Hold the Steam: Sterilization Using Dry Heat

Erin Wall

Sterilization in laboratory animal facilities has recently seen several changes aimed at decreasing costs; increasing workplace safety and comfort; and minimizing wear on cages and equipment.
Traditional steam sterilizing processes use an autoclave – a pressurized device used to transfer heat in order to sterilize the contents. In order for the device to work properly, the autoclave must not be overcrowded, and the lids of bottles and containers must be ajar. Also, the autoclave bears initial cost, operating cost (steam boiler, distribution lines, high volumes of water, and licensing of operators), and maintenance costs.
Though steam heat sterilization has been efficient, dry heat is an alternative method to pass-through autoclaves. Dry heat sterilization kills all pathogens and bacteria and provides an array of economical and technological solutions.

What is dry heat sterilization?
Dry heat sterilization is the process of sterilizing containers using hot air. Dry heat can take the form of still or convection. Still heating uses no blowers or moving air. Convection heating uses blowers or fans to keep the heated air moving, reducing the time it takes to heat the environment to the setpoint. Made of stainless steel, carbon steel, or other suitable metals, typical dry heat sterilizers can be used to sterilize animal cages and biological/chemical contaminants.
How does dry heat sterilization work?
Before the sterilization process, a user runs tests to determine the period of time it will take to heat up the chamber. A temperature test strip is placed in the coolest area of the chamber determining the time taken to reach desired temperature. Hot air then circulates within the system, sterilizing the content for a certain time period at the setpoint temperature. During the sterilization process, the hot air rises and exits from the top of the container. Semi-pierced duct walls with adjustable diffuser panels are used during the process to sterilize the external surfaces of the container.
Approximately 90 percent of the air will re-circulate inside the sterilization system and the other 10 percent will exit from the system. After the cycle is over, the sterilizing system rapidly cools down. The intake and exhaust volume increase to extract the heat from the box. The system will cool to about 140°F during the cooling process. At this temperature, the load may be safely handled.
Benefits of Dry Heat Sterilization
There are many benefits to dry heat sterilization, including:
Cost Reduction
A dry heat convection sterilizer can reduce costs in several ways. Using dry heat can save on each sterilization cycle, since the need for water and the energy used to heat the water is virtually eliminated. Additionally, unlike steam heat sterilization, which can introduce rust and corrosion, resulting in a shorter life expectancy for the cages and equipment, dry heat doesn’t depend on steam or water so the cages last longer, reducing replacement costs.
Decreased Cool-Down Time
Dry heat sterilizers have modules with self-contained, integrated blowers to decrease cool-down time and overall sterilization cycles. A dry heat sterilizing system has two identical filters. The intake filter cleans the air as it is directed into the chamber, while the exhaust filter, a BSL-3, assures that the contaminants stay within the filter and are not released into the atmosphere when the hot air is vented. Typical installations of dry heat sterilizers do not require a higher level than BSL-3. The filters also prevent ambient air from entering the chamber through the exhaust outlet to re-contaminate the contents after sterilization. Another environmental benefit is that the dry heat sterilizers don’t release hot steam from the system into the environment.
Workplace Comfort
Dry heat sterilizers allow for comfortable working conditions with temperatures between 70 and 75°F. This allows for a more comfortable working environment than a worker would experience with a steam sterilizer where temperatures typically range from 80 to 94°F.
Time Savings
Ramping, heating, and cooling are three cycles to the sterilization process. In the ramping process, the chamber will increase heat from ambient temperature to a setpoint temperature, usually 340°F. The time it takes to ramp the temperature depends on the load in the chamber and the ambient temperature in the system. The desired time is between 30 to 40 minutes. The re-circulation blower is sized to deliver a predetermined volume of air to the sterilizing process at a static pressure in the range of one-inch water column.
Control Options
Many dry heat sterilizers provide several control options, including those that allow the user to control and record the temperature of the sterilization cycle, and control various processes such as start and stop, fault indicators, temperature displays, timers, and alarms. Remote controls make it possible for the technicians to operate and monitor the sterilizer and to diagnose issues from outside of the sterilization area. Web-based support and control is also available.
Modular Design
A dry heat sterilizer’s modular design allows for flexible workplace configurations that can be sized for any environment without the cost of a completely custom design. Custom oven features including the control console and modular design are available. The intake airflow blower, intake filter box, electrical junction box, and the re-circulating motor can be mounted on top or on the side of the housings. The sterilizing system can have one or more filters, depending on the user’s specification and application. The arrangement of the components and details of the construction can be carried out in a variety of ways that will meet the users’ needs.
Flexible Throughput
Truck-in designs allow flexible “staging” and efficient throughput of sterilization loads. Small and/or diverse loads may be sterilized in a single cycle in sterilizers that have multiple trucks. Trucks and carts also allow loads to be handled and transported more easily and more comfortably. Transfer carts may have self-contained cooling fans that further reduce sterilization cycle times. Additionally, trucks can be removed from the sterilizer to make room for the next sterilization cycle, saving time.
Summary
Dry heat sterilizers were developed to accomplish workplace goals of high output and low cost while being environmentally friendly. Dry heat sterilization requires no water, and uses less energy and requires less maintenance than traditional sterilization methods. Equipment, including cages, can be reused for a longer period of time, and the washroom environment has been made more comfortable.