FREME and energy saving

Energy saving is always a hot topic and one such system designed to provide savings is the FREME package. First let get the acronym explanation out of the way! FREME stands for Flash Recovery Energy Management Equipment (who makes these things up!). Manufactured by Spirax Sarco it is ideal for condensate return systems which have high condensate return percentages with high flash steam percentages.

The FREME uses the energy in the condensate return to heat the boiler feedwater. Critically the heating of the boiler feedwater takes place on the high pressure side of the boiler feed pumps, so the system can safely heat the water to more than 100 °C without boiling and causing pump cavitation. You could say that the FREME is somewhat similar to a boiler feedwater economiser, but using heat in the condensate return that would be otherwise wasted rather than heat in the boiler flue, to heat the boiler’s feedwater.

Condensate return first goes through a flash vessel in the unit. Of the top of this vessel is a heat exchanger for the flash steam, while off the bottom of the vessel is steam trap and then a heat exchanger for the condensate. The two returning streams are then recombined and sent back to the boiler feed tank. At this point the combined stream is sub-cooled, so it is sufficiently warm to begin heating the cold feed but not hot enough to overheat the feed tank.

The boiler feedwater flows through the secondary side of each of the heat exchangers, the lower condensate one first and the flash steam heated one second. By using the unit temperatures above 100 °C can be achieved going into the boiler from an atmospheric boiler hotwell feedtank. To ensure that there is always a flow of boiler feedwater through the heat exchangers the boiler feedwater pumps must be of a modulating control type. Recovering heat in the condensate return makes perfect sense if it is currently wasted. Not all steam systems can recover the heat in the condensate. This due to:

  • The energy in the condensate would raise an atmospheric feedtank’s temperature to above boiling point. In practice, the maximum boiler feedwater temperature is 85°C or 90°C for an atmospheric feedtank, above this and damaging cavitation can be induced in boiler feed pumps.
  • Up to half of the recoverable energy in the condensate can be lost as flash steam, which is generated as condensate leaves the pressurised steam system and returns to atmospheric pressure.

As a rule of thumb for every 6°C rise in boiler feedwater, 1% is typically knocked off the boiler energy bill using condensate returned from the steam distribution system to heat boiler feedwater.

The FREME system was originally designed for condensate return systems from a corrugated cardboard producing machine. Competing with other systems like the Baviera and CBA to use the energy in the condensate coming off the cardboard forming machine. The FREME has however been used in other industries with success, including laundries, and the food sector. The FREME won the IChemE innovation and excellence award in energy in 2009, and was the runner up in the Cabon Trust Innovation for energy award 2010 and CIBSE low carbon performance award in 2010.

Posted by Admin on 24/03/2015

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