First and foremost it is important to understand that every water restoration project utilizes some amount of heat during the duration of the drying process. Water requires a substantial amount of heat, or energy, to change from a liquid to a vapor. It is also important to understand that a substantial amount of heat is not the same as high temperature. The amount of heat in something is not the same as its temperature.
Heat can be measured just like temperature. One measurement of heat is BTUs (British thermal units). A BTU is about enough heat to raise the temperature of a pint of water 1 degree Fahrenheit, one BTU is not a lot of heat. If you have one gallon of water that you wanted to heat from cool to hot, from 50 degrees Fahrenheit up to around 120 degrees Fahrenheit, it would require adding around 560 BTUs (8 pints X 70 degree Fahrenheit change).
Evaporation requires a considerably more amount of heat. Evaporating a pint of water requires a little over 1,000 BTUs, and that is without changing its temperature. The reason it takes so many BTUs to evaporate water is because the water molecules need to absorb enough heat energy to push away from each other, they need to convert from a liquid (close to each other) to a vapor (far apart). Evaporation does not require a temperature change as evaporation is a phase change. A pint of water will evaporate at 70 degrees Fahrenheit and the water vapor will be at 70 degrees Fahrenheit, all those 1,000 BTUs are used in the phase change needed to convert liquid water into water vapor. A pound of air at 70 degrees Fahrenheit and 25% relative humidity contains about 21 BTUs. A pound of air at 70 degrees Fahrenheit and 90 % relative humidity contains 32 BTUs. It’s the same temperature, but about 60% more heat content. These extra BTUs are referred to as latent heat because they do not show up in the temperature, they only make themselves known by a moisture content reading.
All vapors exert a pressure called vapor pressure. In the restoration industry, references to vapor pressure are assumed to be for water. At any given temperature there is a maximum amount of water vapor that can exist within the air. This maximum is the saturation vapor pressure. Vapor pressure is often measured in inches of Mercury (in. Hg). The weight of the water vapor in the air is measured in grains per pound (GPP). When the temperature of the air is raised, the amount of moisture in the air (GPP) doesn’t change so the vapor pressure doesn’t change, but the maximum possible vapor pressure increases (more grains of water can exist in the warmer air). Liquid water does have a vapor pressure and it is tied to temperature. If you raise the temperature of the water in the wet material, you raise the vapor pressure.
Vapor pressure differential drives evaporation and temperature is one of the main controls over how fast it will happen. The temperature at the surface of a wet material determines the saturation vapor pressure, meaning that the higher the temperature the higher the vapor pressure will be. The vapor pressure in the air can be determined by its temperature and RH (vapor pressure can be found on the psychrometric chart next to Grains per pound). Assuming adequate airflow, the bigger the difference is between the two vapor pressures, the faster the evaporation rate. Conversely, the smaller the difference is, the slower the evaporation rate. If the ambient air is already saturated, the material that is attempting to be dried will not dry. When using heat to dry materials, the goal is to maximize the vapor pressure differential by adding heat to provide sufficient evaporation without excessive temperature rise rather than focusing on controlling the moisture content of the air through dehumidification.
If you have any questions on heat in the drying process, please feel free to reach out to the Gerloff Company’s restoration professionals.
Until next time my friends, be prepared and stay safe.
Reference: “Restoration Drying with Heat: Part 1” by Larry Carlson and Mike Steffe