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Something needs to come out

Publication date: 23 March 2010
Author: Greg Sutcliffe, Burkert Fluid Control Systems

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We have successfully sized the steam control valve, but during commissioning and testing the warm up times are sluggish. Why?

Well is might be a case of needing to get things out, before we can hope to put things in! We know that “new” steam is delivered, as the “existing” steam in the heat transfer area condenses. However, we can not hope to effectively deliver steam energy to the process unless we remove the resulting condensate and any air (and other condensable gases) which might be present.

Air can be extremely disruptive to the heating process. In fact it is entirely possible that 1% of air by volume in steam can reduce heat transfer efficiency by 50%, due to its remarkable insulating effects. Therefore, whenever air is present in a steam system, it needs to be efficiently and effectively removed. The choice of steam trap is crucial, one that removes air and other non-incondensable gases is a major consideration but this is unlikely to remove all the air present, partly due to the steam trap connection position being typically at the bottom of a steam jacket. It is possible that as steam enters the steam jacket, air present will be pushed around to the highest point in the system, establishing a “cold spot” in the vessel. Therefore the fitting of an air-vent device is recommended at the high point of the jacket, ensuring that the insulating air is completely vented.

Another key point is the sizing of the steam trap itself. If the steam trap is too small to pass the rate of condensate produced, condensate will back up flooding the heat transfer surfaces. In fact it is worth noting that although the volume of condensate is significantly less than that of steam, the mass of steam and its resulting condensate remains the same,

i.e. 500kg/hr of steam will result in 500 Kg/hr of condensate.

A problem will arise if your steam trap is designed to pass 400 kg/hr of condensate for example, but the process is producing 500 Kg/hr. The system will start to water-log. The energy in hot water (sensible heat) amounts to approximately 5 times less than that of latent heat energy in steam at the same temperature. Water-logged heating surfaces designed for steam = dramatically reduced heat up times.

So, the moral of today’s story? Yes of course sizing a steam control valve correctly is a crucial element for an effective steam heat transfer process. However remember this; you can’t effectively get the steam in, unless we take the air and condensate out.

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