Laboratory Products
Energy Effi cient Autoclaves -
Misconceptions and Misunderstanding Gareth West, Astell Scientifi c Ltd
Autoclave manufacturers are keen to espouse their green credentials and inform us of how their devices use less energy, but is it possible to make the radical savings that are claimed? Not always. Sterilisers are fi nely-tuned devices, and producing maximum results with minimum impact is often a question of balance.
To understand the energy effi ciency capacity of autoclaves it is crucial to begin by pinning down the essentials: what an autoclave must be to work as it should.
Back to basics - How does an autoclave work?
An autoclave is essentially a highly refi ned tool built for a specifi c job: sterilisation. The goal is to kill all biological life and deactivate biological agents placed within autoclaving chambers, typically by raising their temperature to 121°C for 15 minutes or more [1, 2] (although shorter processes at higher temperatures are also possible [3]). Reliably maintaining and measuring these temperatures is essential for achieving and validating sterilisation, and fl uctuations below the sterilisation temperature invalidates the process, requiring it to be repeated.
For sterilisation to be effective, a fl uid substance must fi ll the autoclave chamber and transfer heat to everything within it. Frequently, this means using steam with specifi c qualities. Pure steam gas that contains no liquid water – known as dry steam – is a poor conductor of heat and ineffective at steam sterilisation [4]. Dry saturated steam that carries 5% by mass of liquid water (or has a dryness fraction of 0.95) is the most effective fl uid for transferring heat to a load within an autoclave. Any dryer and the heat is not effectively transferred to the load, any wetter and the loads will be sodden.
As Anders Celsius eternally reminds us, the boiling point of water is 100°C at standard temperature and pressure (STP), so to reach minimum sterilisation temperature autoclaves must create conditions that make the boiling point of water higher. By raising the pressure in a sealed environment, the boiling point of substances within that environment is also raised (see Figure 1). For example, raising the pressure within an autoclave chamber to 2.068 Bar(a) will cause water to boil at 121°C, while increasing pressure to 2.896 Bar(a) will create an environment in which water boils at 132°C.
To sterilise effectively, both a sealed pressurised environment greater than 2.068 Bar(a) and a temperature of greater than 121°C are required. An autoclave must be able to both withhold these internal temperatures and pressures, and generate dry saturated steam from water (usually via electrical energy).
The invariability of steam generation.
Dry saturated steam generation from water is an inevitable starting point for investigating autoclave energy use. At STP, water requires 2,619 kilojoules of energy per kilogram (kJ/kg) to become dry saturated steam. If the environment in which the steam is produced is at higher pressure, the quantity of energy needed to convert water to steam is less. At the minimum sterilisation temperature and pressure (121°C and 2.068 Bar(a)), 2,619 kJ/kg are required to generate dry saturated steam from water. This process will create steam that will fi ll a 0.841m3
space per kilogram of water used. These are the physical,
unchanging properties of H2O, and provide the limitations on how energy effi cient an autoclave can be.
In the hypothetical ideal autoclave (that wastes no energy) the initial volume of water used to generate steam is the only variable that affects energy consumption. The smallest possible volume of water in the steam generator provides the most energy effi cient system – if water requires a specifi c quantity of kilojoules per kilogram to convert it to steam, the only way to reduce the energy usage is to reduce the mass (and volume) of water (see Figure 2).
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