HOW MUCH AIRFLOW IS NEEDED?

Transcription

1 22 Plover Close, Interchange Park, Newport Pagnell. MK16 9PS TEL: HOW MUCH AIRFLOW IS NEEDED? By Brandon Burton Supporting the UK Restoration Industry SCIENCE THAT WORKS

2 HOW MUCH AIRFLOW IS NEEDED? By Brandon Burton, Restoration Sciences Academy I have felt for a long time that we are taking shots in the dark when we make decisions about how many airmovers to deploy on a drying job. We have some very basic industry standards about airflow, but I believe more specific information is needed. UK water restorers seem to over focus on dehumidification and understate the use of air movement, especially in the early stages of the drying process. The reality is that many restorers are not placing enough airflow at the beginning of a project, and on the same loss, have too much airflow at the end of the project. AIRMOVERS ARE NECESSARY No matter how you look at the drying process, it is easy to understand that airmovers are needed on virtually every drying project. For a basic understanding, consider the drying pie. The drying pie includes three factors for successful drying: low humidity, adequate temperature (in the air and in the affected materials), and of course airflow. All three aspects must be controlled to promote rapid evaporation. A more advanced understanding of the drying process promotes vapour pressure differentials between the wet materials and the air. If you are in doubt about the importance air movement, consider the fact that the mathematical equation for evaporation of a liquid uses airflow as a multiplier! Airflow multiplies the effectiveness of the other tools you use on the project. Airmovers are most important at the beginning of the water project, when evaporation is most rapid. Materials that absorb water quickly (carpet, pad, and drywall, etc.) also release that water quickly. During this phase of drying, airspeed or velocity is critical and more is better. Within the UK restoration industry there is limited technical information and guidance on how many airmovers should be used during a drying project. As such, here are the IICRC s recommendations which could act as an initial guide: Air moving devices used to create a general drying environment, when practical, should be set up so that continuous rapid airflow is provided across wet surfaces. In many structural drying situations, one of the most difficult areas to dry is the lower part of the wall where it meets the floor. Restorers should install one airmover for each 3 to 5 linear meters of wall, with the outlet of each airmover pointing in the same direction. (IICRC S p. 51) It is significant that the word should is used in the previous quote. This means that placing an airmover every 3 5 linear meters is the accepted standard of care to be followed by US professional restorers, question is, what about the UK restoration industry, is the science of drying and the evaporation process any different? Airmovers should be placed in this manner when there are wet walls and floors. The standard goes on to state that small rooms need an airmover as well. Bottom line: when dealing with saturated materials and high evaporation rates, bigger airflow is better. Later I will give specifics on how big this airflow needs to be for good evaporation. Unfortunately, many restorers simply have too few airmovers at the beginning of the process. 2

3 BUT, AS THE PROJECT PROGRESSES the need for rapid airflow decreases. Earlier, I referenced the evaporation equation which states that airflow is a multiplier. But it is important to understand that the evaporation equation assumes that materials are saturated. It assumes that liquid water is continuously available at the surface of the material. In most circumstances, the majority of the surface water is gone within the first hours of a dry down. (If you are not getting these results on your projects, please read up on the extraction process!) So, if surface water is gone, evaporation rates fall, and less airflow is needed. TECHNICALLY SPEAKING The rapid initial phase of drying is called the constant rate period. Airflow is a multiplier to the effect of evaporation during this phase. The basic formula is this: Evaporation = Airflow Vapour Pressure Differential After surface water has evaporated, we enter the falling rate period which is a much slower rate of evaporation. Airflow is no longer multiplying the evaporation. Now, the basic formula is: Evaporation = Vapour Pressure Differential 3

4 Practically Speaking How Much Airflow is Really Needed? In the restorative drying industry, there is no real peer-reviewed science that says how much air movement is needed. Thankfully, much research has been completed on industrial wood products as well as food products. The science from these industries is very clear, and it directly relates to what we do because wood and food products are hygroscopic, just like our wet structures. The research says that more airflow is needed at the beginning of the process, and less is needed at the end. You can see the references for my research at the end of this article. Basically, the studies show the following: During the constant rate period most studies specified airflow rates between meters per second (11 13 Km/Hr). During the falling rate period all studies specified airflow rates between meters per second (2 5.5 Km/Hr). Based on this research, the best approach is to start drying jobs with airmovers placed every 3-5 linear meters. This easily achieves over 3 meters per second along wet surfaces. Of course, flow rates near the outlet of the airmover will be moving faster in some cases as high as 10 meters per second. Achieving the correct airflow can be verified by using an anemometer. Constant Rate of Evaporation Phase m/s needed Rate of Evaporation Surface water is completely evaporated about here Falling Rate of Evaporation Phase m/s needed Time There is a high rate of evaporation on a water loss until all surface water is evaporated. At that point the evaporation rates slow down and less airflow is needed. 4

5 Airmover Adjustments As the job progresses and materials no longer have surface moisture, the airflow can be reduced to as little as one airmover per room or area. This small amount of airflow is needed for circulation in the drying environment. How can a restorer know when materials no longer have surface moisture? Here are a number of ways, listed in order from most difficult (and most accurate) to easiest (and least accurate): Perform an equilibrium relative humidity test on the wet material. If the ERH is below 85%, the material no longer has surface moisture. This test is done with a hygrometer placed on the surface and sealed over with plastic and taped down for several hours. Test the surface of material with a penetrating moisture meter. If the meter shows reading less than 30% moisture content, it no longer has surface moisture. Hold a hygrometer probe near the affected surfaces while airflow is applied. If the temperature and RH near the surface correspond to the ambient conditions, you have enough airflow. If the temperature is cooler and RH is higher, than more airflow is needed. Sensory test. If it looks or feels wet, there is still surface moisture. In my opinion, any of these methods of searching for surface water would be appropriate. Common areas to test for surface moisture include, wall and floor surfaces, on the surface of the carpet, in between the carpet and pad and finally in between the pad and floor. Of all the areas in this list, the space between the pad and floor will be the last to feel dry to the touch. Let me be absolutely clear! I am not saying these areas are dry, only that they are in the falling rate period when they no longer have surface moisture. Boost Your Confidence As I was reading the research from other drying industries, I found that what we consider to be restorative drying best practices matches up well to what the PhDs are saying. The most efficient restorers are going to use proper airflow to start a project and note when surface water is no longer present and reduce airflow. Efficiencies like these are often rewarded with more business, higher profits and greater customer satisfaction. Common Practice Better Practice Best Practice Number of Airmovers Day 1 Day 2 Day 3 Day 4 Day 5 Total Airmover Days Notice that overall total airmover days are similar in each case. NOTE: The numbers used in this are for illustration only and are not intended as an exact air mover requirement. 5

6 It s not just about the science Ever heard a customer complain about the noise generated by airmovers? I m sure we all appreciate that experiencing a flood can be stressful and compounding this by giving your customer a headache is not a great scenario for anyone! However, based on the above, we have a great opportunity to sell the benefits of rapid air movement during the initial stages of drying and then offer some good news that typically within 2-3 days the amount of air movement, and related noise, will be reduced dramatically! Spending a small amount of time explaining the drying process and offering your customers some light at the end of the tunnel is probably your best way of ensuring the drying process stays on track whilst also highlighting to your customer that you really are an expert on the science of drying! REFERENCES Lamb, Dr. Fred M. The Importance of Air Velocity in Drying. Modern Woodworking. July Steiner, Ylva. Optimising the Air Velocity in an Industrial Wood Drying Process. Norwegian University of Life Sciences Dept of Ecology and Natural Resource Management Salamon, M and McIntyre, S Manipulation of Air Velocity Permits Drying Time Savings. Canadian Forest Industries. Volume 89. Simpson, William T. Effect of Air Velocity on the Drying Rate of Single Eastern White Pine Boards. Res. Note FPL RN 266. Madison, WI: US Department of Agriculture, Forest Service, Forest Products Laboratory Xuai-Kang Yi, et al. Thin Layer Drying characteristics and Modelling of Chinese Jujubes. Mathematical Problems in Engineering ABOUT BRANDON BURTON ABOUT DRI-EAZ UK As the Technical Education Manager for Restoration Sciences Academy, Brandon has trained more than 4000 water restoration professionals in the science and application of drying practices. Brandon has been consulted by industry leaders, restoration professionals and insurers on the subject of best practices, cost control and risk management. Brandon has served the cleaning and restoration industry since 1995 and acts as the technical lead for Dri-Eaz UK as well as offering support to the wider UK restorative drying industry. Restoration professionals turn to Dri-Eaz UK for the most reliable, robust and innovative drying solutions and products. Dri-Eaz UK is part of Legend Brands family of companies, which successfully combines over 185 years of experience in providing equipment, accessories, chemicals and training to cleaning, restoration and remediation professionals worldwide. 6