RF Dryers – Finding Hazards in the Workplace
When Radio Frequency (RF) safety is mentioned, it is often in the context of cell phones and cell towers. However, a multitude of RF systems exist in industry, medicine, and laboratories that require an understanding and evaluation of potential radiation hazards. The workplace is subject to OSHA standards including Section 5 under the General Duty Clause which states in part employers are required to provide employees employment and a place of employment which is free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees. However, few safety plans take into account the potential of radio waves from RF generators. Many facility managers and safety directors are not even aware of machines in their workplaces that could pose risks.
One of the more ubiquitous industrial radio frequency machines is the RF dryer. Often imbedded in process systems, these machines can operate at very high power levels and pose serious personnel risks. This essay seeks to introduce managers to these risks and their management.
The invention of the microwave oven in 1946 started the use of RF generators in the food processing industry. Today, RF generators are utilized widely in manufacturing processes including the drying or heating of ceramic materials used for automobile components as well as sealing bags and curing foods. Other applications include the drying of textile yarns, fabrics, and garments as well as post-baking and moisture control of food products.
Most of the RF generators used in drying processes are of a size comparable with a high power radio station. The generator output is connected to two electrode plates which creates an electric field inside the process or drying chamber. The material to be heated is conveyed between the electrodes, where an alternating energy field causes molecules in the material to continuously reorient similar to bar magnets moving to face opposite poles in a magnetic field. Friction resulting from this molecular movement causes the material to rapidly heat throughout its entire mass. An RF dryer offers the advantages of reduced processing times, consistent quality, and simplified process control.
Typically an RF dryer which is one type of generator used in manufacturing processes operates at one of the following frequencies: 13.56 MHz, 27.12 MHz, 40.68 MHz and 2450 MHz. The output power from each of these generators can be high ranging from 5 to 150 kW. As a perspective, a typical AM radio station operates at 5 kW while and FM station radiated power is 100 kW.
A comprehensive measurement survey by a competent engineer of all areas surrounding RF dryers utilized in a manufacturing process requires the use of a calibrated radiation meter and isotropic probe as well as knowledge of the behavior of electromagnetic fields including radiated power density for various objects and surfaces. LBA Group, Inc. studies and reports on a variety of industrial RF applications including RF dryers, cranes and smart meter production facilities. More information can be found at https://www.lbagroup.com/associates/industrialrfintro.php.
Measurements should be performed around all faces of the RF dryer including the waveguide between transmitter and process chamber as well in the areas where factory employees work. Measurements should be taken as a percent of maximum permissible exposure (MPE) for the general population standard because manufacturing employees are not fully aware nor can they control RF exposure. For FCC purposes, the general population standard applies to human exposure to RF fields when the general public is exposed or in which persons who are exposed as a consequence of their employment may not be made fully aware of the potential for exposure or cannot exercise control over their exposure. Therefore, members of the general public always fall under this category when exposure is not employment-related.
Although not likely to be applicable to a manufacturing environment, The FCC also have exposure limits for occupational(controlled) environments where human exposure to RF fields are for persons exposed as a consequence of their employment and have been made fully aware of the potential for exposure and can exercise control over their exposure.
For a typical manufacturing environment with RF dryers, the general population standard applies where an MPE limit of 0.98 milliwatt per square centimeter (mW/cm2) is used for 13.56 MHz emissions, 0.24 mW/cm2 for the 27.12 MHz emission, 0.2 mW/cm2 for the 40.68 MHz emissions and 1 mW/cm2 for the 2450 MHz emissions as prescribed in FCC OET Bulletin-65.
Some of the RF dryers used in manufacturing process can have “leaky” doors where RF radiation can escape through the door seals. Manufacturers of this equipment should be aware of the potential RF hazards and the need for adequate shielding designed into the equipment. Care must be taken to ensure no RF energy from these dryers leak. The initial approach to any RF dryer should be a cautious one where measurements are made at a relatively large distance then moving closer to the equipment while monitoring the radiation meter. Both electric and magnetic fields are a concern in RF dryers that operate in the HF band.
In a recent RF survey by LBA engineers, a large manufacturing facility that supplies automobile materials, containing various RF dryers was found to be in compliance with FCC MPE levels. However, one RF dryer contained a “leaky door” and excessive power density readings were observed 3 to 6 inches from its surface. The power density levels exceeded MPE limits close to the door seal then decreased rapidly and
met MPE limits at 12 inches from the surface. Damaged finger stock in the door seals can leak RF energy and persons who may accidently stand against or near the door could be exposed to excessive RF energy. Touching the door seal with exposed skin where the leak exists can be an RF shock or burn hazard.
A coordinated test and RF measurements of the generator and its leaky door were performed and planned in accordance with the manufacturing process schedule. However, if the leaky door could not have been immediately corrected, the appropriate RF signage should be installed, and workers alerted to the confined “hot spot”.
There are many RF processes in manufacturing, including heat sealers, RF welders, plasma systems, and many more. The hazards associated with these may not be immediately obvious. To assist safety managers, LBA offers a range of RF safety planning and program services, including site audits, industrial hazard evaluations, safety training courses, and radio frequency safety plan development.
LBA’s new OSHA RF Safety Awareness course is oriented to wireless industry workers. However, it is relevant to all trades and types of personnel that may encounter RF exposure in the workplace. Information on the RF safety training is at https://www.lbagroup.com/associates/rftraining.php.
For assistance in RF safety matters, contact Bryan Dixon, Safety Services Director at 252-757-03279 or bryan.dixon@lbagroup.com.
