air dehydrator(macmac).doc

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COMPRESSED AIR PLANT OPERATION AND MAINTENANCE

As an Engineman, you must be thoroughly aware of the operational and safety procedures you must use when you are operating or main-

aining a compressed air system. You must operate any air compressor or air system in strict compliance with approved operating procedurCompressed air is potentially very dangerous. Keep in mind that cleanliness is of greatest importance in all maintenance that requires the

pening of compressed air systems.

AFETY PRECAUTIONS

There are many hazards associated with pressurized air, particularly air under high pressure. Dangerous explosions have occurred in high-

ressure air systems because of DIESEL EFFECT. If a portion of an unpressurized system or component is suddenly and rapidly pressurize

with high-pressure air, a large amount of heat is produced. If the heat is excessive, the air may reach the ignition temperature of the impuritresent in the air and piping (oil, dust, and so forth). When the ignition temperature is reached, a violent explosion will occur as these

mpurities ignite. Ignition temperatures may also result from other causes. Some are rapid pressurization of a low-pressure dead end portion

he piping system, malfunctioning of compressor after-coolers, and leaky or dirty valves. Use every precaution to have only clean, dry air a

ompressor inlet.

Air compressor accidents have also been caused by improper maintenance procedures. These accidents can happen when you disconnect pa

nder pressure, replace parts with units designed for lower pressures, and install stop valves or check valves in improper locations. Impropeperating procedures have resulted in air compressor accidents with serious injury to personnel and damage to equipment.

You must take every possible step to minimize the hazards inherent in the process of compres-sion and in the use of compressed air. Strictl

ollow all safety precautions outlined in the NAVSEA technical manuals and in theNaval Ships Technical Manual, chapter 551. Some of

hese hazards and precautions are as follows:

. Explosions can be caused by dust-laden air or by oil vapor in the compressor or receiver. These explosions are triggered by abnormally h

emperatures, which may be caused by leaky or dirty valves, excessive pressurization rates, and faulty cooling systems.

. NEVER use distillate fuel or gasoline as a degreaser to clean compressor intake filters, cylinders, or air passages. These oils vaporize eas

nd will form a highly explosive mixture with the air under compression.

. Secure a compressor immediately if you observe that the temperature of the air being discharged from any stage exceeds the maximum

emperature specified.

. NEVER leave the compressor station after starting the compressor unless you are sure that the control, unloading, and governing devices

perating properly.

. If the compressor is to remain idle for any length of time or is in an exposed position in freezing weather, thoroughly drain the compress

irculating water system.

. Before working on a compressor, be sure the compressor is secured and cannot start automatically or accidentally. Completely blow dow

he compressor, and then secure all valves (including the control or unloading valves) between the compressor and the receiver. Follow theppropriate tag-out procedures for the compressor control valves and the isolation valves. When the gauges are in place, leave the pressure

auge cutout valves open at all times.

. When cutting air into the whistle, the siren, or a piece of machinery, be sure the supply line to the equipment has been properly drained o

moisture. When securing the supply of air to the affected equipment, be sure all drains are left open.

. Before disconnecting any part of an air system, be sure the part is not under pressure. Always leave the pressure gauge cutout valves opehe sections to which they are attached.

. Avoid rapid operation of manual valves. The heat of compression caused by a sudden flow of high pressure into an empty line or vessel

ause an explosion if oil or other impurities are present. Slowly crack open the valves until flow is noted, and keep the valves in this positio

ntil pressure on both sides has equalized. Keep the rate of pressure rise under 200 psi per second.

UMMARY

n this chapter, we have covered the various types of air compressors you will be required to operate. We have discussed how the air is

ompressed and the requirements and methods of producing oil-free air as well as how moisture is removed from the compressed air. We h

lso discussed some of the safety precautions you must use whenever you are operating or working on a compressed air plant or system. If

re unclear as to any of this information, go back and review this chapter before proceeding to chapter 15


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Desiccant Type of Air Dehydrator (Type II)

A desiccant is a drying agent. More practically, a desiccant is a substance with a high capacity to remove (adsorb) water or moisture. It also

high capacity to give off that moisture so that the desiccant can be reused. DESICCANT-TYPE DEHYDRATORS are basically composeylindrical flasks filled with desiccant. Compressed air system dehydrators use a pair of desiccant towers. One tower is in service dehydrati

he compressed air while the other is being reactivated. A desiccant tower is normally reactivated when dry, heated air is routed through the

ower in the direction opposite to that of the normal dehydration airflow. The hot air evaporates the collected moisture and carries it out of

ower to the atmosphere. The air for the purge cycle is heated by electrical heaters. When the tower that is reactivating has completed the

eactivation cycle, it is placed in service to dehydrate air, and the other tower is reactivated. Another type of desiccant dehydrator in use is t

Heat-Less dryer. These units require no electrical heaters or external sources of purge air. Figure 14-24 shows the compressed air entering

he bottom of the left tower (view A). The compressed air then passes upward through the desiccant, where it is dried to a very low moistur

ontent. The dry air passes through the check valve to the dry air outlet. Simultaneously, a small percentage of the dry air passes through thrifice between the towers and flows down through the right tower. This dry air reactivates the desiccant

Figure 14-24.-Heat-Less desiccant dehydrator.

nd passes out through the purge exhaust. At the end of the cycle, the towers are automatically reversed, as shown in view B.

Refrigeration and Desiccant Type of Air Dehydrator (Type III)

ome installations may use a combination of refrigeration and absorption for moisture removal. Hot wet air from the compressor first enter

efrigeration-type dehydrator where low-temperature R-12 removes heat from the airstream and condenses water vapor from the air. The coartially-dried air then flows into a desiccant-type dehydrator, where the desiccant absorbs additional moisture from the air.

A compressed air dryer is a device for removing water vapor from compressed air. Compressed air dryers are commonly found in a wide

ange of industrial and commercial facilities.

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

elative to free atmospheric air and leads to condensation within pipes as the compressed air cools downstream of the compressor.

Excessive water in compressed air, in either the liquid or vapor phase, can cause a variety of operational problems for users of compressed

These include freezing of outdoor air lines, corrosion in piping and equipment, malfunctioning of pneumatic process control instruments,

ouling of processes and products, and more.

There are various types of compressed air dryers. [1]Their performance characteristics are typically defined by the dew point.

Regenerative desiccant dryers, often called "regens" or "twin tower" dryers

Refrigerated dryers

Deliquescent dryers [2]
http://en.wikipedia.org/wiki/Air_dryer#cite_note-0http://en.wikipedia.org/wiki/Air_dryer#cite_note-0http://en.wikipedia.org/wiki/Air_dryer#cite_note-1http://en.wikipedia.org/wiki/Air_dryer#cite_note-1http://en.wikipedia.org/wiki/Air_dryer#cite_note-0


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Membrane dryers

Water vapor is removed from compressed air to prevent condensation from occurring and to prevent moisture from interfering in sensitive

ndustrial processes.

ontents

1 Characteristics

2 Refrigerated dryer

3 Deliquescent dryer

4 Desiccant dryer

5 Membrane dryer

6 Usage

7 External links

Characteristics

A regenerative desiccant dryer typically delivers adew pointof between -40F(-40C) and -100F (-73C)

A refrigerated dryer delivers a dew point not lower than approximately 35F (2C)

A deliquescent dryer delivers a dew point suppression that fluctuates with air temperature. Typically this suppression is 20F below

compressed air temperature.

Refrigerated dryer

Refrigeration dryers employ twoheat exchangers, one for air-to-air and one for air-to-refrigeration. However, there is also a single TRISAB

eat exchanger that combines both functions. Thecompressorsused in this type of dryer are usually of the hermetic type and the most com

as used is R-134a. The goal of having two heat exchangers is that the cold outgoing air cools down the hot incoming air and reduces the s

f compressor required. At the same time the increase in the temperature of outgoing air prevents re-condensation.

Most manufacturers produce "cycling dryers". These store a cold mass that cools the air when the compressor is OFF. When the refrigeratiompressor runs, the large mass takes much longer to cool, so the compressor runs longer, and stays OFF longer. These units operate at low

ew points, typically in the 35 to 40 degree F range. When selected with the optional "cold coalescing filter", these units can deliver

ompressed air with lower dew points.

ome manufacturers are marketing compressors with built-in refrigeration dryers, but these have had a mixed acceptance in the market.

Commonly a coalesing prefilter is installed immediately upstream of a refrigerated dryer to remove lubricating oil and other contaminants tave the potential to foul the dryer's heat exchangers.

Deliquescent dryer

A deliquescent dryer typically consists of a pressure vessel filled with a hygroscopic media that absorbs water vapor. The media gradually

issolvesor deliquescesto form a solution at the base of the pressure vessel. The liquid must be regularly drained from the vessel and nmedia must be added. The media is usually in tablet or briquette form.

Deliquescent dryers have no moving parts and don't require electrical power for operation. Common applications therefore often involve

emote, hazardous, or mobile worksites. Deliquescent dryers are used for removing water vapor from compressed air, natural gas, and wast

ases such as landfill gasand digester gas.

The performance of a deliquescent dryer, as measured by outlet dew point, is highly dependent on the temperature of the air or gas being

rocessed, with cooler temperatures resulting in better performance.

Desiccant dryer
http://en.wikipedia.org/wiki/Air_dryer#Membrane_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Characteristicshttp://en.wikipedia.org/wiki/Air_dryer#Refrigerated_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Deliquescent_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Desiccant_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Membrane_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Usagehttp://en.wikipedia.org/wiki/Air_dryer#External_linkshttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/R-134ahttp://en.wikipedia.org/wiki/R-134ahttp://en.wikipedia.org/wiki/Landfill_gashttp://en.wikipedia.org/wiki/Landfill_gashttp://en.wikipedia.org/wiki/Digester_gashttp://en.wikipedia.org/wiki/Digester_gashttp://en.wikipedia.org/wiki/Air_dryer#Membrane_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Characteristicshttp://en.wikipedia.org/wiki/Air_dryer#Refrigerated_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Deliquescent_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Desiccant_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Membrane_dryerhttp://en.wikipedia.org/wiki/Air_dryer#Usagehttp://en.wikipedia.org/wiki/Air_dryer#External_linkshttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/R-134ahttp://en.wikipedia.org/wiki/Landfill_gashttp://en.wikipedia.org/wiki/Digester_gas


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The term "desiccant dryer" refers to a broad class of dryers. Other terms commonly used are regenerative dryer and twin tower dryer, and t

esser extent absorption dryer.

The compressed air is passed through a pressure vessel filled with an absorbent media such as activated alumina, silica gel, molecular sievetherdesiccantmaterial. The desiccant can bring the dewpoint of the water vapor in the air down to -40 degrees C (-40 degrees F) or below

This means that the air will not condense (deposition) water until it is cooled to -40 degrees C (-40 degrees F). In practice, two cylinders w

esiccant are used; one is drying the air, while the other vessel is being regenerated. The switching of the vessels and the regeneration sequ

s typically done automatically via solenoid operated valves.

The regeneration of the desiccant vessel can be during three different methods:

Heatless "pressure-swing" drying, which uses part of the dry compressed air coming from the other vessel to dry the desiccant in t

vessel being regenerated at lower pressure.

Heated dryer, which uses a hot air blower, so there is no loss of compressed air.

Heat of compression, which can only be used with an oilfree compressor.

Membrane dryer

Membrane dryer refers to a dehumidication membrane that removes water vapor from compressed air.

Typically, the compressed air is first filtered with a high quality coalescing filter. This filter removes liquid water, oil and particulate from tompressed air. The water vapor laden air then passes through the center bore of hollow fibers in the membrane bundle. At the same time, a

mall portion of the dry air product is redirected along the outside surface of the fibers to sweep out the water vapor which has permeated tmembrane. The moisture-laden sweep gas is then vented to the atmosphere, and clean, dry air is supplied to the application. The membrane

ryers are designed to operate continuously, 24 hours per day, 7 days per week. Membrane air dryers are quiet, reliable and require no

lectricity to operate.

ome dryers are non-porous which means they only permeate water vapor. Non-porous membranes' drying power is only a function of flowate, pressure. The sweep flow is strictly controlled by an orifice and is not a function of temperature.

Porous membranes are modified nitrogen membranes and pass air as well, usually changing the composition of the compressed air by redu

he oxygen content. The only maintenance required is changing the prefilter cartridge twice a year. The performance of porous membranes

ependent on temperature as well as operating pressure and flow.

Membrane air dryers depress the incoming dewpoint. Most dryers have a challenge air dewpoint and pressure specification. So if the inletewpoint is lower than the specified challenge air then the outlet dewpoint is even lower than specified. For example, a dryer could be rated-40F dewpoint with a challenge of +70F dewpoint and 100 psig. If the incoming air has an inlet dewpoint of only 32F, the outlet dewpoin

will be somewhat less. Pressure also plays a role. If the pressure is higher than the rated specification then the outlet dewpoint will be lowe

This lowering of the outlet dewpoint is due to the longer residence time that the air has inside the membrane. Using the spec above, an

perating pressure of 120 psig will yield a lower outlet dewpoint than specified. The extent of the improvement is dependent on the nature

he membrane and could vary among manufacturers.

Dewpoint suppression is not a feature of refrigerated dryers as they chill the incoming air to a fixed temperature, usually 35F. So a lower

ewpoint challenge will not yield a dewpoint lower than 35F. Membrane air dryers are used in pneumatic components, spray painting, lase

lenum purge, air bearings, air spindles, medical equipment, air guns and pneumatic brakes for vehicles & trains.

Usage

Drying air for use in commercial or industrial processes that demand dry air:

o Telecomm industry (pressurizes its underground cables to repel moisture and avoid shorts)

o Painting

o Pneumatic tools

o Pneumatic control systems

o Feed air forZeolitetype OxygenandNitrogengenerators

Truck and Train Air brake systems.
http://en.wikipedia.org/wiki/Silica_gelhttp://en.wikipedia.org/wiki/Desiccanthttp://en.wikipedia.org/wiki/Desiccanthttp://en.wikipedia.org/wiki/Deposition_(phase_transition)http://en.wikipedia.org/wiki/Coalescerhttp://en.wikipedia.org/wiki/Nitrogen_membranehttp://en.wikipedia.org/wiki/Zeolitehttp://en.wikipedia.org/wiki/Zeolitehttp://en.wikipedia.org/wiki/Zeolitehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Silica_gelhttp://en.wikipedia.org/wiki/Desiccanthttp://en.wikipedia.org/wiki/Deposition_(phase_transition)http://en.wikipedia.org/wiki/Coalescerhttp://en.wikipedia.org/wiki/Nitrogen_membranehttp://en.wikipedia.org/wiki/Zeolitehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Nitrogen


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