How Does a Laboratory Autoclave Work?

Steam sterilization is, and should be, an important process within every laboratory. This process is performed in a steam sterilizer and this article will explore how a sterilizer works, the sterilization process, why we do it, and its origins.

The following guide outlines not only the history of steam sterilization, but also explains how it works and why it’s such a crucial component to maintaining a clean and sterile lab.

An Introduction to Steam Sterilization

Before discussing anything related to steam sterilization, it’s important to address the following:


The terms Autoclave and Steam Sterilizer are essentially synonymous and often used interchangeably. “Autoclave” is used more often in laboratory settings, while “Sterilizer” is more commonly heard in hospitals or pharmaceutical settings.


The autoclave, a device that is ubiquitous in almost all scientific settings, is a device that uses steam heat to kill any microbial life that may be present on a contaminated Load. Any load (also referred to as Goods) that has undergone a full sterilization cycle are considered to be sterile and can be used without fear of introducing foreign microorganisms into a sensitive environment — that environment being a laboratory, hospital operating room, food production facility, etc. Different types of goods must be sterilized for different times, at different temperatures. Some autoclaves contain features that others do not, such as vacuum functions, special cycles, and integral electric boilers.

And now a history lesson.

History of the Autoclave

The autoclave was invented in 1879 by Charles Chamberland, but the concept of using steam in an enclosed space for the purpose of preventing sickness had already existed since 1679.

Unlike most processes going on in today’s laboratories and hospitals, sterilization has occurred using very much the same principles and methods for the last 150 years.

Most advances in autoclave technology since that time have revolved around either keeping better track of the process of sterilization to better guarantee the safety of the users or creating new types of sterilization cycles.

Another element of sterilization that hasn’t changed over time is the use of steam as a sterilizing agent.

Why Steam?

In order to kill a cell through heat, its temperature must be raised to the point where the proteins in the cell wall break down and coagulate. Steam is a very efficient medium for transferring heat, therefore it is an excellent way to destroy microbes. Air, on the other hand, is a very inefficient way to transfer heat/energy when compared to steam because of a concept called the Heat of Evaporation.

To bring one liter of water to the boiling point (100˚C) requires 80kcal of heat energy. Converting that liter of water to steam requires 540kcal — this means that steam at 100˚C contains seven times as much energy as water at 100˚C.

It’s that energy that makes steam so much more efficient at destroying microorganisms. When steam encounters a cooler object, it condenses into water and transfers all the energy that was required to boil it directly into it, heating it up far more efficiently than air at similar temperatures.

In short, steam is how we achieve sterility in the sterilization process.

What is Sterility?

Most people have a working understanding that sterile goods are free of microorganisms and are safe to use in medical, food production, or other settings where the presence of germs would be a significant safety hazard.

Exactly how many microorganisms are going to be left alive over time at a fixed temperature is expressed by a logarithmic curve, a function that approaches, but never reaches zero (see Figure 1.)


Figure 1

As the function approaches zero, typically a level of confidence (called Sterility Assurance Level or SAL) is chosen for the odds that the last microorganism present will survive. The general standard for SAL is 10-6, or a one in a million chance of a single viable microorganism.

How long sterilization will take depends on the set temperature and the Sterility Assurance Level desired. Higher temperatures will achieve sterility faster.

How Does an Autoclave Work?

General Process

Whether it’s a small tabletop autoclave or a room-sized bulk autoclave, all autoclaves operate on similar principles that they share with a common kitchen pressure cooker — the door is locked to form a sealed chamber, and all air within the chamber is replaced by steam. The steam is then pressurized to reach the desired sterilization temperature and time, before exhausting the steam and allowing the goods to be removed. Here are the various phases of a sterilization cycle (see Figure 2).

1. Purge Phase: Steam flows through the sterilizer beginning the process of displacing the air; temperature and pressure ramp slightly to a continuous flow purge.

2. Exposure (Sterilization) Phase: During this phase, the autoclaves’ control system is programmed to close the exhaust valve causing the interior temperature and pressure to ramp up to the desired setpoint. The program then maintains the desired temperature (dwells) until the desired time is reached.

3. Exhaust Phase: The pressure is released from the chamber through an exhaust valve and the interior is restored to ambient pressure, although contents remain relatively hot.


Figure 2

Critical Components of an Autoclave

Your typical laboratory and hospital autoclaves are constructed of the following key components.


Figure 3

1. Chamber

The chamber is the primary component of a steam autoclave, consisting of an inner chamber and outer jacket. Laboratory and hospital autoclaves are constructed with “jacketed” chambers where the jacket is filled with steam, reducing the time that sterilization cycles take to complete and reducing condensation within the chamber.

In the U.S., every autoclave chamber is inspected and tagged with an American Society of Mechanical Engineers (ASME) name plate that includes a National Boiler number. Manufacturers are required to hydrostatically test each chamber and apply the ASME name plate, before the autoclave can be put to use. This inspection and ASME name plate are crucial indicators of a properly functioning autoclave.

Lab and hospital autoclave chambers can vary in size (from 100L to 3,000L). Chambers are typically constructed of 316L or Nickel-Clad (for inner chambers) and 316L, 304L, or Carbon Steel (for outer jackets).

2. Controls System

All modern autoclaves are equipped with a controller interface, not unlike your microwave or oven. Autoclave control systems are, however, a bit more sophisticated and complicated than those of your household appliance. A sterilization cycle follows a software “recipe” that takes the process through a series of phases that involve the opening and closing of valves and components in a specific sequence. Therefore, all autoclaves will require some form of controls, whether those be as simple as a “push button” system with a microprocessor or as complex as a Programmable Logic Controller with color touch screen.

3. Thermostatic Trap

All autoclaves will feature some form of thermostatic trap or steam trap, a device designed to allow air and water (condensate) to escape from the chamber. Although various types of traps can be used in a steam delivery system/steam autoclave, they all perform the same function —removing condensate while allowing the passage of dry steam. Most often, steam traps are temperature sensitive valves that close when heated past a certain setpoint. Thermostatic traps are a critical component of any properly designed autoclave.

4. Safety Valve

All autoclaves operate under elevated pressures (14-45 psig), therefore, must be manufactured with an incredibly robust construction and fitted with a number of safety features and devices to ensure that they present no danger to users. One of these safety devices is the safety valve. This is the final fail-safe device for the pressure vessel should all electronic controls fail. It is imperative that the safety valve is inspected, tested and verified to be in proper working condition based on the recommendation of the sterilizer and/or valve manufacturer as well as local inspection and insurance agencies.

5. Waste-Water Cooling Mechanism

Most autoclaves are equipped with a system to cool the effluent before it enters the drain piping. Many municipalities and buildings do not allow effluent above 140˚F to enter the floor drain. In order to avoid damage to the facility’s drain piping, the steam must be cooled before it is finally sent down the drain. The simplest, and oldest, method of cooling the steam is to mix it with additional cold tap water, but the amount of water required can make autoclaves the single biggest contributor to a building’s water use. Some autoclaves come equipped with systems designed to cut down, or even eliminate, this water consumption.

6. Vacuum System (if applicable)

The primary concern for ensuring sterilization is making certain that all the air inside the chamber is replaced with steam. Certain commonly sterilized goods, particularly porous materials like animal bedding or cloth, or containers with small openings like large flasks or goods in plastic bags, tend to retain air pockets when the autoclave only relies on displacing the air by pushing steam into the chamber. If an air pocket is present during the cycle, any microorganisms within that pocket will survive and the goods will not be sterile.

For this reason, many sterilizers will include a vacuum system. Not only does this allow you to forcibly remove air by pulling a vacuum on the chamber before a cycle (also known as pre-vacuum), it also helps the user by pulling a vacuum after the cycle (also known as post-vacuum) to remove the steam remaining in the chamber and dry off the goods inside the autoclave.

7. Steam Generator (if applicable)

The most common steam source for a laboratory autoclave is from a central “house” boiler. However, when house steam is not available or is insufficient for the autoclave, one must resort to an electric steam generator (also known as a boiler). These boilers typically sit integral (i.e. underneath the chamber) to the autoclave and utilize electric heating elements to heat water and generate steam. For further information on steam sources check out this comprehensive guide.



Figure 4

Sterilization Cycles

In general, there are four standard sterilization cycles: Gravity, Pre-Vacuum, Liquids and Flash (also known as Immediate Use). The chart below explains these cycles in greater detail.


Additionally, some autoclaves have the capability to perform specialty cycles aimed at avoiding damage to delicate goods which may need to be sterilized, but for which the normal fast changes in temperature and pressure would damage or destroy expensive products. These include much longer cycles at lower temperatures, steam-air mix cycles with special pressure controls to avoid breaking sealed test tubes or cycles using special instrumentation to ensure full sterilization temperature is achieved.

For more information on sterilization cycles, check out our Definitive Guide to Steam Sterilization Cycles:

For a full list of factors that should be considered when purchasing a new autoclave or replacing an old one:

Emerging Trends

Autoclaves may be ancient devices by the standards of modern science, but this does not mean that innovation is not present, particularly in the areas of controls, cloud connectivity, and ecological impact.

Autoclave controls have advanced greatly in the age of computers, progressing from manual controls and simple timers to computer automation that minimizes the need for user input. Computer control has also led to advances in data control, record keeping, and remote monitoring from mobile devices. Autoclaves with automatic printers that record data for purposes of verifying successful sterilization are now being replaced with new autoclaves that connect to the Cloud to store cycle records on the internet.

Another trend in autoclave design is toward sustainability and reducing the environmental impact from running an autoclave. The autoclave’s role as a major consumer of water and energy (both in the lab and hospital) has driven widespread efforts to reduce their impact. Green autoclaves that cut or even fully recycle the water (going from 1,500 gal/day down to less than 1 gal/day) consumed by a sterilizer are critical for creating a laboratory that is environmentally friendly. Control systems that automatically turn off the autoclave when not in use, also contribute greatly to reducing energy consumption from as high as 80kWh/day down to 20kWh/day.


Whether you’re using an autoclave to sterilize medical or laboratory equipment, it’s imperative you develop a keen understanding of how the sterilization process works, why it is still used to this day and how it is changing.

Consolidated Sterilizer Systems has over 65 years of experience in the steam sterilization industry. Today this rich heritage for manufacturing excellence continues, thriving alongside the company’s steadfast commitment to deliver high quality, high performance sterilization and distillation solutions. Contact us to learn more about the process of steam sterilization, as well as other autoclave-related questions.

17 Questions to Ask Before Buying Your Next Autoclave

We created this 17-question eBook as a framework to help you explore and discover the exact type of autoclave best suited to your needs.

Get eBook
Download Our eBook