We continue our discussion from Part One on the vacuum vapor degreasing method by focusing on the subject of part drying.
Drying in Aqueous Systems
In aqueous systems, heated air-drying is the most common method used for water removal. However, it is fair to say that most aqueous washers do a poor job of effective drying. Generally ambient air is employed, which depending on the relative humidity can contain significant water vapor to begin with. Upon heating the air becomes saturated and if the temperature can be maintained, can evaporate a considerable amount of water. The problem, however, is not usually in the saturation levels of the air, rather in the heat and mass transfer operations required to accomplish the water removal.
The rate of drying is dependent upon the mass transfer driving force created between the water surface and the heated bulk air. The vapor concentration at the water droplet surface is totally dependent upon the temperature to which one can heat the droplet. The bulk air moisture content is totally dependent upon the air source used for drying. It is therefore clear that the initial task in air-drying is to heat the surface of the part or droplet in order to increase the water concentration at the drop surface (that is, the vapor pressure of the liquid).
The heated section of an aqueous washer, or for that matter an air oven, transfers heat to a component part by natural convection. The natural convection method is very slow with respect to the rate of heat transfer (Equation 1) which is given by:
(1) Q = U Δ T
Where U, which is known as the heat transfer coefficient and can be considered a measure of the effectiveness of the heat transfer process, generally falls in a range of between 1 and 5 BTU / (ft2-hr-°F). Delta T (ΔT) is the driving force for heat transfer, which in this case is the temperature of the air minus the temperature of the water drop or solid surface.
The drying process is generally very slow under these conditions even with fans moving the air from the heating source to the part surface. Under forced convection conditions, U generally can be increased to between 55 – 170 W/m2-hr °K (10 to 30 BTU / ft2-hr-°F). Forced convection can increase the heat transfer from 2 to 30 times that of natural convection.
However, the heat transfer is only half the task, during the heating of the water droplets, vapor at the droplet surface becomes more concentrated than the water vapor in the bulk air and begins to diffuse into the bulk air, The rate of transfer is given by (Equation 2)
(2) N = k Δ C
Where k is called the mass transfer coefficient, N is natural convection and Delta C (ΔC) is the water vapor concentration difference between the droplet surface and the bulk air.
As with the heat transfer coefficient, the value can be greatly increased with forced convection versus natural convection. The mass transfer of water from the drop to the bulk air removes heat from the drop since the specific heat of the water vapor, and more importantly the latent heat of the water vapor evaporating from the drop to replace the vacating water vapor, cooled the drop and acts to counter the incoming heat carried by the air.
Drying in Vacuum Solvent Systems
Another method used to dry water from parts is by the use of a vacuum to reduce the bulk concentration of water vapor in air. The vapor at the droplet surface immediately diffuses as the vacuum is reduced and the concentration difference is maintained by constantly removing the evaporating water. Since heat is not transferred, the diffusion process is very rapid. The droplet and parts surface begins to cool as the heat is transferred to the escaping vapor. If the water does not completely evaporate before the part cools below 0ºC (32°F), ice can form and the process will dramatically slow down ((since the ice now must sublime into the surrounding vacuum.) Since the vapor pressure of ice is very low, the driving force Δ C becomes very small even if a total vacuum is attained. Typically at this point it is more beneficial to use forced air convection.
Vacuum Drying Process
The goal in vacuum drying is to remove all the air from the drying chamber. Upon removal, a heated vapor using an insoluble solvent is introduced to the chamber. The condensing vapor acts as a heat transfer medium. This process is similar to condensing steam and can be described by Equation 1 (above). The heat transfer coefficient in this case is 570 – 1700 W/m2-hr °K (100 – 300 BTU /ft2-hr-°F) and thus up to 3 – 30 times faster than forced convection heat transfer.
In a vacuum, the pressure in the chamber can only reach the vapor pressure of a liquid if a liquid phase is present in the chamber. The vapor pressure is depended upon the temperature of the liquid present (Fig. 1) shows.
For insoluble liquids, they exert their own vapor pressure and thus the amount of vapor is actually additive under these conditions (Fig. 2). Under these conditions water droplets heat rapidly and diffuses quickly into the vapor state. If this mixture is continuously removed, the water concentration in the vapor state can be kept low and the diffusion rate high. The continuous addition of solvent vapor can maintain the temperature and can easily be separated from the water after condensation.
The phase diagram for a water-PCE mixture (Fig. 3) show us that for equilibrium conditions at one (1) atmosphere, one can expected that a PCE heated solvent heated will enter the chamber at 120°C (250°F). The exiting vapor can be expected to be rich in water vapor and approach the equilibrium mixture of 63% water up until there is essentially no water remaining on the part, The temperature of the drying is approximately 90°C (190°F), which would be equivalent to drying water at 480 Torr.
If one compares the drying method above to the conventional oven or vacuum drying methods, both the heat and mass transfer can be an order of magnitude higher. This translates into drying times measured in minutes rather than hours as usually encountered in industrial water drying of difficult parts. A phase diagram of an actual system (Fig. 4), which operates under vacuum, can be seen that by controlling the pressure of the system one can vary the actual drying temperature.
A process that provides for the best of both air-drying and vacuum drying is highly desirable. It will provide both a medium to carry heat to the part (as in air-drying) and a faster and more complete method of removing water on the parts (as in vacuum drying). In addition, it requires less of a vacuum to dry at lower temperatures.
1. Wikipedia (www.wikipedia.org)
2. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.
3. Gray, Donald and Joseph P. Schuttert, “Removal of Entrained Moisture from Powdered Metal Parts Using High Temperature Solvent and Vacuum” PM2TEC 2003.
4. Herring, Daniel H., “It’s Time to Clean Up Our Act!”, Industrial Heating, 2008.
5. “Hyperflo Sequenced Solvent Vapor-Spray Cleaning Technique, white paper.
Daniel H. Herring / Tel: (630) 834-3017) /E-mail: [email protected]
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.