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ECEEE 2009 SUMMER STUDY ACT! INNOVATE! DELIVER! REDUCING ENERGY DEMAND SUSTAINABLY 1045

Cogeneration in industrial steam systems

with multiple-disk turbines

Dr. Ferenc LesovitsBudapest University of Technology and Economics (BME)

Department of Energy Engineering

Hungary

[email protected]

Keywords

energy eciency, cogeneration, steam turbine, industrial steam

systems, pressure reduction, saturated steam expansion

AbstractTere are a lot o industrial and communal heat supply systemsthat are operated with saturated or hardly superheated steam.

Steam production generally happens according to the highest

pressure demand, meanwhile more or less quantity is used at

lower pressure. Pressure reduction is done by throttle-valves.

Tis is an unused potential or co-generation. Multiple disk

turbines may be the expansion equipment which is suitable or

realising co-generation in small- and medium scale heat supply

systems. Since the construction is simple it can be produced at

reasonable prices, so co-generation can be economical even at

low power rates. Te investigation o advantages and disadvan-

tages o multiple disk turbines has become the subject o my

presentation. Furthermore the investigation o operation con-

ditions at dierent boundary conditions in case o cogeneration

applications and the determination o optimal operation condi-

tions. Te same construction may be applied to a wide range o

operation parameters without any modication. With variation

o nozzle cross section it can be adjusted to dierent mass ow

rate demands. Te turbine can be operated even with saturated

steam. Since this turbine is not sensitive to steam quality, it can

be operated even with steam used in normal steam systems.

Tere is not needed to install an extra water treatment system.

Considering design and operation eatures o these types o

turbines it can be stated that these machines may be suitable or

perorming economical co-generation in small- and mediumsized heat supply systems. In this way the use o co-generation

method may be expanded signicantly.

Introduction

Rising prices o ossil uels, dwindling resources and eorts

in the eld o environmental protection orce society to use

energy resources as ecient as possible. Te best solution or

ullling above mentioned requirements is to build and oper-

ate combined heat and power generation (co-generation) sys-tems. At small power rates, internal combustion engine driven

cogeneration systems are used nowadays. But the utilization

o these systems is limited. In one way they can be operated

only with clean uels (petrol, diesel oil or gas). In another way

the temperature level o the heat supplied by these systems is

limited. At wide-spread small power rate steam systems (e.g.

chemical and ood processing plants, textile industries and

hospitals) where only saturated or hardly superheated steam is

generated there is no economical possibility or co-generation

today. A study o industrial technologies [1] has shown that a

wide eld o applications can be ound where low or medium

pressure, saturated steam is available but or the next technol-ogy a pressure reduction has to be carried out. In most o the

cases steam is generated at the highest pressure demanded in

the system. (Te highest pressure is generally determined by

the highest saturated steam temperature demand in the sys-

tem, because heat utilization happens at steam condensation at

saturation temperature.) Te highest pressure in these systems

is in the range rom 7 up to 17 bar (absolute). But more or less

steam is consumed at lower pressure. Generally there are sev-

eral (4 or 5) pressure level applied in case o a large chemical

or ood processing industry. Generally rom 25% up to 50%

o generated steam is consumed at signicantly lower pressure

than it is generated. Nowadays throttle valve are generally used

in these systems or pressure reduction. Multiple disk turbine

is intended to use or utilization o pressure drop.

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1046 ECEEE 2009 SUMMER STUDY ACT! INNOVATE! DELIVER! REDUCING ENERGY DEMAND SUSTAINABLY

PANEL 5: ENERGY EFFICIENCY IN INDUSTRY

Expansion Behaviour of Saturated Steam

Following cases are typical cases o expansion:

Trottling:

In this case no power is gained rom expansion.

Pe = 0

Eciency o this expansion equipment would be 0%,

T = 0Afer this expansion steam will be slightly superheated.

h hin throut

=

Tis is the case o expansion at throttle-valve pressure

reducers.

Isentropic expansion:

Tis is the expansion, where the theoretical maximum

available power is gained rom the pressure reduction.

Eciency o this expansion equipment would be 100%.

T

= 100%

Isentropic (theoretical) power can be derived as:

P m h his steam in isout

= ( )

Afer this expansion steam will be in saturated vapor-

liquid mixture area.

Real expansion:

Tis is the real expansion line using a real expansion

equipment, e.g. a turbine. Power o the turbine can be

derived as:

P m h hreal steam in realout

= ( )

Eciency o the turbine can be derived as:

T real is real isP P h h= =

/ /

Afer this expansion steam can be slightly superheated, satu-

rated, or in saturated vapor-liquid mixture area, depending

o expansion eciency. At about

= 30% turbine eciency

exhaust steam will be saturated.

During the expansion, when the expansion line has crossed

the saturated steam-line and expansion happens in the vapor-

liquid mixture area, a change o phase should begin to occur. At

this point the random kinetic energy o the molecules has allen

to a level which is insucient to overcome the attractive orces

o the molecules and some o the slower moving molecules

coalesce to orm tiny droplets o condensate. When the expan-

sion process is rapid, and ow velocity is very high, this process

does not have time to occur. Te achievement o equilibrium

between the liquid and vapor phases is thereore delayed, and

the vapor continues to expand in dry state. Tis state is called

supersaturated, or supercooled[5]. Because these states are not

states o stable equilibrium, they are called metastable states.

Te name is originated rom the idea that this is not an equi-

librium state, but it cannot be called unstable as well, because

an innitesimal disturbance will not cause a major change o

state. Tis metastable state, depending on velocity and pres-

sure level, can exist till 3%-5% o liquid content o steam. Te

delay in condensation leads to a build-up o molecular cohesive

orces which nally results in sudden condensation at manypoints. Tis condensation occurs suddenly, with an increase o

both entropy and pressure. Afer the beginning o condensa-

tion saturated steam consists o increasing amounts o water

droplets during expansion.

Expansion Utilization Possibilities

Reciprocating steam engines cannot be used because increas-

ing water content in the steam can cause serious damage at

the crank mechanism or at the piston, when the water volume

is greater than the clearance volume o the cylinder, because

liquid is practically not compressible.

Bladed turbines are sensible to liquid content in steam. In

one way turbine eciency is decreasing, and in another way

droplets can cause erosion on the turbine blades. In order to

avoid serious erosion problems at turbines used in nuclear

power stations, where a large part o steam expansion happens

at liquid-vapor mixture area, dewatering is used between every

turbine stage. Tis is a very expensive solution.

Both solutions have a higher quality demand or water than

needed or normal boiler operation.

Demands or a small power rate expansion machines oper-

ated with saturated steam:For a small power rate operation a plain and robust construc-

tion is needed [6].

o avoid large amount o water droplets in the steam, end

state should be close to the saturation line. Tis means that

turbine eciency should not be higher than

= 50%. Tis

will not cause higher losses, because heat is utilized afer ex-

pansion.

o avoid erosion the streamline should not bend sharply.

At those pressure drops where our turbine is intended to op-

erate, this maximum available power is about 5-15% o the total

energy content o the steam. When turbine eciency is about:

= 30% - 40%. Generated energy by this turbine is about 2-5%o total energy content o steam. Friction-, heat- and energy

conversion losses give about 0.5-1% that are not recoverable as

useul heat. Extra boiler loss ratio to the extra uel power is the

same as original boiler loss over original uel power and equal

with the applied boiler eciency. Fig. 2 shows the energy ow

(Shankey) diagram o our turbine-generator set applied or

pressure reduction. Fig. 3 shows eciency variation o cogen-

eration rom pressure reduction against additional ring power

in the unction o nominal power o the system [2].

Description Of An Adequate System

Te main goal o the system is to replace throttle valve pres-sure reducers with expansion utilization equipment. Reducing

steam pressure is a common task in these systems. Nowadays

generally throttle valves are used or this purpose. In this case

the possibility or generating electricity is not used. We have

developed a system or exploiting the power generation poten-

tial by pressure reduction in industrial heat supply systems. Te

purpose was not to reach the highest possible eciency, but it

was to reach economical operation at above describe circum-

stances or a wide range o working medium parameters. We

have kept in view the exible applicability. Tis system works as

a pressure reducer installed into a heat supply system, while it

generates electricity and eeds electricity back to mains.

We have chosen a special radial type turbine, which is dier-

ent rom conventional bladed turbines. Te turbine rotor con-

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Figure 1. Saturated steam expansion cases in h-s chart

Figure 2. Energy ow diagram o pressure reduction with and without cogeneration

Figure 3. Efciency variation o cogeneration rom pressure reduction against additional fring power

in the unction o nominal power o the system

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1048 ECEEE 2009 SUMMER STUDY ACT! INNOVATE! DELIVER! REDUCING ENERGY DEMAND SUSTAINABLY


sists o a number o thin, smooth, at, parallel disks arranged

perpendicular to a shaf and astened rigidly to it with small

spaces between the disks. Tis is the multiple disk (ESLA)

turbine, where the working medium is accelerated in a nozzle

and streaming radially in between smooth, at, parallel disks

which orm the turbine rotor. Expanded steam is streaming out

along the turbine shaf. Fig. 5 shows design principal o mul-

tiple disk turbine.

Te multiple disk turbine was patented by Nikola esla in

1913 [3], [4].

Tis turbine is a one stage radial type expansion equip-

ment.

wo main parts are the nozzle and the rotor.

In the nozzle happens the acceleration o the medium.

Te medium leaving the nozzle ingresses into the rotor,

which is built up rom smooth disks installed with a certain

gap between each.

Te simple construction o the rotor ensures that the turbineis tolerant to pollution, and has relatively low manuacturing

costs. Te eciency o the turbine is not too high, it is about

30-40% at nominal load (depending on the parameters o the

working medium). o limit eciency is advantageous rom the

point o steam humidity. When inlet steam is saturated, outlet

steam will contain only a small amount o humidity. Te turbine

contains a nozzle which accelerates the working medium and

leads it to the rotor in the appropriate direction. Te turbine

construction ensures that nozzles can be replaced with another

one, varying its cross section, or variable nozzle may be applied.

Tis ensures a exible adjustment to dierent pressure and mass

ow rate demands. Nominal mass ow rate can be adjusted to

actual demand (in a certain range). A urther advantage o ap-

plication o this system compared to other type o expansion

equipment acilities is that it does not need urther purication

o boiler eed-water. Normal eed-water quality, which is ad-

equate or e.g. shell type boiler is adequate or this system.

A system has been developed according to above mentioned

conditions. A side view and the unctional connection o the

system can be seen in the Fig. 6.

In a urther step we have analyzed economical application

conditions o this system in case o dierent nominal power

and dierent utilization actor. Investigation is based on exist-ing heat supply system, where steam-pressure has been reduced

by throttling. It was investigated advantages o the installation

Figure 4. Functional diagram and connection o the system

Figure 5. Cross-section o the multiple disk turbine

Figure 6. Side view and unctional connection o turbine generator set.

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5025 LEzSOVITS

o pressure reducer turbine instead o throttling valve. Costs are

categorized to constant and proportional part. Constant cost

is given mainly by installation and maintenance cost. Propor-

tional is given mainly by extra uel cost. Income is calculated

according to the average price o electricity. Results o this

analysis can be seen on Fig. 7 and on Fig. 8 or dierent nomi-

nal power level as the unction o utilization hours.

Summary

Signicant expansion o utilisation o co-generation may be

achieved with implication o small- and medium scale heat

supply systems. Multiple-disk turbine may be the expansion

equipment which is suitable or realisation o co-generation

in small- and medium scale steam supply systems. Since the

plain construction it can be produced at reasonable price, so

co-generation can be economical even at low power rate. Te

turbine can be installed instead o throttling valves into exist-

ing steam generation systems without signicant modication.

In this way scope o co-generation method may be expanded

signicantly, which helps energy saving and reduction o CO2

emission.

References

[1] U.S. Department o Energy: Steam Pressure Reduction:

Opportunities and Issues

http://www.nrel.gov/docs/y06osti/37853.pd

[2] Ferenc Lezsovits, Modelling o energy transer process o

the multiple-disk turbines and application or operation

with steam PhD dissertation 2006. Budapest University o

echnology and Economics

[3] Fluid propulsion US patent # 1,061,142, issued to Nicola

esla in 1913.

[4] urbine US patent # 1,061,206, issued to Nicola esla in

1913.

[5] Georg Gyarmathy: Grundlagen einer Teorie der Nass-

dampfurbine Juris Verlag Zrich 1962

[6] Application o Solar echnology to odays Energy Needs

Chapter IX. Energy Conversion With Heat Engines

http://www.princeton.edu/~ota/disk3/1978/7802/780214.

PDF

Figure 8. Net cost variation o electricity generation rom pressure reduction with application o our system at dierent power rate and

in case o dierent utilization hours [2]

Figure 7. Proft variation at dierent power rate and in case o dierent utilization hours [2]

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