The function of a powder recovery system is to collect the overspray material and render it suitable for recycling and at the same time to remove the powder particles from the exhaust air stream before discharge into the atmosphere
There are two types of collectors:
(there are more designs of collection systems which use these two principles).
The input to the cyclone is connected to the booth while the output is connected to a suitable exhaust fan. The overspray powder arrives at the cyclone inlet at a velocity of about 20 metres per second.
On entering the cyclone chamber tangentially the air/powder mixture is given a rotary motion which creates a centrifugal force on the particles. The larger and heavier particles tend to be ejected to the outside walls of the chamber and fall to the bottom where they are collected. The lighter fractions will stay suspended in the air stream which on reaching the bottom is deflected by a conical tail air/powder mix into a rising spiral which is then carried through the central stack to a filter collector.
For a standard powder the recovery efficiency can be as high as 95%. For lines that have a high % of particles <10µ in the recovered powder the recovery efficiency will be reduced (as low as 85%). Inevitably therefore a cartridge filter is used in conjunction with a cyclone solely to prevent discharge of the fine powder to the atmosphere.
Despite these limitations cyclones are popular for powder recovery, especially if colour changes are involved, as cleaning is comparatively easy compared with cartridge filters. Also in automated systems at high throughput rates, cyclones enable oversprayed material to be collected continuously at the required speed.
An additional advantage of cyclone recovery, with particular reference to colour change, is that due to frictional contact of powder particles, one with another and ‘bounce-back’, little or no adherence of powder particles occurs on the cyclone wall. This means that in many cases only the powder collection hopper need be thoroughly cleaned between colour changes. In many cyclones cones are removable and substitutes can be made as required if spares are held in stock. The contaminated cone can then be cleaned while the replacement is in operation.
The recovered powder is removed from the cyclone by means of a rotary valve and is then passed through a sieve to remove any agglomerates and foreign matter. The recovered powder is then blended with the virgin material in predetermined proportions.
As cyclone efficiency depends on maintaining a high particle size velocity through the cyclone, the cartridge filter following the cyclone must be designed to maintain the stability of the required velocity throughout the system.
The filter media should permit easy and frequent cleaning. The fabric filters which historically were used collected powder on the inside of the bag which does not fit with high production requirements as the bags have to be periodically cleaned down.
A superior method is to arrange a series of cartridge filters within a metal enclosure so that the powder collects on the outside of the filters and are then cleaned by a reverse compressed air flow which operates about every 30 seconds to provide an air counter current to the powder air flow. The total resistance of this multi cartridge system can be balanced with that of the cyclone so that the cyclone efficiency can be maintained.
In this technique the overspray powder from the application booth arrives at an enclosure containing a number of cartridge filters.
Typical filter materials:
The cartridge filters separate the powder from air by causing the powder/air mix to pass from the outside of the cartridge to the inside through a layer of filter material which retains the powder and allows the air to permeate through and on to the atmosphere.
As filtering continues the retained powder accumulates on the upstream side of the cartridge and forms a powder layer which, being permeable to air flow, increases filtration efficiency albeit at the expense of increased resistance to air flow. This powder layer must be continuously removed to control filter resistance. The retained powder particles are periodically removed from the outside of the filters by reverse air jet blowing. The high speed, high-pressure reversing jets operate for less than 0.2 seconds at 30 second intervals and, because they are applied to only part of the filter for a brief dwell time, they have no practical effect on the main air flow, thus giving a continuous filtration characteristic.
The powder particles released from the cartridge filter then drop into a hopper to be sieved and returned to the system.
Cartridge filters are extremely effective being up to 99% efficient. The degree of efficiency depends on the type of filter employed and the regularity of its cleaning.
Attached to the material discharge of either cyclone or cartridge filter recovery system must be a dust tight seal, ie. a rotary valve, with which the reclaimed powder can be metered after passing through an inline sieve into the virgin material.