Exploring Autoclaves and Their Temperature Profiles #1

In this article, we will understand autoclaves and their fascinating Role. Specifically, we will delve into their various types, classification systems, and uncover essential insights that will enhance your knowledge.

Principle

Before we talk about autoclave classification, its crucial to grasp some important background information. Autoclaves operate on the principle of moist heat sterilization, utilizing steam as the primary sterilization agent. This method ensures effective elimination of harmful microorganisms, making it a cornerstone of sterilization practices.

Physical Design

When it comes to their physical design, autoclaves commonly adopt a cylindrical shape, which holds significant advantages over other shapes like boxes or cubes. The cylindrical form demonstrates superior durability and resilience, enabling it to withstand and distribute high pressures more efficiently. The homogenous pressure distribution throughout the chamber guarantees reliable and consistent sterilization results, which is critical in ensuring product safety and efficacy.

To illustrate the effectiveness of the cylindrical shape, refer to Figure 1, where the pressure distribution is visually highlighted.

Phases of sterilization

Moist-Heat Type has 3 main phases as it shown below in Figure 2 which could be differ slightly according to the type of every cycle in the autoclave:

1. Purge Phase: During this initial stage, steam takes charge, displacing air from the autoclave chamber. As this displacement occurs, both temperature and pressure begin their rise. The removal of air sets the foundation for a sterile environment.
2. Sterilization phase :With the chamber purged of air, the exhaust valve remains closed, allowing temperature and pressure to rapidly rise to the desired levels. This phase is the heart of the sterilization process, ensuring the destruction of harmful microorganisms.
3. Exhaust phase: As the sterilization phase concludes, the pressure is gradually released from the chamber, although the temperature remains relatively high. This controlled release of pressure ensures a safe and efficient environment for the operator to access the sterilized contents.

Moist-Heat sterilization cycles

In accordance with ISO 17665, autoclaves can be classified into different cycles for moist heat sterilization, with some autoclaves capable of accommodating multiple cycles. These Cycles are:

1. Saturated steam sterilization — Vented systems
2. Saturated steam sterilization — Active air removal
3. Steam-air mixtures
4. Water spray
5. Water immersion

We will delve into each of these moist heat sterilization cycles through our articles, examining their mechanisms, advantages, and applications.

Saturated steam sterilization — Vented systems (Gravity Displacement)

According to ISO 17665, the first cycle we will discuss is commonly known as Saturated steam sterilization - Vented systems or Gravity Displacement. This cycle operates on the principle that cold air within the autoclave chamber is heavier than the incoming steam. Consequently, the cold air tends to sink to the bottom of the chamber as steam enters.

During the process, the air is pushed out through the drain located at the bottom of the chamber and exits along with the condensate via a steam trap. The effectiveness of this process in removing air depends on the proper operation of the steam trap and the appropriate distribution of steam. Steam is injected into the sterilizer chamber through a baffle or spreader bar, such as a perforated pipe, to ensure even distribution.

It is important to note that if steam is introduced too rapidly or not distributed properly, pockets of air may become trapped near the top of the load. Conversely, if steam is added too slowly, the air can become heated, diffuse into the steam, and become more challenging to remove. It is worth mentioning that gravity displacement sterilizers are not as efficient as other designs when it comes to air removal and are not recommended for items that pose air removal challenges.

Air vs Steam

Now, lets explore why air is heavier than steam. The explanation lies in chemistry, specifically the Ideal Gas Law, which states:

????=?????? (Equation 1)

In this equation, ?? represents pressure, ?? is volume, ?? is temperature, ?? is the amount of substance, and ?? is the ideal gas constant.

We can further break down the amount of substance (??) using the equation:

??=??/?? (Equation 2)

Here, ?? represents the total mass in kilograms, and ?? is the molar mass.

By substituting Equation 2 into Equation 1, we arrive at:

??=????/???? (Equation 3)

In this equation, ?? denotes the density of gas, ?? represents pressure, ?? is volume, ?? is temperature, ?? is the molar mass, and ?? is the ideal gas constant.

From Equation 3, we can infer that density increases with an increase in molar mass and vice versa.

For steam (H2O), the molar mass is calculated as follows: 2(atomic weight of hydrogen) + atomic weight of oxygen = 18.

On the other hand, air has an average molar mass of approximately 28.96.

In an ideal scenario where temperature and pressure are the same inside the steam chamber, we conclude that air has a higher molar mass than steam. Consequently, air is denser than steam.

As a result, the steam effectively displaces the air, and due to gravity, the air is pushed and removed from the chamber, as depicted in Figure 3.

The Phases of Gravity Displacement

The sterilization cycle encompasses three primary phases, as depicted in Figure 4:

1. Heating (Conditioning) Phase: During this phase, the vent is open, allowing steam to enter the chamber. The steam displaces air until the desired conditions, typically determined by temperature measurement, are achieved. Subsequently, the vent closes (a), and steam continues to be introduced into the chamber until the sterilization temperature and corresponding saturated steam pressure are reached.
2. Sterilization Phase: In this phase, the sterilization temperature is maintained within the chamber by the continuous presence of steam for the prescribed duration. The sustained exposure to the designated temperature ensures effective sterilization.
3. Cooling Phase: The cooling phase may vary depending on the type of product being sterilized. One approach involves supplying air to the chamber through the vent to facilitate the release of residual steam and condensate, gradually reducing the chambers pressure. Alternatively, for solutions in sealed containers, filtered compressed air can be introduced into the chamber to prevent rapid depressurization. This controlled cooling process continues until the pressure inside the chamber reaches atmospheric pressure. In the case of sealed containers, it also concludes when a safe temperature is reached within the containers.

Stay Tuned

In our forthcoming article, we will delve into the remaining cycles, aiming to enhance our understanding in a very easy way.

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