Tony Collins explains why effective autoclave loading is the key to effective sterilisation
Much has been written about why research institutions and industrial laboratories use autoclaves, the whole purpose being that of sterilisation. The aim is to destroy any microorganisms; improperly autoclaved items can result in contamination, lost time, wasted money and – in extreme circumstances – much worse. In steam sterilising the sterilising agent is, by definition, saturated steam. If the items in the autoclave do not come into contact with saturated steam then they will not be sterilised properly. Therefore effective loading of the autoclave chamber to aid steam flow and penetration and air removal is essential.
Research grade and laboratory autoclaves in the main are available with cylindrical, rectangular or square chambers. When making a purchase, chamber size has a major influence on selection.
Where higher daily throughputs are required many may opt for a rectangular chamber, as straight sides mean that more items can be loaded into the sterilising chamber, from wall to wall, top to bottom. But, when fully loaded, is the rectangular chamber design more efficient? Does it provide effective sterilisation? What about overall cost of ownership? Could a cylindrical chamber be a more viable alternative?
The sterilising environment
To ensure that sterilisation takes place, the autoclave chamber (a sealed pressure vessel) must contain dry saturated steam held at temperatures, typically, up to 140°C and a pressure of 2.4bar, and this steam must come into contact with the items being sterilised.
To create this environment, air must be removed from both the load and the chamber. This can be accomplished in a number of ways. Air removal from high mass, low surface area items (ie, comprising mainly solid areas that contain little or no air pockets, such as bottled media) will require little air removal, and can be facilitated by automatic air purging. Here, air is allowed to leave the chamber through a vent as steam enters the chamber either from an integral source (upward displacement) or an external supply (downward displacement) with the vent only closing when all of the air is removed from the chamber.
This method can be further advanced by ‘freesteaming’ when the vent is allowed to stay open for a set length of time once the autoclave is above 100°C. The turbulent steam then passes through the vent, forcing any trapped air out of the autoclave. This helps with air removal and is useful in a different way for liquid loads as it effectively holds the autoclave chamber at 100°C so that the liquid load temperature can catch up with it.
In some systems there is the option to pulse the ‘freesteaming’ by repeatedly allowing pressure to build and then releasing it to assist with air removal from laboratory waste loads, in particular. Freesteaming of pulsed steaming is not suitable for liquid loads however.
For more stubborn loads that contain a number of air pockets, such as wrapped instruments or fabrics, especially where large or tightly packed loads are concerned, air removal can be further assisted by using a vacuum system at the start of a cycle to remove air from within the autoclave chamber and load, which can then be replaced actively with steam as the autoclave heats up.
A false economy
Having established what is required within the chamber environment (dry saturated steam), it is important to appreciate that care is required when loading. Sterilisation can only take place if the steam is allowed to flow freely within the chamber so that it comes into direct contact with items to be sterilised – waste, glass bottles, etc.
Laboratories looking to invest in a new autoclave and requiring a higher throughput may initially look to purchase a rectangular chamber design. Size for size in chamber capacity, it would seem to offer better loading than that of a cylindrical machine and look to be the right choice; more items could be crammed into the chamber. Is it a better buy? Is it false economy?
In actual fact, a cylindrical chamber autoclave often offers a lower cost of ownership and is more energy efficient.
Rectangular chamber autoclave Cylindrical chamber autoclave
More expensive, could be up to 50% more costly Less expensive, simpler manufacturing process, quicker customer delivery
More metal, greater density – longer to heat-up Less metal, heats up more quickly
Uses more energy More efficient, lower energy costs
Extended heating time reduces. Faster heat-up means more sterilising cycles per day.
Potential for easily overloading the chamber Cylindrical chambers by their nature tend to prevent overloading
number of sterilising cycles per day.
For more information, visit www.scientistlive.com/eurolab
Tony Collins is MD of Priorclave.
