Poof, there goes a dollar, and another, and another – in seconds, constantly. No I’m not talking about filling up the gas tank and watching the dollars rack up faster than The Flash, a very fast early 90’s superhero. I’m talking about your disappearing steam. If you have a medium or high pressure steam system with an atmospheric return system, you may know the culprit – flash steam.
High pressure steam systems with atmospheric return systems all have flash steam. It originates at the steam traps, which, if in good working order, only release condensate into the condensate return pipe. Once the trap opens and allows the condensate (which is under pressure) into the atmospheric condensate pipe, the liquid condensate immediately lowers its pressure. That’s when the magic happens – FLASH – a portion of the condensate flashes to steam as pressure drops below its boiling point. This occurs because this expansion is an adiabatic process and the higher pressure liquid has more enthalpy than lower pressure liquid. When the pressure is removed, an energy release happens in the form of a phase change.
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The answer is simple: you have to capture that renegade steam and use it. I have seen several successful ways of doing this. The two most common methods are to use a vent condenser or vapor recompression system. Vent condensers are easy to implement and are less costly, but vapor recompression, if it will work for your system, can produce greater savings.
A vent condenser is a heat exchanger that is installed on the vent of the condensate receiver tanks. This has two useful outputs. It returns your condensate to the boiler and produces warm water that can be utilized for other uses such as boiler make-up water or process make-up water.
Vapor recompression requires much more engineering and cannot be implemented on every system. This actually uses centrifugal compressors to compress the steam back into medium pressure steam. This would be more useful for facilities and/or processes that have little and/or inconsistent need for relatively low temperature heat.
You may be asking if your system is a good candidate for either of these technologies. As with every good engineering question the answer is “it depends.” It depends on how your specific system is set up and the load it supplies. A good rule of thumb to follow is to check if you can see steam venting out of your condensate tank fairly continuously, and if there is a need for heat nearby such as make-up water. The magical disappearing dollar could very well reappear in a cost effective manner.