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Many factors should be considered when purchasing a new electrical resistance-heated lab/test furnace or kiln. One of the first factors should be the desired operating temperature that will be used most often. Many new lab and test furnace purchasers often assume that one furnace can operate through a very broad temperature range, such as 1000-1800°C. While this is certainly possible, requesting such a range can lead to sticker shock, which can be discouraging to new furnace purchasers. Upon further consideration, the user may realize that 95% of their lab/test processes only require a maximum temperature of 1100°C, which would be more realistic and certainly more cost effective at purchase time.
Lab and test furnaces can be divided into three temperature ranges according to their heater technology. The first range, based on wire heating element technology, extends to a maximum of 1300°C, although some special use applications can reach up to 1400°C. The second group is based on silicon carbide (SiC) heating elements and generally has a useful upper range of 1550°C. The third group uses molybdenum disilicide (MoSi2) heating elements that can easily reach 1750°C and, with care, can be used to up to 1800°C.
Of course, as the temperature increases, so does the cost. A rough rule of thumb is that if a furnace with a maximum temperature of 1300°C costs one unit, a furnace with maximum temperature of 1550°C will cost two to three units, and a furnace with a maximum temperature of 1750-1800°C will be three to four units. (This rough order of costing assumes the same general heated chamber geometry for each unit.)
Expansion PotentialAnother point to consider when buying a new lab/test furnace or kiln is future expansion. It makes sense to order a furnace with specific working dimensions for existing projects. However, if planning indicates that future projects will require a larger unit, it might be wise to consider a larger unit for the initial purchase (depending on timeframes and cost restrictions).
Unfortunately, due to a range of variables, it is impractical to estimate how much more a larger unit will cost without getting an actual price quote. In some instances, doubling the working volume adds less than twice the cost and delivery time; in other instances, the cost could more than double and the delivery could be significantly extended.
If future projects dictate that a higher temperature unit will be required, several additional factors must be evaluated. The external size of the unit can increase with temperature because of the need for more insulation. For this reason, the current existing normal operating temperature parameters should be studied with respect to issues of maintenance, reliability, and temperature uniformity as compared to a higher temperature unit.
Furnaces are designed to operate most efficiently and give the best uniformity at their specified operating temperature. Purchasing a 1600°C rated furnace based on possible future applications and then using it for daily operation at 600°C only creates operational issues related to process control and temperature uniformity. This would be the equivalent to someone buying an F1 racecar to drive to the supermarket, with the remote possibility that they may have a chance to drive in the Monte Carlo Grand Prix someday. Capital costs, reliability, low speed performance, and maintenance issues might make a used VW a wiser purchase.
Box vs. TubeThe next point to consider should be furnace geometry. Should you purchase a box unit or a tube unit? While a box unit is great for loading samples in a batch operation, a tube unit is generally better for continuous applications such as gas conditioning or material characterization testing that can take place inside of a process tube.
Each style also has additional options to consider. Box units feature several types of doors, such as a simple front door (either vertical or hinged side swing). Special process applications might require a bottom-loading or elevator-type unit in which the samples are loaded on a base that is then raised into the bottom of the furnace. Elevator units can typically be made more uniform and can be loaded “hot.” In addition, if designed correctly, they have a faster recovery time and can be more efficient. Some drawbacks include a significantly higher purchase price and possible higher maintenance costs due to moving parts.
With tube furnaces, options include either a solid tube or a split tube type. If the application requires repeated access to the internal heated chamber, a split tube is the best option. The solid tube unit offers a generally flatter section of radial uniformity and costs typically 20% less.
Sometimes the process requires that samples be shielded from direct radiation from the heaters. In this case, the solid tube unit can be designed with a diffuser built in as an integral part of the heating structure, while the split tube will have to have a separate unit installed and supported. This diffuser will also somewhat negate the advantages of quick access to the samples being processed, unless the diffuser is used as a carrier that is loaded ahead of time then placed in the furnace for processing.
In other cases, rather than having a flat uniform temperature in the working area, it is required to have a known temperature gradient across the work area. A tube furnace would be the most beneficial and simply adjusted solution under those circumstances.
UniformityUniformity is another issue that must be addressed when looking at a new lab/test furnace or kiln. A general rule of thumb states that the center 80% of the working dimensions of a furnace will exhibit a ±5° temperature variation. Should a greater uniformity be required, several options exist. For lower temperature units (approximately 700°C or lower), stirring fans or recirculating air heating systems would be necessary.
For higher temperatures the furnace may need to be larger in order to achieve the required temperature uniformity or “flat zone,” perhaps a different heater configuration may be recommended, or the addition of multiple heat zones may provide the solution. Unfortunately, no hard or fast rule covers all situations. It is often the case that the design requirements are specific to the user’s uniformity and operating requirements.
Process requirements may dictate the need for forced or controlled cooling. Many options are available, ranging from the introduction of cooled gases to vents, fans or a combination of all the above-including special programming of the temperature controllers to achieve a controlled cool-down cycle.
ControlsAnother important consideration is the type of controls desired. Will a standard single set-point temperature controller work? With these controllers, the unit ramps at an uncontrolled rate to a set process temperature and stays there until manually shut down. Or will a programmable unit be required? Programmable units allow for adjustable ramp rates with hold times, soak times, and shut down after completion of the process.
In addition, a data logging and/or computer interface may be desired, or an over-temperature control to ensure that the unit does not self-destruct. Of course, with increasing degrees of sophistication and technology, the price will also increase. Thanks to advances in technology and electronics, many of these options are significantly less expensive than they were just a few years ago and, in some cases, one controller can perform many different functions.
AtmosphereAnother consideration should be the atmosphere that will be used in the unit, as this can have a significant bearing on purchase and maintenance costs. In general, if the unit is going to be operating in an air atmosphere, no special considerations are required. Should the process generate off-gassing of volatile compounds, efforts must be made for venting and possibly protecting the inside of the furnace from chemical attack, depending on the types of gases released.
If an atmosphere is required, and it is a simple “blanketing gas” such as nitrogen or argon, then a gas inlet and exhaust port might be necessary in an otherwise standard furnace. The spent gas can then be safely exhausted from the work area via an exhaust hood or exhaust manifold piping. It should be noted that when using nitrogen with the higher temperature classes, special care must be taken to prevent damage to the elements due to interaction of the nitrogen and compounds used in the silicon carbide and molybdenum disilicide heaters.
Should the atmosphere be some sort of “forming gas” or explosive in nature such as hydrogen, various safety features will be required and the use of a retort may be needed. A retort is simply a sealed containment vessel that serves to protect the furnace from attack while containing hazardous compounds. A retort can significantly add to the purchase price of the unit, as well as additional operational, safety and maintenance issues.
AestheticsFinally, cosmetic issues should be considered. Will the vendor’s standard color scheme work, or must the unit be painted a special color to match existing equipment or standards? Instead of a painted exterior, is it necessary for the unit to have a stainless steel exterior?
While many companies offer both versions based on style and unit type, the stainless units often carry a premium. In addition, converting a standard painted unit to a stainless exterior will cause significant increases in cost and delivery.
Planning is KeyStandard products are available from most vendors of lab/test furnaces and kilns, and some manufacturers offer standard units as well as fully customized units that can be specified and engineered to exact customer requirements. Careful consideration of the topics covered in this article will put you ahead of the curve when the time comes to begin the search for your new lab/test furnace or kiln.
For more information, contact Thermcraft, Inc. at 3950 Overdale Rd., Winston Salem, NC 27107; call (336) 784-4800; email firstname.lastname@example.org; or visit www.thermcraftinc.com.