selecting right air dryer

8/2/2019 Selecting Right Air Dryer

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TypesofAirDryers

1. Desiccant typical outlet dew point range: -40F to -100F

2. Refrigerated - typical outlet dew point range: 38F to 50F3. Deliquescent - typical outlet dew point range: 40F to 80F4. Membrane - typical outlet dew point range: 20F to 60F

5. Point of Use - typical outlet dew point range: -40F to -100F

UnderstandingAirDryersandHowTheyWorkA compressed air dryer is a device for removing water vapor

from compressed air. Compressed air dryers are commonly

found in a wide range of industrial and commercial facilities.

Most compressed air users understand that air dryers areessential in any air system. The question is not so muchwhether an air dryer is needed, but which is the best, most

cost-effective solution for a particular application.

The process of air compression concentrates atmosphericcontaminants, including water vapor. This raises the dew point of the compressed air

relative to free atmospheric air and leads to condensation within pipes as the compressed

air cools downstream of the compressor.

Excessive water in compressed air, either in the liquid or vapor phase, can cause a varietyof operational problems for users of compressed air. These include freezing of outdoor air

lines; corrosion of equipment; fouling of processes and products.

There are 3 forms of water in compressed air:

Liquid water

Aerosol (mist)

Vapor (gas)

Air-water separators remove liquid water aerosols. They remove up to 99% of theliquid water mist and none of the water vapor.

Water in Aerosol or Vapor form is more difficult to remove and requires the use of a

Compressed Air Dryer. For every 20F drop in compressed air temperature, themoisture holding capacity of air is reduced by 50%. Drying prevents liquid waterforming downstream where it can contaminate or damage the system causing

operating problems, costly maintenance, repairs and product spoilage.
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SelectionConsiderations

1. Flow Rate (scfm)

2. Operating Pressure (psig)3. Air Inlet & Dew PointTemperature4. Ambient Temperature

5. Application & Environment

1. Flow Rate: Determine the maximum capacity based on your system. This is normally inscfm and can be determined by your air compressor horsepower if you dont know. The

approximation of the air compressor horsepower times 4 will get your close to the flow rate

needed. (Example: 25 HP x 4 = 100 scfm flow)

2. Operating Pressure: Determine the minimum/maximum operating pressure based onyour system. Dryers are rated at 100 psig. If your pressure increases, the moisture load is

decreased, reducing the strain on the refrigeration system. As a rule of thumb, an increasein capacity of approximately 20% for every 25 psig increase from 100 psig can be

estimated. For every 25 psig decrease from 100 psig, your capacity is reduced by

approximately 20%.

3. Air Inlet & Dew Point Temperature: Determine the minimum/maximum operating air

inlet temperature based on your system. Then determine the dew point requirements forthe application, and then select which dryer type will produce that dew point. (desiccant,

refrigerated, deliquescent, or membrane). Choose a dew point temperature below thelowest ambient temperature the compressed air system will be exposed to. Consider air

airlines located in air-conditioned or unheated areas running underground or betweenbuildings.

To calculate your dew point temperature, take the lowest air temperature and lower it by20. (Example: Lowest ambient temp. 58F 20F = 38F dew point) Typical applicationscan utilize a refrigerated dryer. More critical applications that require low moisture in the

air-line require a desiccant dryer. (Desiccant dryers have % to 0.5% relative humidity in

the dry air). The lower the dew point, the dryer the air.

4. Ambient Temperature: Determine the minimum/maximum operating ambient air

temperature based on your system. During hot summer months, higher ambienttemperatures result in higher inlet temperatures and can result in overheating a

refrigerated dryer. If your ambient temperature is over 100F, this may exceed themaximum inlet temperature of dryer. Therefore, you may need to go to the next size dryeror select a high temperature dryer.


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5. Application & Environment: All of the above specifications will help narrow the gapbetween what type of dryer the system requires to fit its application and environment.Other factors like indoors/outdoors, weather, and location all can change these factors.

How the air is used in the application is the most critical part in selecting the type of dryer.

Refrigerated dryers have some moisture present in the outlet air, about 10% to 20%

relative humidity. Most applications can use a refrigerated dryer based on their needs.Desiccant dryers have very low moisture present in the outlet air, less then % relativehumidity. They are used in higher quality air instrumentation applications, which provide a

pure stream of air.

DesiccantDryersRegenerative desiccant dryers adsorb moisture on soliddesiccants, such as activated alumina, silica gel, or

molecular sieves. Two desiccantpacked towers operate

alternately; compressed air flows through the desiccantin one tower while the desiccant in the other tower isdried, or regenerated.

Regenerative desiccant dryers supply the lowest dew

point, usually 40F but as low as 100F. They are also

the most expensive, so specify a regenerative desiccantdryer only when the application calls formaximum dew

point performance.

Regenerative dryers are often used to protect

instrumentation and control systems, laboratoryequipment, and moisturesensitive processes and

materials. They also protect outdoor lines from freezing

in severe weather conditions.

There are three different methods to regenerate the saturated desiccant bed: by a purge ofdry air from the operating tower, by internal heaters, or by an external heat source. Here

are the pluses and minuses of each:

Heatless dryers use a purge of compressed air from the active tower, expanded toatmospheric pressure to dry the desiccant. It is a simple process, with the purge air

vented to atmosphere. The units require little maintenance and, because they do

not deal with the high voltages or high temperature, are quite safe. Heatless dryersare also the least expensive regenerative dryers, so they are a good choice for

smaller systems with sufficient excess air capacity. Unfortunately, the purgerequires 15 to 18% of air capacity, which means high operating costs in larger


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systems. Often the expense associated with lost compressor horsepower restrictsheatless dryers to systems smaller than approximately 2,000 scfm. This isincreasingly the case as energy costs rise and companies strive to minimize power

consumption.

Heated regenerative desiccant dryers use electrical resistance or steam heat todry the desiccant bed, so only about 4.5% purge air is needed to drive off thewater. These units initially cost more than heatless dryers because they have

additional components and more complex controls. Skilled personnel are alsorequired for maintenance. Heated dryers are predominantly used in the 500 3,000

scfm range. However, the specific application always dictates the best choice.

Blowerpurge compressed air dryers use an external blower to force heatedatmospheric air through the regenerating tower. This dries the bed withoutrequiring compressed air. However, blower units require the highest initial

investment among regenerative dryers. For two reasons, they are used almostexclusively in large systems above 3,000 scfm. One, the cost of purge air required

by the heatless type dryer is proportional to the size of the system, making theexpense of purging prohibitive in large installations. Two, conducting heat evenly

through an Internally Heated type dryer with a large desiccant bed is a technicalchallenge that requires many embedded heaters; to regenerate a large desiccantbed it is much simpler to evenly heat the bed with a blower.

The desiccant in heatless dryers lasts ten to fifteen years with an oil-free compressor andfive to ten years with a lubricated compressor and a good prefilter. Oil contamination

destroys the sensitive desiccant in about two to three weeks. Heated dryers tend toaccelerate the aging process of adsorbent desiccant. Typically, new desiccant material is

needed every three to five years.

RefrigeratedDryersRefrigerated dryers operate much like a household

refrigerator. They cool the compressed air sufficiently to

condense the entrained moisture and separate it from the airsupply. An advantage refrigerated dryers hold over

singletower deliquescent dryers is that they produce a

constant dew point regardless of fluctuations in inlettemperature. The dryers typically deliver a dew point of

either 38F or 50F depending on the design. For the bestperformance when operating in high ambient temperaturesor in dirty environments, a watercooled condenser is

recommended.


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Most refrigerated dryers run continuously; others cycle on and off. Continuous operationeliminates start/stop current inrush and extends compressor life. It also permits

instantaneous response to airflow demands to produce the required dew point.Cyclingtype refrigerated dryers do not respond as quickly. On the plus side, however,

they may use less electrical power during extended periods of low air consumption. Whileboth types are dependable, servicing generally requires a qualified technician with

refrigeration recovery/recycling equipment.

The major limitation with refrigerated dryers is their dew points. Because they operate by

condensing water in a heat exchanger, the lowest practical dew point reached is about38F. Any lower and there is the danger of water freezing and plugging or cracking theheat exchanger. While this limits them to indoor applications, refrigerated units are the

most commonly used dryers.

DeliquescentDryers(Note: deliquescent salt tablets are not desiccantadsorbents). Single tower deliquescent dryers are filled

with an absorbent deliquescent salt in tabular form. This

material attracts moisture and gradually dissolves ascompressed air passes through the deliquescent salttablet bed. Single tower dryers are simple, durable

devices that have no moving parts, consume no power,

and require no special control systems.

One benefit of single tower deliquescent dryers is that

operating costs are directly proportional to air use. Ifthere is no air demand, no desiccant is consumed. The

dryers are simple to maintain as well. Draining the liquid

out of the dryer vessel daily and periodically refilling itwith deliquescent tablets are the only requirements. Forthese reasons, single tower dryers are generally the most economical in terms of both

initial cost and operating expense.

Single tower deliquescent dryers produce a compensating dew point that is directly related

to the inletair temperature. In other words, they reduce the dew point by a fixed amount

below the inlet temperature. Outlet dew point, therefore, varies as inlet temperaturechanges. The deliquescent tablets are mixtures of hygroscopic salts compressed into

nuggets. Standard materials reduce the dew point by 20F; more aggressive desiccantsreduce the dew point more than 55F below the inlet temperature, but they are also moreexpensive.


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Single tower deliquescent dryers appear simple, but they encompass considerabletechnology, from ensuring optimum airflow through the vessel to the science of producinghigh quality desiccant tablets.

However, single tower deliquescent dryers are suitable for some applications where

refrigerated dryers will not work, particularly in harsh environments that quickly foul theaircooled or watercooled condensers on a refrigerated unit. Also, because no power is

required, deliquescent units are wellsuited for mobile applications and in hazardous areas

with electrical restrictions. Single tower deliquescent dryers can be used outdoors, as well

as with aircooled aftercoolers.

MembraneDryersMembrane dryer refers to a dehumidication membrane that removes

water vapor from compressed air.Typically, the compressed air is first filtered with a high quality coalescingfilter. This filter removes liquid water, oil and particulate from the

compressed air. The water vapor laden air then passes through the

center bore of hollow fibers in the membrane bundle.

At the same time, a small portion of the dry air product is redirected alongthe outside surface of the fibers to sweep out the water vapor which haspermeated the membrane. The moisture-laden sweep gas is then vented

to the atmosphere, and clean, dry air is supplied to the application.

The drying power of the membrane is controlled by varying thecompressed air flow rate and pressure. The membrane air dryers are designed to operate

continuously, 24 hours per day, 7 days per week. Membrane air dryers are quiet, reliable

and require no electricity to operate. The only maintenance required is changing theprefilter cartridge twice a year.

Point-of-UseDryersPoint-of-Use Dryers deliver clean dry air exactly where

needed. They offer a compact, lightweight design, simple plug& play installation and low maintenance and upkeep.

These air dryers can go 12,000 hours between desiccant

replacements, often paying for themselves in a matter ofmonths. The only maintenance required is changing the

prefilter cartridge often once a year, or depending on thesystem usage.


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