The lab needs a new lab furnace. There are a number of points that need to be considered while purchasing a new electrical resistance heated furnace. This article provides a guide to buying the right furnace for your application.
Factors to Consider while Buying Furnace
The various factors that need to be considered when puchasing a new laboratory furnaces. They are listed below:
- Operating Temperature
- Furnace Size
- Furnace Geometry
- Furnace Atmosphere
- Furnace Controllers, Data Loggers and Computer Interfaces
Firstly, one needs to decide on what will be the operating temperature that will be used frequently. It is definitely possible to obtain a furnace that spans a broad temperature range especially for laboratory purposes but then cost factors have to be considered.
One needs to research a bit more and the user may accept that most of their processes require only a particular maximum temperature, which is more practical and definitely more economical.
Temperature Ranges Based on Heating Technology
Test and lab furnaces can be classified into three temperature ranges depending on their heater technology.
- This temperature range is based on wire heating element technology and extends to a maximum of 1300°C, although certain special use applications claim up to 1400°C.
- The second group is based on silicon carbide (SiC) heating elements and has an upper range of 1550°C.
- The third group is based on molybdenum disilicide (MoSi2) heating elements that can reach temperatures of 1750°C and can be used with caution up to 800°C.
It should be noted however, that with an increase in temperature, the prices also increases.
Clarification Between Furnace and Kiln
The terms furnace and kiln can be used interchangeably in a broad sense. To be more specific, the term kiln is used in the cement and ceramics industry while the term, furnace is used in general heating, metal characterization and metallurgical processing applications. Hence based on the industry, the term kiln or furnace may refer to the same unit.
When purchasing a new laboratory furnace one must consider whether there is a chance of future expansion in the near future. A furnace may be ordered with specific working dimensions for current projects, but future plans may require a larger unit.
Keeping in mind, cost restrictions and time frame, it is wise to consider buying a large unit during the initial purchase itself. It is possible that by doubling the working volume, the cost may drastically increase or maybe not as much but you can never be too sure unless one gets a price quote.
In case future projects need a higher temperature unit then several factors must be evaluated. The cost involved with a increased temperature range, a larger size unit due to the need for insulation, and also one must consider the present normal operating temperature parameters with regards to issues of maintenance, consistency and temperature uniformity when compared to a higher temperature unit.
Furnaces are designed to operate with high efficiency and uniformity at their specified operating temperature. It is again not a wise decision to purchase a high-temperature furnace when you actually need a low- temperature one as this will create operational issues with temperature uniformity and process control.
The furnace geometry is the next point to be considered. A box unit is ideal for loading samples in a batch operation while a tube unit is ideal for a continuous application such as gas conditioning or material characterization testing that takes place inside of a process tube.
There are additional options to be considered with each style. A box unit may have a simple front door either vertical or hinged side swing or for certain applications an elevator type or a bottom loading unit may be required in which the samples are arranged on a base, which is raised into the furnace bottom.
With tube furnaces, either a split tube of a solid tube can be chosen. In case the application requires repeated access to the internal heated chamber, then a split tube is the right choice. The solid tube unit will provide a flatter section of radial uniformity and will cost 20% less.
In cases where the samples have to be shielded from direct radiation from the heaters, the solid tube unit can be made with an in-built diffuser as an integral part of the heating structure, while the split tube must have a separate unit installed and supported.
Instead of a flat uniform temperature in the working area, a known temperature gradient is essential across the work area. In this case, a tube furnace is highly advantageous.
It is important to address uniformity. The standard rule states that the central 80% of the furnace’s working dimensions exhibits a +/- 5°C temperature variation. In case more uniformity is needed, for lower temperature units, approx. 700°C or lower, stirring fans or recirculating air heating systems will be needed.
For higher temperatures a larger furnace is required in order to achieve the required temperature uniformity or “flat zone”, a different heater configuration may be suggested or multiple heat zones may need to be added. The design requirements are specific to the user’s uniformity and operating requirements.
Due to process requirements, controlled or forced cooling may be needed. The options include the introduction of cooled gases, to vents, fans, or all of the above including special programming of the temperature controllers to achieve a controlled cool down cycle.
Furnace Controllers, Data Loggers and Computer Interfaces
Another key consideration is the type of controls needed.
There are several choices available, which are listed below:
- A single set point temperature controller which ramps at an uncontrolled rate to a specific process temperature and remains there until manual shut down is initiated.
- A programmable unit with controlled heating and cooling rates, soak times, hold times and shut down after process completion
- Data logging or computer interface or over-temperature control to make sure the unit does not overheat to protect against damage to the furnace.
Purchase and maintenance costs of laboratory furnaces are substantially affected by the atmosphere.
For an air atmosphere, no special considerations are needed. In case the process generates off gassing of volatile compounds, then provisions must be made for venting and as well as protection of the furnace interior from chemical attack, based on the types of gasses released.
Blanketing Gas Atmosphere
In case a blanketing gas atmosphere such as argon or nitrogen is needed then providing a gas inlet and exhaust port to the existing furnace will suffice. There must also be a way to exhaust the used gas from the work area either via an exhaust hood or exhaust manifold piping. While using nitrogen with the higher temperature furnaces, it must be ensured that elements are not damaged because of interaction between nitrogen and its compounds used in the silicon carbide and molybdenum disilicide heaters.
Explosive or Forming Gas Atmosphere
In case the atmosphere is an explosive or “forming gas” then various safety features are needed and the use of a retort may be required. A retort is a sealed containment vessel that helps protect the furnace from attack as well as contains hazardous compounds. A retort may add to cost as well as safety, operational and maintenance issues.
The final item to be considered are cosmetic issues. Will the standard color scheme of the vendor be fine or must the unit be painted a special color to match existing equipment or standards? Once we try to customize the furnace the cost proportionally increases with lead times. Instead of a painted exterior one may need a stainless steel exterior. While many companies offer both versions based on style and type of unit, the stainless versions are more in demand and taking a painted standard unit and converting to a stainless exterior will add considerable increase in cost and delivery.
Standard products are available from most vendors of laboratory furnaces, while some manufacturers such as Thermcraft offer standard units as well as completely customized units that can be specified and engineered to meet customer requirements. By carefully considering the factors listed, it will help in choosing the ideal lab furnace.
Thermcraft Inc. manufacturers custom industrial ovens, laboratory furnaces, and other furnace related supplies and is a recognized leader in high temperature heating equipment. Their state-of-the-art facility and design software will allow their staff to meet your requirements for any furnace or oven configuration that you may need.
This information has been sourced, reviewed and adapted from materials provided by Thermcraft.
For more information on this source, please visit Thermcraft.