A hot air oven, aka forced air circulating oven, is a laboratory instrument used to sterilize the laboratory equipment and other materials using the dry heat sterilization technique.
The dry heat sterilization method can be employed for that material that cannot be wet, or that will not melt when subjected to high heat.
Materials that can be sterilized are surgical dressings, rubber items, plastic material, also glassware, material that contains oil, metal equipment, etc.
The major advantage of a hot air oven is also that it can destroy heat-resistant spores just like an autoclave but with a hot air sterilization technique.
The most widely used temperature-time combinations are 170 degrees Celsius for 30 minutes, 160 degrees Celsius for 60 minutes, and 150 degrees Celsius for 150 minutes.
The technique was developed by Louis Pasteur.
Principle of Hot Air Oven
The hot air oven is based on the principle method of dry heat sterilization. Since conduction is the basis of dry heat sterilizations, thus the temperature first reaches the surface of the material to be sterilized, and then it gradually moves towards the core of the material. Thus, dry heat sterilization makes sure to sterilize every part of the material. Then, the whole material gets a uniform supply of heat, and if this heat is employed for a certain amount of time, then it helps in the sterilization of all different kinds of microorganisms, such as bacteria, viruses, fungi, and even the resistant Endospores, which escape most of the sterilization procedures.
Dry heat sterilizes the material by inducing oxidizing the particles inside it and damaging their primary component, which results in the ultimate death of the organism. Usually, the temperature that is set for efficient sterilization is about an hour.
Since hot air is lighter than cold air, thus increasing the temperature inside the chamber results in the flow of hot air up to the roof of the chamber while cold air comes down. Thus, it facilitates the circulation of hot air inside the chamber.
Parts/ Components of Hot Air Oven
- External cabinet: It is made up of stainless steel, and it also covers the inner chamber.
- Glass wool insulation: Glass wool is fitted between the inner and external chamber, which provides insulation to the system.
- Inner cabinet: This is also made up of stainless steel, and the material to be sterilized is kept inside it.
- Tubular air heaters: Heat inside the inner chamber is created using tubular air heaters, and they usually present in a pair of two on each side of the inner chamber.
- Motor-driven blower: It helps in the uniform circulation of the hot air within the chamber.
- Temperature sensor: It is a temperature display present on the screen which measures the inside temperature of the chamber.
- Tray slots: They are used to hold multiple trays.
- PID temperature controller: during the entire cycle of circulation of hot air, this temperature controller helps in maintaining the accurate temperature. It also helps in controlling the temperature and displays it on screen.
- Load indicator: Indicates the overload of material inside the chamber.
- On/off switch: helps turn the hot air oven on/off.
- Safety thermostat: aka over-temperature protection device, which helps keep the oven and specimen inside it safe in case of any malfunctioning, thus avoiding damage.
Types of Hot Air Oven
- Gravity convection:
- Air in this type of oven is distributed by spontaneous convection.
- Since the hot air is lighter than cold air, thus it rises up, and with the hold of temperature, it is moderately uniform inside the container and provides optimum sterilization.
- Forced convection ovens
- Limited air circulates in this type of oven due to the presence of a fan within the heating container.
- It is a fast and efficient method that heats the air fast and provides faster restoration times, which, when mixed with low-temperature differences inside the chamber, provide full sterilization.
- It is a conventional sample-drying oven with flexible vents and a semi-forced exhaust delivery system.
- Mechanical convection:
- It is a gravitation convection oven served with a re-circulating fan in a working container.
- Forced exhaust ovens:
- Air is circulated inside the working container by a fan and scattered through an adaptable vent.
- It is mostly required where vapors and fumes are required through the heating process, and the vapors and fumes generate immediately and continuously discharge from the working container.
- These have huge advantages over traditional convection ovens. Also, an additional air channel and a flexible outlet to achieve larger forced exhaust velocities.
- These are more expensive than a normal convection oven.
- Side draught ovens:
- These types of ovens produce air that flows from one side to another, from left to right.
- These have a huge advantage in the way that they provide speedy heat up and restoration time that is essential for plastic clothes or any profession where sheets and plates are used.
- Connect the machine to the power cable.
- By turning on the machine, the display on the screen start showing the present value (PV) and set value (SV).
- One can also set the temperature and time for sterilization.
- Can be used to sterilize heat-sensitive or corrosion-sensitive materials like oils, glassware, etc.
- Water is not required for sterilization.
- Easy to manage and safe to work with as it does not create pressure as much as an autoclave.
- It is a more compatible fit in a laboratory environment.
- They are much smaller in size as compared to autoclaves and if far more effective in sterilization than other sterilizing types of equipment.
- It works faster also and can achieve a high temperature.
- Operating a hot air oven is simpler than other means.
- It is unable to sterilize some living organisms, such as prions, because it uses dry heat rather than using wet heat because it uses the principle of thermal inactivation by oxidation.
- Surgical dressings, rubber items, or plastic materials are some of the materials that do not fit with a hot air oven because they can melt down even at lower temperatures.
Application and Uses
- It has huge applications in dry glassware, sterilizing N95 masks, general instruments, and packaging items in the microbiology laboratory.
- It also has applications in the pharmaceutical, chemical, food, textile, and beverage industries.
- Used in curing, drying, baking, and annealing because it can eliminate moisture from material.
- Used for measurement of mixed liquor suspended solids (MLSS)
- It can also be used to store material at a constant temperature.
- Always use a pair of gloves.
- Open hot air oven with care.
- Since the machine draws a high current, thus there are huge chances of damage to the heating coil, so it is advised to use the lowest optimum temperature setting to sterilize as it will increase the life span of the heating coil.
- 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