Regenerating powders and extracted air in coating with electrostatic sprayers

Abstract

A description is given of a powder pigment recovery system used at a factory for making protective coatings. The system provides higher throughput by the use of two series-connected ITsN cyclones and an additional fine-finishing filter. Tests have given the overall performance in dust trapping for various particle fractions.Coatings made with powder pigments include a number of operations: preparing the surface for coating, depositing the powder composition, and solidification. This technology avoids the use of liquid solvents and provides a given film thickness with the minimum consumption of coating material, while improving the ecological state of the process and the working conditions. All these advantages are particularly important on depositing powder coatings by means of electrostatic sprayers.In such coating schemes, there is a system for recovering the powder (in one or two stages) consisting of cyclones with various designs and sleeve filters of explosion-safe design [1,2]. According to GOST 9.410-88 [3], the performance in cleaning the air in the recovery system should not be less than 99.8%. However, the dust-trapping devices used in these systems sometimes do not provide such performance or do not meet the required MPC standards for the air in the working area. Therefore, searches are continuing for the most effective and economical methods of cleaning the air extracted from the deposition chamber to remove suspended powder particles by the use of new models of cyclones and sleeve filters.For example, at the Stekloprof plant, a recovery system has been built containing two ITsN-2 cyclones and an FRBN sleeve filter, in which various modifications were made in the design of some elements (which adversely affected the ratingplate characteristics), which resulted in reductions not only in the throughput and total pressure but also in the speed of the extracted air in the working parts of the chamber. This reduced the nominal planar velocity in the cyclone and consequently influenced the periodicity in regenerating the sleeve filter.In the upgraded system, the basic original designs were restored, which not only raised the performance of the cyclones and provided easier conditions for the filter operation but also enabled one to reduce the discharge of pollutants to the atmosphere in accordance with the required standards.However, the use of sleeve filters in the second stage increased the capital costs of the recovery to a substantial extent, and additional equipment was required to regenerate the sleeves, which increased the working costs. It was therefore decided to further upgrade the system by using plant of Upor type intended to process 2000 m 3 /h of air drawn off from the deposition chamber in the powder recovery and air treatment units. The speed of the extracted air in the working and transport sections is 0.7 m/sec, and the dust is not ejected through. The powder consumption attains 15 kg/h (deposition on the workpieces up to 50%). The technological scheme is given in [4]. The powder size distr...