The use of condensing technology in steam and hot water systems and its potential for reducing running costs and CO2 emissions – part 2

Which systems are suitable for condensing technology?

Most condensing boilers and gas boilers with low outputs are made entirely out of stainless steel. For technical reasons and due to the high costs, hot water boilers with higher outputs for heating large buildings and building complexes are not made of stainless steel. These boilers are equipped with special stainless steel flue gas heat exchangers for condensing technology. These heat exchangers are already integrated on the boiler ex works or are installed as a separate module on-site.

In steam boiler systems, a two-stage flue gas heat recovery concept enables condensing technology to be used. Rather than having integrated systems, the stainless steel flue gas heat exchangers are only used as a separate module and connected downstream of the boiler on the flue gas side. This topic is discussed in more detail below in “Areas of application for condensing technology in steam boiler systems”.

As a separate module, the flue gas heat exchanger is ideal for retrofitting existing systems. The hot water boiler shown is designed as a flame-tube smoke-tube boiler in a three-pass system with a completely water-flushed rear flue-gas reversing chamber. Thanks to the large radiative heating surface of the flame tube and the large convection heating surface in the second and third flue gas passes, the flue pipes can reach an efficiency of 95 % without the need for downstream heat exchangers or maintenance-intensive turbulators.

If a dual-fuel burner is used with natural gas and fuel oil, the heat exchanger for condensing must be equipped with a flue gas bypass in order to protect the system from unwanted condensation when operated with light fuel oil.

Areas of application for condensing technology in hot water systems

The use of condensing technology in homes has been commonplace since the early 1990s. However, this technology is also tried-and-tested commercial and industrial applications and often makes sense from an economic and environmental perspective. In particular, with a suitably low temperature level in the heating return – around 30 °C to 50 °C – condensing technology can be used for local heating supplies.

In contrast, the system return temperatures in high-pressure hot water generators for process and district heating are mostly above 100 °C – in other words, far higher than the flue gas dew point and therefore not suitable for condensing technology. In this case, flue gas heat exchangers for “dry” operation are an ideal solution and they can reach a boiler efficiency rate of up to 98 %. It is possible to use condensing technology in these instances if there is a low-temperature secondary circuit in the system.

Hydraulic interconnection of condensation heat exchangers in hot water boiler systems

It is possible to make maximum use of condensing technology with the largest possible difference in temperature. The network return with the lowest temperatures (below the flue gas dew point for the fuel) flows through the condensation heat exchanger and this starts condensation at the heating surfaces of the heat exchanger. The flue gases cool, the low-temperature heating circuit heats up and is fed back to the hot water network.

The return flow temperature maintenance mixes the network return to the boiler with flow water before entering the boiler. This ensures compliance with the required minimum water inlet temperature of 50 °C in the boiler. A special injector on the top of the boiler means that there is an effective flow and mixing in the boiler across the entire control range of the modulating burner. Moreover, even when the burner is in a low or small load range, this leads to long burner runtimes with low flue gas temperatures and optimal use of condensing technology. The return flow temperature maintenance prevents boiler temperatures from falling below the flue gas dew point for the fuel, which would otherwise cause corrosion in the boiler.

Areas of application for condensing technology in steam boiler systems

Steam generators with medium temperatures, typically between 150 °C and 200 °C, are supplied with deaerated feed water and temperatures of between 85 °C and 105 °C. Due to the physical processes, the flue gas temperatures for these steam boilers are approx. 80 °C higher than the boiler temperature. This is a considerable amount of heat and, if flue gas or condensing technology is not used, this energy is lost via the roof. A flue gas heat exchanger that uses the feed water as a heat sink (also known as an “economiser”) considerably reduces the flue gas losses. The flue gases cool down to around 130 °C, which is still in the “dry” range and is significantly higher than the dew point.

For high-pressure steam generators, adding a second heat exchanger stage with low-temperature consumers also allows condensing technology to be used. As with all downstream flue gas paths and drainage lines, this flue gas condenser is made of corrosion-resistant stainless steel.

Unlike building heating systems with clearly defined system and return flow temperatures, a wide range of steam application and heating systems are used in industry, meaning that there are various competing energy-saving and heat recovery systems. The basis for finding the most economical solution is to perform an up-front detailed analysis of all heat suppliers and heat consumers.