A hot air oven is a piece of laboratory equipment used to sterilize laboratory objects and materials using dry heat (hot air). Dry heat sterilization is another name for this method of sterilization.
This heat treatment process was developed in the late 1800s by French scientist Louis Pasteur, who utilized dry heat for a short period of time to destroy unwanted germs in wine without affecting its taste. Keep reading to learn the basics of how hot air ovens work, the different types available, and tips for using one.
Hot air ovens are electrical devices that sterilize using dry heat. Louis Pasteur invented them. Typically, it uses a thermostat to regulate the temperature.
A hot air oven uses convection heating to circulate hot air around the oven chamber. It works by heating the air inside and circulating it around the oven.
How Does a Hot Air Oven Work?
A hot air oven works by circulating hot air around the chamber using a fan. The hot air oven operates on the dry air sterilization concept of convection, conduction, and radiation. The heating components heat the air inside the chamber, which is then circulated evenly within it by fans, exposing the sample surfaces to hot and dry air. This exposure causes the external surface of items to heat up, and the heat is transmitted to the center of the item via conduction.
Similarly, heat causes water inside microorganisms to evaporate, resulting in oxidative damage to cellular components, protein denaturation, the toxic effect of high electrolyte levels, and, finally, the death of the microorganisms.
Major Parts and Components of a Hot Air Oven
The major parts of a hot air oven include Mechanical and electrical components.
Fiberglass: Glass wool insulation fills the area between the outer cabinet and the interior chamber. There are two types of fiberglass in a hot air oven, namely, brown fiberglass and yellow fiberglass. The latter is less hazardous than the former. Brown glass promotes respiratory system inflammation, whereas yellow glass causes skin sensitivity. As a result, when working with it, it is best to wear hand gloves. It also prevents heat loss from the device’s interior to the exterior.
Coat/Cabinet: The external shield is made of aluminum or stainless steel, both of which are resistant to mechanical shocks and oxidation. It also acts as an insulator between the interior and external environments, preventing heat loss.
Chamber: The rectangular-shaped chamber is composed of aluminum or stainless steel and provides space for ribs to hold the shelves at the proper levels.
Shelves (Mesh): These are aluminum plates-holding items. Their number might vary depending on the number and size of things, as well as the oven capacity. When they are put on the ribs, they aid in the passage of air by lifting particular areas. Some shelves may also have aeration openings.
Door: A single door is mounted on sturdy hinges on one side. The side asbestos door gasket is used to reduce heat loss during operation.
Motorized fans/blowers: The fan is powered by a motor and is used to distribute heated air uniformly across the chamber.
Fuse: A fuse protects the electrical system from harm caused by excessive current during short circuits or high loads.
Power supply: A 220V-50Hz transformer and rectifier are used to power the system.
Heater: Heat is generated by passing an electric current through a conductor after the temperature rises. High resistance, electrical insulation, and high thermal conductivity are the three fundamental characteristics of the heating element. One-sided circular heaters, one-sided U heaters, one-sided wave heaters, one-sided square heaters, three-sided heaters, and four-sided heaters are the various types of heaters used in hot air ovens. Temperatures range from 50 to 300 degrees Celsius.
Temperature indicator: The internal temperature of the oven can be determined using a thermometer or a thermocouple.
Thermostat: A heat sensor that is directly connected to a heater and has a strong negative temperature coefficient to withstand excessive heat. It makes it easier for users to achieve the correct temperature in the hot air oven and avoids temperature overshoot.
Control Panel: This is the area where the user may adjust various parameter settings such as temperature, time, and so on. It also features an indicator power bulb (typically green), an indicator heater lamp (usually red), and a switch knob.
Timer: There are two types of timers, electrical and mechanical, and they can run for 5-60 minutes depending on the sterilizing time.
Different Types of Hot Air Ovens
There are three main types of hot air ovens:
Natural convection oven/ Gravity convection oven
It works on the idea of natural convection, in which warm air at the bottom of the device rises to the ceiling, begins to cool, and then drops to the floor, allowing the cycle of heating and cooling to continue naturally within the device. It has a non-uniform temperature distribution and is suited for powder samples.
Forced convection oven
The forced convection oven uses a motorized fan/blower to force hot air to distribute uniformly throughout the chamber’s interior. Because the sample is constantly in contact with heated air, it absorbs heat and dries faster.
Side draught oven
It has a one-way flow of air from one side to the other, allowing samples to absorb the most heat in the shortest amount of time. It is great for preheating plastic garments, smooth sheets, or plates in hospitals.
What does a Hot Air Oven Heat to?
Hot air ovens can reach a wide range of temperatures, typically from room temperature of 50°C to 300°C. To kill the bacteria, the system employs an oxidative mechanism. The dry heat of the oven oxidizes the cellular components of bacteria and spores, killing them.
Key Applications and Uses of Hot Air Ovens
Hot air ovens have many useful applications, especially in industrial settings.
- It can be used to test food items, pharmaceutical products, and other consumable materials to verify temperature stability during shelf life.
- It can be used for research in biological, chemical, and material science.
- It can be used to heat treat and dry samples like metals, soil, alloys, and other materials.
- It is used to sterilize glassware (flasks, pipettes, Petri plates, and test tubes), culture medium, metal items (forceps, spatula, scalpel, scissors), non-volatile substances (zinc and starch powder, sulfonamide), and other oil-containing materials.
What items are Sterilized by Hot Air Oven?
Hot air ovens are used to sterilize a variety of items in laboratories, hospitals, and industrial settings. Some of the common items sterilized in hot air ovens include:
- Glassware like test tubes, beakers, and pipettes
- Metal instruments such as scissors, tweezers, and spatulas
- Plastic ware such as culture plates, Petri dishes, and pipette tips
- Paper products like filter papers and weighing boats
- Glass slides and cover slips used for microscopy
- Packaging materials like bottles, vials, and flasks before filling them
- Powders and granules that can withstand high heat without deteriorating
The hot air oven uses dry heat to kill microbes, so any items that can withstand temperatures of 160°C or higher without melting or charring can be sterilized in these ovens. The hot circulating air penetrates materials to eliminate bacteria, viruses, fungi, and spores.
- Allow the oven to cool completely after use before cleaning or servicing any parts. The heating elements and interior can remain extremely hot for some time after operation.
- Exercise caution when removing parts like the fan guard or fan blade for cleaning. The blades can be sharp, and the oven may still be warm.
- Never operate the oven without the fan guard in place. The fan blade can cause injury if accidentally touched while spinning at high speeds.
- Keep the oven in a well-ventilated area away from any flammable objects. Hot air ovens produce and exhaust hot air, and the exterior of the unit also becomes very hot during use.
- Never leave the oven unattended during operation. Always monitor in case of overheating or fire hazards.
- Allow the oven to cool for at least 30 minutes after use before storing to prevent fire risk. The exterior remains dangerously hot for some time after the oven is turned off.
- Follow all instructions carefully when servicing electrical components like heating elements, thermostats, or timers. Incorrect installation or repair can lead to shock or fire.
- Keep the oven clean and free of built-up debris like dust, grease, or grime, which can become a fire hazard if overheated. Perform regular maintenance to ensure safe working conditions.
- Disconnect the power supply before attempting any repairs or servicing of electrical components. Always exercise caution due to the risk of electric shock.
- Never operate a damaged or malfunctioning oven. Have repairs made by a qualified technician to avoid the risk of injury or fire.
Advantages of Hot Air Ovens
Hot air ovens offer many benefits for industrial processing, which includes the following:
- Safety. Its lower pressure promotes safety throughout the operation.
- Rapid heating. Hot air circulates to reach the target temperature quickly.
- Uniform heating. The circulating hot air evenly distributes heat to all parts of the oven.
- It has an adjustable temperature. A wide range of temperatures is possible by controlling the heating elements and fan.
- It is energy efficient. The circulating hot air efficiently transfers heat to the materials.
- Metals or other sharp objects are not corroded or rusted by dry heat.
- It is versatile. Able to process many types of materials like plastics, ceramics, glass, and wood.
- Easy loading and unloading. Materials can be loaded on trays or carts and rolled in and out.
- Minimal pollution. The hot air ovens have little emissions or byproducts. They are a “green” technology.
- Unlike an autoclave, it does not require water for sterilization.
- It is inexpensive and simple to use.
- The oven’s small size requires less space and makes installation easier.
- It is not poisonous. There will be no dangerous chemical residues dumped.
- Dry heat can penetrate deeply into thick items, assisting in the achievement of an in-depth sterilizing effect.
Disadvantages of Hot Air Oven
The main disadvantages of hot air ovens are:
- It requires constant electrical supply to operate, which increases the running cost.
- It may not kill heat-resistant endospores and prions since they employ dry heat rather than moist heat.
- It has a limited temperature range (usually up to 300°C), which restricts its use for various applications.
- It takes a longer time to attain the desired temperature, which reduces its effectiveness. It takes longer than steam, flame, chemical sterilization, or radiation.
- Has lower thermal efficiency as a major amount of heat is lost to the surroundings.
- It has an uneven temperature distribution, which leads to non-uniform heating of materials.
- Due to its low melting point, it is not suited for sterilizing products such as rubber, plastics, surgical dressings, and so on.
- Requires frequent maintenance and repair, which increases the downtime.
Is Autoclave and Hot Air Oven Same?
No, an autoclave and a hot air oven are not the same. While they are both used for sterilization, they operate in different ways:
An autoclave uses high-pressure steam to sterilize equipment and supplies. The steam is produced in a closed chamber, allowing it to reach higher temperatures than boiling water. The steam kills bacteria, viruses, and other microbes by disrupting their physical structures. However, autoclaves are fast and penetrate materials thoroughly. A hot air oven can operate at greater temperatures and a faster rate than an autoclave.
In conclusion, the hot air oven is an essential instrument in scientific study and medical treatment. Because of its capacity to sterilize using dry heat, it is essential for treating heat-resistant materials. The hot air oven, which has its origins in Pasteur’s pioneering work, is a testament to the value of scientific innovation.