Safety, Hazard Containment & Sterilising Equipment
Cycle Times Matter – Cooling Options Explored Steve Foreman, Astell Scientifi c Ltd
When considering the amount of time required to run a typical steam sterilisation cycle, it’s a common misconception that the total cycle time is made up almost entirely by the sterilisation period itself. Whilst quick cycles might be the case with certain load types (e.g. glassware, plastics, discard (waste) loads and instruments), many published cycle times are based on calculations using a certain number of wrapped instruments, or an ‘average’ amount of plastic/glassware. However, when it comes to bottled fl uid/media loads, standard autoclaves with no cooling system have dramatically longer cooling periods (and therefore overall cycle times) due to the signifi cantly larger thermal mass of liquid loads compared to, for example, ‘solid’ loads such as glassware, dry discard etc.
Therefore understanding the different cooling options available for autoclaves, and how they can affect cycle times can help you in your decision to buy the right autoclave that meets your daily cycle quota. As with a car where you are able to choose anything from a basic ‘no frills’ vehicle right through to a luxury top-end model, the range of cooling options available for autoclaves is the same, and can dramatically affect the speed of cooling – in some cases by up to 70% compared to a standard machine without a cooling system. In a society that’s conscious about effi ciency and making the optimal use of time, this article considers the various cooling options available and how they can benefi t your sterilising effi ciency. All of the following scenarios are based on the cycles which take the longest time to cool down; which are fl uid cycles.
To understand the impact of cooling options, we must fi rst start by looking at a standard autoclave with no cooling options fi tted. In this scenario, at the end of a cycle, once the sterilisation process is complete, the autoclave will naturally cool. On a standard unit with no cooling options fi tted, the cooling time is based on how long it takes for the temperature and pressure in the chamber to reach a safe level (normally 80°C) relative to the ambient temperature outside the autoclave. This period is particularly signifi cant when dealing with fl uids/media (and in particular sealed or partially sealed bottles) where the length of cooling time can present a major inconvenience if the load is left to cool down naturally.
The reason the cooling period is necessary (and why the door can’t be opened immediately following sterilisation) is because at the end of the cycle, the temperature in the chamber (and thus the pressure) is at a much higher level than that of the atmospheric pressure outside the chamber; so whilst mandatory safety features prevent the door from being opened, if in theory you could open the door before safe conditions are achieved, this difference in pressure could potentially cause liquids to boil over and, in extreme circumstances, glass containers to shatter. The latter, of course, could feasibly result in lab personnel being scalded by media splashes, so compliance with health and safety standards is as always an important factor. It is therefore important with all cooling methods for fl uid loads that the cooling is either gradual (so as not to cause a sudden change in pressure or temperature), or for rapid cooling systems that there are measures taken to compensate for the rapid change in temperate/ pressure.
laboratory can have a greater adverse effect on the effi ciency of fan cooling than it can on coiled water cooling. Many laboratories will select one of these options since though the improvement in cycle times is only moderate, the additional cost is low.
Internal Cooling Methods
Autodrain - Whilst it is not always classifi ed as a cooling method per se, for autoclaves where the heaters are located in the chamber itself (and thus the water that is used for the sterilisation process), autodrain effectively drains the chamber of all remaining water at a pre-selected pressure during the cooling phase of the cycle. By removing all this hot water (and in conjunction with one of the external cooling methods previously described), cycle times can be substantially reduced. Autofi ll and Autodrain are key features on the Astell 33-63 litre ‘Autofi ll’ range, ensuring that cycles are as quick as possible, and are also available as options on larger models.
Air Ballast – Whilst air ballast is not a cooling method in itself, it is integral to many of the fast cooling methods used for fl uid loads. Air ballast works by using an external compressed air supply to compensate for changes in pressure within the chamber and is recommended for loads that would be affected by sudden pressure changes. When autoclaves are fi tted with faster cooling methods (such as jacket water cooling and internal fan cooling), a sudden decrease in steam pressure inside the chamber must be compensated for with air ballast. In simple terms, if the pressure change is not compensated for, media can boil over, load deformation may occur, and depending on structural integrity certain items may spill, crack or shatter. Introducing fi ltered compressed air to counter the sudden change in chamber pressure prevents these adverse effects; whilst at the same time ensuring that cooling systems can be effective immediately after sterilisation. In other words, air ballast ensures that there is no need to wait for loads to initially cool down naturally to atmospheric pressure before applying a rapid, effective cooling method.
So with this in mind, let’s take a look at the various cooling methods available and their application:
Simple External Cooling Methods
The simplest methods of cooling are external fan cooling or a coiled water cooling system. Both of these methods introduce either air or water (as applicable) in direct contact with the outside of the pressure vessel. Whilst these cooling options are better than a unit with no cooling, because the process is external to the pressure vessel, cooling is a fairly slow and gradual process. Both methods are about equal in terms of increasing the speed of the cooling phase, although coiled water cooling has a slight edge over fan cooling due to a larger surface area of coils. Additionally, the ambient temperature within the
Full Water Jacket – Effectively this is an advanced version of the external cooling coils. The water cooled jacket is more effi cient than its coiled counterpart for one reason – the amount of surface area it covers. Whilst a coiled water system covers about 25% of the chamber’s external surface, a jacketed vessel normally covers the whole outside circumference of the chamber. Cold water comes into direct contact with the vessel, cooling it by conduction. Warmed water in the jacket is constantly circulated and replaced with cold water for maximum effi ciency. This option is normally used in conjunction with air ballast for best effect.
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