HIGH EMISSIVITY COATINGS AND METHODS OF HIGH · PDF fileHigh Emissivity Coatings and Methods...

$: HIGH EMISSIVITY COATINGS AND METHODS OF HIGH · PDF fileHigh Emissivity Coatings and Methods of High-Temperature Testing Conference on Refractories, Furnaces and Thermal Insulation,$
High Emissivity Coatings and Methods of High-Temper ature TestingConference on Refractories, Furnaces and Thermal In sulation, Nový Smokovec, 2010High Emissivity Coatings and Methods of High-Temper ature Testing

Conference on Refractories, Furnaces and Thermal In sulation, Nový Smokovec, 2010

MPO TIP project / recipient BG SYS HT s.r.o. 1/16

HIGH EMISSIVITY COATINGSAND METHODS OF

HIGH-TEMPERATURE TESTING

Doc. Ing. Milan Honner, Ph.D. - Ing. Ondřej Soukup, New Technologies Research Centre,

University of West Bohemia in Pilsen, Univerzitní 8, 306 14 Plzeň

Ing. Vlastimil Bartel - Ing. Milan Mauer, BG SYS HT s.r.o., Holubova 389, 530 03 Pardubice




Contents

• Emissivity and its impact on radiation heat transfer

• BG HitCoat® coatings and benefits of application

• New methods of high-temperature testing of coatings

• Conclusions• Acknowledgement and contact




Emissivity of Material Surface

Defined as the ratio of the intensity of surface radiation and the intensity of radiation of black body radiation at the same temperature.

- Spectral emissivity - emissivity value at certain radiation wavelength - Band emissivity - effective value in the considered range of wavelengths- Total emissivity - defines the total radiation output across all

wavelengths

Since emissivity is typically greatly dependent on wavelength and temperature, it is necessary to take these values into account for the purposes of practical application of heat-resistant materials and evaluation of heat radiation transfer.

Emissivity values stated in various printed and electronic medialack the temperature and wavelength specifications and are therefore worthless and often misleading.




Emissivity and Radiation Heat Transfer

It is the total emissivity that matters for heat transfer between two solid surfaces which act as grey bodies with continuous spectrum of distribution of radiation intensity at wavelength, as the heat flow is proportional to total emissivity and the difference of biquadrate of absolute temperatures of these surfaces.

charge (2)

furnace (1)

heat flow

Increase in the emissivity of the furnace wall by using a high emissivity coating for instance by 10% will result in 10% increase of the heat flow from the wall to the charge.

( )42

412121 TTq −=↔ εεσ

(i) without combustion gases





Heat transfer gets more complex with the impact of combustion gases:- heat flow between combustion gases and charge surface- heat flow between combustion gases and furnace walls- heat flow between furnace walls and charge surface

Application of high emissivity coating will result in - more intensive absorption of combustion radiation into the furnace

- more intensive radiation from the furnace wall

furnace

combustion gases

(ii) with combustion gases

charge





While heat transfer between combustion gases and solid surfaces takes place only in narrow absorption bands of combustion gasses, the transfer between solid surfaces takes place mainly in “free slots“ in between the absorption bands.

Spectral redistribution takes place on the wall surface and the overall efficiency of heat transfer is more intensive at higher emissivity.

(ii) with combustion gases

2 4 6 8

wavelength (um)

Radiation intensity

Furnace wall

combustion gases




BG HitCoat®CoatingsInorganic sillicium-based composite systems developed for protection of heat exchange surfaces subject to heat, clinkering and caking, abrasion and high-temperature corrosion.

These materials carry two features which are essential for application in demanding application of various industrial devices:– they create an amorphous ceramic matrix without the necessity of being heated

– resistance against repeated heat cycles showing no signsof crystallization

Applicability on metals, ceramics (refractory concrete, fireclay) or heat insulation from mineral fibres. Water dilutable, dry at regular temperatures.

Composition may be adjusted to various types of surfaces. Other properties, including emissivity, may be modified by variability of filling agents and active agents.

Details - BG SYS HT s.r.o., Pardubice

spray application of BG HitCoat directly

in economizer




Benefits of Application of BG HitCoat®Coatings

The primary benefit of application of coatings with high emissivity are energy savings for heating and combustion which are achieved by increased efficiency of radiative heat transfer which results in elimination of thermal loss.

Due to the fact that emisisivity of BG HitCoat® coatings is above the value of 0.92 in the whole temperature range up to 1650 °C, unlike other commonly used construction materials either metals or lining, (most linings have the value of 0.85 at room temperature and it drops with increased temperature and may even drop under 0.3), significant measurable savings may be achieved within the range of several percent.


(i) saving energy for heating and combustion





Energy savings for heating and combustion go hand in hand with adequate reduction of CO2

either in the heat installation itself, in case of fossil fuel combustions, or indirectly on the energy source.

Additionally high emissivity coatings also have interesting effect on reduction of NOx in combustion gases when burning fossil fuels as the increased efficiency of radiant heat also leads to decrease in combustion gases temperature and decrease in creation of NOx.


(ii) reductions of CO2 and NOx emissions

Burner protected by BG HitCoat® coating





Severalfold increase in service life of both linings and fibrous insulation as well as metal parts of the furnaces and boilers.

Surface reinforcement of fibrous insulation play a big role as well. This insulation materials typically have several great properties but they are limited by low mechanical resistance. Spray application of ceramic BG HitCoat®increase their mechanical resistance and which will prevent them from defibering and will significantly eliminate the initial shrinkage.


(iii) increase in service life of heat installations

Crucible furnace with BG HitCoat®

applied after 100 meltings, blocks of fibrous insulation protected by

BG HitCoat® after 170 000 hours of operation in hydrogen flame.




New Methods of High-Temp. Testing of Coatings

(i) equipment used

heating: Coherent Highlight ISL 4000L diode laser (4,3 kW - 800 nm)

on an industrial robot

measuring: infrared cameras FLIR SC2000 and A325 (7.5-13 um), broad spectrum output

analyzer, thermocouples,

Overall workplace layout

detail of the laser head and infrared cameras




(ii) measuring emissivity and thermal conductivity

thermocouple (1mm coat.type K)

- total heating at app. 800 °C – 4 kW laser 15x - 20s ON / output 26-31%

+ 10s OFF, - cooling off to 200 °C

substrate

referential coating

analyzed coating

infrared camera (micro bolometer, 7,5-13um)

layout A -Emissivity -measuring

layout B - Thermal conductivity

measuring

infrared camera (micro bolometer, 7,5-13um)

diameter 25 mm

thickness5 mm

Experimental method layout

Temperature field of the sample





(iii) measuring heat flux

- total heating at 1050 °C – 4 kW laser ON- 4min/output 23% - 3min/output 24% - 3min/output 25%

- cooling off to 550 °C

infrared camera (microbolometer, 7,5-13um)

substrate

referential coating

analyzed coating

Broad spectrum heat flux analyzer (0,5 - 20um)

thermocouple(1mm coat.type K)






(iv) service life and resistance against thermal shocks

4 kW laser - local heating at 2000 °C - increasing output 23 - 40% on 25 cm trajectory- various number of repeated moves

infrared camera - one stationary, - second with laser

substrate

analyzed coating

surface condition after test completion






ConclusionsIt is essential to know the emissivity values of construction materials at high temperatures and to use them in order to use the energy supplied to thermal installations more efficiently by means of intensification of radiation heat transfer. Application of composite coatings with high emissivity can save as much as 10% of energy and shorten the heating and melting period and yet increase the service life of the devices. New measuring methods for the purposes of development of new coatings for various application conditions are being developed for measuring thermal and spectral emissivity dependencies and thermal conductivity as well as high-temperature testing methods for testing service life and resistance of coatings against thermal shocks.




Acknowledgement

This contribution was carried out with financial support from the state budget of the Czech Republic via the Ministry of Trade and Industry within the project FR-TI1/273 “Research and Development of High Emissivity Composite Coatings for High—Temperature Applications”

BG HitCoat Coatings

BG SYS HT s.r.o., Pardubice

laser technologies, measurement of material properties

New technologies Research Centre, University of West Bohemia in Pilsen

Contact

www.ntc.zcu.cz

www.bgsysht.eu

HIGH EMISSIVITY COATINGS AND METHODS OF HIGH · PDF fileHigh Emissivity Coatings and Methods...

Documents

Transcript of HIGH EMISSIVITY COATINGS AND METHODS OF HIGH · PDF fileHigh Emissivity Coatings and Methods...