This article discusses the TransTemp Transparent Tube Furnace and some of the factors which need consideration when specifying a new electrical resistance heated furnace.
- Furnace Geometry
- Operating Temperature
- Type of Controls
- Future Expansion
- Standard vs Custom
- Cosmetic IssuesFurnace Operating Temperature
One of the first things to consider is the preferred operating temperature that will be utilized most frequently. “Most often’ because many new industrial and lab furnace buyers often believe that one furnace can operate through a wide temperature range.
For example, a common query would be, ‘I need a furnace that can be used from 1000 °C to 1800 °C” While this request is certainly possible the requested temperature range crosses all three temperature limits and price ranges. This is a usual mistake which leads to sticker shock which can be discouraging to new furnace buyers. Upon further deliberation, the user can escape to the fact that 95% of their procedures only require a maximum temperature of 1100 °C, which will be more cost effective and realistic at purchase time.
Vertical Split Tube Furnace industrial and Lab furnaces can be split into three temperature ranges, according to their heater technology. The first uses a wire heating element technology which extends to a maximum of 1300 °C, although some special use applications claim up to 1400 °C. The second category employs silicon carbide (SiC) heating elements and usually has a working upper range of 1550 °C. The third category is based on molybdenum disilicide (MoSi2) heating elements which can easily attain 1750 °C, and with care can be utilized up to 1800 °C.
Obviously, with higher temperatures, the pricing also rises. A rough rule of thumb suggests that if a furnace with a maximum temperature of 1300 °C costs one unit, a furnace with a maximum temperature of 1550 °C will cost 2 to 3 units, and a furnace with a maximum temperature of 1750-1800 °C will be 3 to 4 units. This rough order of costing assumes the same common heated chamber geometry for every unit.
The term “kiln” is most often utilized in the cement industry and ceramics processing, while the term “furnace” is used most often in general heating, metallurgical processing, and material characterization applications. According to the type of industry, the exact same unit can either be referred as a furnace or a kiln. In general, these terms may be used interchangeably.
Another point for consideration when purchasing a new lab furnace is future expansion. It may be required that a furnace is ordered with specific dimensions for ongoing projects, but plans may suggest that future projects will need a larger unit. As per the cost restrictions and time frames, it may be wise to seriously deliberate a larger unit in the initial purchase.
Unfortunately, because of a wide array of parameters, there is no quick way to calculate how much more a larger unit may cost without obtaining an actual price quote. In a few cases, doubling the volume can add twice the delivery time and cost, in other cases; the delivery might be significantly delayed, and the cost may be more than double.
If future projects indicate that a higher temperature unit would be needed, various factors must be assessed. The external size of the unit and the cost for increasing temperature range can rise with temperature because of the requirement for more insulation; you must also account for the existing usual operating temperature criteria pertaining to issues of Compact Split Tube Furnace reliability, maintenance, and temperature uniformity in comparison to a higher temperature unit.
Furnaces are created to work efficiently and provide the best uniformity at their indicated operating temperature. Buying a 1600 °C rated furnace considering potential future applications and then utilizing it for day-to-day operation at 600 °C only may pose operational challenges in terms of temperature uniformity and process control. This is like buying an F1 race car driving to town for grocery shopping with the least possibility that you may get a chance to drive in Monte Carlo grand prix someday. Reliability, capital costs, low speed performance, and maintenance issues would be such that may be a used VW could have been a prudent purchase, not to mention upgradation in technology which may happen, making the unit somewhat obsolete.
The next point to note is furnace geometry. Should you buy a tube unit or a box unit, the box unit is great for loading specimens in a batch mode while a tube unit is usually better for a continuous application like material characterization testing or gas conditioning which may occur inside a process tube.
Each style also has other options to contemplate. With a box unit there are many types of doors like a simple front door (either hinged side swing or vertical) or probably because of special process utilities an elevator type or a bottom loading unit may be needed in which the specimens are loaded on a base which is then elevated into bottom of the furnace. The elevator unit generally can be made more uniform. LAB-TEMP Chamber may be loaded “hot”, and if created appropriately has a quicker recovery time and might be more competent but at the drawback of a considerably higher buying price and probably higher maintenance costs because of moving components.
With tube furnaces, you have an option of either a split tube or a solid tube type. If the utility needs repeated access to the internal heated chamber, then a split tube is the preferable choice. The solid tube unit will provide a usually flatter section of radial uniformity and would cost approximately 20% less.
In some instances, the process needs that specimens be covered from direct radiation from heaters. In such case, the solid tube unit can be designed with a thermal diffuser built-in as an integral portion of heating structure, while the split tube will need a separate unit installed. This thermal diffuser will also to some extent reduce the benefits of quick access to the specimens being processed, unless the thermal diffuser is also utilized as a carrier which is loaded before time and placed in furnace for processing.
In few instances, instead of having a flat temperature in the working area, a known temperature gradient is needed across the work area. In such case, a tube furnace could be the most advantageous and simple solution.
Uniformity is another point which must be addressed when looking at a new lab furnace. A general rule of thumb suggests that the center 80% of working dimensions of a furnace will show a +/- 5 °C temperature fluctuation.
If a higher uniformity is needed, various options exist. For lower temperature units (around 700 °C or lower), recirculating air heating systems or stirring fans would be essential. For higher temperatures a larger furnace might be needed for achieving the “flat zone” or necessary temperature uniformity, probably a different heater configuration might be advised, or multiple heat zones may give the solution.
Unfortunately, no rule addresses all scenarios. It is frequently the case that the design criteria are specific to the operating and uniformity requirements of the user.
It may be needed to add some kind of controlled or forced cooling because of process requirements. Several options are available including the vents, introduction of cooled gases, fans, or a combination of all with special programming of temperature controllers to attain a regulated cool down cycle.
Another important consideration is the preference of type of controls. Will a programmable unit be needed (adjustable ramp rates with soak times, hold times, and shut down post completion of the process) or will a usual single set point temperature modulator work (unit ramps at an uncontrolled rate to a set process temperature and remains there until manually closed)? Additionally, some kind of computer interface and/or data logging may be preferred, or an over temperature modulation to make sure that the unit does not self-damage.
Of course, with rising technology and degrees of sophistication, the cost will also increase Thanks to advances in electronics and technology, man options are relatively less expensive compared to just a few years ago, and in few instances, single regulator may carry out many different functions with Lab Split Tube Furnace.
Another aspect is the atmosphere which will be utilized in the unit as this have a substantial bearing on buying and maintenance expenses. Usually, if the unit will be functioning in an air atmosphere, no special deliberations are needed. If the process generates de-gassing of volatile materials, then arrangements must be made for expelling and probably protecting the inside of the furnace from chemical attack, according to the types of gasses released.
If an atmosphere is needed and it is a simple “blanketing gas” like argon or nitrogen, then all that may be needed is to add an exhaust port and a gas inlet in an otherwise regular furnace with the user arranging a way to safely remove the used gas from the work area either through an exhaust manifold piping or exhaust hood.
Due to the interaction between nitrogen and the compounds employed in the molybdenum disilicide and silicon carbide heaters, special care must be exercised to avoid damage to the elements while employing nitrogen with the higher temperature classes.
In case the atmosphere is explosive or in nature like hydrogen or some sort of “forming gas”, then several safety features will be needed and the use of a retort may be advised. A retort is basically a sealed vessel which works to save the furnace from attack and hazardous components. A retort can substantially add to the buying price of the unit, not to mention safety, operational, and maintenance issues .
A final point to be deliberated, although it may seem trivial at first, is cosmetic problems. Should the unit be colored in a special color to match existing standards or equipment? Will the service provider’s regular color scheme work? Be mindful that the moment the term “custom” come into the equation, buying prices as well as lead times start to rise.
Moreover, in place of colored exterior, is it required for the unit to have a stainless steel exterior? While many manufacturers provide both versions depending upon type and style of unit, the stainless types often carry a premium, and buying a colored standard unit and changing to a stainless exterior will significantly add to the cost and delay the delivery.
Standard vs Custom Furnaces
Standard products are offered by majority of lab furnace vendors, while certain manufacturers like Thermcraft provide standard units along with fully customized furnaces which can be engineered to meet the specific customer needs.
Cautious contemplation of the topics covered in this write-up will put you in a better position while searching for your new lab furnace.
This information has been sourced, reviewed and adapted from materials provided by Thermcraft.
For more information on this source, please visit Thermcraft.