Q&A: What are the most common consumable costs when operating a low-energy EB system?
by Anthony Carignano, Director, Sales – Americas
Electricity, Nitrogen, Tungsten Filaments, Titanium Foils
During the past decade, interest has grown substantially in potential uses for low-energy electron beams with regard to inline curing and cross-linking applications. For example, with increasing market demand globally for flexible consumer product packaging, EB has become an energy-saving processing method option both for improving the physical performance of polymeric films and enhancing indirect contact print safety of consumer product packaging. As part of the vetting process to determine if EB is right for you, estimated below are a few common equipment consumable costs associated with operating a low-energy electron beam system.
Electricity (Energy Consumption)
Several studies completed in the past decade compare the energy requirements and efficiency of operating a low-energy electron beam versus conventional drying methods. On average, the total energy required to EB cure one gram per square meter of acrylate chemistry is 30 joules. Hot air drying for organic solvent and waterborne systems requires a significantly larger amount of energy to dry the same product coat weight, as shown in Table 1.
Table 2 shows the total cost requirement estimate per year for operating a 125kV EB system at 3MR for 100 percent solid coil coatings cured at room temperature. Organic solvent-based coil coatings require water quench tanks and thermal oxidizers. Based on energy cost savings alone, the return on investment of an EB coil coating line can be less than 18 months.
Nitrogen gas is used for a wide range of industrial applications, from precision laser cutting to welding and even eye surgery. Most common EB-curing processes require nitrogen inerting at purity levels at or above 99.9995 percent. For surface-curing applications, a nitrogen inerting level of less than 200 ppm oxygen is typically used. Width, height and the web gap of the irradiating area are major factors in EB system nitrogen consumption. Beam absorber shielding designs typically require high volumes of nitrogen when in operation, compared with shield roll designs. Higher irradiating line speeds and porous substrates that entrain air also have a major effect on nitrogen use. Formulation selection also can impact nitrogen consumption.
For EB-curing applications, high-purity nitrogen gas is by far the largest consumable cost associated with operating a low-energy electron beam system. A 61-inch irradiating width EB coil coating line running at 460 feet per minute for 5,000 hours per year can be expected to consume about 140 cubic meters of nitrogen per hour, resulting in a total high-purity bulk nitrogen cost of $80,000 per annum. Your elected EB system supplier can help identify a suitable and reliable nitrogen source and configuration options.Tungsten Filaments & Titanium Foils
Multifilament linear cathode EB systems, also referred to as actively “pumped systems,” are commonly used for inline crosslinking and curing applications and can range in irradiating width from 30 to 108 inches. The filaments and foils found in actively pumped systems typically require replacement at least once per year, if not more often, and can represent around 15 percent of the total consumable cost of operating an EB system. The current, which is generated by the EB system high-voltage power supply (HVPS), illuminates the tungsten filaments and generates the release of electrons through the cathode vacuum chamber and titanium foil. After either the mass of the filament is depleted through its release of electrons, or because there is poor vacuum inside the vacuum chamber, filaments eventually fail and must be replaced. Under optimum conditions, tungsten filaments will last up to 10 months.
Titanium window foils cover the full irradiating width of the electron beam and are supported by a copper cooling grid (Figure 1). Certain EB system designs require the replacement of one continuous foil. Other systems require the replacement of two foils per system. Titanium foil replacement typically is completed by a technician who is well trained in the art of not creasing foils that can range in total thickness from 10 to 15 microns. The average life of the titanium window that faces the shielded surface of the EB is four to six months. Improper installation, quality of consistent vacuum, efficient transfer of heat away from the foil window and arcing caused by temporary lapses in voltage are variables that can affect foil life. To minimize downtime and maximize EB performance, the filaments and foils on pumped systems are typically replaced at the same time and can cost approximately 10 percent of the total operating cost, depending on the width of the irradiating width of the electron beam.
Unlike pumped systems, sealed “EB lamp” systems contain a single cathode filament tube. Much like a light bulb, the entire lamp is replaced when spent and thus considered a consumable. Depending on usage, sealed EB lamps have an average life of greater than 8,000 continuous hours and generate electrons under the same principles as pumped systems. It is anticipated that the cost of sealed EB lamps will decrease substantially over the next three years as demand increases, driven by surface sterilization, narrow web finishing and ultralow migration inkjet printing applications.
The conclusion to this discussion is that EB systems represent a highly energy efficient method for instantaneously surface curing and crosslinking polymeric substrates. The EB curing requires a source of high-purity nitrogen for surface inerting to minimize the potential for oxygen inhibition during the free radical polymerization process. Nitrogen is the major consumable cost in operating EB but can be minimized through optimized system design. Tungsten filaments and titanium foils also are consumable items required to operate a low-energy electron beam and also are affected by EB equipment design and performance.