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Kiln cooling by colorful insulation
Do paint coatings have thermal insulating properties along with decorative ones? 

Experts say, ‘Yes’!One square millimeter of a special paint cools down the heated surface by more than 22ºC. Tests carried out by GCE Group showed that proper choice of thermal insulating paint and thickness of paint coating  bring high energy savings.

GCE Group experts studied paints used mostly at cement and pulp and paper plants - the most energy intensive industries facing challenges with thermal insulation.The target of GCE Group experts was to analyze modern thermal insulating paints efficiency with respect to rotary kilns insulation.‘We developed a test unit which allowed carrying out tests in conditions close to realty’, GCE Group expert Andrew Pestovskii comments. 

According to his words, the test unit is a one cubic meter metal tank with a tube heater and a metal plate installed inside. During tests, the plate heating temperatures varied from 50 to 250 ºC. Selected temperature range helped to test different types of thermal insulating paints and coating methods. Manufacturers confirm that thermal insulating paints reduce thermal energy flow rate and energy losses. ‘During tests we recorded quantity of thermal insulating paint coatings, metal plate temperatures, ambient air temperature and temperature of metal surface of  non-insulated sections’, Chief Engineer Energy Efficiency Department Michail Chaika clarifies. 

Tests showed that average temperature difference (54.7ºC) is obtained with eight thermal insulation paint layers (2.4 mm) and one insulating paint coating reduces surface temperature by 7 ºC. Consequently, application of 1 mm of thermal insulating paint reduces temperature by 22.7 ºC. We would like to draw your attention to the fact that different manufacturers of similar thermal insulating paint say that one millimeter coating reduces surface temperature by 63.3 ºC However, practice showed that temperature reduction is three times lower. 

Consequently, commercial proposal for potential users shows only desirable results which are far from the realty. This fact must be considered by engineers during equipment coating by thermal insulation paint. GCE Group experts have carried out main experiments in Limited Liability Company ‘Cement’ in the city of Slantsy, Leningrad region. Rotary kiln surface was coated by thermal insulating paint after shutdown, when the kiln body cooled down to 40 ºC. The kiln surface was cleaned by metal brushes and degreased before coating. Experimental area of 1.4 m² was separated into five sub-areas. 

Experts varied quantity of coatings for each area. Ten kilograms of paint were used during tests. Thermal insulation was spread by brushes, paddle rollers and paint blowers. One area remained unpainted for further comparative measurements.   Expert conducted thermal imaging of painted surfaces after the kiln startup and gaining operational parameters. Next thermal vision survey was conducted three days after the kiln startup.  No damages or cracks were detected on the painted surface. GCE experts used three types of metering gauges during tests – thermal imaging camera, infrared pyrometer and contact pyrometer. 

Experimental results showed real thermal insulation properties of paint coating and optimal thickness of insulation.  Experimental conditions didn’t let to identify precise impact of factors (s.a. overall temperature pattern of the kiln which may affect thermal imaging results and errors during determination of coating thickness) on actual values deviation from values specified in the commercial proposal of manufacturers. Why studying thermal insulation paint properties was target of this experiment? ‘It is known that cement plant calcining kilns have a form of a tube with the diameter of  2.5-4.5 meters and length of  30-120 meters. Cyclones used for exhaust gas treatment are installed at exhaust gas outlet. Temperature of exhaust gas entering cyclones is approximately 800ºC.

External surface temperature of cyclones varies from 100 to 250ºC depending on insulation. Modern kilns are equipped with clinker cooler. On the one hand, clinker temperature after the kiln is approximately 1000ºC, and it is obvious that hot clinker transportation and processing are practically impossible. On the other hand, we can increase efficiency of the kiln and reduce fuel consumption, if clinker thermal energy returns to the kiln.  Clinker thermal energy is partially recovered by clinker cooling. Clinker is cooled by the air used for fuel combustion in the kiln. We propose to insulate kiln end and cyclones. 

Hence, we will reduce thermal losses and fuel consumption. It is not correct to insulate parts of kiln body which are close to the flare, as there will be a risk of excess heating and kiln lining will deteriorate faster. Cyclones have conic shape and insulating by traditional mineral wool or galvanized steel seems complicated. Application of thermal insulating paint is the most optimal option for cyclones’, Michail Chayka says.It is important to note, that application of other insulation materials instead of thermal insulating paint is practically impossible, as operational staff must monitor all possible damages of kiln body. 

If kiln surface is insulated, operational staff will not detect cracks and other defects in time. Consequently, we propose to use thermal insulating paint for rotating equipment which require visual inspection, or have complicated geometrical shape. Is is feasible to choose traditional insulation materials for stationary equipment. One of experimental targets was to determine optimal thickness of the paint layer, as energy savings benefits should exceed the price of paint coating.  The fact is that thermal insulating paint cost is much higher in comparison with traditional mineral wool, galvanized iron, polyurethane, etc. Experts agree that presented insulation material is an innovative technology and is not widely used. At the same time, among the advantages are low wear, water resistance, low weight of painted coating, etc.  GCE Group experts have carried out similar experiments at other industrial plants. 

The target was identification of temperature influence on paint properties when the thermal insulation paint is spreaded on equipment under operation.  Such experiment has been carried out at Itambe cement plant (Brazil). Energy survey results showed saving perspective of more than 60 ths. USD, if the plant uses thermal insulating paint.  Payback period is less than a year.  What other areas of thermal insulation paint application? GCE experts consider that thermal insulating paint can be used in ferrous and non-ferrous metal industries and for rotating units installed on-site of ore enrichment plants. 

The experiment resulted in several important findings. First, thermal insulation paint can be used for rotary kiln surface with average temperature of 270ºC. Test showed solidness of thermal insulation paint, as no cracks or damages were detected on painted surfaces.  Second, thermal insulating paint layer of 1 mm reduces surface temperature by 22.7ºC. Experts recommend considering this figures during technical and economical feasibility study of thermal insulation paint implementation. Nowadays, GCE Group experts are planning additional tests of thermal insulation paint properties and quality. 

Written by: Irina Krivoshapka

Source: Newspaper «Energy and Industries of Russia » No. 07(16), February 2015
 
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