., and it is simply blowing around the entire room, am I adding a significant pressure difference (energy difference) between the wet wood and the surrounding environment so as to efficiently drive the moisture out of the wood?"
- "If I heat the air in a room, but the plywood remains cool, am I adding energy to the water trapped in the plywood?"
If you answered "No!" to each of these questions - you are well on your way to understanding how we can alter our drying strategy to effectively dry difficult materials. Now, remembering MPH, let's rephrase these questions toward a positive response:
- "Since trying to transfer momentum to water found deep within wood is a lost cause, then let's look at the other benefit of airflow - sweeping away boundary layers. If I use high CFM, low pressure airmovers, will I be more effective and energy efficient in sweeping away the layer of vapor gasses found at the surface of the wet materials?"
- "If I use LGR dehumidifiers that are able to reduce the vapor pressure to approximately .15 Hg., or even better, a desiccant dehumidifier that can reduce the vapor pressure to almost 0 Hg., and I intentionally place that very dry air adjacent to the wet material, am I adding a significant pressure difference (energy difference) between the wet wood and the surrounding environment so as to efficiently drive the moisture out of the wood?"
- "If I (responsibly!) heat the plywood to 30ºF - 40ºF warmer than normal, am I significantly adding energy to the water found deep within the plywood?"
"Drying Godzillas" effectively manipulate these laws of physics. Now you know the secrets too.
Measuring the moisture in these materials will be discussed in a future article.
|
|