Preventing moisture problems in your dust …

1 This article discusses how moisture can hinder dustcollector performance and the causes of moisture indust collection systems. The article also explains howmonitoring airstream temperatures in your dustcollection system can help you prevent moistureproblems and describes methods for dryingcompressed air for reverse pulse-jet cleaning systems. moisture can be very disruptive to a dustcollection system. Dust collector filter mediatends to become plugged with material whenwet, particularly if the dust is hygroscopic and absorbswater. The dust-liquid mixture clogs the pores in thefilter media, creating an impermeable coating that canresist most types of filter cleaning systems. The pluggedfilter media resists airflow through the dust collector,causing the pressure drop across the media to rise andthe dust collector s process airflow rate to fabric bag filters exposed to intermittent mois-ture can sometimes regain permeability if the moistureis removed and the filters are allowed to dry.

2 With cel-lulose cartridge filters, however, moisture can ruin thecellulose filter media even if the dust isn t hygroscopic,causing the filter to permanently lose its permeability. moisture often goes undetected in a dust collection sys-tem until the damage has already occurred and beginsto reveal itself through high differential pressure read-ings and low airflow rates. Solving a dust collectormoisture problem typically involves providing a mech-anism to keep the moisture in vapor form or removeexcess moisture from the air before the vapor can con-dense into its liquid state. Before discussing solutionsto moisture problems , however, it s important to un-derstand what causes excess moisture in a dust collec-tor and how to identify the moisture s source. How to recognize an intermittent dust collectormoisture problemA dust collector can experience moisture problemseven if moisture is only occasionally present. Themoisture may appear only during certain times of theday, during certain seasons of the year, or as a result ofspecial occurrences affecting the process airstream.

3 Ifthe dust collector is located outside and exposed todaily and seasonal weather changes, moisture maycondense on the collector s inside surfaces when theoutside air temperature drops below the dew dew point is the temperature at which an air-vapormixture becomes saturated and incapable of holdingmore water vapor. The dew point temperature variesdepending on the amount of moisture in the air at anygiven time, and hot air can hold more water vapor thancooler air. As the air temperature falls below the dewpoint, water vapor begins to condense out of the airPreventing moisture problems in your dustcollectorBrian Mathews Scientific Dust appeared in March 2017 Copyright CSC Publishingand form liquid water droplets. Examples of thisphenomenon include fog forming in very humid air,dew forming on grass, and frost forming on a carwindshield. To determine whether moisture is intermittentlypresent in your dust collector, check inside the dirty airchamber when the possibility of moisture is high andthe dust collector isn t operating.

4 Inspect the insidewalls of the dust chamber for drops of moisture , wetdust clumps, or areas of rust formation. While wearingappropriate protective clothing, carefully removesome dust and place it onto a paper towel. Squeeze thepaper towel with the dust sample inside and see if anymoisture or oils transfer to the paper towel. If anynoticeable amount of moisture transfers, a moistureproblem is present and should be corrected. Don tperform this test if your dust is toxic, however. Toxicdust should only be handled by professionals trained inhandling such materials. And remember, while themoisture may be intermittent and can disappear beforeanyone notices, the damage may be permanent,especially to cellulose cartridge of moisture in dust collectorsKey factors that can cause moisture to be present in adust collector include:Hygroscopic dust being collected may behygroscopic and contain moisture . Sawdust, forexample, typically has a moisture content of 19 or or aerosol sprays may beintermittently or continuously added to the processairstream.

5 For example, machine cutting tools oftenrequire coolant sprays to prevent overheating duringoperation. A hood collecting dust generated by thecutting tool may also draw moisture from the coolantspray into the dust collection changes caused by winddirection or other weather conditions can increase themoisture content of the process airstream air temperature at thesystem s dust collection points may be considerablyhotter than in the dust collector, particularly if the dustcollection point is located over a heat source, the dustcollector is located outside the building in an area withcold weather conditions, or the process airstreamtravels a long distance between the dust collectionpoint and the dust air compressed air for thepulse-jet cleaning system may contain excessivemoisture. A pulse-jet cleaning system directsintermittent high-speed pulses of compressed air at theclean side of the dust collector filters to dislodge cakeddust and prolong filter life.

6 If the compressed aircontains excessive moisture , the filter media canbecome soaked, Preventing caked dust fromdislodging from the filter during the cleaning cycle andcausing problems for both bag and cartridge airstream temperatureYou should monitor your dust collection system sprocess airstream to identify times or events that causesudden changes to the airstream moisture . For example,if a dust collection hood protecting an additional factorymachine begins to add a cool secondary airstream to thesystem s warmer main airstream. At these moments ofchange, measure the temperature at different points inthe duct and in the collector to understand the change seffect on each part of the dust collection system. A largetemperature differential (approximately 15 F or more)between the dust collection point and the dust collectormay indicate a potential condensation risk. Thetemperature differential that will produce condensationwill vary, however, depending on the amount ofmoisture in the pulse-jet cleaning systems may also contributeto process airstream cooling.

7 The compressed cleaningair passes through a narrow orifice as it enters the dustcollector, causing the air to expand and cool. Forexample, if the compressed -air pressure is 85 psig andthe compressed -air temperature is 70 F, this expansionwill cause the airjet to cool to ?19 F as the air enters thedust , the low pressure created by the fast-movingairjet draws at least 4 cfm of surrounding process air intothe airjet for each cfm of compressed cleaning air. Thiswarmer process air mixes with the compressed air,raising the airjet temperature from ?19 F to about 52 additional heat is regained because of turbulencein the airjet, so the net effect is that the overall airjettemperature is 5 to 10 F cooler than the compressedair s original temperature. If this temperature is belowthe airjet s dew point, condensation can occur. Also, the cool airjet mixing with the warm processairstream can potentially lower the overall airtemperature in the dust collector. If the warm processairstream is carrying significant moisture and is closeto its dew point, then any significant cooling by theclean airjet could lower the temperature of the mixtureand cause condensation to appear inside the example, in some areas of North America, wintertemperatures can drop below 0 F.

8 If a dust collector isCopyright CSC Publishinglocated outside in those conditions, the temperature ofthe compressed air in the pulse-jet cleaning system willtend to closely mirror the ambient temperature. Theprocess airstream temperature, however, will likelyremain fairly constant and warm year round. A greatertemperature differential will exist between thecompressed air and the process air during the wintermonths, which may cause more moisturecondensation in the dust collector. Taking periodictemperature readings at the dust collection points andinside the dust collector at different times of day andseasons of the year will help you anticipate potentialcondensation moisture from compressed airA successful pulse-jet cleaning system requires clean,dry compressed air. A filter can be used to dry the airbut isn t nearly as effective as an air dryer. Thestandard method for drying saturated compressed airis to use an air dryer in addition to a filter. The twomain dryer types are: refrigerant dryersand refrigerant dryer, as shown inFigure 1, is a relatively low-cost drying solution thatuses a combination of heat exchangers and filters toremove moisture from compressed air.

9 The dryingprocess in a refrigerant dryer follows four , saturated air from the compressor passesthrough an air-to-air heat exchanger where it s cooledby the cold, dry air leaving the cooler air travels through an air-to-refrigerant heat exchanger where it s cooled to the dewpoint by refrigerant flowing through a constantlycirculating refrigeration refrigerated air passes through amulti-stage moisture filter (or separator) that removescondensation created during the refrigeration step also commonly includes a high-efficiencycold coalescing element immediately following themoisture separator to remove any oils or solid particleslarger than 3 cold, dry air flows back through theair-to-air heat exchanger and is warmed by theincoming hot, saturated air from the compressor. Thisreheating step increases the air volume and preventsthe downstream piping from desiccant dryer uses a combinationof filters and desiccant (or adsorbent) material, such asactivated alumina, molecular sieve, or silica gel, toremove moisture from compressed air.

10 The three typesof desiccant dryers are: heatless, heated, and heatedblower. In a heatless desiccant dryer, as shown inFigure 2, is the simplest of the desiccant dryers andtends to have a lower initial cost but a higher operatingcost than the other types. The drying process followsfour , saturated air from the compressortravels through a coalescing filter, which removes oiland some moisture . To prolong the life of the desiccantmaterial, a two-stage coalescing filter system is saturated air flows through an automaticdiverter valve that channels the air into one of twoFigure 1 Refrigerant dryerSaturated-airinletAir-to-air heatexchangerAir-to-refrigerantheat exchangerRefrigerationsystemRefrigerants upplyRefrigerantreturnMoistureseparatorC old coalescingelement(optional)Dry-airoutlet Figure 2 Heatless desiccant dryerSaturated airDry airRegeneration airSaturated-airinletCoalescingfilterReg enerationtowerDivertervalveAfter-filterD ry-airoutletExhaust valve(open)Exhaust valve(closed)DryingtowerDivertervalveCop yright CSC Publishingdrying towers (also called beds).