How to Choose a Steam Source for Your Autoclave
Autoclaves, by nature, require steam to operate. However, there are several possible autoclave steam generator sources to choose from when it comes to laboratory sterilizers; everything ranging from building steam (also known as house steam) to steam from an on-board electric steam generator. The following guide explores some of the most common steam sources for laboratory autoclaves.
The most common steam source for a laboratory autoclave is from “the house” boiler. House steam is steam derived from the building’s boiler system (usually located in the basement). In most situations, standard house steam is adequate if it is available. There are, however, two scenarios where the house steam source may not be adequate:
- Insufficient steam pressure: There should be a minimum of 50psi available to operate a laboratory autoclave. If this is unavailable, then refer to the other options discussed below.
- Poor steam quality: If the house steam has too many impurities, either from poor water quality or old plumbing, then an inlet filter may be required to clean the steam. There is also the issue of steam saturation. Steam should have a dryness of 0.95 (steam by weight) and contain less than 3.5% (gases by volume) of non-condensable gases.
Electric Steam Generator
An electric autoclave steam generator (or boiler) utilizes electric heating elements to heat water and generate steam. When it comes to laboratory autoclaves, there are two types of electric generators:
- Integral Boiler: Integral boilers are mounted within the footprint of the autoclave (typically, underneath the chamber) and are usually at or below 45kW. Integral boilers are available in carbon steel or, if clean steam* is required, stainless steel construction.
- Remote Boiler: Remote boilers are required for larger autoclaves (24x36x60” chambers or larger) and are a bit larger in size. Their larger size (greater than 45kW) means they cannot fit underneath the autoclave and must be remotely mounted either next to the autoclave or in another room. Like integral boilers, remote-mounted boilers are available in carbon steel or stainless steel construction.
Electric boilers are typically available in 208, 240, 380, and 480 voltages and in single or three-phase connections, with capacities ranging from 20kW to 180kW. They are constructed of carbon steel or stainless steel.
- Carbon steel boilers require tap, softened, or RO water with resistivity under 26,000 ohms (recommended 2,000-6,000 ohms).
- Stainless steel boilers can be used to generate clean steam and, per the ASME pressure vessel code, require high purity DI water of >1 megohm•cm.
(See below for feed water requirements for stainless and carbon steel boilers.)
An automatic autoclave steam generator blow-down is an option available for electric boilers that will help extend the life of your heating elements by flushing away minerals left behind from the feed water source. This feature is highly recommended for facilities that have poor water quality. Even if the automatic blow-down feature is not provided with the boiler, most have a manual blow-down valve, which allows the user to manually drain the boiler to reduce scale and preserve the life of the heating elements. The automatic blow-down feature is not necessary on stainless steel boilers because the feed water is already purified.
Steam-to-steam boilers, also commonly referred to as indirect heat exchanger boilers, are used to generate what is known as clean steam. Available in either integral or remote-mounted configuration, these boilers are made of stainless steel construction and are fed with high purity water (RO/DI) of greater than 1 megohm•cm. With this type of boiler, house (dirty) steam is used to indirectly heat water within a system of heat exchangers; ensuring clean steam is produced and fed to the autoclave free of contaminants. There is no opportunity for precipitation or scale to form within the generator. Because these boilers require very little maintenance (i.e. there are no heating elements or electrical components) they are the most economical to operate. The downside is that they provide less production capacity than an electric autoclave steam generator and have a much higher capital cost.
*Clean Steam is considered to be steam that is free of contaminates. It is derived from high purity (DI) water. Clean Steam can be created using either an electric steam boiler or a steam-to-steam heat exchanger.
Choosing Your Next Steam Source
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Water Feed Requirements, Carbon-Steel Steam Generators
The table below shows the recommended feed water requirements for a standard steel boiler. If water quality fails to meet maximum condition requirements listed below, then your water will require purification by an RO or water softening system. If you are unsure of your facility’s water quality, please contact Consolidated to arrange for an initial assessment.
|Carbon Steel Steam Generators (1)||General Vacuum Device & Quench|
|Characteristic||Recommended Condition||Maximum Condition||Recommended Condition||Maximum Condition|
|Temperature [°F (°C)]||As Supplied||140 (60)||40-60 (4-16)||70 (21)|
|Total Hardness (mg/L)||17||85||10-85||171|
|Total Dissolved Solids (mg/L)||50-150||250||50-200||500|
|Total Silica (mg/L)||0.1-1.0||2.5||0.1-1.0||2.5|
|Resistivity (Ω•cm) (2)||2,000-6,000||26,000||2,000-26,000||500,000|
* If water supplied is greater than 26,000 ohm•cm contact Consolidated for recommendation.
Water Feed Requirements, Stainless Steel Generators
Stainless-steel generators require deionized water >1 megohm•cm.
If you have additional questions about steam sources for laboratory autoclave or any other questions related to autoclaves in general, please contact us for more information.
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Read this comprehensive 31-page guide about when and how to use the top 12 steam sterilization cycles. This guide outlines how today’s steam autoclaves can be configured with specific parameters suitable for nearly any load and application type.