Controlling Temperatures to a Fraction of a Degree
Ovens are particularly common in chemistry, biology, medical, materials, and forensics labs. Applications range from low-tech glassware drying to sample drying and incubation, equipment sterilization, evaporation, hardening/ curing, tempering, stability testing, aging, baking, annealing, brazing, sintering, burn-off of organics, melting, heat-treating, and hot-pressing.
Most basic lab uses employ oven temperatures from just above ambient to several hundred degrees Fahrenheit, although ovens used for materials processing reach temperatures in excess of 1000ºF. Kilns, specialty ovens used to process ceramics, may reach 2400ºF.
Basic components common to all general- purpose lab ovens are an electrical heating coil, insulation, temperature measurement and/or recording, and a circulation mechanism that provides even temperature distribution. Advanced features include double doors, digital control, and temperature recording (useful for regulated industries requiring documentation).
Oven configurations include bench- or cabinet-style, conveying, and vertical. Cabinet ovens are used for batch processing, while conveyor designs— common with medium-to-industrialsized process applications—provide continuous heating of many samples.
Circulation ovens (the most common in labs) come in two types: gravity convection or mechanical (forced) draft. The former often suffer from temperature inhomogeneities and stagnation, which is why ASTM and AASHTO standards call for forced draft ovens.
Throughput and types and breadth of applications are the principal factors influencing oven purchases. Larger labs primarily interested in glassware drying are better served by large ovens with customizable configurations than by high-tech units with advanced controls. Materials testing or pharmaceutical development groups involved in drying or curing should focus on temperature stability/uniformity and perhaps automated recording and diagnostics. Users should modestly overbuy on temperature range to ensure that their applications will easily be covered.
However, for a given heat rating, oversized ovens consume considerably more energy than compact designs, have a larger footprint, and may require specialized electrical hookups. Smart buyers whose oven volume and application needs vary often purchase several smaller ovens rather than one large one. Lab ovens range in size up to capacities of 25 cubic feet, but most applications employ units of 6 cubic feet and smaller.
Other features to consider are general location, exhaust capabilities, mounting (floor or tabletop), fire/explosion protection, ambient or inert atmosphere, and controls/displays. Location is connected with unit size, ease of use, compatibility with other equipment, exhaust, and access to electric utilities.
PID [proportional-integral-derivative] controllers add precision, accuracy and uniformity to temperature control, notes Frank Brombley, general manager at JEIO Tech (Woburn, MA). PIDs provide step programming in user-defined increments and times and are desirable in precision applications like materials curing, biology, chemistry, or drying. “PID controllers minimize errors between a measured process variable and a desired set point by calculating, and then outputting, a corrective action that can adjust the process accordingly and rapidly,” says Brombley.
Inexpensive controllers act like home thermostats that simply turn the heater on and off, resulting in temperature cycling. Even precise thermostating results in whole-oven temperatures varying over time by several degrees. Temperature variations, in turn, cause the system to expand and contract, compromising the integrity of the seals, which adds even more temperature fluctuation. This may not be an issue for glassware drying ovens but may introduce variability for materials curing or biological cell culture.
Uneven temperature distribution often arises with lower-cost ovens in which the heating element is in contact with the outer envelope of the main oven compartment. This design causes contact points to heat more rapidly and stay hotter than the rest of the oven for a given overall temperature. Binder has pioneered a double outer chamber system comprising an insulating air jacket that prevents contact between the heating element and the oven chamber. The result is low-fractionsof- a-degree variability throughout.
Trends in lab ovens
Uwe Ross, executive VP at Binder (Great River, NY) notes that in recent years, users’ preferences have shifted from gravity ovens without fans to fanbased forced-air units. Fans distribute heat more rapidly on startup, and “people are becoming less willing to wait for units to heat up,” Ross observes. Fans are suitable for most applications, with one notable exception: powders.
Fans provide more even heating by minimizing temperature variability within the oven, to the point where temperature distribution becomes a selling point. ASTM, for example, specifies an oven’s temperature deviations by measuring at nine locations inside the oven, while the newer DIN (Deutsches Institut für Normung) standard uses 27 points. Vendors supply temperature specifications, which vary from fractions of a degree in high-end ovens to several degrees. “Users will tell you that an application works great—on the middle shelf in the rear left corner—but nowhere else,” comments Ross.