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  2. Combustion efficiency
  3. Flue gas temperature or flue gas loss
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Flue gas temperature or flue gas loss

Economiser

The flue gas temperatures at the boiler outlet are normally roughly 60K above the temperature of the product inside the steam boiler.

Fig. „Integrated economiser in UL-S“

At an operating pressure of 10 bar, which corresponds to a saturated steam temperature of 185°C, the flue gas temperature is therefore roughly 245°C. This corresponds to a flue gas loss of roughly 11%. As shown in the graphic (Fig. „Efficiency gain for various exemplary sizes of economiser“), the flue gas loss can be reduced by roughly 1 percentage point, or the boiler efficiency increased accordingly, with each 20°C reduction in flue gas temperature.

Using an integrated or downstream economiser, the flue gas temperature can be reduced to 120 – 140°C, depending on the economiser design, which reduces the flue gas loss significantly. During this process, the heat in the flue gases is transferred to the boiler feed water flowing in countercurrent. The heat removed from the flue gas flow is fed to the boiler via the heated feed water. This increases the combustion efficiency by 5 – 7%.

Simplified flow diagram of a steam boiler system with integrated economiser

Simplified flow diagram of a steam boiler system with integrated economiser

Efficiency gain for various exemplary sizes of economiser (increasing from I to III)

Efficiency gain for various exemplary sizes of economiser (increasing from I to III)

ƞ

ECO size 3
 

ECO size 2
 

ECO size 1
 

without
     

tA

ECO size 3
 

ECO size 2
 

ECO size 1
 

without

Unregulated economisers achieve the optimum efficiency gain because in partial load optimum use is made of the available heating surface to cool the flue gas.

Info on Economiser

If the permissible minimum temperature of the chimney needs to be taken into account, economisers can be designed individually for the various flue gas inlet and outlet temperatures.

On the one hand, in order to achieve a high degree of economic efficiency via a low flue gas temperature and also to comply with a permissible minimum flue gas temperature for the chimney on the other, a stepless feed water controller and a water-side bypass control are necessary boiler components. Nowadays the integrated economiser is a standard feature of a steam boiler in more or less every application. It normally pays for itself within a few months.

Condensing heat exchanger

When using condensing technology, not only the sensible heat which is directly linked to the temperature but also the condensation heat latent in the water vapour is partially removed from the flue gas. Liquid flue gas condensate is produced which must be removed from the flue gas path, neutralised and introduced into the sewer system.

This is made possible without causing long-term corrosion damage using corrosion resistant materials in heat exchangers, moisture-resistant flue gas systems and chimneys made of stainless steel.

Given the right framework conditions, an additional improvement in efficiency of up to 7% is possible. To do this, the condensation economiser is always connected downstream of the dry economiser on the flue gas side.

As an example, the following graph shows the boiler efficiency as a function of boiler load.

Efficiency curve as a function of the boiler load of a boiler without economiser, boiler with economiser and boiler with economiser and additional condensing heat exchanger

Efficiency curve as a function of the boiler load of a boiler without economiser, boiler with economiser and boiler with economiser and additional condensing heat exchanger

Steam boiler with economiser
and upstream condensing heat exchanger

Steam boiler with economiser

Steam boiler without economiser
Simplified flow diagram of a steam boiler system with integrated economiser and downstream condensing heat exchanger

Simplified flow diagram of a steam boiler system with integrated economiser and downstream condensing heat exchanger

Integrated economiser (steel)

Flue gas bypass flap

Condensing heat exchanger
(stainless steel)

Make-up water

Steam boiler

Chimney

Water service module WSM-V

To operate a condensing heat exchanger efficiently, a sufficiently large (>30% of the boiler steam output) and cool (temperature <35°C) water flow is required as a low temperature heat sink. This should be available when the steam boiler is in operation.

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.

Air preheater

The air preheater can be used as an efficiency-enhancing measure in new systems to be installed together with economisers.

During this process, combustion air at ambient temperature is heated in a heat exchanger upstream of the burner to roughly 80°C. The feed water which is cooled down to 65°C now flows through a second economiser bundle and the flue gas temperature then falls to around 80°C.

Bosch offers air preheating for single or double flame-tube boilers with duoblock burners. As the burner is specially designed for this and the installation expenditure for the combustion air heat exchanger, additional economiser and pipework must be considered, air preheating is particularly attractive from an economic standpoint when the boiler is operated for many hours at full load, i.e. roughly 4,000h/year, or if the boiler output is roughly 5 tons steam/hour or more. Then the amortisation time for the additional costs involved in preheating are often only 1.5 – 2 years. It also generally makes sense however, to retrofit this equipment to existing systems with a high number of hours run annually, i.e. roughly 6,000h/year.

Luftvorwärmer

Air preheater

Combustion air preheating at steam boiler according to Bosch patent

Combustion air preheating at steam boiler according to Bosch patent

Steam boiler

Heat exchanger, combustion air

Fan

Combustion air

Flue gas heat exchanger stage 1

Flue gas heat exchanger stage 2

3-way valve

Feed water

Chimney

Temperature controller (TIC)

Feed water cooler

The flue gas temperature is decisive for the combustion efficiency of steam boilers without flue gas condensation. The feed water used in the economiser to cool the flue gases in systems with thermal deaerating is however not colder than 103°C. This can only be used to reduce the flue gas temperature economically in the economiser to roughly 120°C.

Feed water cooling module

Feed water cooling module

Feed water cooling module

Feed water cooling module

Feed water cooling module

Economiser

Steam boiler

Water service module

Make-up water

3-way valve

However, if heat consumers with a temperature level below 100°C are present in the overall system, e.g. heating of the make-up water of the steam boiler system, a building heating system or process water heating system, the feed water temperature can be reduced from 103°C down to 65°C using a simple inexpensive plate heat exchanger. Owing to the larger difference between the flue gas and feed water temperature, the flue gas can now also be further cooled to roughly 85°C without requiring further investment in the economiser. This increases the combustion efficiency and achieves up to 1.8% fuel savings.

This efficiency-enhancing measure can also be retrofitted to existing systems for a relatively small investment outlay.

Summary

Optimising the reduction of flue gas losses is a high-priority task in the planning and also operation of steam boiler systems. The following question often arises in relation to this:

What measure or combination of measures will achieve the best heat recovery?

The following diagram shows the measures for reducing flue gas losses described in previous sections using a steam boiler as an example.

Which technology is most suitable for optimum economic efficiency of a steam boiler system depends on the application in each case. The “size” and temperature level of a low temperature heat sink are particularly decisive when it comes to choosing the efficiency-enhancing measure.

Example:

  

Condensate accumulation rate

c = ṁCo / ṁS

Make-up water rate

z = 1 – c

UL-S

10,000 x 16

System steam output

10,000 kg/h with pavg = 13 bar

Blowdown rate

5 %

Case

Component

Efficiency
   

Components

total

1

Boiler

88.9 %

---

2

Boiler + economiser

88.9 % + 6.5 %

95.4 %

3

Boiler + economiser + condensing heat exchanger
(with z = 0.3 / α = 12%)

88.9 % + 6.5 % + 2.8 %

98.2 %

4

Boiler + economiser + condensing heat exchanger
(with z = 0.5 / α = 20%)

88.9 % + 6.5 % + 3.8 %

99.2 %

5

Boiler + economiser + condensing heat exchanger
(with z = 1 / α = 34%)

88.9 % + 6.5 % + 7.6 %

100.9 %

6

Boiler + economiser + air preheating
(20°C to 65°C)

88.9 % + 6.5 % + 1.7 %

97.1 %

7

Boiler + economiser + feed water cooling
(with z = 0.3)

88.9 % + 6.5 % + 0.6 %

96.0 %

Case studies for combinations of measures for the best heat recovery

The sinks used for the condensing heat exchanger are generally make-up water for the production of steam, central heating backup or process water heating. The same applies for the feed water cooler which is an especially low cost alternative to the condensing heat exchanger if the water flow to be heated up is relatively small or, as in the case of heating, the heat cannot be used continuously the whole year round.

If no heat sink exists (or only one that is characterised by strong time-based fluctuations), use of air preheating is recommended.

Temperature-efficiency diagram for steam boiler systems with measures for increasing efficiency

Temperature-efficiency diagram for steam boiler systems with measures for increasing efficiency

Boiler (not shown)

      

Boiler + economiser + condensing heat exchanger
(with z = 1 / α = 34%)

Boiler + economiser

  

Boiler + economiser + air preheating
(20°C to 65°C)

Boiler + economiser + condensing heat exchanger
(with z = 0.3 / α = 12%)

  

Boiler + economiser + feed water cooling
(with z = 0.3)

Boiler + economiser + condensing heat exchanger
(with z = 0.5 / α = 20%)