With electricity-hungry autoclaves sterilising items across sectors, from the laboratory to the processing plant, we ask if this key piece of hardware can be made more energy efficient.
Autoclaves come in a variety of shapes and sizes. Is one type more energy efficient?
Why are autoclaves traditionally so energy intensive?
The basics of how an autoclave uses energy are pretty fixed. An autoclave sterilises by heating objects in their chamber to typically 121°C for 15 minutes or more. (There are hotter, faster sterilisation cycles, but 121°C is the norm). This combination of temperature and time is shown to destroy anything biological: bacteria, spores, viruses, prions etc. If the temperature drops below 121°C during this time, sterilisation cannot be assured, so the process has to be repeated.
Heating objects using just air is inefficient, so autoclaves use steam containing a specified quantity of liquid water droplets (5% by mass). This mixture is perfect for carrying heat without making the load in the autoclave too wet.
The problem with using water is that, at room pressure, it boils at 100°C. The autoclave produces conditions where water boils at 121°C by creating a sealed, pressurised environment of above 2.068 Bar(a). Filling the autoclave chamber with 121°C steam at 2.068 Bar(a), and maintaining these conditions for 15 minutes is fundamental to what an autoclave does, and accounts for the majority of its energy use.
So, steam generation takes the lion’s share of energy consumption, but how can process become more efficient?
Turning water to steam is a process governed by the laws of physics. At the pressure required for sterilisation (2.068 Bar(a)), water needs 2,619 kJ/kg of energy to boil. For each kilogram of water heated, this process creates enough steam to fill a 0.841m3 space.
There are some claims that using a lower wattage heating element will mean an autoclave uses less energy. This isn’t true – the autoclave will still need to add 2,619 kJ/kg of energy to turn water into steam at 2.068 Bar(a). If you half the size of the heating element, you double the time it takes to boil the water. By heating the water more slowly, you increase the opportunity heat has to escape the autoclave, increasing energy loss and overall energy use. One change that cuts energy use is reduce the amount of water the autoclave heats to make steam – halve the volume of water you are heating, and you halve your energy use. We applied this rule first on our heaters-in-chamber models, which have now up to 16kw heating elements in a smaller volume of water, while our autoclaves with integral steam generators have up to 72kw heating elements.
Three of the most common autoclave chamber shapes
Autoclaves come in many shapes and sizes. Which is the most energy efficient?
The autoclave’s chamber, and the vessel that contains it, are the biggest determinants of the shape and size of an autoclave. The shape of the autoclave chamber is important to saving energy, but there is no simple answer to which is best. Because its shape distributes pressure more evenly, a cylindrical chamber allows you to use thinner metal for the vessel. This thinner construction takes less energy to heat. The problem with a cylindrical chamber is that there is less usable space inside it. If you have a cylinder on its side as your chamber, you have a rounded base and rounded walls. This means you need to build a shelf to compensate for the round floor, and you can’t fill all the space beside the walls because they curve away from the floor. This can result in 36% of the chamber space being empty, and a 36% increase in wasted energy.
The alternative to a cylindrical chamber is a cuboid one. With a flat floor and flat sides, filling a cuboid chamber is much easier; the fuller the chamber, the less energy is wasted. The problem is that you need thicker walls than with a cylindrical chamber to hold the pressure. The thicker your walls, the more energy they take to heat. We have companies asking for all kinds of cuboid, horizontal cylinder, and upright cylinder chambered autoclaves. By discussing how their operators use the autoclave, we can help find the most energy efficient chamber for each organisation.
The geometry of a cylindrical autoclave chamber: Usable and unusable spaces
What other steps can be taken to further reduce energy use?
We often recommend a heating and cooling jacket is added to the autoclave. A jacket wraps around the outside of the vessel, and can be filled with hot or cold water. This allows for better temperature control, and reduces the chance of fluctuations below the sterilisation temperature. Such temperature fluctuations can cause the sterilisation process to be invalid. If sterilisation is invalid, the process needs running again, doubling the energy use.
Regular servicing, calibration, and validation are also very important to make sure the autoclave is running efficiently, at accurate temperatures, and that its loads sterilise fully and correctly.
Where specifically can the biggest energy savings be made?
Making sure the autoclave is only used when it is full to capacity is the best way to make energy savings; If your autoclave can hold thirty Duran bottles and you only autoclave one bottle, around 97% of your potentially useful energy is wasted. This also highlights why you don’t want an autoclave that is too big for your needs. At the other end of the spectrum, if the autoclave is too small you might need to run more loads; More loads mean more energy is wasted heating and cooling the autoclave.
In short, organisations need autoclaves that are designed to meet their specific requirements. The more “one size fits all” the design of the autoclave, the less it has been designed for the company’s needs. The less the autoclave is designed to fit a business’s specific needs, the greater the possibility of inefficient use. This is why we provide so many variations in models, options, and chamber sizes.