Today we explore some basic principles of using an autoclave with liquids and how your choice of machine, could save you lots of money in energy savings.
Autoclaving Liquids Properly
When autoclaving liquids in the laboratory it is important to achieve correct sterilization temperature for the correct period of time. In the case of sterilizing liquid waste the primary goal is to neutralize all living organisms so the waste can be disposed of safely. With liquid wastes it is therefore important to be sure the liquids reach 121oC for a specified period of time; usually 15 - 20 minutes. Generally, over sterilizing (i.e. remaining at 121oC for longer than 15 - 20 minutes) is not a problem as the waste will just be disposed of. When sterilizing other liquids such as growth media - over sterilizing can present a problem as going above 121oC or remaining at 121oC for longer than 15 - 20 minutes could have deleterious effects on the performance of the media.
Accurate control of the autoclave process is of paramount importance in order to reproducibly achieve desirable results. The vast majority of autoclaves being used in labs today are unable to accurately and reproducibly perform liquid sterilization cycles. This is mainly due to the fact that most autoclaves utilize a single temperature probe that determines the absolute temperature within the sterilization chamber; but does not directly measure the temperature of the liquids being sterilized. Autoclave operators therefore often either over sterilize, or under sterilize liquids and usually "guess" at how long an autoclave cycle should be to correctly sterilize their liquids. Often times the volume of liquid being sterilized is ignored when considering the duration of the cycle - this is a critical misstep as higher liquid volumes require more energy and hence longer heat up times to reach 121oC.
The only way to safely and accurately perform a liquid sterilization cycle is to directly measure the temperature of the liquid throughout the entire heating, sterilizing and cooling phase of the cycle. The latest generation of high quality laboratory autoclaves utilize a flexible PT100 temperature probe that can be directly placed in the liquid being sterilized or in a reference vessel with water (containing the same volume as the liquid being sterilized). The flexible temperature probe provides feedback directly to the microprocessor controlling the sterilization cycle meaning that the sterilization phase will not begin until the liquid reaches 121oC and the sterilization time will only begin to countdown once the specified sterilization temperature has been attained. Only laboratory autoclaves with the design features described ensure liquids reach the correct temperature for the correct period of time. Additionally, during the cooling phase accurate temperature measurement ensures boil over will not occur as the pressure release can be controlled based on the liquid temperature. This level of control and monitoring prevents the operator from opening a properly equipped autoclave containing super heated liquids as the microprocessor will prevent door opening until the liquids reach a safe handling temperature (usually 80oC).
Laboratories performing liquid sterilization should confirm they are using autoclaves that provide the level of control and sophistication to directly monitor and control liquid temperature, and sterilization vessel pressure. Relying on a one size fits all sterilization time and "slow exhaust" setting for liquids will not provide accurate, reproducible liquid sterilization results.
Why choose round or cylindrical autoclave chamber vs. rectangular?
The pressure inside an autoclave chamber naturally attempts to make the chamber round. Therefore a cylindrical chamber can have thinner walls and weighs less than a rectangular chamber. Since cylindrical chambers have less metal; heat up and cool down occurs more efficiently with less energy consumption and faster cycle times. Cylindrical chambers provide better steam circulation on front loading models. Due to the circular profile of the working chamber and the ‘square’ profile of most autoclave loads there will normally be space around the sides of the load for steam circulation. If the steam cannot get to the load or the air cannot escape sterilizing will not be effective. Extra care and skill must be employed when loading a rectangular chamber to ensure that steam circulation is not restricted by overfilling the autoclave chamber. Again, if steam cannot get to the load or air cannot escape then sterilizing will not be effective.
Cylindrical autoclaves offer more efficient steam circulation and air removal, combined with lower operating costs due to increased efficiency in heat up and cool down times. Long term savings can be realized with lower operating costs, more reliable sterilizing and faster cycle times. See below how a Systec Autoclave from Microbiology International can be an energy efficient autoclave which could save you thousands per year in electricity.
|Other Autoclaves||Systec Autoclave|
|Autoclave Steam Generator (Kw)||30||18|
|Estimated hours per day autoclave will run||4||4|
|Estimated days per week autoclave will run||3||3|
|Estimated weeks per year autoclave will run||48||48|
|Utility costs (Dollars / kWh)||$0.13||$0.13|
|Estimated Annual Energy Cost||$2,246.40||$1,347.84|
|Estimated annual savings: $898.56|