7/28/2019 Drying Your Compressed Air System Will Save Real Money

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Drying Your Compressed Air System Will Save RealMoney

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The cost of replacement parts, labor, standby inventory, and downtime can have a devastating effect on a plant's bottom line. Eliminating even one of them bydrying a system's compressed air will offset the cost of installing and operating the equipment.

When pneumatic components wear or become corroded as a result of moisture, they consume more compressed air - and lose energy efficiency. When thiswear or corrosion becomes great enough, components must be repaired or replaced - increasing operating expense.

Types Of Dryers

Dryers remove water vapor from the air, which lowers its dew point - the temperature to which aircan be cooled before water vapor begins to condense. In broadest terms, there are four basic typesof industrial compressed air dryers: deliquescent, regenerative desiccant, refrigeration, andmembrane.

Deliquescent dryers contain a chemical desiccant which absorbs moisture contained in the air,whether the moisture has already condensed or is still a vapor. The desiccant is consumed in thewater-removal process and must be replenished periodically. The solution that must be drainedfrom these dryers contains both liquid water and the deliquescent chemical, so disposal may be a

problem. Local environmental regulations should be checked before disposal of this solution.

Deliquescent dryers reduce the dew point of the air 15 to 25 F below the inlet air temperature. Ifthe incoming air has a dew point of 90 F, it will leave a deliquescent dryer with a dew point ofabout 65 F. Depending on operating conditions, some deliquescent dryers can produce dew pointsas low as -40 F; new deliquescent chemicals may produce even lower dew points.

Two important points: desiccant level should not be allowed to fall below that recommended by the dryer manufacturer, and inlet temperature should be limitedto 100 F or less to prevent excessive desiccant consumption.

Regenerative desiccant dryers remove water from air by adsorbing it on the surface of a microscopicallyporous desiccant, usually silica gel, activated alumina, or molecular sieve. The desiccant does not reactchemically with the water, so it need not be replenished. However, it must be dried, or regenerated,periodically.

Heatless regenerative dryers use two identical chambers filled with desiccant. As wet air moves up throughone chamber, a portion of the dry discharged air is diverted through the second chamber at close toatmospheric pressure, reactivating its desiccant. The moisture-laden purge air is vented to atmosphere.Some time later, air flow through the chambers is reversed.

Standard industry dewpoint ratings for these dryers is -40 F at pressure. By adjusting the flow rates andvolume of purge air, -100 F pressure dewpoints can be achieved. These dryers, as with all desiccant dryers,should be supplied with oil-free air. Oil will greatly reduce the life expectancy of the desiccant.

Heat regenerative dryers also use two identical chambers. In this type, however, air flows through onechamber until its desiccant has adsorbed all the moisture it can hold. Then air flow is diverted to the second

chamber. Internal heating elements or an external source of heat (steam or electricity) then dries thesaturated desiccant in the first chamber. Because desiccant's adsorption capacity decreases as temperatureincreases, the dried desiccant bed must be cooled from the temperature it reaches during regenerationbefore it can be used again. The regeneration cycle in these dryers usually lasts several hours - 75% heatingand 25% cooling.

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Regenerative desiccant dryers can produce pressure dew points as low as -100 F. The type of desiccant used has a definite e ffect on the final dew point.

Refrigeration dryers condense moisture from compressed air by cooling the air in heat exchangers chilled by refrigerants. These dryers produce dew points in arange from 35 to 50 F at system operating pressure.

Most 20-scfm and larger refrigeration dryers reheat the cooled air after it has been dried, usually by routing it through heat exchangers in contact with the hot

incoming air. Reheating the cooled air prevents condensation from forming on the exterior of air lines downstream from the dryer and also precools incomingair.

Standard refrigerating dryers should not be used where ambient temperature can drop below 40 F because lower temperature can freeze condensate. This willblock air passages and could damage the dryer's evaporator. Dryers may be equipped with heat tracing packages for operating in ambient temperatures as lowas 50 F.

Refrigeration dryers should not be operated in conditions where the incoming air and ambient air heat load is 15 to 20% of the rating - a 100-scfm rated dryer(100 F inlet and ambient) can freeze up if operated at 20 scfm and 40 F.

Refrigeration Dryer Types

Refrigeration dryers can be further classified into three types:

Tube-in-tube refrigeration dryers operate by cooling a mass of aluminum granules or bronze ribbon thatin turn cools the compressed air. As the tube-to-tube refrigeration dryer cycles, a thermometer in thegranule mass senses its temperature. As the temperature rises, a switch turns on the refrigeration unit.When the temperature drops to a cut-off point, refrigeration stops. These dryers are designed toproduce dew points of 35 or 50 F.

Water-chiller refrigeration dryers use a mass of water for cooling. An extra heat exchanger is needed tomaintain chilled water flow through the condenser, as well as a water pump. Dew points can bebetween 40 and 50 F. Water-chiller dryers cycle as they operate.

Direct-expansion refrigeration dryers use arefrigerant-to-air cooling process to producepressure dew points of 35 F below standardoperating conditions. (100 F temperature atcompressor inlet, 100 psig, 100 F ambient -from the NFPA standard). No recovery period isnecessary, so direct-expansion refrigerationdryers run continuously. The cost differencebetween cycling and continuous operation isdifficult to calculate. The difference in electrical

power consumption between cycling and non-cycling refrigerated dryers is outweighed by thevalue of continuous operation of the plant'spneumatic equipment.

Membrane-Type Dryers

Membrane-type dryers are gas-separation devices. They consist of miniature membrane tubes made of plastic materials compounded to allow water vapor topass through when there is a vapor pressure differential. They work as your lungs do, venting water vapor each time you exhale.

Typically this membrane material is formed into bundles of thousands of individual fibers from one end of the dryer to the other. Water vapor escapes throughthe walls of the fiber to a sweep chamber from where it is continually vented to atmosphere as a gas. A fraction of the dried air is routed through the sweepchamber to continuously purge and exhaust moisture vapor.

Industrial-grade membranes can be used for years to dry air continuously. They respond spontaneously to any change in inlet conditions. They perform attemperatures between 40 and 150 F (ambient or inlet), and handle pressures from about 60 to 300 psig. They will deliver a consistent outlet dew-pointreduction anywhere between these extremes. The inlet flow rate and pressure determine the outlet dew point suppression. In other words, membrane air dryers

deliver a consistent level of drying protection that follows the rise or fall of the inlet dew point temperature, and can easily be sized to follow the ISArecommended 20 F pressure dew point suppression below ambient. Outlet pressure dew points can also be selected as low as -50 F. Flow capacities arerelatively low, but modules can be installed in parallel for higher flows.

Prefilters mounted immediately upstream from the membrane dryer keep out liquids and solids to allow an almost unlimited service life. Because water vaporpasses right through the membrane material, it does not accumulate there, so membranes do not become saturated and never need to be regenerated.Membranes have no moving parts to wear out. They are non-electric and suitable for most hazardous locations. They require no RF shielding or protection. Theyuse no refrigerant gas or potentially dusty desiccants.

They make no noise. And, they can be mounted in any orientation. Their low-mass components are inherently vibration-resistant. Because they are static, inertdevices, they never need service or adjustment and don't require monitoring devices. Made of plastic and aluminum, they do not rust or corrode and don't need

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painting. They have almost no pressurized volume, so most pressure code restrictions do not apply.

Note: membrane gas separators will remove other gases too. Some membrane-type compressed air dryers can reduce outlet oxygen concentrations (or notpermeate oxygen at all). Consult the manufacturer to determine if membrane can be used for breathing air.

Importance of Dew Point

As pointed out earlier, wet air adds to plant operating expenses through the cost of:

repair parts,

repair labor,

product damage, and

production downtime.

The economic advantages of reducing or eliminating thesedetriments of moisture build a strong case for installing adryer. Once the decision to install a dryer has been reached,two questions arise: how dry must the air be, and what type ofdryer should be used?

The most important criterion in choosing an air dryer is thepressure dew point that it must produce. The required dewpoint of an air system determines how dry the air must be andto a great extent, which type of dryer to use. Dew point varieswith pressure. For example: the dew point conversion chart atleft shows that air at atmospheric pressure with a dew point of-12 F has a pressure dew point of 35 F at 100 psig. Dryermanufacturers may specify the dew point that a particularmodel can attain at atmospheric pressure or at a typicalsystem pressure, such as 100 psig. If performance is specifiedat atmospheric pressure, use a chart like ar right to find whatthe minimum dew point will be at the system's operatingpressure.

The required dew point varies with each application. Ifpreventing condensation in compressed air lines is the mainconcern, then the lowest ambient temperature to which airlines will be exposed will be the controlling factor. However, forsome applications, dew point requirements will be morestringent, possibly as low as -100 F at line pressure. Anexample might be the air used for spraying a powderysubstance. Even the slightest trace of moisture in such aircould condense and cause particles to stick together.

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If all the compressed air will be used inside a building where temperature is maintained at a stable level, then the required dew point can be fixed within a fewdegrees. But if some or all of the compressed air is subjected to outdoor temperature variations, the required dew point can change from day to day, or evenhour to hour.

Do not be too aggressive by estimating an unjustifiable margin for error. Stating a dew point much lower than that actually required wastes money. A rule-of-thumb margin for error is about 20 F maximum.

Extremely low dew points may be required at only a few isolated locations. If this is the case, consider using individual small heatless regenerative dryers atlocations requiring pressure dewpoints below 35 F. A less-expensive dryer to dry the air to less-stringent requirements can then be installed for the rest of theair system.

Evaluating Flow Capacity

An air dryer not only must dry compressed air to the required dew point, but also must be able to handle the required air flow without causing excessivepressure drop. Flow capacity of a dryer depends on:

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operating pressure,

inlet air temperature,

ambient air or cooling water temperature, and

required dew point.

When any of the above conditions changes, flow capacity of the dryer also changes. Dryer manufacturers can supply performance curves that show therelationship of their dryer's flow capacity to these four factors. Evaluating characteristics of the different types of dryers will help indicate which is best for aparticular application. This is where cost finally can be considered. Purchase price of the dryer is only one factor to evaluate when choosing an air dryer. Adeliquescent chemical dryer, for example, has a relatively low initial cost, but its chemical must be replaced periodically, adding to the operating cost. This costis offset somewhat because the deliquescent chemical dryer requires no external power source.

Other dryer types may cost more initially , but have lower operating costs because they can run for long periods with little or no maintenance required. It shouldbe clear, then, that cost analysis should be conducted based on manufacturers' specifications as they relate to an individual application's physical andeconomical requirements.

Installation and Maintenance

Location can affect how well an air dryer performs. The site for an air-cooled dryer should be well ventilated, so heat can be carried away, and readily accessibleto aid maintenance. The maximum ambient temperature for a refrigerated dryer is about 100 to 120 F. Higher temperatures prevent the dryer fromexchanging heat with its surroundings and keep it from operating properly. Dryers with water-cooled condensers can tolerate higher ambient temperaturebecause they transfer heat to the cooling water instead of to the surrounding environment. Refrigerant dryers, whether air- or water-cooled, should not beexposed to ambient temperature below 32 F unless optional low-ambient-temperature controls are installed.

If a deliquescent dryer is used in a central compressed-air system, bypass piping should be installed around the dryer to maintain air supply whenever the dryeris taken off line to add desiccant. There should also be no set of operating conditions that permit system pressure to drop low enough to allow high, turbulent

air flow through the dryer that might carry chemicals into system air lines. It is important to shut off the water in water-cooled aftercoolers when the air systemis shut down. A leak in the aftercooler could flood the deliquescent dryer and fill downstream piping with desiccant, making all pneumatic componentsinoperable.

Refrigeration and deliquescent dryers should be drained regularly, depending on the volume of liquid accumulated. Most refrigeration dryers have automaticdrains, at least as an option.

It should be noted that dryers remove water vapor, while filters remove liquid water. A good drying system always has a filter with an automatic drain installedupstream from the air dryer. Air dryers of all types are not stand-alone components. The cost of adequate prefilters, both particulate and oil coalescing, is awise investment to protect the more expensive dryers. Postfilters are necessary for several reasons. For refrigerated dryers, a coalescing filter can catch any oilfrom a refrigerant leak. For deliquescent dryers, a particulate filter downstream will catch any carryover of the corrosive desiccant. For regenerative dryers, a0.5-m postfilter is necessary to catch desiccant dust, which is common to all adsorptive desiccants.