Navigating the requirements of drying, curing and baking temperature-sensitive components can be challenging for the process engineer. When raising the temperature is not an option, lowering the pressure can be an effective process tool. Utilizing a low-pressure condition in a vacuum oven can offset the confines of heat restrictions by causing trapped moisture and gases to evacuate at lower temperatures. For optimal offgassing and drying, a combination of heat and vacuum is required.
Vacuum drying and offgassing are common in the pharmaceutical, electronics, medical device and aerospace industries where fragile organic compounds, plastic components, resins and polymer devices must be dried and cured without the damaging effects of high-heat processing. While vacuum oven processing is effective, it is also fraught with obstacles. Vacuum is such a poor conductor of heat that it is the most effective insulator on the planet. In other words, the vacuum that is assisting the drying process is also impeding the transfer of heat to your product.
Ten Tips for Vacuum Ovens
1. Adding Heat is Vital to Vacuum Drying, Offgassing
Exposing components to a vacuum at ambient temperature quickly results in below-ambient temperatures on the samples themselves. As the pressure drops, molecules volatilize and are swept out by the vacuum pump. The phase change from liquid to gas consumes heat energy, which reduces the temperature of the sample. As the offgassing continues, the temperatures continue to drop and create pockets of ice. Once water has frozen, it is difficult to remove it from the samples. For this reason, vacuum processing alone is seldom adequate to fully outgas or dry samples. A vacuum oven that utilizes increased heat and reduced pressure is the key to consistent, thorough processing of these temperature-sensitive samples.
There are three main methods of heat transfer: convection, conduction and radiation. A campfire provides an excellent illustration of all of the three ways heat moves. Most process heating applications rely on convection, just like toasting a marshmallow over a campfire. Ask any Eagle Scout or thermodynamicist and he will tell you to hold the marshmallow in the heated air over the flames so that it is perfectly toasted and not burned. It is not the fire itself but the heated air above the fire that is toasting the soon-to-be s’more.
If you hold a metal skewer with one end in the fire for a few minutes, you will notice that soon the handle is hot to the touch. The metal itself transfers the heat along the entire length by the second-most common method of heat transfer: conduction. Finally, sitting around the campfire, the warmth you feel emanating out in all directions is due to radiation.
The most efficient and effective method of heat transfer is convection. In a drying application, hot air is circulated around the product, heating it and causing trapped moisture and gases to volatilize over time. Convection ovens have wire racks or perforated shelving to promote air circulation because this is the primary heat source of heat transfer. This method of heat transfer is not at play in vacuum processing. The lack of atmosphere prevents convective heat transfer. With convection off the table, the vacuum process relies on conduction and, to a lesser degree, radiation to transfer heat to the samples.
2. Not Just a Lot of Hot Air
Conduction is the second-most effective form of heat transfer. Conduction requires two bodies that are in contact with each other so that the heat will transfer from one body to another. In the case of vacuum oven processing, heat is transferred from the shelf surface to the samples. To maximize the efficiency of conductive heat transfer, it is important to carefully select the correct metal for shelf construction. Thermal conductivity is expressed as the quantity of heat transmitted through a unit thickness due to a temperature gradient. In other words, thermal conductivity is a calculation of how evenly and efficiently heat can move across any given material. Granite has low conductive properties at 1.7 W/(m·K). On the other end of the spectrum, diamond is the best conductor of heat at 1000 W/(m·K). Metals such as silver, gold and copper conduct heat well.
For efficient — and affordable — heat transfer in a vacuum oven, aluminum is the best choice for shelf construction. The worst choice would be stainless steel. High-quality vacuum ovens are always equipped with solid aluminum shelves to promote maximum heat transfer.
The thermal conductivity value is not only important for heat transfer but for protection of sensitive products as well. Materials that do not have high thermal conductivity properties — like stainless steel — will transfer heat unevenly, which results in hot spots, temperature spikes and uneven batch processing.
The final method of heat transfer is radiation. Unlike convection and conduction, which rely on atoms to transfer heat, radiation uses electromagnetic waves along the infrared spectrum to move heat. The sun transfers heat through the vacuum of space to earth by radiation. Our sun is a massive nuclear reactor that weighs 2x1030 kilograms and emits 3.86 x 1027 kilowatts of power per second. It is difficult to reproduce this level of heat transfer in a small metal box known as a vacuum oven. Even though all of the heat energy from the sun is transferred to the surface of the earth by radiation, it is primarily by conduction and convection that the heat is transferred to the air.
Although the least efficient and slowest means of heat transfer, radiation is a small part of the equation in vacuum processing. Heat will transfer by radiation from both the actual heat sources — typically radiant heaters placed evenly outside of the vessel — and the other surfaces in the chamber. This is where things get tricky. Relying on radiation to consistently heat your sample requires that you understand the exact heat transfer coefficient of the sample and everything touching it. If the sample in the oven — a circuit board for example — is on a solid shelf, it will be shielded from thermal radiation on the bottom surface. This means the top will be processed at a different rate. If the circuit board is on a wire rack — like a convection oven shelf — the board will be exposed more than it would be on a solid shelf, but the places that are in contact with the wire rack will not receive radiation and will be heated unevenly. Are the samples and the shelves black, white, polished or matte? All of these variables will affect the heat transfer.
Changes in the sample over time will also have an effect. In the case of a phenolic resin, the heat transfer coefficient will change as the samples go from a liquid to a solid state. Highly conductive shelves can mitigate the damaging effect of radiative heat transfer by evenly conducting the heat across the shelves.
3. Optimal Placement of Heat Sources
Convection ovens have the luxury of using volumes of hot air circulating around the chamber to evenly drive heat to samples. The rapidly moving air in the chamber makes heater distribution and placement a secondary concern to overall heater wattage and airflow. Purely in terms of temperature uniformity, one larger heater is just as effective as multiple, smaller heaters. Yet, in a vacuum oven, heater distribution and placement is critical. Because we are relying almost entirely on conduction, it is important that the heaters are well distributed around the chamber and evenly transfer heat to the shelves. Heater wattage should be evenly distributed on both sides of the vessel because horizontal transfer across the shelves is the primary vehicle for heating. Larger vessels will sometimes utilize supplemental heaters on the bottom and top of the vessel to drive heat to the product.
Even though hot air pooling at the top of the chamber is not a concern because of the lack of atmosphere in the chamber, top mounting of the heaters is the least optimal placement. A top-mounted heater that is warming the vessel above the samples will not be able to effectively drive heat evenly across multiple shelves. Further, the radiation from the heater could have an adverse effect on the uppermost sample while not providing any heat at all to the lower shelves.
4. Be a Control Freak
The proper shelves, heater placement and heater wattage are all just chatter if the control loop is not effective. A standard temperature probe will be ineffective in a vacuum oven because there is no air to warm it. Ideally, the probe should measure off of one of the conductive surfaces. By placing the temperature probe in close proximity to a shelf, we can ensure that the control loop is tracking actual internal temperature. PIDs should be utilized within the controller to prevent temperature spikes and fluctuations. The slow response time due to the vacuum will make auto-tuning useless. The control parameters have to be engineered by hand. The correct control loop will have an internal probe in contact with a shelf and a robust controller with PID values designed for vacuum applications.
In conclusion, good vendors do more than supply you with quality equipment and materials. Seek partnerships with vendors who understand their products well enough to provide in-depth process analysis and suggest solutions. Use key vendors as part of the product development cycle and when building a new production line so that you can benefit from their specific knowledge. You do not need to be an expert on vacuum ovens; you just need to know where to find one.