Hot air oven Definition, Principle, Uses, Parts, Application, Procedure.

Hot air oven introduction

A hot air oven is a laboratory appliance that is used to dry, sterilize, or heat materials. It works by circulating hot air inside the oven chamber to evenly distribute heat to the materials being processed. Hot air ovens are often used in a variety of settings, including research laboratories, industrial settings, and educational institutions.

Hot air ovens typically have a temperature range of ambient temperature to 250°C or higher and are equipped with a thermostat to maintain a consistent temperature. Some hot air ovens also have a fan to circulate the hot air inside the oven chamber, which can help to ensure that the materials being processed are heated or dried evenly.

Hot air ovens are commonly used to sterilize equipment and materials, as the high temperatures can kill microorganisms. They are also used to dry materials, such as chemicals, biological specimens, and glassware. In addition, hot air ovens can be used to heat materials for a variety of purposes, such as activating chemicals or heating materials to a specific temperature for testing.

It is important to follow the manufacturer’s instructions for your specific hot air oven and to use caution when working with hot materials.

Hot air oven Definition / What is hot air oven?

• A hot air oven is a laboratory instrument that uses dry heat to sterilize laboratory equipment and other materials. This method of sterilization is used for equipment that cannot be wet or material that will not melt, catch fire, or change form when exposed to high temperatures.
• The hot air oven is also known as a forced air circulating oven, which means that the air inside the oven is constantly circulated, providing even temperature distribution.
• Some examples of materials that cannot be sterilized by a hot air oven include surgical dressings, rubber items, or plastic material. However, there are several materials that can be sterilized, such as glassware (like petri dishes, flasks, pipettes, and test tubes), powder (like starch, zinc oxide, and sulfadiazine), materials that contain oils, and metal equipment (like scalpels, scissors, and blades).
• A hot air oven provides extremely high temperatures over several hours to destroy microorganisms and bacterial spores. The widely used temperature-time relationship in hot air ovens to destroy microorganisms are 170 degrees Celsius for 30 minutes, 160 degrees Celsius for 60 minutes, and 150 degrees Celsius for 150 minutes.
• Most of the medical industries use hot air ovens to sterilize laboratory instruments and material due to its simple standard operating procedure and low price. It also provides quick-drying processes.
• The process of dry heat sterilization using a hot air oven was originally developed by Louis Pasteur. The temperature range of a hot air oven is 50 to 300 °C, and it can be controlled by using a temperature regulator.
• The forced air circulation provided by the oven ensures the temperature uniformity throughout the oven. In a hot air oven, first, the surface of the material is sterilized, and then the temperature slowly enters the center of the item.

Working Principle of Hot air oven/Hot air oven principle

Utilizing convection, conduction, and radiation, a hot air oven operates according to the dry air sterilisation procedure. The heating components heat the chamber’s air, which can be evenly circulated with the use of fans to expose sample surfaces to hot, dry air. This exposure causes the external surface of an object to heat up, and the conduction mechanism transfers the heat to the object’s core. Similarly, heat causes microorganisms’ internal water to evaporate, resulting in oxidative damage to cellular components, denaturation of proteins, a toxic consequence of high electrolyte levels, and, finally, death.

Dry Sterilisation

It is originally used in a hot air oven. This method is proper for items like glassware, powders, oil containing materials, metal material. Hot air oven acts in a manner so that items stored inside the oven don’t grab fire or meltdown. It primarily acts on the principle of structure where heat moves on the outside of the substance and then to the core of the item. It is termed a dry sterilization process because the procedure is performed by employing the hot air. 

While the air shifts hot, it becomes thinner and transfers towards the ceiling of the chamber. If air hits the roof of the chamber it travels towards the ground of the chamber. It helps in the circulation of air within the chamber. This current flow assures the proper and uniform heating everywhere in the chamber. This is approaching a longer method than autoclaving or moist sterilization. Hence, a few organizational facilities are needed if it is practiced in the medical laboratory, so that workers can create a schedule of the sterilization method.

Features of a Hot air Oven

• Hot Air Ovens are intended to generate heat via blower-assisted air recirculation within the chamber at temperatures ranging from ambient +10°C to 150°C, 200°C, or 250°C.
• They are commonly used to dry glassware, sterilise N95 masks, general tools, and packaging materials, and are also referred to as forced air circulating ovens.
• They are also utilised in life sciences, microbiology laboratories, MLSS analysis, and numerous applications based on research.
• There are numerous industries that employ hot air circulation ovens, including chemical and pharmaceutical, food and beverage, textile, electronic component, and steel manufacture, etc.
• These ovens have an operational temperature range of 150°C, 200°C, and 250°C that is controlled by a microprocessor-based PID temperature controller. It demonstrates both the set value (SV) and the process value (PV).
• It precisely measures the temperature using an RTD PT100 temperature sensor. Utilizing a motor-driven blower system, forced air circulation is achieved.
• Sheathed air heaters are used for heating. Each unit is equipped with an overtemperature prevention system (safety thermostat) that safeguards your oven and samples in the event that the controller fails.
• The primary function of a hot air oven is to remove moisture from the material; as a result, it is employed in applications such as curing, drying, baking, annealing, Measurement of mixed liquor suspended solids (MLSS), and even storage of certain products at constant temperature in laboratories and hospitals, etc.
• The interior chamber of our hot air ovens is made of thick gauge stainless steel sheet (SS 304/SS 316) in order to meet the requirements of a hot air oven’s design, durability, and ease of cleaning. The shape of the corners makes cleaning a simple task for the operator.
• Shelves are removable stainless steel wire mesh cable shelves that allow for height adjustment. Depending on the size of the internal chamber, the number of shelves might range from two to five or more.
• External cabinet is constructed of either powder-coated GI sheet or stainless steel, per customer specifications. Large hot air ovens are outfitted with caster wheels with brakes for convenient transportation.
• A number of optional accessories, such as an explosion-proof heater, digital timer, PLC-based HMI controller, RS485 computer interface, 21 CFR software, data logger and circle chart recorder, and HEPA filter air inlet, are also available for your hot air oven.

Hot air oven Parts and Functions

1. External cabinet: The External cabinet is made of stainless sheets. It covers the inner chamber.
2. Glass wool insulation/Fibreglass: The space between the inner chamber and external cabinet is filled with Glass wool. It provides insulation to the hot air oven.
3. Inner chamber: The rectangular-shaped chamber is built of aluminium or stainless steel and provides space for ribs to maintain shelf levels.
4. Tubular air heaters: They help to generate heat within the inner chamber. Two Tubular air heaters are located on both sides of the inner chamber.
5. Shelves (Mesh): These objects are constructed of aluminium and hold plates. Their number may vary based on the quantity and size of the articles, as well as the oven’s capacity. When they are put on the ribs, raising certain places facilitates the flow of air. Additionally, certain shelves may have ventilation holes.
6. Motorized fans/blowers/Motor-driven blower: The motor-driven fan is utilised to evenly distribute heated air within the chamber.
7. Door: A single door is mounted on the strong hinges on one side. A side door gasket made of asbestos is utilised to reduce heat loss during operation.
8. Temperature sensor: It measures the temperature within the hot air oven and displays it on the controller screen.
9. Tray slots: The inner wall of the chamber contains several try slots that hold the trays.
10. PID temperature controller: It maintains the accurate temperature during the entire cycle. It also controls the temperature and also displays the temperature values.
11. Load indicator: it indicates the hot air oven is overloaded.
12. Mains on/off switch: It helps to turn on/ turn off the hot air oven.
13. Heater: With the passage of an electric current through a conductor, heat is produced due to the temperature increase. High resistance, electrical insulation, and high thermal conductivity are the three primary characteristics of the heating element. The various types of heaters utilised in hot air ovens include one-sided circular heaters, one-sided U heaters, one-sided wave heaters, one-sided square heaters, three-sided heaters, and four-sided heaters. The heater’s temperature range is between 50 and 300 degrees Celsius.
14. Safety thermostat: It is also known as an over-temperature protection device. It keeps your oven and specimen safe in case of controller malfunction. It is a heat sensor directly attached to a heater that can withstand intense heat and has a high negative temperature coefficient. It enables users to reach the desired temperature in the hot air oven and prevents overshooting.
15. Timer: There are two possible types of timers, electrical and mechanical, that can function for 5 to 60 minutes given the sterilising time period.
16. Fuse: The role of the fuse is to prevent electrical harm caused by excessive current during short circuits or heavy loads.
17. Control Panel: This is the section that lets the user to adjust various parameter settings, such as temperature, time, etc., and contains an indication power bulb (often green), an indicator heater lamp (typically red), and a switch knob.

Types of hot air oven

There are present different types of hot air oven such as;

1. Gravity Convection

• Gravity Convection Air is distributed by spontaneous convection. As hot air flows up, a gentle flow holds temperatures moderately uniform inside a container and wholly uniform in any distinct position.

2. Forced Convection Ovens

• These ovens carry a fan that gives limited air circulation within the heating container.
• This method provides very fast heat up and restoration times, mixed with especially low-temperature differences inside the working chamber. 
• Flexible vents and semi-forced exhaust deliver it a conventional sample-drying oven. 

3. Mechanical Convection

• Mechanical Convection is a gravitation convection oven served with a re-circulating fan in a working container.

4. Forced Exhaust Ovens

• In these ovens, air is pushed into the working container by a fan and scattered through an adaptable vent. 
• This variety of oven is especially helpful in purposes where the heating process provides vapors or fumes that require to be immediately and continuously discharged from the working container. 
• All of the forced air ovens consume at a higher percentage than a convection oven. Though, much larger forced exhaust velocities can be accomplished by adding an air channel and a flexible outlet.
• This adjustment takes an extra $100 and is totally achievable with forced convection ovens.

5. Side Draught Ovens

• Certain ovens produce airflow from one side to the other i.e. left to right. 
• Speedy heat up and restoration time make this type of oven prototype for preheating plastic cloths(hospitals, etc.) or any profession where smooth sheets or plates are used.

Operating Procedure of Hot air oven / Hot air oven procedure

A hot air oven is a laboratory appliance that is used to dry, sterilize, or heat materials. Here is a general operating procedure for using a hot air oven:

1. Connect the power cable to the machine: To start, connect the power cable to the hot air oven. Make sure the machine is properly grounded.
2. Preheat the oven to the desired temperature: Next, preheat the oven to the desired temperature. This can typically be done by turning the temperature dial to the desired setting and waiting for the oven to reach the desired temperature.
3. Place the materials inside the oven: Place the materials to be heated or dried inside the oven. Make sure the materials are placed on a shelf or tray so that they are not in direct contact with the oven floor. Use only materials compatible with dry heat sterilisation.
4. Wrap the items or equipment: Wrap the items or equipment in paper or newspaper and place them in a cardboard or metal container. Metal containers can also be used for wrapping. Test tubes, flasks, and pipettes can be plugged with cotton wool.
5. Arrange the objects in the chamber: Arrange the objects in the chamber in a manner that permits the free flow of air.
6. Turn on the hot air oven: After proper placement, close the door and turn on the hot air oven. The temperature will then begin to climb.
7. Check the time required: Check the time required to maintain the material at the proper temperature once it reaches the desired temperature. The sterilizing holding time is dependent on the temperature of the hot air oven. The typical time is 120 minutes at 160°C.
8. Allow the materials to heat or dry: Close the oven door and allow the materials to heat or dry for the desired amount of time.
9. Turn off the oven: Once the materials have been heated or dried for the desired amount of time, turn off the oven and allow it to cool down before opening the door.
10. Remove the materials from the oven: Carefully remove the materials from the oven using oven mitts or tongs to protect your hands from the heat.

It is important to follow the manufacturer’s instructions for your specific oven and to use caution when working with hot materials.

Sterilization Control of Hot air Oven

To evaluate the efficacy of any instrument, quality control testing is necessary. Biological indicators are utilised to test the hot air oven’s functionality.

Clostridium tetani spores from a nontoxigenic strain are employed as the microbiological test for quality control. 106 spore-impregnated paper strips are placed in envelopes and inserted into proper packaging. After sterilisation, the strips are removed, infected with thioglycollate or cooked beef medium, and incubated at 37 degrees Celsius for five days for a sterility test under strict anaerobic conditions. If there is proper sterilisation, the spores will be eradicated and there will be no growth.

Chemical control: Browne’s tube No. 3 is green following 60 minutes of sterilising at 160°C ( colour changes from red to green).

Physical control: Thermocouples and a temperature chart recorder are employed for C) physical control.

What can be Sterilized by using a Hot Air Oven?

A hot air oven can be used to sterilize a variety of materials, including:

1. Glassware: Glassware, such as test tubes and beakers, can be sterilized in a hot air oven.
2. Laboratory equipment: Laboratory equipment, such as pipettes and Petri dishes, can be sterilized in a hot air oven.
3. Medical instruments: Medical instruments, such as scalpels and forceps, can be sterilized in a hot air oven.
4. Fabric: Fabric, such as lab coats and towels, can be sterilized in a hot air oven.
5. Chemicals: Some chemicals can be sterilized in a hot air oven by heating them to a high enough temperature to kill any microorganisms present.

It is important to follow the manufacturer’s instructions for your specific hot air oven and to use caution when sterilizing materials. Some materials may be sensitive to high temperatures or may release harmful fumes when heated. It is also important to allow the materials to cool down before handling them after they have been sterilized.

Maintenance of Hot air Oven

Here are some general maintenance tips for a hot air oven:

1. Clean the oven regularly: Regularly clean the inside and outside of the hot air oven to prevent a buildup of dirt or debris. This can be done by wiping the inside of the oven chamber with a mild detergent solution and drying it thoroughly.
2. Check the seals and gaskets: Make sure the seals and gaskets on the door of the hot air oven are in good condition and sealing properly. If the seals are damaged or not sealing properly, heat may escape from the oven and the oven may not function properly.
3. Check the thermostat: Make sure the thermostat is functioning properly and accurately maintaining the desired temperature.
4. Check the heating element: Inspect the heating element regularly to make sure it is functioning properly and not damaged.
5. Check the fan: Make sure the fan is functioning properly and not damaged. The fan is responsible for circulating hot air inside the oven chamber.

By performing regular maintenance on your hot air oven, you can help to ensure that it is operating safely and efficiently. If you notice any problems with your hot air oven, it is important to address them as soon as possible to prevent further damage or risk of injury.

Technical Specification of a Hot air oven

Temperature range	Ambient +10°C to 250°C
Temperature accuracy	±05°C or better
Temperature sensor	RTD PT100
Temperature controller	PID temprature controllerDisplay of SV & PV
Construction	Double walled
Inner chamber	Stainless steel 304
Exterior	Powder coated steel sheet
Insulation	Ceramic wool insulation
Shelves	Chrome plated wire mesh cable trays (removable)
Door	Insulated solid door with spring latchp
Door gasket	High temperature silicone rubber gasket
Air circulation	Motor driven blower assembly
Vent port	fume vent port
Safety	Over temperature thermostat
Power supply	220 Volts / 50 Hz
Optional	Horizontal air flowPass through (two side doors)Port hole for sensors & data monitoringDigital timer 99 hours 99 minutesProgrammable controller (Ramp & Soak)Toughened glass windowStainless steel exterior (GMP model)Data logger with USB interfaceHMI & PLC automationEUROTHERM controllerCaster wheels with lock

Advantages of Hot air oven Pros

• No need to water to sterilize the material.
• Endotoxins from bacteria are inactivated.
• Only the hot air oven can sanitise oils and powders. They cannot be sterilised in autoclaves because moisture may cause clumping.
• Non-corrosive for metals and things with sharp edges.
• Simple to instal and inexpensive to operate.
• Not much pressure is created like autoclave which creates it easy to manage and also makes it safer to work with.
• In a laboratory environment, it is more fitting to use as compared to other sterilizers.
• Hot air oven is much smaller in size as compared to autoclaves and also more effective.
• It can operate at a greater temperature and is quicker than autoclave.
• The operating procedure is simple as compared to other sterilization methods.
• Its price is low as compared to autoclave. 

Disadvantages of Hot air oven

• According to the principle of thermal inactivation by oxidation, it can not slaughter some living organisms, such as prions due to the use of dry heat rather than wet heat.
• Most of the materials are not fit with hot air ovens such as surgical dressings, rubber items, or plastic material; they can be a meltdown at low temperatures.
• Occasionally, prions may be resistant to the dry heat method of sterilising.
• Due to the high temperature required for sterilising, glassware may get smoky.
• Plastics and rubbers cannot be sterilised in a hot air oven due to their inability to withstand temperatures over their melting point.

Hot air oven Application/Hot air oven uses

• It is used to dry glassware, sterilize N95 masks, general instruments, and packaging items in life science, microbiology laboratory.
• It is also used in chemical and pharmaceutical industries, food and beverage industries, textile industries.
• It helps in the elimination of moisture from the material thus it is used in curing, drying, baking, and annealing.
• It is also used for the Measurement of mixed liquor suspended solids (MLSS).
• In certain laboratories and hospitals, it is used to store materials at a constant temperature.

Precautions

There are several precautions that should be taken when using a hot air oven:

1. Wear protective equipment: When using a hot air oven, it is important to wear appropriate protective equipment, such as oven mitts or tongs, to protect your hands from the heat.
2. Use caution when opening the oven door: The oven chamber and the inside of the door can become very hot during use. Use caution when opening the door to avoid burns or injury.
3. Follow the manufacturer’s instructions: It is important to read and follow the manufacturer’s instructions for your specific hot air oven. This will help to ensure that the oven is used safely and correctly.
4. Do not overheat materials: Overheating materials in a hot air oven can cause them to catch fire or release harmful fumes. Be sure to monitor the materials being heated or dried and remove them from the oven as soon as they have reached the desired temperature or dryness.
5. Do not leave the oven unattended: It is important to supervise the hot air oven while it is in use to ensure that it is operating safely. Do not leave the oven unattended while it is in use.
6. Keep flammable materials away from the oven: Do not store flammable materials near the hot air oven or place them in the oven chamber. The high temperatures of the oven can cause flammable materials to ignite.
7. Keep the oven clean: Regularly clean the inside and outside of the hot air oven to prevent a buildup of dirt or debris that could potentially catch fire.
8. Keep Dry: Ensure the goods stored in the hot air oven for sterilising are dry.
9. Types of Material Used: Do not utilise rubber products, textiles, or combustible or volatile substances in the oven.
10. Maintain Space: Do not overfill the oven. Maintain space for air circulation.
11. Wrapping: Wrap the outside of glassware such as Petri dishes and pipettes well.
12. Before opening the door of the hot air oven, cool it to 60 degrees Celsius. It keeps glassware from shattering.

By following these precautions, you can help to ensure the safe and effective use of your hot air oven.

Hot Air Oven Working Principle Video

FAQ

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.3340325
• Types of Ovens, ProSciTech Pty Ltd, 2014-06-05, Available at https://laboratoryresource.com/?navaction=getitem&id=204
• Use of Hot air oven in laboratories, prestogroup, available at https://www.prestogroup.com/blog/use-of-hot-air-oven-in-laboratories/
• https://en.wikipedia.org/wiki/Hot_air_oven
• https://www.isotechpl.com/blog/post/use-hot-air-oven
• https://www.slideshare.net/TayyabTariq8/hot-air-oven-instrument