|Fig. 1. Vacuum Bagging (for composites)|
This article continues the series of Five Reasons that vacuum is used in science and industry.
1. To Provide a Working Force
2. To Remove Active and Reactive constituents
3. To Remove Trapped and Dissolved Gases
In a typical industrial vacuum system it is possible to trap pockets of atmospheric air when manufacturing the system and each time a cyclic system is unloaded and reloaded. One example is fixtures that are bolted to brackets on the chamber wall. If the securing bolts are placed in blind tapped holes, gas can be trapped in the bottom of the hole. With a regular bolt in a blind tapped hole, the only escape for gas molecules is along the thread form where there is space in the root and crown areas of the thread. If a blind hole is unavoidable, bolts that have a hole through them will allow the air to escape easily during the evacuation part of the process. There is a company in the USA that specializes in these types of bolts and screws. Using tapped through holes eliminates this problem.
|Fig. 2. A small urethane degassing system|
If a fixture is held in place with a bolt and a nut or nut and washer, air can be trapped in the space around the bolt in the clearance hole. Again the air will slowly bleed away during the evacuation and extend the pump down time. Even mating surfaces can trap air under them which will cause a slower evacuation because the trapped air will only release slowly.
Another example of trapped gases in the heat treat industry is when porous insulation materials are used. Air will be adsorbed into the porous material, which has a very large internal surface area, at atmospheric pressure and will then be slow to release during the evacuation due to low conductance of gas from the very tiny spaces in the material.
In the refrigeration and air conditioning industry small bore tubing will have water vapor adsorb onto the inside of the tubes during manufacturing or when assemblies such as cooling coils are stored in the shop. This water vapor must be removed after assembly and before the refrigerant is charged into the system. Any water vapor not removed may be enough to freeze at the expansion valve and cause the system to stop working. The process of expanding the refrigerant uses heat which causes the refrigerator to get cold or the air conditioner to cool the air blown over its coils. Any water in the refrigerant will freeze at the point where the expansion takes place and block the flow. Every time a refrigeration device is serviced it has to be dehydrated before the new refrigerant is added.
|Fig. 3. Degas system with motor driven impeller|
This water vapor is said to be trapped because it is difficult to remove it easily and quickly. By evacuating the tubing the water vapor slowly releases off the interior surfaces and is pumped away. Water has a vapor pressure of about 18 Torr at room temperature. This process can take several hours due to low flow conditions (low conductance) inside the small bore tubing at low pressure.
A process called Vacuum Bagging is used to remove vapors and condition fiberglass and other layered composite material moldings. The layers of material can trap bubbles of air and vapor is given off the wet materials as they cure. It is used to make composite items for boats, airplanes, cars, bicycles and many other products where strength and light weight parts are needed. The “layup” of materials to be vacuum bagged is covered with a porous material and then with a rubber or plastic cover which is sealed at the edges. The porous material allows the trapped air and vapors to move through it to reach the one or more vacuum connections in the pliable bag and be pumped away. This process can take days to complete, depending on the size of the layup and the materials used. Fig. 1 shows a cross section through a typical vacuum bagging layup.
|Fig. 4. A simplified Urethane Degassing System|
Note that vacuum is also used for drying products such as sterilized medical items, some foods and industrial parts after wet processing. Another drying process called freeze drying is used for drying temperature sensitive products such as vaccines, flowers and other foods. In this process the product is pre frozen and then placed under vacuum. In the vacuum the ice will “sublime” to water vapor without going through the liquid phase. The water vapor is collected on a cold condenser located between the main chamber and the vacuum pump. These processes may also be classified as “Reason 2, Removing Active and Reactive Gases”.
Dissolved Gases in Liquids
A number of liquids are used in industry for different applications and they generally need to be pure or the process may be affected. During mixing and pouring operations gas can be trapped in the liquid as small bubbles. Some bubbles of gas may be visible and others may dissolve into the liquid and be invisible to the naked eye.
|Fig. 5. Transformer Oil Dryer and Degasser|
Urethanes are often used for small molded products such as plugs, caps/covers, grommets, bumpers and even tires for fork lift trucks. Bubbles of gas in the first four items will make them appear to be of poor quality. Bubbles in the fork lift truck tires may cause accelerated wear and a shorter life.
Resins are used for casting parts and decorative figures as well as for potting small electronic components. A company I once visited was molding Dungeon and Dragon figurines.
Single stage rotary vane vacuum pumps are often used for degassing urethane and other somewhat viscous liquids prior to pouring them in the mold. The product will degas once the vacuum is below about a half atmosphere (380 Torr) depending on the viscosity of the liquid. The amount of liquid degassed at one time can vary from a few ounces to many gallons depending on the product but the process is quite similar.
|Fig. 6. Large Vac./Press. Impreg. System|
A small molding system is shown in Fig. 2. In this system a container with urethane in it would be placed in the vacuum chamber and the acrylic lid replaced on top. The see-through lid lets the operator watch the process and control how fast it degasses. Degassing resin, varnish and urethane can result in a lot of frothing. As the gas bubbles in the liquid move to the top of the liquid, expand and burst they release the trapped gas into the chamber where it is evacuated by the vacuum pump. To reduce the frothing the rate of pressure drop is controlled by a valve such as a ball valve in the vacuum line outside the chamber. Due to the frothing the container holding the liquid often has a high depth to diameter ratio so that the frothing doesn’t spill out of it. After the first high rate of frothing has diminished the operator can visually see when there is no more gas escaping from the product.
In larger systems with gallons of liquid to degas, a stirring mechanism is often added to the chamber to slowly move liquid from the bottom of the chamber to the top. The gas bubbles will escape from the liquid easier when they are near the top of the liquid with less weight of liquid pressing on them. The chamber shown in Fig. 3 has a 55 gallon drum loaded in it and a motor driven stirrer mounted through the lid. Fig. 4 shows a line drawing cross-section through a typical urethane degassing system.
|Fig. 7. A simplified Vacuum Impregnation System|
To prevent arcing the special oil used in electrical transformers is degassed to remove entrained air and absorbed moisture before it is used to fill a transformer case. (Fig. 5) The system heats the oil uniformly to help moisture to evaporate and then sprays it over a series of thin metal plates in the chamber to degas. The oil runs over the plates in a thin film which improves the efficiency of the degassing.
Vacuum and Pressure Impregnation (VPI)
Varnishes and resins are used for potting and also for insulating items such as electric motor and transformer windings. The liquid is degassed and then used in a two chamber unit called an impregnation system (Fig. 6). The windings are placed in a vacuum chamber and placed under vacuum. Molecules of air are trapped between the layers of copper wire and it takes some time for this air to escape from the small spaces and be pumped away. Once the air is removed the liquid is introduced to the vacuum chamber from a storage vessel. With the air molecules removed the liquid is able to penetrate every small space between the windings and create a layer of insulation. A positive pressure is applied to the top of the liquid to help force the liquid into the tiny voids. Fig. 7 shows a typical VPI system.
Many metal castings often have voids in them when they solidify. These castings are often impregnated with liquid resin to seal any porosity and imperfections. Once the casting is impregnated it is heated to cure the resin. The process is similar to VPI described above and is especially important in the automotive engine industry to seal aluminum castings.
Howard Tring is the owner of Vacuum and Low Pressure Consulting, a company that supplies vacuum pump accessories such as reconditioned inlet traps and exhaust filters and new replacement elements for exhaust filters. Howard also offers on-site vacuum technology and oil sealed vacuum pump repair training and consulting services, customized to the needs of the client. Howard is a member of ASM International and the Heat Treat Society, the AVS, the SME, the SVC and the American Society for Training and Development.
Copyright May 2014, Tring Enterprises LLC – Comments on this article are welcome. I do not profess to know everything about any specific vacuum related subject. However, I have worked in the vacuum pump industry a long time and have seen good, bad and ugly. Please contact me with any comment or question. All messages related to the content of the article will be answered.