Compressors and the Compressed Air System

Introduction

Compressed Air Usage

Compressed air is a major prime mover in many industries. People say air is free but compressed air is not free and is even costly. The compressed air provided for use in plants is designated ‘Instrument air’, ‘Plant air’ or ‘Process air’. Instrument air is cleaned and dried compressed air, used to prevent corrosion in some instruments. Plant air is compressed air but is usually neither cleaned nor dried, although most of the condensate and oil, are removed by a separator near the compressor, especially if adequate cooling can take place. Plant air is used for cleaning, power tools, blowing out vessels, etc: if used for air-powered tools exclusively, some suspended oil is advantageous for lubrication, although filter/lube units are usually installed in the air line to the tool.

Process air is compressed air, cleaned and dried, which may be used in the process stream for oxidising or agitation. The trend is to supply cleaned and dried air for both general process and instrument purposes. This avoids run-ning separate lines for process and instrument air.

Process and instrument air for some applications requires to have an oil content less than 10 parts per million. As almost all oily contaminants are present as extremely small droplets (less than 1 micron), mechanical filtration may be ineffective; adsorption equipment can efficiently remove the oil.

Costing of Compressed Air (Example)

(1) Single stage compression at 100 psig

- 25 BHP is consumed to produce 100 cfm.
- It consumes 150 units of power in every shift = 150 x 5.0 = 750 Rs / shift.

(2) Two stage up to 100 psig

- 19 BHP for 100 cfm
- = 114 units of power in a shift
- = 114 x 5 = 570 Rs /shift.

(3) For two stage compression upto 215 psig (15 kg/cm^2g)

- 28 BHP / 100 cfm of air
- 168 units / shift

The above does not include maintenance and spares, water cost for compressor headers, intercoolers and aftercoolers and operating costs for cooling water pumps.

Units of Air Flow

Air flow specified for any compressor is based on the air flow at suction conditions and not in compressed conditions.

Compressors

Compressors are the heart of the system. This is the equipment used to compress the atmospheric air to a higher pressure which in turn is used as a source of power in pneumatic tools or to operate various machines, or used as process/instrument air.




Two Main Types of Air Compressors are


(1) Positive Displacement Type

(2) Dynamic Compressors

1) The Positive Displacement Type Compressor is one in which successive volume of air/gas are sourced into a closed space and elevated to higher pressure. Hence the discharge is in batches. e.g. Reciprocating, Screw, Sliding Vane, etc.

2) Dynamic Compressors are fast moving rotary compressors in which high speed rotating parts accelarate the gas as it passes through the element. The velocity head is converted to the pressure head by the diffuser/casing. e.g. Centrifugal compressors/Axial flow.





Following are the Three Main Types of the Compressors Used in Indian Industry:


(1) Reciprocating Compressor:


• Most common
 
• Positive displacement type
 
• Lubricated and Non Lubricated versions are available.
 
• No economical substitutes in lower ranges.
 
• Available in 50 cfm to 20000 cfm
 
• Discharge pressure high as 60000 psig (4250 kg/cm2g)

(2) Screw Compressors :

Basically twin helical lobe in which both lobes rotate simultaneously to compress the air/gas between

• Available upto 20000 cfm
 
• Not for vey high discharge pressure
 
• In case of lube type, carry over of oil is very high.
 
• Screw compressors are best suited for relatively high flow and low pressures
 
• Advantages are economy with regard to

(a) lesser space requirement

(b) less expensive foundation

(c) Low power cost

(d) low maintenance cost.

(3) Centrifugal Compressors :


• Specially meant for large air flow.
 
• Maintencance is very low.
 
• Heat generated is very less.
 
• Air flows upto 650000 cfm and pressure up to 550 psig
 
• These are sensitive to dust/dirt.
 
• Care to be taken to provide dirt free air at suction.
 
• These are oil free.
 
• Impellers are rotated at very high speed 20000 to 60000 rpm.
 
• Consumes less space compared to equivalent reciprocating compressors.
 
• Power consumption is also less

Contaminants in Compressed Air Lines

(1) Water


• Corrodes internls surfaces of air piping, valves, fittings, pneumatic tools etc.
 
• May damage end products like food/pharama/paint etc.
 
• Air dryers must be installed at the outlet to remove the water.
 

• Instrument air lines dew point at line pressure should be at least 10 deg C below the minimum local recorded ambient temperture.

(2) Dirt


• Act as tiny missiles which attack the internal parts of an system such as valves/pipes/pneumatic cylinders, etc.
 
• Choke small nozzles
 
• Contaminates end products
 
• Use Filters
 
• The Maximum particle size acceptable in instrument air line is 3 micron.

(3) Oil :

This causes

• Coagulation of dirt
 
• Causes pressure drops and flow restrictions
 
• Contaminate end products
 
• Use Filters
 
• Instrument air lines oil content max 1 ppm acceptable limitations excluding non condensables.

Accessories in Compressed Air System

Intercoolers and Aftercoolers

• Cool the air between stages.
 
• Generally attached with dampner and Moisture separator.
 
• Perform as

(1) Dampening pulsations thus allowing smooth flow to the next stage avoiding uneven loads.
 
(2) Cooling the air thus ensuring the second stage receives cooled air. This results in low power consumption and cheaper grade material can be used.
 
(3) Cooling causes moisture to be separated. If this is allowed in next stage, it causes damage to piston ring/cylinders. Hence ADT is used.
 
• Available in air/water cooled versions.
 
• Water-cooled are preferred as efficiency is very high.
 
• Pressure drop of air side to be low.

Pulsation Vessels/Dampners

• Pressure vessels designed to provide a buffer in the compressed air line so as to reduce the air pulsation by the reciprocating action of compressors.
 
• Generally installed after every stage of compressor and after final discharge.

Moisture Separator and Oil Separators

Moisture separator: -

(a) Impingement Baffle type
 
(b) Centrifugal type
 
(c) Demister Pad type

Moisture separator should be located after the:

(i) outlet of Intercooler and aftercooler

(ii) bypass line of dryers

(iii) after bends in pipelines

(iv) at regular intervals.

Installation ADT is preferred afterwards.

Air Dryer

The air dryer shall be used to remove the moisture from the air. Air dryers are of following types:

(i) Refrigerated air dryer : (Moisture is removed by cooling the air) This type of dryer can be used up to 3 deg PDP (Pressure Dew Point)

(ii) Desiccant type air dryer : This type of the air dryer shall use the principle that the materials (Desiccant) like activated alumina, CMS, silica gel shall be used to remove the moisture from the air but these desiccant are required to be regenerated and based on the regeneration method these are categorised into the following types

• Purge Type
• No loss split flow (Electrical heater type)
• Heat of compression type
• Blower reactivated type

Air Receiver

The air receiver shall act as a surge vessel and act as a buffer and try to reduce the pressure fluctuation at the outlet as per IS standards. Its volume shall be 1/6 or 1/10 of the flow per hour. The accessories for the air receiver shall be ADT (Auto Drain Trap), pressure indicator, and pressure switch for low-pressure alarm, etc

Air Filter

The air filter shall be installed to remove dust particles the more than 5- micron size. If the compressor is lubricated then oil level at the outlet of air shall be in the range from 3-5 PPM and this shall reduce up to 1 or 0.1 PPM level by means of submicro glass filters. These filters are replaceable filters and to be replaced appox. after 5000 working hours. These filters shall have the DP indicator which shall show the clogging of the filter element.

Compressor House and Piping Layout

• If the compressor is handling a gas heavier than air, eliminate pits or trenches in the compressor house to avoid a suffocation or explosion risk.

• Provide air entry louvres if a compressor takes air from within a compressor house or other building.

• Provide maintenance facilities, including a lifting rail or access for mobile lifting equipment. Allow adequate floor space for use during maintenance. Additional access may be required for installation.

• Prevent transmission of vibration by providing a foundation for the compressor, separate from the compressor-house foundation.

• Consider the use of noise-absorbing materials and construction for a compressor house.

The vendor’s drawings should be examined to determine what auxiliary piping, valves and equipment covered in the design points are to be supplied with the compressor by the vendor:

• Install the compressor on a concrete pad or elevated structure. Piling is often a necessary part of the foundation. Large reciprocating compressors are often installed on an elevated structure to allow access to valves and provide space for piping. Provide a platform for operation and maintenance of such an installation.

• Keep piping clear of cylinders of reciprocating compressors and provide withdrawal space at cylinder heads.

• Use long-radius elbows or bends, not short-radius elbows or miters.

• If the compressor and the pressurised gas are cooled with water, route cooling water first to the aftercooler, then to the intercooler (for a two stage machine), and lastly to the cylinder jackets (or casing jacket, if present, in other types of compressor).

• Arrange an air compressor, associated equipment and piping so that water is able to drain continuously from the system.

• Pipe a separate trapped drain for each pressure stage. Ensure that the pressure into which any trap discharges will be lower than that of the system being drained—less the pressure drop over the trap and its associated piping. Do not pipe different pressure stages through separate check valves to a common trap.

• If a toxic or otherwise hazardous gas is to be compressed, vent possible shaft seal leakage to the suction line to avoid a dangerous atmosphere forming around the compressor.

• Do not overlook substantial space required for lube oil and seal oil control consoles for compressors.

Suction Piping for Air Compressors

(1) To reduce damage to a compressor by abrasion or corrosion, the air supply needs to be free from sotids and water (water in the air intake does not affect the operation of liquid-ring air compressors). Air intakes are best located where the atmosphere is uncontaminated by exhaust gases, industrial operations or by traffic.

(2) For efficiency, the air supply should be taken from the coolest source such as the shaded side of a building, keeping to building clearances.

(3) If the air supply is from outside the building, locate the suction point above the roofline, and away from walls to avoid excessive noise.

(4) Keep suction piping as short as possible. If a line is unavoidably long and condensate likely to form, provide a separator at the compressor intake.

(5) Provide a rain cover and screen.

(6) Filters must have the capacity to retain large quantities of impurities with a low pressure drop, and must be rugged enough to withstand pulsations from reciprocating compressors.

(7) Avoid low points in suction lines where moisture and dirt can collect. If low points cannot be avoided, provide a clean-out.

(8) If the suction line is taken from a header, take it from the top of the header to reduce the chance of drawing off moisture or sediment.

(9) A line-size isolating valve is required for the suction line if the suction line draws from a header shared with other compressors.

(10) Consider pickling or painting the inside of the suction piping to inhibit rust formation and lessen the risk of drawing rust into the compressor.

Discharge Piping for Air Compressors

(1) Discharge piping should be arranged to allow for thermal movement and draining. Anchors and braces should be provided to suppress vibration. The outflow from the after cooler will usually be wet (from the excess moisture in the suction air) and this water must be continually removed.

(2) Provide an unloading valve and bypass circuit connected upstream of the discharge isolating valve and downstream of the suction isolating valve, so as to ensure circulation through the compressor during unloading, and to permit equalising pressure in the compressor.

(3) A more efficient layout for compressed air lines is the ring main with auxiliary receivers placed as near as possible to points of heavy intermittent demand. The loop provides two-way air flow to any user.

(4) Compressed air system distribution lines and risers should originate from a separate outlet connection on air receiver and sloped back to air receiver.

(5) Individual connections (Service Branches) should be taken off from the top of headers.

(6) Compressors lines headers from vertical risers should be sloped down 1/16" per feet minimum in the direction of flow.

(7) Use butt welding fittings wherever practical.

(8) Maximum Equipment Nozzle loading to be checked by the piping engineer.

(9) Design the lines with velocities 8 to 12 m/s at discharge & 15 to 20 m/s at the headers.

(10) Install isolation valve and check valve in the discharge line near the compressor.

Summing Up

Air distribution piping design and layout is an important responsibility of piping engineering. The air distribution network is important for all process industry. Therefore it is important to see to the proper installation, functioning and efficient piping, piping element selection and routing. Layout and routing, supports, vent and drain, controlling element, safety of the system and safety to the utility points are important aspects and can be easily understood from this article.

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