The electrical devices which are called as hot air oven are used in sterilization by providing the dry heat.
They were originally developed by Pasteur. The oven uses dry heat to sterilize articles. Generally, they can be operated from 50 to 300 degC (122 to 572 degF) . There is a thermostat controlling the temperature. These are digitally controlled to maintain the temperature. Their double walled insulation keeps the heat in and conserves energy, the inner layer being a poor conductor and outer layer being metallic. It is used in the sterilization of pharmaceutical products and other materials. It is double walled chamber made of steel.
The principle of operation is based on a fine gravity air convection in a an electrically heated chamber of the unit. A two jacket design of the chamber and an automatic control unit ensure a homogenous temperature profile in the chamber, precise course of processes and short recovery time (return to the preset temperature) after door opening. This line of drying ovens is characterized by its economical operation. It is suitable for simple process of drying and heating of common materials. The units work noiseless.
- All external cases of hot air ovens are covered with a thick layer of stoved epoxy polyester paint, made of mild steel and with strong seamless welding.
- This provides a strong, highly screech and heat resistant surface for a long lasting attractive finish to the Hot air ovens.
- Internal surfaces (including the doors) of Hot air ovens are manufactured from either aluminium or stainless steel, with great care in order to eliminate sharp edges.
- The double walled chamber of hot air ovens is filled with high grade insulation to prevent heat loss. Elements of the hot air ovens are of the sheathed black heat type are put inside beads and placed at the bottom and in both the side ribs are placed for uniform temperature.
- The door of hot air oven closes on to an asbestos gasket the shelves slide neatly into the fixed runners. Insulation materials such as glass fibres of asbestos is filled between the two walls of the oven to avoid heat loss.
- The door is also double walled having asbestos gasket on its inner side. There is also an air filled space in between to aid insulation.
- These are fitted with the adjustable wire mesh plated trays or aluminium trays and consist of fan for distribution of air uniformly.
- The capacities of these ovens vary. Power supply needs vary from country to country, depending on the voltage and frequency (hertz) used.
- Temperature sensitive tapes or other devices like those using bacterial spores can be used to work as controls, to test for the efficacy of the device in every cycle.
- Two or three perforated shelves are fixed in side the oven to place the materials for sterilization. An electrical fan is also fitted to ensure the uniform circulation of hot air in the oven in order to maintain the required temperature in all the shelves.
- Heating elements are fitted on the bottom of the oven and it is thermostatically controlled. A thermometer is fitted in the oven to note down the temperature in side the oven.
- The materials to be sterilised are placed on the perforated shelves of the oven.
- To prevent slipping and this protects the bench surface the hot air oven is mounted on four rubber foots. The scale is calibrated in 5degC steps.
The working of the hot air oven is based on the hot air inside the chamber of oven by the forced circulation. As it is a universal scientific fact that in any chamber hot air rises above, So by utilizing this principle when the hot air reaches the top of chamber it is circulated back to bottom by a fan installed inside the chamber and hence optimum amount of heat is achieved gradually inside the hot air oven.
After heating the content of the oven for two hours at 160 @c, the articles are allowed to remain there, till the temperature comes down to 40 @c. then the sterilized materials is then removed from the oven.
- It is mainly used for the sterilization of glasswares such as pestle and motar, petridishes, flasks, pipettes, bottles, test tubes etc.
- It is used for the sterilization of powders such as sulphacetamides, sulphadiazenes, kaolin, zinc oxide, starch etc.
- Injections where fixedoils is used as the vehicle are sterilised by the dry heat method
- Example: injections of progestrone, injection of testosterone propionate and injections of oestradiols dipropionate.
- It is also used for sterilisation of scalpels, scissors, spatula, blades and glass syringes.
- The chemicals, glassware in laboratories, research institutions, industries, hospitals and R&D centers used hot air ovens are suitable for temperature upto 250degC.
- Hot air ovens suits to various applications like heating, drying, sterilizing & baking.
- It is used for the sterilisation of those substances which gets spolied during moist heat sterilisation. Eg: oily materials and powders.
- The method is suitable for sterilisation of assembled equipments such as all glas syringes due to expose to high temeprature for a long time.
- It is not so damaging to glass and metals equipments as moist heat.
- Dry heat will not corrode or rust instruments or needles.
- Dry heat will sterilize instruments containing many parts that can not be disassembled
- This method is not suitable for the surgical dressings.
- This method is not suitable for the most of the medicaments, rubber and plastic good because the articles are exposed to a very high temperature for a long period.
- Dry heat penetrates slowly and unevenly.
- Dry heat requires long exposure times to effectively achieve sterility.
- Dry heat requires higher temperatures that many items cannot be safely exposed to.
- Dry heat requires specialized packaging materials that can sustain integrity under high heat conditions.
- Dry heat may require different temperature and exposure times, depending on the type of item being sterilized.
- Glass apparatus must be wrapped with the clean cloth or filter paper and containers must be plugged with non assorbants cotton wool.
- The article and substances which are to be sterilised should not be placed at the floor of the oven as it receieves direct heat and becomes much hotter.
- The oven should not be over loaded with the materials meant for sterilisation.
- There should be sufficient space in between the artilcles, so that there is uniform distribution of heat.
- Mehta RM, Pharmaceutics-I: sterilisation (1996), Page no: 204 - 205.