As restorative drying professionals, we are aware that humidity, airflow, and temperature (HAT) are necessary for evaporation. We often consider each component of HAT as equal in creating the ideal conditions for evaporation; however, they are never equal, but rather, balanced in importance based on the drying cycle. Let’s take a look at each of these components and see how they interact to influence the rate of evaporation.
Low relative humidity (RH) is necessary for drying as moisture in materials and air seeks equilibrium. The lower the RH of the air, the quicker the wet materials will give up moisture to become equal with the moisture in the air. Equilibrium relative humidity (ERH) is the air equal to equilibrium moisture content (EMC) moisture in materials. Lower RHs are more important than low humidity ratios (grains per pound [GPP]), as moisture seeks equilibrium based on RH, not GPP.
On the first day of drying, we need buckets for dehumidification due to massive amounts of water that will be evaporated from wet materials. The dehumidifier formulas recommended in the ANSI/IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration, using the simple calculation, get us to the goal of 40% RH on the second day of drying l . However, I see tremendous value in getting RHs in the low 30% or below within 24 hours and keeping it there. Very low RHs can reduce drying times by a day or more. So, add more dehumidification.
The argument I see in the field from third-party administrators or others (franchise and insurance auditors) is “we will only pay for the minimum IICRC recommendation for dehumidifiers” (e.g. large versus extra-large or only one dehumidifier instead of two). As long as materials are drying and you maintain low RHs (40% or below better yet in the low 30% range), you are doing your job and saving your client and our insurance partners money by reducing drying times.
During declining rates of evaporation, when materials don’t look wet (day two and beyond), low vapor pressures provided by LGR or desiccant dehumidifiers are necessary for efficient drying.
Air movement is the workhorse of drying by displacing high RH at the surface of wet materials with lower RH. Circulation airflow moves wet air to our dehumidification systems (either mechanical or ventilation), allowing us to manage RH and water vapor in the air. Sounds simple, but not so, as airflow alone will not dry anything and needs to be balanced with heat (energy) and humidity control. The velocity of airflow, measured in feet per minute (fpm), during the initial stages of drying, known as constant rates of evaporation, ideally should be above 600 fpm 2
It is essential during this stage of drying that all wet materials are exposed to high-velocity airflow (speed, not CFM is most important). The air mover calculation in the S500 is spot on and should forever end the argument that the restoration contractor put too many airmovers on the job3 . During constant rates of evaporation, when materials look wet (liquid water in material), capillary action brings
water to the surface of materials, and there is a significant difference between the vapor pressure of the air and wet materials. Physics is working in our favor.
All of that good stuff changes somewhere around the second day of drying. During declining rates of evaporation, high-velocity laminar airflow becomes a deterrent to drying as heat becomes more important. As is pointed out in Chapter 7 of the S500, “The velocity of airflow has a diminishing return as the water available for evaporation at the surface reduces” 4 .
Reducing airflow to under 150 fpm allows water in the material to gain energy (heat), increasing the vapor pressure of the water in the material. At this point, energy brings the big stick to the fight. It seems counter intuitive to remove or reduce air movement when materials are not dry, but reducing air movement will actually speed up the drying at this time. Going to a circulation model instead of directed airflow reduces the cooling effect of direct air flow, allowing water in the material to gain energy more efficiently.
We often get stuck on the idea that high temperatures are needed for evaporation (e.g. water boils at 212 degrees Fahrenheit). But energy is what we need, measured as enthalpy or BTUs. Heat is necessary to break the chemical bonds that hold individual water molecules that form liquid water together. For evaporation to occur, energy must be added to the water. That energy (heat) comes from the surface of the wet materials. Evaporation’s net effect to the surface temperature is cooling as energy from the material is added to the water. Cooler surface temperatures on the first day of drying is good since temperatures can be near or below the ideal temperature for mesophilic mold and bacteria growth 5 .
So, what is the ideal drying temperature? That depends on the timeline of drying. On day one, 70 to 90 degrees Fahrenheit is best for a number of reasons. This temperature range is comfortable; it is the optimum performance range for refrigerant dehumidifiers; and there is very little risk of overheating liquid water in materials and causing damage.
As the job progresses and moves into declining rates of evaporation, heat has a greater influence on evaporation than airflow. Our equipment is throwing off BTUs and increasing temperatures at just the right time. By adding energy to the water in the material, we can increase the vapor pressure of the water in the material, thereby increasing the vapor pressure differential between the materials and the air. The greater the difference, the faster the drying.
Maximizing equipment efficiencies
As an industry, we are often criticized for putting too much equipment on the job. Here is the deal: A lot of equipment reduces drying times, period. The client can take his or her pick: a lot of equipment and short drying times or a little equipment and long drying times. Pick one. You don’t get short drying times with limited amounts of equipment. Unfortunately, by doubling up on equipment, you don’t reduce drying time by 50%. If it were that simple, it would be an easy sell.
Keep this in mind: High-velocity air movement is necessary on all wet materials on day one. Reduce air flow (slower fan speeds and/or pull airmovers) during declining rates of evaporation and add heat. Towards the end of the job and after you have pulled some airmovers, consider turning off one of your dehumidifiers for an hour. If the RH goes up, document and turn the dehumidifier back on.
Remember: Low RH is more important than grain depression (water removal). Towards the end of the job, I commonly have only one airmover in each room (where the drying goal has not been met) and all of my dehumidifiers running.
By maximizing our efforts, we can bring value to our clients and insurance partners by reducing the time to dry wet contents and materials. Better value to