Dry Saturated Steam – A contradiction?

Many are confused with the term dry saturated as it seems to be a contradiction. How can something be both dry and saturated?

The answer lies in the terminology that we use – in the context of steam, dry has the meaning that we are all familiar with – the absence of moisture, not wet etc. If we look up “saturated” in a Thesaurus it will give us: soaked, drenched, flooded, inundated, soaking wet etc. etc. which confirms the contradiction.

However, in steam engineering the term “saturated” has a different meaning and in this context refers to the condition where boiling occurs. A quick look in steam tables will show the temperature column labelled t with a suffix s, where the “s” relates to saturation. Thus the temperature at which boiling occurs is known technically as the saturation temperature. The saturation temperature will vary with pressure.

For steam to exist at a given pressure, it must be at the saturation (boiling) temperature for that pressure or above. If at a lower temperature, only water can be present and if at a higher temperature, the steam will be superheated.

Therefore, the steam that we use for sterilization, sanitation, CIP/SIP etc. and for plant applications such as space heating, heating of hot water via calorifiers, humidification etc. is known as saturated steam. It is not a liquid, it is a vapour and it is not superheated.

The term “dry” is the steam engineering context has a specific meaning – it describes steam that has no entrained moisture present. When steam is produced in a boiler or steam generator it will usually have some moisture present and which comes from the water from which it is generated. The presence of any such moisture is sufficient to render the steam “wet”. When we observe a boiling kettle we can see steam issuing from the spout as a white vapour. What we are actually seeing is a mixture of dry steam, which is a clear, colourless vapour and moisture. The droplets of water reflect the light and make it appear white in colour.

Therefore the term “dry saturated steam” refers to steam that is not superheated and has no moisture present in it.

Most plant steam boilers/generators produce wet steam, which typically has between 1 and 5% moisture present by mass and in general, the dryer the steam the higher the quality is deemed to be. The wetness of the steam will vary as a result of the boiler/generator design and the demand steam at a given time. We want our systems to generate dry steam, as any moisture present will have less energy than the steam and will have to be removed via separators and steam traps and will not even usually reach the process. The wetter the steam, the lower the system efficiency. As steam leaves the boiler/generator, its dryness will deteriorate as a result of energy loss through the steam pipe and again the condensate will have to be removed. However, the use of separators and steam traps coupled with good design can result in steam reaching the point of use with an equal or better quality than when leaving the boiler.

The amount of moisture present in steam is normally expressed as a “dryness fraction” where 1.00 represents 100% steam and 0% moisture. 0.98 represents 98% steam and 2% moisture. In steam quality, we usually use the expression “dryness value” instead of “dryness fraction” and the reason for this will be the subject of another blog.

The clean/pure steam generators used in the Pharma industry are designed to produce “dry” steam, a clear, colourless vapour with no entrained moisture. They achieve this by the use of sophisticated separation techniques and often are fitted with control systems that prevent their performance being affected by excessive demand. The reason that we wish these to produce such high quality steam is not associated with good steam engineering. Our objective is to ensure that no contaminants that may be present in their feed water is allowed into the steam distribution system and onto either our products or equipment that is used to manufacture our products. They therefore operate in the same fashion as a still by effecting a phase change of water to steam at a molecular level, to leave behind contaminants. The only difference between a still and a PSG is that the still condenses the steam back into water. Many stills are capable of being used as pure steam generators and are sometimes designed to operate in a dual capacity.

As far as we are able to measure these generators produce “dry saturated steam.” That is to say they produce steam with a dryness fraction/value of 1.00 and the absence of carryover of water into the steam supply can be verified by spiking the feed water with endotoxins and measuring their absence/presence in condensed steam.

The benefits of dry saturated steam for sterilization applications (as opposed to wet steam) is that pound for pound it has more energy and will heat up the load with less condensate being generated.

Does this matter as we need the presence of moisture to sterilize?

This will be the subject of another blog!

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