Cleaning & Restoration Association News

Understanding Drying Systems

October 12, 2004
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Q: I'm interested in diversifying into water-damage restoration. What additional equipment do I need to get started? Do I need any special equipment for sewage or mold jobs?

A:Before we get into this discussion, I would like to stress that equipment is not, by itself, the answer to efficient drying of wet buildings and contents. We have some great tools available today, with new ones constantly emerging. However, far more important than the tool is the knowledge of how to use it properly and of how to combine it with other tools in a synergistic way. I highly recommend you attend schools associated with the IICRC, ASCR and other industry organizations to acquire this knowledge.

Section 6 of The S500 Standard and Reference Guide for Professional Water Damage Restoration states that, "There are four general principles used in drying water-damaged structures and materials. ...The four principles of drying include Excess Water Removal, Evaporation, Dehumidification and Temperature Control."

Excess water removal is usually efficiently accomplished with pumping and extraction equipment. These machines aren't really part of a drying system, although this step must be completed before a drying system is installed.

There are two basic categories of drying systems. There are systems that dry the structure as a whole, and there are those designed to spot-dry specific assemblies or materials. Different types of equipment are used in conjunction to set up these systems.

Systems for Drying the Entire Structure
Once the excess water has been removed, the remaining water must be evaporated from materials into the air. Rapid air movement across the surface of the materials is the most efficient way to accelerate this process. A number of specialty air movers are available to contractors to accomplish this.

"Traditional" air movers use centrifugal "squirrel-cage" fans like those in your home's HVAC system. These air movers are extremely versatile, as they can be used to just move air around in a room (free-air), or they can be used to "float" carpet for quicker drying. Attachments are available that allow them to be used in a structural cavity drying system mode. Different models vary significantly in the volume of air they move, their static pressure capability, and the amount of electricity they use.

In recent years, several types of specialty air movers have become available. Many of these are axial fans that may be somewhat less versatile than traditional air movers, as they are usually not well suited to floating carpet, but they move a significantly larger volume of air in "free-air" mode than traditional air movers. A major advantage of these axial fan air movers is that they usually require significantly less electricity than traditional air movers, which allows the contractor to safely plug more units into a single circuit. One new type is designed so it can be safely pressure-washed for more efficient decontamination. Another new model allows "daisy-chaining" of the power supply and is designed to focus airflow directly on the juncture between wall and floor for faster drying of walls.

As moisture evaporates from materials into the air, the humidity of the air goes up, which eventually slows evaporation. For rapid evaporation to continue, the air must be dehumidified.

Refrigerant dehumidifiers use the principle of condensation, cooling the air below the dew point so that water vapor condenses on the coils. Low-grain refrigerant (LGR) dehumidifiers are refrigerant dehumidifiers specially designed for the restoration market. Unlike conventional refrigerant dehumidifiers, LGRs continue to remove significant amounts of water vapor below specific humidity of 40 grains per pound. They are much more efficient and versatile than conventional refrigerants, and can be used to efficiently dry difficult assemblies or materials such as lath and plaster walls, hardwood floors, and soil in crawlspaces.

Properly used, desiccant dehumidifiers are even more efficient than LGRs at creating very low specific humidity and vapor pressure, which allows them to effectively pull moisture out of difficult-to-dry materials. Most work on the principle of adsorption. Heat for the regeneration process can be provided by electricity, fuel oil or propane. The regeneration air stream generally must be vented to the exterior, so setup is more complex than for refrigerants. Desiccants generally use a lot more energy than refrigerants and are a good deal more expensive to purchase. They are available in sizes from about 150 cfm to more than 10,000 cfm. The larger sizes are very efficient for drying large commercial buildings.

Temperature control is the last principle of drying. Since water vapor is a higher energy state than liquid water, evaporation requires heat to take place efficiently. You can easily overdo the heat when using refrigerant dehumidifiers, since most refrigerant dehumidifiers lose efficiency very rapidly when the temperature climbs into the upper 80s.

Another application of temperature control is to use cool or cold outside air, heated on its way into the building, as a source of dry air. Increasing the air's temperature 20°F very roughly cuts its relative humidity in half. Thus 30°F/60 percent RH air heated to 70°F will enter the structure at less than 14 percent RH. Air in this state will dry materials very rapidly. There are a number of systems available on the market, operating on propane or fuel oil, to accomplish this heating. The heat exchanger types that do not exhaust combustion gases into the structure are obviously preferable. Carbon monoxide is a combustion gas, as is water vapor, which is what you are trying to get out of the air in the first place.

In recent years, some have developed high-heat drying systems in which outside air, heated to very high levels (120°F and up), is injected into the structure to dry it. Manufacturers claim they are effective even when outside conditions are hot or humid. Using these various types of equipment, as well as the environmental controls built into the structure, it is possible for a knowledgeable technician to devise and install a drying system that will very effectively dry most materials in the building.

Drying Systems for Specialty Materials and Assemblies
There are certain types of material or assemblies, however, that can be very slow to dry, even in a building with an efficient structural drying system installed. This is usually because the warm, dry, rapidly moving air in the building does not come into direct contact with the wet material, including:

  • Drywall walls incorporating vapor barriers, insulation, paneling, vinyl wallpaper or low-permeance paint finishes.
  • Hardwood flooring.
  • Behind and below cabinets and other built-ins.
  • Lath and plaster walls.
  • Structural cavities and voids of various sorts.

    Several manufacturers have produced drying systems that address these special needs. They generally work by directly injecting or extracting air into these areas or pockets, replacing the moisture-laden air in contact with the wet surface with drier air that encourages evaporation.

    Some structural-cavity drying systems (SCDSs) are attached to traditional-type air movers. The baseboard is removed, holes (1/2 inch to 11/2 inches) for air entry are drilled into the wall cavity, and the SCDS is installed over the holes. When the air mover is turned on, it positively pressurizes the cavity with warm, dry room air. Cool, damp air is forced out. Unfortunately, this positive pressure can also flush out any contaminants, like mold, that might be present in the cavity. This may create a liability situation for the restorer.

    Other SCDS designs use long lengths of manifold hose, up to 80 feet, and small 3/16-inch injectors. These designs operate at higher static pressures than the air mover-based units, and are highly efficient. One way of making them even more efficient when used in a positive pressure mode is to duct the exhaust of a desiccant or LGR dehumidifier into the intake of the SCDS, thus injecting extremely dry air directly into the cavity. They can also be used in a filtered negative-pressure mode to prevent spread of contaminants from cavities.

    Negative pressure mats are available as attachments for these high-pressure SCDSs. Placed over strip hardwood flooring, they suck air from underneath the strips. They are highly effective at drying wet hardwood floors, especially when used with other systems injecting air under the floor at the edges. In many cases, even "cupped" floors will lay back down, possibly not even requiring refinishing.

    Of course, to be effective, all SCDSs and other specialty material drying systems must be installed in a building with a good structural drying system in place.

    Modifying Drying Systems in Contaminated Environments
    Mold and other contaminants present in a building can greatly complicate drying. The massive turbulent airflow used in standard drying systems can actually make the mold problem worse by spreading it. However, if you know how to modify your drying system appropriately for such environments, it is still possible to set up safe, effective drying systems.

    One specialized piece of equipment that is a must for a contaminated environment drying system is an air filtration device, or AFD. An AFD consists of a fan and a HEPA filter that is housed in a chassis. The filters remove particulate contaminants, such as mold, from the air. AFDs can be used to "scrub" the air, or to generate negative air pressure in the contaminated area to keep contaminants from escaping into the rest of the building.

    AFDs, combined with appropriate modifications in the way other equipment is installed, can be used to create containment-drying systems that dry structures and materials effectively while preventing the spread of contaminants. Some additional specialized training is needed to learn how to perform this kind of drying.

    All the equipment in the world, however, will do you no good if you don't know how it works. Understanding how to use the equipment properly is much more important than the equipment itself. Get good training from knowledgeable instructors and you can begin designing and installing highly effective drying systems.

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