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Utilities - Instrument Air
September 18, 2002

Introduction
Air Quality 

        Dew Point
        Oil Content
        Particulate
        Temperature
Control Philosophy
Compressor
Inlet Air Filters
After Coolers
Air Receiver
Oil Filters

Air Dryers
        Heatless Regenerative Desiccant Dryers
        Heat Regenerative Desiccant Dryers
        Refrigerated Dryers

After Filters
Associated Links

Introduction

The instrument air system supplies dry and oil free air to the instrument air system of the sulphuric acid plant.  A typical instrument air system consist of the following components:

The quality of instrument air is what distinguishes it from a compressed or service air system.

Air Quality 

The quality of the air is important to ensure that instrumentation will function properly and reliably.   The most important parameters in specifying air quality are:

The Instrument Society of America sets quality standards for instrument air in ISA S7.3.

Dew Point

The dew point temperature or saturation temperature can be defined as the temperature at which condensation or moisture begins when moist air is cooled.  This temperature can be determined by observing, either visually or by a photoelectric cell, condensation of water vapour on a polished metal surface that is slowly being chilled.  Indirect methods for measuring moisture include wet and dry bulb psychometry, adiabatic expansion, electrical resistivity and methods that depend on the hygroscopic properties of various materials. 

When an air-water vapour mixture is compressed: 

          1. its ability to hold water is decreased;

          2. water vapour will start to condense at a higher temperature; and

          3. the dew point of the mixture at elevated pressure will be higher than that of the same mixture at atmospheric pressure.

The dew point required for an instrument air system is generally set by the minimum ambient temperature to which the instrument air system will be exposed.  The dew point at line pressure shall be at least 10°C (18°F) below the minimum local recorded ambient temperature.

In cold climates, a -40°C dew point is typically used.  In warmer climates the dew point temperature can be increased appropriately.

Under no circumstances should the dew point at line pressure exceed 2°C (35°F).

In is important that the dew point temperature be specified at the operating pressure of the instrument air system and not atmospheric pressure.  This will ensure that no condensation of water will occur anywhere in the system.

Oil Content

Oil free air is generally required for an instrument air system.  In an oil free system, an oil content of less than 0.01 ppm is generally specified.

Where a instrument air consumer requires air with oil for lubrication purposes, individual oilers are installed specifically for the consumer.

Particulate

Particulate in the air may plug the small passages in some valves and instruments.  Instrument air is generally filtered to remove particulate matter to a level less than 0.02 mg/m³ (size 100% < 0.01 micron). 

Temperature

When any gas is compressed its temperature will increase.  For and instrument air system this increase in temperature is undesirable.  After coolers are generally provided immediately after the compressor to cool the air and remove the heat of compression.  A typical outlet temperature is a maximum of 40°C.

Control Philosophy

In some systems, two compressors are provided to ensure greater reliability and availability.  One compressor unit is designated the operating unit while the other is placed on automatic standby.  If the operating unit should fail, the standby unit comes on line to ensure uninterrupted supply of instrument air. 

A typical means of controlling when a compressor load/unloads is as follows::

                                        Duty                  Standby

                                    Compressor       Compressor

 

High/High Pressure            Unloads                   -

High Pressure                         -                    Unloads

Low Pressure                      Loads                    -

Low/Low Pressure                   -                     Loads 

 

Compressor

Air compressors generally come as an integrated unit consisting of the following components:

a)      Compressor

b)      Inlet air filter

c)      Drive motor

d)      Lubrication system

e)      Oil separator

f)        After coolers

g)      Control system and panel

 

All components are housed in an enclosure to minimize noise.

Compressors must be sized to deliver the maximum quantity of air at the specified pressure.  Various types of compressors are available, including:

Compressors of the non-lubricated type are commonly used to prevent problems with oil or lubricant contamination of the air.  Where lubricated compressors are used, provision must be made to adequately remove these contaminants from the air.

Inlet Air Filters

Generally, the instrument air system intake should be located outside and in the coolest area.  The lower the air intake temperature, the greater the compressor efficiency.  If contamination exists in the compressor intake area, the air should be taken from an elevated or remote location free from contamination.

Inlet air filters are essential to remove grit and dust that are present in practically all plants.  Removal of these substances is important in order to achieve the desired air quality and to protect the compressor, since any solids in the air will cause wear on the moving parts.  These filters must be regularly inspected and replaced.

After Coolers

The compression of air results in a rise in the temperature of the air.   After the compressor the air is passed through an after cooler to reduce its temperature.  Upon cooling the air temperature may be below its dew point, so a mechanical moisture separator is provided.

After coolers are generally water or air cooled.  Water-cooled after coolers are generally sized to achieve a 5.5°C to 8.5°C (10°F to 15°F) approach to the cooling water supply temperature.  Air-cooled after coolers are generally sized to achieve a 14°C to 17°C (25°F to 30°F) approach to the ambient air temperature.

Air Receiver

The air receiver is sized to store enough air to handle system demand surges, to allow time for moisture separation and to provide a reserve for orderly or emergency shutdown.  The receiver pressure is usually used for loading and unloading the compressor.  A receiver that is too small will result in a compressor that is constantly loaded which will shorten the life of the equipment.

The receiver ambient temperature is typically lower that the dew point of the air entering the receiver.  This causes moisture to condense inside the receiver.   An automatic liquid drain device must always be furnished on an air receiver to dispose of any moisture.airfilter.jpg (8528 bytes)

Oil Filters

Oil and lubricants are major contaminants in instrument air systems.   They enter the air stream in liquid, aerosol (mist) and vapour form.  Oil and lubricants are most troublesome when they combine with moisture and solid contaminants to form a sludge that can clog instruments.  Oil can also contaminate desiccants in air dryers.

Oil filters are generally installed after the air receiver and before the air dryers.  These filters will generally be installed in a duplex fashion allowing one filter to be maintained while the other is in operation.

Air Dryers

Water in an instrument air system can cause rust in distribution pipes or freeze in exposed outdoor lines.  It is important to dry the air prior to distribution to prevent problems in the system.

Air drying is generally done in one of two ways:

a)   Heatless Regenerative Desiccant Dryers

b)   Heat Regenerative Desiccant Dryers

c)   Refrigerated Dryersairdryer.GIF (6618 bytes)

Heatless Regenerative Desiccant Dryers

Heatless regenerative dryers utilize the principle that expansion of a compressed gas from a high pressure to a low pressure reduces the partial pressure of all constituents in a proportion equal to the ratio of absolute pressure change.  Thus, a water saturated adsorbent will give up its water vapour to a lower vapour pressure environment.  In practice this is achieved by taking a portion of the high pressure dry air and expanding it to essentially atmospheric pressure and passing it through the desiccant bed to strip off the water content.

The disadvantage of heatless type regenerative desiccant dryers is the loss of a portion of dry air used to regenerated the desiccant bed.

The heatless dryer works by passing compressed air through a tower filled with desiccant.  Moisture from the air stream is adsorbed onto the desiccant thereby producing dry outlet air.  While one tower is on line performing the drying function, the second tower is off line being regenerated by a portion (approximately 10 to 20%) of the dry discharge air.

This operation is controlled by valves (V1 and V2) which divert the flow of air to the on-line tower and away from the regenerating tower.  At the same time, a purge valve (V3 or V4) allows the regenerating air flow entering the off-line tower to escape and thereby purge the moisture adsorbed from the desiccant to the atmosphere.airdryer2.jpg (12527 bytes)

Once the regeneration process is complete the off-line tower is re-pressurized to prevent fluidization of the desiccant bed.  The purge valve (V3 or V4) will close so that the off-line tower is allowed to re-pressurize.

After re-pressurization is complete, the flow valves (V1 and V2) will divert the drying air flow to the previously off-line tower.  Simultaneously, a purge valve (V3 or V4) will open allowing the previously on-line tower to de-pressurize and begin regeneration.  The sequence is repeated continuously ensuring a constant supply of dry air to the system.

Heat Regenerative Dryers

Heat regenerative dryers utilize the principle that a hot gas can hold a larger amount of water vapour than a gas at a lower temperature.  Thus, a water saturated adsorbent will give up its water to a gas at a higher temperature.  Hot air, heated by an electric or steam heater passes through the desiccant bed removing the water from the adsorbent and regenerating the bed.airdryer3.jpg (12317 bytes)

Refrigerated Dryers

Refrigeration type dryers utilize the principle that the lower the dry bulb temperature of the air, the lower the water vapour content.  Typical, refrigeration type dryers are capable of achieving a dew point temperature of -1.7°C.

After Filters

The after filter provides final cleaning of the compressed air stream by removing particulate matter from the dryer discharge.  After filters are required on all instrument air systems, particularly desiccant-type dryers to prevent desiccant dust from passing downstream.  These filters will generally be installed in a duplex fashion allowing one filter to be maintained while the other is in operation.