Hot Air Oven: Definition, Principle, Instrumentation and Application

In many laboratories, a hot air oven is one kind of heat sterilizing apparatus. Hot air ovens eliminate bacteria and bacterial spores by heating food to extremely high levels for several hours. By heating an object’s exterior surfaces, which the object then absorbs and transfers the heat into its interior, the ovens use conduction to sterilize objects. It functions by distributing heat to the items being processed uniformly across the oven chamber using hot air circulation.

Hot air ovens are usually fitted with a thermostat to keep the temperature constant, and they can withstand ambient temperatures up to 250°C. The hot air inside the oven chamber is circulated by a fan in certain hot air ovens, which can assist in ensuring that the materials being treated are heated or dried uniformly.

Hot air ovens, which provide controlled conditions for a variety of purposes, have evolved into essential instruments in many industries and laboratories.

What is Hot Air Oven?

An electrical device specifically made for the use of dry heat sterilization is a hot air oven. This instrument is today a necessary component of scientific and medical labs, having its roots in the groundbreaking work of the eminent scientist Louis Pasteur. Its principal purpose is to remove microbiological life—including bacterial spores—from a variety of heat-resistant materials.

• A hot air oven is an electrical device that uses dry heat to sterilize materials that can resist heat, thereby removing microorganisms, such as bacterial spores, from the objects that are placed inside its chamber.
• One of the most important tools for both scientific and medical study is the hot air oven. It is essential for treating heat-resistant materials since it can be sterilized using dry heat. The hot air oven, which has its origins in Pasteur’s revolutionary work, is a monument to the lasting importance of scientific advancement.
• It is the procedure or approach employed for killing the microbes.
• We may perform experiments without contamination because of the sterilization techniques. Physical methods of sterilization include those that involve the use of heat, radiation, or other physical agents. On the other hand, chemical methods of sterilization refer to the techniques that involve the employment of chemical agents. Instruments used in autoclaves and hot air ovens are examples of physical sterilizing techniques.
• Heat conduction is the basic principle behind how a hot air oven works. This process guarantees that the material is sanitized gradually, layer by layer, from the outside to the inside. Microorganisms are neutralized through oxidation of their biological components caused by the dry heat produced inside the oven chamber. Materials that are resistant to dry heat and thermal stability, such as some metals, glassware, and particular powders, are most suited for this mechanism’s sterilization.
• Temperature-sensitive tapes or biological indicators are frequently used by the oven to track and confirm the success of the sterilizing procedure. These indicators, which are frequently based on bacterial spores, offer a concrete assessment of the sterilizing effectiveness of the device while it is in use.

Principle of Hot Air Oven

• The dry heat sterilization principle explains the hot air oven’s functioning, which combines convection, conduction, and radiation mechanics. According to scientific studies, the process starts when the chamber’s air is warmed by the heating components. Fans move this heated air around to guarantee even heating of the samples or objects inside, exposing all of their surfaces to the same high temperatures.

• Conduction is the main factor behind the sterilizing mechanism. Heat is gradually transmitted within objects layer by layer as their exterior surfaces absorb it until the item as a whole reaches the required sterilizing temperature. Even the item’s core is sufficiently disinfected thanks to this meticulous heat transfer.

• At the microscopic level, heightened temperatures exert a significant influence on microorganisms. The elevated heat prompts the expulsion of water molecules from these organisms, resulting in dehydration. This dehydration, accompanied by oxidative harm to cellular components, triggers the alteration of crucial proteins.
• Proteins, vital for the survival and operation of microorganisms, undergo a loss of their natural structure and function due to this denaturation process. Furthermore, the rise in electrolyte concentrations induced by heat poses a harmful impact on cellular integrity. The collective consequences of oxidative damage, protein alteration, and electrolyte imbalance ultimately lead to the effective eradication of microorganisms.

The basic idea behind the hot air oven is to sterilize objects by using dry heat. By methodically applying convection, conduction, and radiation, it guarantees that the objects are not only sterile on the outside but also devoid of microbes deep within. This technique emphasizes how crucial it is to maintain consistent heat dispersion and exact temperature control in order to achieve complete sterilization.

Features of Hot Air Oven

• The temperature range in which hot air ovens can generate heat is +10°C to 150°C, 200°C, or 250°C. This enables flexible application in various heat treatment, sterilizing, and drying procedures.
• A blower-assisted air recirculation system is used in hot air ovens to ensure uniform heat distribution throughout the chamber. This forced air circulation helps to evenly dry or sterilize goods and maintain a constant temperature.
• Application areas for hot air ovens include research, life sciences, microbiology, MLSS analysis, and laboratories. They are also used in the manufacturing of steel, textiles, chemicals, pharmaceuticals, food and beverage products, and electronic components.
• A PID temperature controller based on microprocessors is installed in the ovens. This controller shows the process value (PV) as well as the set value (SV), enabling accurate temperature management. An RTD PT100 temperature sensor is used to measure temperature.
• Safety precautions are built into hot air ovens to safeguard both the oven and the samples. They have a safety thermostat or overheat prevention mechanism that turns on in the event of a controller failure to stop overheating and possible damage.
• Hot air oven interior chambers are usually built from thick gauge stainless steel sheets. This material selection guarantees longevity, ease of maintenance, and adherence to the oven’s design specifications.
• The ovens have detachable shelves made of stainless steel wire mesh cables. The height of these shelves can be changed to suit various materials or sample sizes. The size of the internal chamber affects how many shelves there are.
• Hot air ovens’ external cabinets might be composed of stainless steel or powder-coated GI sheet, depending on what the customer wants. For easy mobility, larger ovens frequently have caster wheels and brakes.
• Hot air ovens include additional features that can be added to improve functionality and fulfill certain needs. A digital timer, PLC-based HMI controller, RS485 computer interface, explosion-proof heater, data logger and circle chart recorder, 21 CFR software, and HEPA filter air inlet are a few examples of these.

Instrumentation of Hot Air Oven

The multiple mechanical and electrical components of the hot air oven are responsible for its performance.

Figure: Hot Air Oven (https://link.springer.com/protocol/10.1007/978-1-4939-9861-6_5)

• Cabinet: The oven’s outermost layer of protection, known as the coat or cabinet, is usually made of stainless steel or aluminum and is resistant to oxidation and robust against mechanical impacts. The cabinet has two functions: it keeps heat loss from the oven to a minimum while operating as a barrier between the interior and exterior elements.
• Fiberglass insulation: There is a strong layer of glass wool insulation between the inner chamber and the exterior cabinet. Brown and yellow fiberglass are the two main varieties used. Yellow fiberglass may induce skin sensitivity, but brown fiberglass may cause lung inflammation. This insulation’s main function is to stop heat loss, which keeps the oven operating efficiently.
• Chamber: This rectangular chamber, the inner sanctuary of the oven, is made of either stainless steel or aluminum and has ribs that allow shelves to be positioned at different levels to accommodate objects of varying sizes and numbers.
• Shelves: These aluminum-built shelves are used to store the objects that need to be sterilized.
• Motorized Fans/Blower: The fan, an essential part that is driven by a motor, makes sure that the hot air is distributed evenly throughout the chamber. For sterilization to be constant and efficient, this equal dispersion of heat is essential.
• Door: The oven has a single door that is supported by sturdy hinges. During the sterilization procedure, this door is essential for maintaining a sealed environment and gaining entry to the chamber.
• Power supply: The transformer and rectifier that supply the oven’s power are 220V-50Hz. This ensures a steady and reliable energy supply for the oven’s functions.
• Heater: The heater, which is essential to the operation of the oven, produces heat when an electric current flows through a conductor. The heating element’s great thermal conductivity, high resistance, and good electrical insulation are its key characteristics.
• Thermostat: The thermostat is connected directly to the heater and functions as a heat sensor. It has a strong negative temperature coefficient and is made to tolerate extreme temperatures. With the use of this part, users can regulate the oven’s temperature to their preferred level while making sure it stays within the predetermined range.
• Temperature indicator: A temperature indicator is used to keep track of the oven’s internal temperature. This can be a more sophisticated thermocouple or a conventional thermometer; both can give precise temperature measurements.
• Timer: The oven may have an electronic timer or a mechanical timer, depending on the model. Depending on the sterilization needs, these clocks can be adjusted for periods of five to sixty minutes.
• Fuse: Safety is the priority, and the fuse is essential to this. In the case of an excessive current flow, such as during short circuits or heavy loads, it is intended to break the circuit, averting possible electrical harm.
• Control panel: This is how the oven’s user interface looks. Temperature and time are two of the many parameter settings that can be accessed through the control panel. Typically, it has a switch knob for setting adjustments, an indicator power lamp (usually green) to show when the oven is powered on, and an indicator heater bulb (usually red) to show when the heater is engaged.

Types of Hot Air Oven

In many scientific and medical contexts, hot air ovens are essential tools, mostly utilized for sterilizing. These ovens sterilize and destroy bacteria using dry heat. Hot air ovens fall into various categories according to their design and working mechanisms:

• Gravity Convection Ovens: To distribute air evenly throughout the oven chamber, this oven does not utilize a fan. Instead, as it expands and becomes less dense than the air around it, the hot air inside the chamber will naturally ascend. The air will cycle through this process again as it rises, loses heat energy, and descends. When sterilizing powdered materials, they work perfectly.
• Forced Convection Ovens: The fans in these ovens move heated air around the chamber. The oven’s forced air circulation system offers exceptional temperature consistency and quick heat recovery. They are employed in lab settings for sample preparation or culturing.
• Mechanical Convection Ovens: They offer even heat distribution, stability, and consistent airflow for optimal temperature management. They heat up more quickly and disperse the heat evenly throughout the chamber. Ovens with mechanical convection are used in production and testing.

Procedure of Hot Air Oven

Mostly used for sterilization, the hot air oven is an essential tool in scientific and medical labs. It is important to adhere to a methodical operating process in order to guarantee its efficient functioning and attain ideal sterilizing outcomes. This is a comprehensive guide on how to use a hot air oven:

• Initialization: To start, plug the oven into the electrical outlet and flip the switch to turn it on.
• Preheating stage: Before adding anything to be sterilized, preheat the oven for thirty minutes. By taking this step, you can be confident that the oven will attain a consistent temperature that is ideal for sterilizing.
• Setting temperature: Mark the desired temperature with the temperature gauge. The volume and type of materials that need to be sterilized determine the precise temperature setting.
• Loading the oven: Place the objects to be sterilized on the trays or shelves of the oven. It’s imperative to keep enough distance between each piece. By allowing for effective heat circulation, this spacing makes sure that every object is uniformly exposed to the sterilizing temperature.
• Securing the oven: After everything has been put in its proper place, shut the oven door. Make sure the included screws are firmly inserted. The oven’s interior temperature will start to rise after the door is closed.
• Monitoring Temperature and Holding Time: It is critical to have a thorough understanding of the holding time and sterilizing temperature needed for a given item. Check the thermometer frequently to see if the oven has reached the desired temperature. Once this temperature is reached, keep it there for the designated holding period to guarantee proper sterilization.
• Completion and cooling: Turn off the oven and let it cool after the allotted holding time has passed. You must hold off on opening the oven door until the oven has cooled down sufficiently. By taking this precaution, possible thermal injuries are avoided and safety is guaranteed.
• Carefully open the oven door after it has cooled. Take the sterilized things off the shelves or trays using safety gear like oven mitts or tongs.
• Final Step: Once everything has been retrieved, shut the oven door to keep the interior tidy and ready for many uses.

Application of Hot Air Oven

A multipurpose tool with many uses, the hot air oven is especially useful in scientific and research environments. Some of the major applications are:

• The oven is widely used in the fields of biology, chemistry, and material science research. It is used by researchers for a variety of experiments that call for regulated temperature settings.
• Numerous laboratory instruments are frequently sterilized using the hot air oven. Glassware like flasks, pipettes, Petri plates, and test tubes fall under this category. Its effectiveness in guaranteeing the sterility of these instruments is crucial for precise and clean experimental outcomes.
• Hot air ovens are an efficient way to sterilize metal instruments like scissors, forceps, spatulas, and scalpels. This guarantees that when these instruments are utilized in surgical or experimental operations, they are free of microbiological contamination.
• Sterilizing non-volatile materials such as zinc, starch powder, and sulfonamide is another use for the oven. During the sterilizing procedure, the dry heat makes sure that these compounds stay stable and uncontaminated.
• Life science research uses the hot air oven extensively because of its accuracy and dependability. It guarantees the accuracy of the results and the preservation of the experimental conditions.
• The baking, curing, and annealing processes can all benefit from the use of hot air ovens. This makes it appropriate for these applications because of its capacity to deliver steady, regulated heat.
• Food, medication, and other consumable goods are tested in hot air ovens to ensure that their temperatures remain stable over time. Also referred to as a hot sterilizer, the hot air oven is essential to the sterilizing process. It will assist you in keeping your lab space sterile when working with pathogenic bacteria.

Advantages of Hot Air Oven

• The purchase and operation of the hot air oven are both economical. It uses less energy and requires less maintenance because of its straightforward design and mechanism.
• The hot air oven uses dry heat instead of steam and water to sterilize objects, saving water. This is in contrast to autoclaves. With the help of this technology, materials are sterilized without running the danger of difficulties from moisture.
• Because the hot air oven can operate at higher temperatures than autoclaves, it can sterilize materials more quickly and effectively, removing microbiological pollutants quickly and effectively.
• The hot air oven is a great option for laboratories with limited space because of its tiny size and minimum footprint. Its small size also makes installation simple.
• The hot air oven provides increased safety by lowering the possibility of mishaps or malfunctions due to its low-pressure build-up during operation.
• By using dry heat, metal instruments and sharp objects are protected against rust and corrosion, prolonging their lives.
• Because a hot air oven sterilizes without leaving any chemical residue behind, the sterilized products stay safe to use and uncontaminated.
• Even for dense materials, the oven’s dry heat may reach deep inside thick objects to provide thorough sanitation.
• Powders and oils are only two of the many things that can be sterilized using a hot air oven. These materials are adequately sterilized without clumping, even if they might in damp settings.
• A hassle-free option for many labs, the hot air oven has a simple design and operation that requires little maintenance.

Limitations of Hot Air Oven

• Sterilizing materials such as rubber, polymers, and surgical dressings is not a good use for the hot air oven. Because these materials have low melting points, the high temperatures employed in dry heat sterilization may cause them to melt or distort.
• The ineffectiveness of the hot air oven in eliminating some heat-resistant endospores and prions is one of its major drawbacks. Since these microorganisms can tolerate the dry heat, it is difficult to completely eradicate them.
• Not every kind of tool or equipment can be sterilized using dry heat. The high oven temperatures can have a negative impact on certain materials.
• Compared to other techniques like steam sterilization, burning, chemical sterilization, or radiation, dry heat sterilization in a hot air oven takes a lot longer. This longer time frame could be problematic in situations where quick sterilization is necessary.
• The dry heat sterilization procedure may not be able to completely remove some microorganisms, especially prions. For some medical and research uses, this could be dangerous.

Precautions

In the field of science and laboratory practices, hot air ovens are frequently used. Nevertheless, a few safety measures need to be closely followed to guarantee its secure and efficient functioning. Here are some of the precaution measures:

• The oven should only be used for items that are resistant to sterilizing by dry heat. Making sure the materials are resistant to heat damage is crucial.
• It is strictly prohibited to place combustible objects, especially volatile substances, inside the oven. Their presence may give rise to dangerous circumstances.
• It’s important to let the oven cool to about 40°C before opening the door after sterilization. This chilling procedure guards against glassware shattering from abrupt temperature swings.
• Sterilization-related items should be properly wrapped, ideally with paper or newspaper. These things are best kept in cardboard or metal containers. Cotton wool works well as a stopper for objects like pipettes, flasks, and test tubes.
• Make sure all glassware is dry before putting it in the oven. Putting paper around them can provide even more security.
• It is best to let the oven cool to about 60°C after sterilizing it before opening the door. By taking this precaution, you can avoid glassware breaking or cracking from sudden temperature changes.
• The effective circulation of hot air might be impeded by overcrowding. To ensure the best possible sterilization, make sure there is enough space between the products.
• Enough space should be left between articles while placing them on the shelves. This guarantees unrestricted hot air circulation, which results in even heating.

References

• Alkadhim, Saif Aldeen Saad, Hot Air Oven for Sterilization: Definition & Working Principle (December 14, 2018). Available at SSRN: https://ssrn.com/abstract=3340325 or http://dx.doi.org/10.2139/ssrn.3340325https://biokimicroki.com/working-principle-and-construction-of-hot-air-oven/
• https://microbiologynote.com/hot-air-oven/
• https://www.plantcelltechnology.com/blog/all-about-hot-air-ovens/
• https://www.prestogroup.com/articles/what-is-the-use-of-hot-air-ovens-in-the-microbiology-industry/
• https://medium.com/@prestogrouponline/hot-air-oven-a-brief-overview-working-principle-uses-2573266baa44
• https://microbenotes.com/hot-air-oven/