In steam boiler systems, this can be the make-up water used to replenish the feed water vessel.
This especially applies to systems with direct steam heating where no or very little condensate (<50% of the steam output) is recovered (e.g. when manufacturing expanded polystyrene or bread and also for humidification or drying). In addition, water losses as a result of surface blowdown, bottom blowdown, re-evaporation and leaks in the steam system must always be balanced out.
The quantities lost vary considerably depending on the specific system. They can be much more than half the quantity of steam produced and must also be replaced with make-up water. The maximum temperature of the make-up water after water treatment is normally 15°C which makes it highly suitable for pre-heating in the condensing heat exchanger.
The low water inlet temperature allows extensive flue gas condensation and therefore optimum use of condensing technology. With this application, the diversity factor between waste heat availability and heat energy demand is also available during routine operation which means this benefit always exists.
With high condensate flow rates however, the required make-up water flow rate is small which means that a condensing heat exchanger is not always cost-effective.
The condensing technology can however still be used providing a suitable low temperature water circuit is available. The condensation heat which is released can, for example, be used for process water heating, especially in the food industry, or as central heating backup.
In contrast to building heating systems which have clearly defined system and return temperatures, the industry is characterised by an extremely wide range of steam application systems and heating systems. A wide diversity of energy-saving and heat recovery systems therefore compete with one another.
A thorough analysis of all waste heat suppliers and heat consumers is required in order to find the most economical solution. To ensure the condensing technology is utilised to optimum effect, close collaboration between operators, planners and boiler manufacturers is particularly indispensable when determining which measures from the countless available options are the most efficient.
If a suitable heat consumer for the condensation heat in the flue gas is not available, air preheating is one measure which can be used to increase efficiency and is described in the following chapter.