vacuum sterilization

Steam Sterilization Cycles, Part 1: Gravity vs. Vacuum

Arthur Trapotsis
Written by: Arthur Trapotsis

MS Biochemical Engineering, MBA, Consultant

Steam sterilization relies on three parameters to eliminate microbes and organisms: time, temperature, and pressure. These parameters can be manipulated into different cycles to meet the sterilization requirements of various types of loads, including red bags, media, glassware, cages, animal bedding, and pipette tips.

Although it’s possible to sterilize the vast majority of common laboratory loads using one of three basic cycles — gravity, vacuum (or prevac), and liquid — you’ll want to ensure that your sterilizer offers the appropriate cycles for all of your load requirements.

In this article, we’ll explore the first two basic steam sterilization cycles — gravity and vacuum — including how they work and when to use them. Then, in future articles, we’ll discuss additional cycle types, such as liquid cycles, air-overpressure cycles, and a myriad of other, more advanced cycles for specialized applications.

Steam Sterilization Basics

For a brief overview of how steam sterilization works, we recommend reading our blog post on the subject or watching this short video:

 

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Gravity Sterilization Cycle

The traditional gravity sterilization cycle — or simply gravity cycle — is the most common steam sterilization cycle. During a gravity cycle, steam is pumped into a chamber containing ambient air. Because steam has a lower density than air, it rises to the top of the chamber and eventually displaces the air. As steam fills the chamber, the air is forced out through a drain vent. By pushing the air out, the steam is able to make direct contact with the load and begin to sterilize it.
At the end of the cycle, steam is discharged through the autoclave’s drain vent. At this point in the gravity sterilization process, the load may still be hot and possibly wet. To address this issue, gravity autoclaves can be equipped with a post-cycle vacuum feature to dry the load. If an autoclave is equipped with this feature, it will run a normal gravity cycle and, once the load is sterilized, a vacuum will pull the steam and condensation through the autoclave’s drain vent. The longer the vacuum system runs during the dry phase, the cooler and dryer the goods will be when you remove them from the chamber.

It’s common to use a gravity sterilization cycle on loads such as glassware, biohazardous material (red bag waste), vented containers, and certain types of unwrapped instruments.

Vacuum Sterilization Cycle

There are certain applications, such as sterilizing porous loads or partially vented containers, where air cannot be easily displaced from an autoclave chamber, rendering gravity air displacement ineffective. These loads — which may include cages with animal bedding, wrapped goods, and surgical packs — require the use of a vacuum sterilization cycle, also known as a vacuum cycle or prevac cycle.

A sterilizer configured to run a vacuum cycle will be equipped with a vacuum system. A typical vacuum cycle begins with a series of alternating steam pressure injections and vacuum draws (also known as pulses) to dynamically remove air from the chamber.

Using pulses to remove ambient air from the autoclave chamber allows steam to be sucked into areas it would otherwise have difficulty penetrating. As noted in “Principles and Methods of Sterilization in Health Sciences” by John J. Perkins, the absence of air within the chamber allows “steam to penetrate the load almost instantaneously,” resulting in more reliable sterilization and shorter sterilization cycle times.

Once sterilization is complete, you can program a post-cycle vacuum to enhance and expedite the drying process. Consolidated’s sterilizers come with two vacuum system options: a water ejector with optional booster pump and a liquid ring vacuum pump.

Common Vacuum Systems

A water ejector system utilizes a simple Venturi effect to create a vacuum within the chamber. A booster pump is included if the facility cannot provide adequate water pressure (> 45 psig dynamic). This system has few moving parts, so it’s easy and inexpensive to maintain. It is also less expensive than the liquid ring pump, but will consume more water over its lifetime.

A liquid ring pump has a higher upfront cost, but uses less water and produces the deepest vacuum of all available vacuum system options.

Gravity Sterilization vs. Vacuum Sterilization: Air Removal Process

See a visual representation of how both gravity cycles and vacuum cycles remove air from an autoclave chamber in this brief video:

Gravity Sterilization vs. Vacuum Sterilization: At a Glance

Basic Cycles Description Typical Application or Load Type
Gravity The most basic sterilization cycle. Steam displaces air in the chamber by gravity (i.e. without mechanical assistance) through a drain port. Glassware, unwrapped goods, waste, utensils, redbags.
Pre-Vacuum Air is mechanically removed from the chamber and load through a series of vacuum and pressure pulses. This allows the steam to penetrate porous areas of the load that couldn’t otherwise be reached with simple gravity displacement. Wrapped goods, packs, animal cage bedding, cages, porous materials, redbags.

Gravity and vacuum cycles are the two most commonly used sterilization cycles because they can accommodate a wide variety of load types and applications. However, some loads — such as syringes, contact lenses, and certain types of media — require special cycle configurations that employ pressure or temperature ramping. Fortunately, today’s advanced controllers can manipulate sterilization parameters to accommodate these loads.

At CSS, we recognize that many of our customers need assistance in identifying and creating the proper sterilization cycle for their load type. As such, we hope this article helps you better understand how gravity and vacuum sterilization cycles work and what types of loads they’re appropriate for. We encourage you to read the rest of our steam sterilization cycles series. In the meantime, if you have any questions, we’re always happy to help — contact us today to speak to a specialist.

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Sterilization Cycle FAQs

Q: What is a gravity sterilizer?

A: A gravity sterilizer is any sterilizer that uses the gravity displacement method for air removal. Gravity sterilizers perform what’s known as a gravity cycle to sterilize loads.

Q: What is gravity steam sterilization?

A: Gravity steam sterilization refers to the process by which loads are sterilized by pumping steam into an autoclave chamber containing ambient air. The steam then displaces the ambient air by gravity — that is, without mechanical assistance — through a drain port.

Q: How long does a gravity sterilization cycle last for?

A: How long a gravity cycle lasts depends entirely on the load you wish to sterilize. The CDC advises that:

  • Wrapped instruments be exposed for 30 minutes at 250°F (121°C) and 15 minutes at 270°F (132°C), with a dry time of 15–30 minutes
  • Textile packs be exposed for 30 minutes at 250°F (121°C) and 25 minutes at 270°F (132°C), with a dry time of 15 minutes
  • Wrapped utensils be exposed for 30 minutes at 250°F (121°C) and 15 minutes at 270°F (132°C), with a dry time of 15–30 minutes

Q: What is a pre-vacuum sterilizer?

A: A pre-vacuum sterilizer is any sterilizer equipped with a vacuum system — either a water ejector or a liquid ring vacuum pump — to ensure air removal from the autoclave chamber. Pre-vacuum sterilizers are used to perform what are known as vacuum (or prevacuum) sterilization cycles.

Q: How does a pre-vacuum sterilizer work?

A: Prevacuum sterilizers run vacuum (or pre-vacuum) cycles. Vacuum sterilization cycles remove air from the autoclave chamber and load mechanically through a series of vacuum and pressure pulses. This allows steam to penetrate porous areas of the load that couldn’t otherwise be reached using gravity air displacement.

Q: How long does a gravity sterilization cycle last for?

A: The CDC recommends that wrapped instruments, textile packs, and wrapped utensils be exposed for 4 minutes at 270°F (132°C). Wrapped instruments should have a dry time of 20–30 minutes, textile packs should have a dry time of 5–20 minutes, and wrapped utensils should have a dry time of 20 minutes.

Article Sources

  1. Centers for Disease Control and Prevention, “Steam Sterilization, https://www.cdc.gov/infectioncontrol/guidelines/disinfection/sterilization/steam.html.”
  2. Consolidated Sterilizer Systems, “Steam Sterilization Cycles, https://consteril.com/resources/sterilization-cycles/.
  3. Consolidated Sterilizer Systems, “How Does a Laboratory Autoclave Work, https://consteril.com/how-does-a-laboratory-autoclave-work/.”
  4. Thermo Fisher Scientific, “What Is Ambient Air?, https://www.thermofisher.com/blog/mining/what-is-ambient-air/.
  5. The Washington Post, “Why dry air is heavier than humid air, https://www.washingtonpost.com/news/capital-weather-gang/wp/2013/08/05/why-dry-air-is-heavier-than-humid-air/.”
  6. John J. Perks, “Principles and Methods of Sterilization in Health Sciences, https://books.google.com/books/about/Principles_and_Methods_of_Sterilization.html?id=-lYpPQAACAAJ.”
  7. Consolidated Sterilizer Systems, “Consolidated Smart Options, https://consteril.com/products/smart-options/.
  8. SimScale, “What Is the Venturi Effect?, https://www.simscale.com/blog/what-is-venturi-effect/.”
  9. Centers for Disease Control and Prevention, “Minimum cycle times for steam sterilization cycles,    https://www.cdc.gov/infectioncontrol/guidelines/disinfection/tables/table7.html.”

 

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