TI 405_eng - ERK vs Schnelldampferzeuger

No. : TI-405_eng Rev. : 1

Page : 1 of 1 TECHNICAL INFORMATION

Date : 14.10.09

SCHNEIDER-Eckrohrboiler (natural circulation) vs. One Through Steam Boiler (forced circulation)

Performance during sudden peak loads pressure drop - recovery time - steam quality

Doc.: TI 405_eng - ERK vs Schnelldampferzeuger.doc

SCHNEIDER International GmbH Email / Internet Hildburghauser Strasse 79 [email protected] 12249 Berlin, Germany www.schneider-kessel.com Tel.: +49-30-75 44 93 99-0 Fax: +49-30-75 44 93 99-21

To evaluate the performance of a steam boiler during sudden peak loads two situations dur-ing operation with two different boiler systems shall be analyzed. These two boiler systems are the natural circulation type with the SCHNEIDER-Eckrohrboiler as a sample and the forced circulation type or similar systems with the One Through Boiler as a sample.

Situation 1

The burner is operating within its turn down ratio.

Situation 2

The burner is switched off since the steam load is below minimum capacity of the burner.

Each steam boiler reacts with a pressure drop on each peak load hence the questions raised on this are listed below

how big is the pressure drop

how long does it takes until the boiler is back on its regular operation pressure

what happens with the steam quality

any other impact on the boiler performance

Provided the function of the two boiler systems are known in general the impact of sudden peak loads shall be analyzed with the SCHNEIDER-Eckrohrboiler as a natural circulation type and the One Through Boiler as a forced circulation type.

For both boiler systems the same criteria are relevant

Note

the bigger the water content of the boiler

the clearer the separation of the internal water circulation and the external steam / condensate circula-tion

the bigger the boiler turn down ration is which means the lower the lowest steam load on which steam can be extracted from the boiler

the bigger the burner turn down ratio

the shorter the running time for the boiler compound control is until ignition

the lower is the negative impact on the boiler performance, such as pressure drop and other perform-ance data, during sudden peak load.

Under above criteria the performance of the two boiler systems during sudden peak loads shall be evaluated.



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The SCHNEIDER-SCHNEIDER-Eckrohrboiler

1 Dampfentnahmeventil / main steam valve 13 Druckbegrenzer / pressure limiter

2 Niveauelektrode, max.. / level probe, high 14 Druckwchter / pressure monitor

3 Niveauelektrode, min. / level probe, low 15 Probenahmekhler / sample cooler

4 Sicherheitsventil / safety valve 15a Absalzeinrichtung / continous blow down (TDS)5 Entlftungsventil /

venting valve 16 Abschlammventil / intermittend blow down

6 Speiseabsperrventil / feed stop valve 16a Absperr-, Entleerungsventil / stop valve

6a Speiserckschlagventil / feed check valve 17 Schauloch / view hole

7 Wasserstandanzeiger, dir. / water level gauge, direct 18 Brenner / burner

8 Wasserstandanzeiger, elektr. / water level gauge, electr. 19 Mannloch / man hole

9 Wasserstandregelung / water level control 20 Besichtigungsffnung / inspection opening

10 Manostatenbalken / pressure control manifold 21 Ablassstutzen / condensate drain

11 Manometer / pressure gauge 22 Abgaskammer / flue gas chamber

12 Druckregler / pressure control * optional



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water content

There is no doubt about that the most important condition of being not sensitive against sudden peak lo-ads is a large water content since the water content of a boiler is a thermal energy reservoir. Peak loads result in pressure drop which means the enthalpy of the water drops accordingly and this enthalpy differ-ence means additional steam generation.

Example:

water content up to low water level : 2.0m

operation pressure at begin of pressure drop : 65.0barg (~281C) operation pressure at end of pressure drop : 60.0barg (~276C) enthalpy difference : 26.82kJ/kg

steam generation out of enthalpy difference : 34kg

On first sight it seems not to be that much but on second sight it looks different. Assuming an extreme peak load occurred over 1 minute has resulted in this pressure drop of 5.0bar these 34.0kg of steam would be equal to an hourly steam generation of 2040kg/h and that is actually impressive. Does this peak load and pressure drop occurs during regular boiler operation of 100% load this would mean that a boiler of a regular capacity of 7.5 t/h has a short time capacity increase of almost 30% up to 9.74t/h

This large energy reservoir is the guarantee for a short reaction time when peak loads occur even during switch-off times of the burner.

Bottom Line :

The large water content of the SCHNEIDER-Eckrohrboiler secures low pressure drops and short recovery time when peak loads occur.

Since any pressure drop goes along with an increase of water and steam volume the following issues are of high importance.

clear separation of internal water circuit and external steam / condensate circuit

Because the SCHNEIDER-Eckrohrboiler has a defined and controlled water level due to the clear sepa-ration of the internal water circuit and external steam / condensate circuit the sudden increase of volume during a sudden peak load results in a slightly and temporary lifted water level. Due to the large designed steam drum there is never the risk of any carry over hence even during extreme peak loads absolute dry saturated will be delivered to the consumer.



Date : 14.10.09





In closed consumer circuits (heat transfer in heat exchangers) this means ideal condition for the heat transfer even during peak loads

Surface heat transfer coefficient for dry saturated steam ~ 70 kW/mK

In open consumer circuits (steam gets in direct contact with the product, i.e. splitter) absolute dry satu-rated steam is the main and essential condition for high production ratio and short production time. The risk of contamination of the product due to carry over does not exist with the SCHNEIDER-Eckrohrboiler since there simply is no carry over.

Another unique advantage of the SCHNEIDER-Eckrohrboiler and the clear separation between the in-ternal water circuit and the external consumer circuit is the possibility of steam extraction at full operation pressure even when the burner has stopped

Bottom Line

The clear separation between the internal water circuit and the external consumer circuit in connection with the large water content as energy reservoir are further reasons for the trou-ble free operation even during sudden and extreme peak loads.

boiler turn down ratio

Because of the clear separation between the internal water circuit and the external consumer circuit the SCHNEIDER-Eckrohrboiler has a boiler turn down ratio of 1:100 since steam extraction is possible even when the burner has been switched off by the automatic pressure control system.

In One Through Boiler or similar boiler systems the feed water pump starts synchronously with the burner and initiates an additional pressure drop due to the fact that cold water is fed into the boiler. In the SCHNEIDER-Eckrohrboiler this is totally different. The water level in the boiler is kept constant and the entire water content of the boiler is on evaporation enthalpy which means as soon as the burner ignition occurs steam is generated at full operation pressure.

Bottom Line

The unbeatable advantage high turn down ratio of the SCHNEIDER-Eckrohrboiler secures the lowest possible regeneration time after sudden peak loads

burner turn down ratio

A high burner turn down ratio is of high importance for the boiler performance during peak loads. How-ever the burner turn down ratio shall not exceed the boiler turn down ratio in order to avoid possible boiler damages due to no sufficiently cooled heating surfaces. For the SCHNEIDER-Eckrohrboiler this risk does not exist as a result of the clear separation between the internal water circuit and the external con-sumer circuit. Hence there is no limitation set for the burner turn down ratio for the SCHNEIDER-Eckrohrboiler and the higher the better applies.



Date : 14.10.09





Bottom Line

The higher the burner turn ratio is the faster is the boiler back on full load and full operation pressure after a sudden peak load.

sequence control running time

The time it takes for the burner to be back on full capacity after the pressure control has switched it off depends on the running time of burner sequence control. This running time compiles of

a. purging time to clean all boiler passes of gaseous remains of the fuel from burner inlet to stack.

b. checking time of tightness control of solenoid valves.

c. running time of compound control on-off before ignition.

d. ignition

e. running time of compound control from ignition load to full load after ignition

The purging time can be adjusted in accordance with local situation since it depends on the boiler design and the distance between boiler and stack. The running time of the compound control depends on the burner brand. The remaining sequences of the running time however are fixed and cannot be changed or adjusted. Bottom Line

The shorter the running time of the sequence control the faster is the boiler back on full load and full operation pressure after a sudden peak load.



Date : 14.10.09





The One Through Boiler



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water content

One Through Boiler have an extreme small water content which normally is less than 25% oft the water content of the SCHNEIDER-Eckrohrboiler which means the energy quantum which can be accumulated is small as well.

Bottom Line

The small water content of the One Through Boiler constitutes no energy reservoir as a buffer during peak loads independent if the burner is in operation or not.

clear separation of internal water circuit and external steam / condensate circuit

This is the biggest difference between the two boiler systems under evaluation and it has the biggest im-pact on the boiler performance during peak loads. The One Through Boiler has no separation between the internal water circuit and the external consumer circuit at all since there is only one circuit from the boiler to the consumer from where it ends up in the condensate system. The feed water is forced by spe-cial circulation pumps into the boiler coil where is evaporated to almost 100%. An evaporation ratio of 100% is not possible due to the boiler system which means a steam / water mixture is leaving the boiler with the ratio of the mixture depending on the boiler load. During a constant medium boiler load the eva-poration ratio is rather high at full load or during peak load this ratio is rather low in other words the carry over is varying with the boiler load. Main separation of water and steam occurs in a small separation ves-sel outside the boiler. Part of the separated water is extracted via a continuous blow down device and hence lost. The other part of the carry over reaches as steam moisture the consumer. This moisture is approx. 5% during medium constant load but increases during peak loads to 25% and more.

But there another significant disadvantage of this design. Due to this design of a coil as heating surface it is not possible to install a blown system at the One Through Boiler in order to flush and remove chemi-cal or other remains from the boiler. Hence these impurities remain in the carry over or in the boiler itself meaning these impurities either reach the consumer or cover the water side heating surfaces where they interrupt the heat transfer resulting boiler damage.

The impact of the steam moisture on the consumer might be described as follows

In closed consumer circuits (heat transfer in heat exchangers) this high content of carry over or high steam moisture will extend the production time since the surface heat transfer coefficient is much lower than for dry saturated steam.

Surface heat transfer coefficient for water is only ~ 4 kW/mK

Which means the higher the steam moisture the worse is the heat transfer

In open consumer circuits (steam gets in direct contact with the product, i.e. splitter) high steam mois-ture results in product contamination since the impurities in the carry over will come in direct contact with



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the product. Furthermore high steam moisture results in decreased production efficiency due to tempera-ture fluctuation in the splitter.

For both circuits apply the high water and hence energy loss due to the blow down ratio of the steam/water separator and the steam moisture.

Bottom Line

Due to the common steam/water/condensate of the One Through Boiler system the negative impact on the boiler performance and hence on the production efficiency and product quality during peak loads is significant

boiler turn down ratio

The boiler turn down ratio of the One Through Boiler is determined by the min. water flow necessary to cool and to protect the heating surfaces in order to avoid boiler damages. This basic condition limits the boiler turn down ratio of the One Through Boiler to approx. 30:100. With every boiler/burner start the forced circulation pump will start simultaneously too and feeds cold feed water through the coil which results in an additional pressure drop.

Bottom Line

The One Through Boiler does not allow any steam extraction when the burner system is swit-ched off. Operating the burner with closed main steam valve in order to keep the boiler in operation condition is not possible as well.

burner turn down ratio

The burner and the boiler turn ration are exactly the same. Synchronization of burner-boiler-circulation pump is mandatory for the One Through Boiler

Bottom Line

Influencing the performance of a One Through Boiler during peak loads by increasing the burner turn down ratio is not possible

sequence control running time

For the One Through Boiler the same statements apply than for the SCHNEIDER-Eckrohrboiler



Date : 14.10.09





CONCLUSION

The SCHNEIDER-SCHNEIDER-Eckrohrboiler The SCHNEIDER-Eckrohrboiler is proof against sudden peak loads. Only slight and momentary pressure drops occur with the steam quality remaining unchanged. The reasons for this performance are

large thermal energy reservoir because of large water content

absolute dry saturated steam because of clear separation of internal water circuit and external steam / condensate circuit

short recovery time after sudden peak loads with unchanged high steam quality.

no carry over even during extreme peak loads.

steam extraction at operation pressure level secured even when the burner is switched off which means boiler turn down ratio 1:100.

Burner can be operated to keep the boiler on operation pressure level even with closed main steam val-ve.

no limitation for burner turn down ratio which is only depending on the burner brand. This assures high influence on the boiler performance.

all boiler loads can be covered by 1 boiler due to the boiler turn down ratio of 1:100.

The One Through Boiler The One Through Boiler is not proof against sudden peak loads which results in significant pressure drops along with worsening steam quality. The reasons are as follows:

no thermal energy reservoir because of negligible water content.

high steam moisture due to carry over.

high energy loss due to carry over along with high blow down ration in the separation vessel.

lifetime limited because no blown down system for the coil can be installed.

no steam extraction possible when burner has switched off.

boiler turn down ratio limited to 30:100. In order to overcome this disadvantage more units with smaller capacities must be installed.

TI 405_eng - ERK vs Schnelldampferzeuger

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