Water Jackets or Air Ducts?

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Water Jackets or Air Ducts?

11.05.14

What Is The Best Way To Heat An Ethylene Oxide (EO) Sterilizer?

Someone recently asked me, “is it better to heat an ethylene oxide sterilizer with air or hot water?”  The straight forward answer is water.  Hot water is an efficient heating medium, whereas air is better known as an insulator. 

The thermodynamic properties of water make it the better choice for heating an ethylene oxide sterilizer.


60o C Specific Heat Capacity
(kJ/kgK)
Thermal Conductivity
(W/mK)
Air 1.005 0.0271
Water 4.185 0.6546

The following facts about water as a heating medium demonstrate its superiority over air.

     1) Heat transfer – As evidenced by the table above, water is a much better mechanism for heat transfer.

     2) Rate of changing temperature – In an appropriately designed system, water can change the temperature of a sterilizer much faster in both directions (heating and cooling).  In order to heat/cool a chamber with air, the system has to be scaled up to overcome the inefficiencies of this medium, which increases operating costs of a hot air system.

     3) Lighter weight – Amounting to the weight of a few pallets of product, the weight savings of an air heated system is inconsequential in comparison to overcoming the inefficiencies of air as heat transfer medium.

     4) Maintenance - A well designed water jacketed system will have the same life expectancy as the sterilization chamber - many decades.  To ensure this:
          a. Avoid the collection of air in the jacket by making sure that it’s properly vented, pressurized and completely filled.
          b. Test water quality and treat it if necessary.  Most medical device manufacturers have water treatment built into their manufacturing and treated water is readily accessible.  Furthermore, boilers used for sterilization heating and steam will require treated water to maximize life expectancy.

     5)  Energy Losses - Air heated systems require large ducts vs. the small pipes required for water heating system.  The significantly larger ducts have more surface area which results in significant energy losses which increases operating costs.    This loss of energy explains anecdotal reports and claims that air heated systems cool rapidly.

Cosmed Group has many water jacketed sterilizers in service.  Some of those chambers have been in continuous service for  more than 25 years and continue to operate as originally designed.  For those customers with specific concerns about water quality at their manufacturing sites Cosmed Group offers stainless steel jackets as an option.