Know-a-Compressor » Difference between screw and reciprocating air compressors

 

General difference between Screw and Reciprocating (Piston Type) Air Compressors

by Rajkaran Chawla

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 Screw Air Compressors
Reciprocating (Piston Type) Air Compressors 
Rotary screw air compressors use two meshing helical screws, known as rotors or air-end, to compress the air.
Helical-Screws 
Reciprocating (Piston Type) air compressors use pistons driven by a crankshaft to compress the air.
Piston-Open-Compressor 

As compressor has only two moving parts which are not in contact with each other so no wear & tear, hence there is less possibility of breakdown. [*1]
(In some brands the screws do touch each other but it is only a contact touch, no friction involved.)

There are a lot of moving parts like piston, piston ring, crankshaft, connecting rod, valves, etc. causing wear & tear. Hence lesser reliability & more chances of breakdown. [*1]
The internal air-end (screw component) operating temperature is around 80 to 99 Deg C. This is possible because of no friction between the screws and oil / lubricant forms a non-wearing seal between rotors and casing thereby removing heat through a thermostatically controlled fluid circuit.
The internal temperature at which the pistons operate is around 150 to 200 Deg C. This is because of the friction generated by the contact of piston rings against the cylinder walls.
These above reasons make the screw compressors suitable for 24 hours (continuous operation). Therefore they can be sized exactly close to the CFM (Cubic Feet per Minute - Air Flow) requirement.

These above reasons make the reciprocating compressors suitable for intermittent use only. Therefore they need to be sized at least 50% higher than the actual required CFM (Air Flow).

Sizing them 50% higher allows the compressor to cycle from time to time and will prevent excessive heat generation and wear and tear.
As there is only one piston type inlet valve Carbonation is less hence shutdown for cleaning the valves is not required.This has around 8 or more valves in total & hence will require frequent cleaning due to high Carbonation. High Maintenance.
As wear & tear is very low, Stocking of spare parts is not necessary. Hence less spares inventory. 
High wear & tear of moving parts like piston rings, valves etc. Stock of spares is a must.
Space required is very less resulting in a compact machine. Now almost all brands are providing machines with inbuilt air dryer or air dryer mounted on the same horizontal air receiver / tank thereby making the machines even more compact.Space required is more. None of brands are offering air dryer mounted on the same air receiver / tank.

In a screw compressor the screws (air-end) rotate in one direction thereby causing less noise, heat and vibration. Furthermore Screw Compressor is skid mounted therefore foundation is not required. No additional cost of foundation.

These compressors require foundation as well as grouting for large machines due to high unbalanced forces & high vibration. Here the pistons rise and fall (vertical machine) or move back and forth in the cylinder thereby causing too much vibration. Hence it involves additional foundation cost.
Screw compressors are available in silenced version [noise levels are within 75 dB(A)±3 which can be kept in a shop floor near the point of application. Piping cost from compressor to application can be less.

These compressors require separate compressor room or a sound isolating box as noise levels could go as high as 100-dB (A). So keeping it near the point of application is a disturbance.

Therefore the Extra piping cost is involved.
Low Oil Content in outlet air as these machines have a 3 stage oil separation system. Oil Content is usually less than 3 ppm (particles per million).No oil separation system therefore oil content in outlet air is relatively much higher compared to screw compressors. This oil content increases when piston rings and oil rings wear out allowing oil to move up to the compression side.

The microprocessor control & monitoring system which is on the compressor panel provides prior-indications of any service requirements, so that the maintenance schedule can be planned.

Microprocessors have various functionalities like Running Hour Meter, High Temperature trip, Serviceability alarm, etc.

(Please check the functionalities before purchasing in their Scope of Supply)

Generally No PLC Control available. The PLC based system will be at an additional cost & will not be on the compressor but near the compressor, which will occupy more Space.

Some brands are giving PLC control as standard in Large Machines.

(Please check carefully before purchasing if any microprocessor is included in their Scope of Supply)

As all the indications will be given by the Microprocessor. Manual Monitoring is not a must. A periodic check-up will do.As wear & tear is more, manual monitoring is a must in case of No PLC control on the machine.
In the Screw Compressor there is no loss / very minor loss in Capacity over a period of time.In reciprocating compressor there is De-ration in Capacity over a period of time due to wear and tear in cylinder crank case, piston rings, pistons and valves.
Cooling system- As screw compressors can be offered as AIR-COOLED, WATER cooling is not required in most of the cases.Small compressors are air cooled but large machines from 30HP and above are only Water Cooled Systems- As these compressors are only water cooled, cooling water is required which adds to operating / maintenance / capital cost. 
Reliability: Reliability of this machine is more due to less maintenance. No standby required. [*1]Reliability: In case of small machines lower than 30 HP the reliability is quite good but in larger machines it is Poor reliability, heavy maintenance required. Always standby recommended. [*1]
CFM (Air Flow) is only measured as FAD (Free Air Delivery).Some brands may mention the Piston Displacement as CFM which is a mathematical calculation and the Free Air Delivery is about 20 to 25% less than this figure.
Maximum pressure that some brands provide with this technology is 14 Bar (g). If more is required then only option available is Reciprocating Piston Type Air Compressors.Maximum pressure that some brands provide with this technology is 200 Bar (g). There are High Pressure Compressors from 35 to 70 Bar (g) and then there are breathing air compressors which use this technology to give high pressure for deep sea divers at around 200 Bar (g).
Recommended to purchase Screw Compressors at 20HP and above. But this depends on the application and how you want to use it.Recommended to purchase these type of compressors lower than 20HP.
Your existing machine operators / maintenance personnel in plant may not know how to maintain this. We at CompressorWale are planning to bridge this gap with Videos and description in Hindi for your maintenance personnel to view and learn the basics in maintaining these machines. Till then a per day visit charge of a service engineer for Screw Compressor service is quite high.These machines are basic and are widely used therefore most of the maintenance personnel can figure this machine out with ease and maintain the machines themselves. No cost of outside personnel thereby saving on labor charges.
Energy Efficiency: More air flow at less power consumed. The difference is more beneficial and viable at 20HP and above machines.Energy efficiency is less of above 20HP models.
Initial cost is very High. Around twice the cost of reciprocating piston compressors.Half the initial cost compared to Screw Air compressors.
Single Stage machines are more commonly used. Two Stage machines are available only above 100HP and in some Oil Free machines. Single Stage machines are available only up-to 5HP. Two Stage machines are more commonly used.
Generally Screw Compressors have either direct drive or Poly-Belt drive, which eliminates transmission loss or has very less transmission loss.Generally belt driven (V-belt) which has transmission loss as high as 5%.

 [*1] – Breakdown is possible in both the Screw and Reciprocating Compressors if Proactive Maintenance is not observed as per the guidelines mentioned in the manual.

 

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