Lab Ovens and Muffle Furnaces Review Apr. 21, 2015

Introduction


Laboratory ovens are used for high-forced volume thermal convection applications. These ovens commonly provide consistent temperatures throughout. Process applications for lab ovens can be for annealing, die-bond curing, drying, polyimide baking, sterilizing, and other industrial laboratory functions. Common sizes range from one cubic foot to 0.9 cubic metres (32 cu ft) with temperatures that can range from room temp. +5 to over 340°C.

Lab ovens can be used in numerous different applications and configurations, including clean rooms, forced convection, horizontal airflow, inert atmosphere, natural convection, and pass through. Some of the industries that typically use lab ovens: the healthcare industry, technology industry, and transportation industry.


How lab ovens work

A lab oven heats its contents via the principle of convection. The heating element is not located within the specimen chamber of the oven, but in a separate external envelope. This prevents radiant heat from affecting the specimen, but the resulting temperature of the oven walls is enough to heat and dehydrate a specimen.

Convective heat transfer is achieved by gravity or mechanical convection. In the former, cooler air is displaced by warmer air and directed towards the heating element until the entire chamber is up to t°. Since this method has poor uniform heat distribution, energy waste, and a longer preheat time than mechanical convection, mechanical convection lab ovens are favorable. These types heat quicker and more evenly due to blowers and baffles in the oven chamber.

 

Air intakes and exhausts can be adjusted to withhold or release humidity, and are necessary to expel VOCs and fumes. Insulation reduces the rate of thermal transfer and is responsible for the energy efficiency of the oven. The oven itself is typically steel in construction which helps prevent radiant heat from the oven exterior. A locking door with robust gaskets provides user access to the oven chamber.

The three basic  functions of a laboratory oven are:

  • Drying: removing the moisture from the specimen and chamber as efficiently as possible.
  • Baking: heating a substrate without dehumidifying it.
  • Curing: the sample is physically or chemically altered by a slow baking and drying process.

Types

Lab ovens are common equipment in most clinical, research and forensic laboratories. The common types are available in gravity/natural convection, mechanical/forced convection and vacuum pump for conventional heating and drying applications.

Convection ovens are used for applications such as drying, baking, sterilizing, pre-heating, and, incubation.

Forced convection ovens use a fan to circulate air around the inside of the chamber.

Natural convection relies on t° differences within the oven, to transfer heat to samples.

Vacuum drying ovens intake dry air into the chamber within a short time, which absorbs humidity and is immediately extracted by the vacuum pump. Vacuum drying ovens are used for delicate drying processes.

When choosing an oven, consider applications, dimension, capacity and lab budget. Search for characteristics that ensure maximum sample and operator protection.

 

Common applications for lab ovens include:

  • Sample testing: the oven is used in conjunction with other equipment to determine material tensile strength, fatigue, deformation, and resiliency.
  • Annealing: metal components undergo surface treatments to improve ductility and malleability.
  • Solvent/water removal: moisture or solvents are removed from industrial compounds or chemicals, such as desiccants and catalysts.
  • Polymer curing: a polymer resin is dehydrated to form a hard-set plastic.
  • Polyimide baking: thermosetting polyimides are cured.
  • Sterilizing: all microbiological life is exterminated from the oven’s heat.
  • Evaporation: samples, such as food, are dried to measure its total water content.
  • Glassware drying: one manufacturer cites 60% of lab oven usage for this purpose.
  • Electronics burn-in: Defects are detected in integrated circuits via dynamic and static exercise.


Specifications


Size

Oven volume and mounting are of critical importance. The internal cubic volume of the oven chamber must suit the spatial needs of substrates, and can ideally accommodate multiple samples during a single oven cycle. An adjustable shelving system can be invaluable to utilizing all of the available space. Control mechanisms are easily located and articulated by an operator.

Additionally, lab ovens are manufactured in three mounting varieties:

  1. Benchtop: the lab oven is installed upon a workbench or counter for easy access. These models often include hardware for stacking several lab ovens upon each other.

  2. Cabinet: the oven is a free-standing item of laboratory furniture, typically with a larger capacity or a specialized application.

  3. Truck-in/walk-in: the oven is large enough where substrates are carried into the chamber by an operator or with material handling equipment.

Temperature

Uniform temperature throughout the entire oven chamber is the most important aspect regarding operation. Standards bodies assess spatial temperature accuracy by testing the oven chamber at 9 or 27 separate points within a time frame.

Most laboratory ovens have a temperature range of just above ambient temperature to 300° C; those ovens which exceed this range are application specific. Ovens also have a temperature tolerance, which references the accuracy to which the oven preheats; increments of fractions of a single degree to tens of degrees are common.

Temperature is regulated by one of three technologies.

  1. Thermostat: a simple on/off switch engages the heating element to maintain a set temperature. Typically, temperature briefly exceeds this value before settling, and ultimately lines up below the threshold.

  2. Proportional control: a thermostat monitors oven temperature, but eases heating as it approaches the preheat value to prevent any overheating.

  3. Proportional integral derivative (PID) control: a processor calculates the amount of energy needed to maintain a temperature, and considers it against the rate of heat loss. This prevents over- and underheating, and is typically the most energy efficient means of temperature regulation.

Controlled Atmosphere

Some lab ovens are compatible with equipment which inserts a specific gas or gas compound into the lab chamber, or removes the atmosphere of the oven chamber altogether (vacuum oven). These ovens typically have flow meters and solenoid valves to control fluid flow, as well as internal sensors to register atmosphere particulate. It is most common to insert an inert gas such as argon or nitrogen into the oven chamber at a pressure of five to ten times the volume of the oven. This blankets the specimen and prevents oxidation and condensation from forming in the chamber.

Energy Efficiency

A lab oven requires considerable quantities of energy to heat the chamber. It is common for ovens to be left on for long periods, sometimes for days, to reduce preheating times. For clear reasons, reducing thermal egress from the oven can result in measurable cost savings. Ovens with a boost feature incorporate an accelerated preheat to limit long operating periods.

The oven shell is typically steel in construction and underlying layers of insulation reduce thermal transfer rates. An efficient oven quickly reaches uniform temperature by thorough circulation. Door gaskets help retain heat around door tolerances. Many ovens are able to compensate for ventilation of the chamber. Recovery time is how long it takes an oven to recuperate to the preheat value after the chamber has been ventilated, such as by a door opening.


Maintenance

On average, lab ovens are obsolete every seven years. This is in part due to design innovations, but mostly because defective lab ovens can be costly and difficult to repair. Many lab ovens never return to specification after the malfunction is addressed, and a new lab oven typically costs less than refurbishment. Oven maintenance should be conducted at routine intervals. Ventilation ports, gaskets, sensors, controls, heating elements, and blowers should be reviewed for faults. The oven should also be temperature calibrated on occasion.


Muffle Furnaces

A muffle furnace is a piece of oven-type equipment that can reach high temperatures. It usually works by putting a high-temperature heating coil in an insulated material. The insulating material effectively acts as a muffle, preventing heat from escaping.

It is used for high-temperature applications such as:

  • Fusing glass
  • Creating enamel coatings
  • Ceramics
  • Soldering
  • Brazing

They are also used in many research facilities, for example by chemists in order to determine what proportion of a sample is non-combustible and non-volatile. Advances in materials for heating elements facilitate more sophisticated metallurgical applications.

 

Scientific environments might use a laboratory muffle furnace for nutritional analysis by calculating the amount of fat, carbohydrate, protein, and water content in a particular food. Labs may also identify organic and inorganic materials by the rate of combustion and the by-products obtained afterward. Industrial uses might include determining the combustion temperatures of specific materials.

The term muffle furnace may also refer to another type of oven constructed on many of the same principles as the one mentioned above, but takes the form of a long, wide and thin hollow tube used in roll-to-roll manufacturing processes.

Both of the furnaces are usually heated to the needed temperatures by:

  • Conduction
  • Convection
  • Blackbody radiation from electrical-resistance heating elements

There is usually no combustion involved in the temperature control of the system, which allows for much greater control of temperature uniformity and assures isolation of the material being heated from the byproducts of fuel combustion.

A muffle furnace is also known as a retort furnace.

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