69


Applying insulating material to the outside of the vessel is an effective method of reducing this loss, but a well-insulated vessel alone will not cool quickly when required - it will require a jacket. The jacket is a cavity between the vessel and insulation that can be fl ooded with steam or water, providing heating and cooling as required. Alongside allowing for vessel insulation, the jacket enhances chamber temperature control, decreasing the possibility of a failed sterilisation cycle and improving processing times. A failed sterilisation cycle is the greatest potential source of wasted energy, effectively doubling power consumption by necessitating a repeat cycle.


There is often a misconception that, for electrical systems, using a smaller and lower wattage element will mean lower energy consumption. In truth, the same quantity of energy will be required to heat the water and produce steam, the process will just take longer (see Figure 3). Given identical conditions, the longer water takes to boil, the greater its heat loss to the external environment. To generate the most energy-effi cient steam for an autoclave, it is therefore essential to have the smallest possible water volume within the steam generator with the biggest possible heating element.


Working out the volume of steam required for sterilisation, and thus the volume of water, is unfortunately not as simple as calculating the air space available in the autoclave chamber. To reach and maintain the correct temperature throughout the autoclaving process, additional hot steam is added and cooled steam removed from the chamber to counter heat transfer. As such, it is the vessel that contains the autoclave chamber that has the potential for being the key source of thermal energy loss.


Although effective autoclaves are designed to maintain a constant temperature to avoid a failed cycle, there is one commonly overlooked attribute that can help mitigate this energy-intensive occurrence - thicker vessel walls. The thicker a vessel’s walls, the greater its thermal mass. This thermal mass provides the vessel with greater ability to counteract temperature fl uctuation, vastly reducing the possibility of a failed cycle. Thicker vessel walls can also retain higher chamber pressures, which provides additional energy-saving benefi ts.


A square hole or a round hole. Vessel and chamber shapes.


As gases exert forces equally in all directions, thinner-walled vessels are limited to a cylindrical shell structure to retain chamber pressures. While in top-loading autoclaves this design proves less detrimental, in front-loading devices such a design is a source of much wasted energy.


The functional sterilising area of a front-loading cylindrical-chambered autoclave can be simply described as a cuboid within the boundaries of a cylinder. This cuboid will always be 64% of volume of the cylinder, no matter the cylinder’s size (see Figure 4). As such, to fi ll a cylindrical autoclave can take up to 36% more steam, and therefore 36% more energy to generate that steam, than its cuboid equivalent. If the top of the cylinder is used for sterilisation, and the bottom section is fi lled to remove the air space, still 18% more steam is required to fi ll the curved sides. While octagonal-fronted shelving units attempt to better fi ll its circular face, the cylindrical chamber provides a challenge to maximise chamber space use.


With thicker walls able to withstand higher pressures, a chamber shape that provides even pressure distribution is less of a concern. Therefore, with thicker walls a more space effi cient chamber and vessel shape can be used - the cuboid. Cube-shaped vessels, when sized and shelved correctly, can be fi lled with loads that leave minimal air space. As such, they require less steam to fi ll their chambers than their cylindrical equivalents - less wasted steam means a lower energy requirement.


Keeping heat and pressure in one place: The Vessel


As the component that contains the autoclave chamber, the vessel is integral to maintaining necessary heat and pressure and must be designed accordingly. To retain pressure, materials with a high tensile strength are required - with variants of steel or aluminium alloys being popular choices. However, the thermal conductivity of these materials is high, facilitating heat transfer from the chamber - via the vessel - to the external environment.


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