ROTARY SCREW COMPRESSOR

The rotary screw compressors consists of two mating rotating screws, one helical lobed male and one helical grooved female, a housing with helical timing gear to maintain clearances. 

ROTARY SCREW COMPRESSOR

SECTION – 6.1

ROTARY SCREW COMPRESSOR

6.1.1 Construction of Rotary Screw Compressor

The rotary screw compressors (figure 6.1) consists of two mating rotating screws, one helical lobed male and one helical grooved female, a housing with helical timing gear to maintain clearances.

Figure 6.1 Screw compressor with timing gears

The compressor consists of

Male rotor                                A

Female rotor                             B

Timing gear                             C

Rotor bearings                         D

Drive shaft                               E

Compressor casing                   G

Figure 6.2 Rotor profile of the four – lobe male and the six lobe female

6.1.2 The Function of the Compressor Components

3. The two rotors: Rotates to perform pumping, the male lobe is the driver. If the timing gears were used, the power is transimited via timing gears.
4. Timing gears: transmits the power from the driving shaft (power shaft) to the driven shaft (idler shaft) without direct contact between the two rotors. The timing gears maintain proper clearances between the two rotors and avoid interference between them.
5. Rotor bearings: To carry the rotors and keep them in their correct positions.

It could out – friction bearings for small and medium sizes.

Slide surface bearings are standard in the high pressure machines

Thrust bearings are utilized to center the rotors in the housing.

4. Shaft seals: It is necessary to seal the shafts to contain the air or gas handled. Several types are shown in figures 6.3 and 6.4

The simple labyrinth is for air in low pressure compressors and floating ring controlled labyrinth or carbon ring for high pressure.

Figure 6.3 Low pressure haft sales

Figure 6.4 High pressure shaft seals

SECTION – 6.2

HOW IT WORKS?

6.2.1 How it Works?

The helical screw compressors are divided into two groups.  They are:

• Dry helical screw compressors.
• Rotary helical screw oil injected compressors.

Both of them from mechanical point of view are similar with minor changes.

Rotary screw compressors employ two intermeshing rotors with helical lobes.

The inlet is located at one end of the rotor.

1. As the rotors revolve, the space between the unmeshing lobes increase allowing inlet air or gas to fill up the intervening space, until the male lobe is disengaged from the female lobe along its whole length.

The helices of the male and female rotors are designed to permit complete charging of the inter – lobe space before they remesh.

3. On completion of the filling operation the inlet ends of the male and female rotors pass the inlet port and become sealed in the casing.
4. With continuing rotation, the male and female lobes begin to re-engage each other, the volume of this space is reduced and compression begins. This compression increases with continuing rotation until the built – in pressure ratio is reached and the air (or gas) is discharged through the outlet port, see figure 6.5

Figure 6.5 Working principle of rotary screw compressor

4. The absence of valves and unbalanced mechanical forces enables screw compressors to run with high shaft speeds.

6.2.2 Dry Type Screw Compressor

In dry type screw compressors the rotors are driven by external timing gears.

Clearances between the male and female lobes, between lobes and casing, and between the ends of the rotors, are kept to the minimum, so that the minimum of slip or leak – back from the high pressure side to the inlet side sis obtained.

The helical screw compressor has the advantage over the reciprocating compressors of mechanical simplicity, as there is no contact between the male and female rotors and / or the casing.  Therefore no internal lubrication is required, and oil – free air is obtained.  Inlet and discharge valves are not required, maintenance is greatly simplified.

A particular limitation of dry screw compressors, however, is that very high speeds are needed to maintain compressor efficiency at low capacities.

6.2.3 Oil – Flooded Rotary Screw Compressor

This type is semillar to the type screw compressor.  The only difference is that:

In the oil – flooded rotary screw compressor the intake air is sprayed with oil at a ratio of one part per thousand.

This oil is used to Perform three jobs:

1. To fill the clearance space to seal the internal clearances.
2. To lubricate the rotors of which can drive the other rotor (i.e. eliminating the timing gear).
3. Provide cooling medium for the air (or gas) during compression. The oil may be either air or water cooled using suitable heat exchanger.

A typical oil – flooding system is shown in figure 6.6

Figure 6.6 Oil / gas flow typical

Injected oil is reclaimed in two stages, and circulated.  Injection is normally done by using the discharge air pressure or may be provided by a separate pump.

Figure 6.7 shows oil – flooding system for a rotary screw compressor.

Figure 6.7 Oil – floding system for a rotary screw compressor (simple drawing)

Figure 6.8 The piping diagram

6.2.4 Capacity Regulation

Capacity regulation can be provided by a valve that controls the air (or gas) supply to the compressor or more simply by varying the speed.

The delivery pipe is normally fitted with a check valve that the system is automatically closed when the compressor is stopped or idling.  In the latter air is conducted from the high – pressures side (discharge side) to the intake side via a suitable channel to prevent compressor working as a motor.

SECTION – 6.3

DRIVING SYSTEM

Drive is accomplished, with a suitable prime mover, using a directly coupled flexible coupling.  The most common prime mover are electric motor and internal combustion engines.

• On compressors without timing gears the input (drive) shaft is an integral extension of the male rotor. The prime mover is connected directly to the male rotor.  The female rotor is driven by the male rotor because the rotors are meshed.
• Compressors with timing gears use an independent input shaft mounted on roller and / or special deep groove ball bearings. The male rotor is driven by the gear mounted on the input shaft and a mating gear on the male rotor (see figure 6.1).

SECTION – 6.4

ACCESSORIES

The helical screw compressor operates at moderate speeds (800 to 21,000 rpm) and may be electric motor, gas or steam turbine or engine driven, direct or through gear or V-belt for the best design.  The low pressure units are normally sized for direct connected motor speeds.  A speed-up gear is used when the driver and compressor speeds do not match and the horsepower is too great for V-belt application.

In the present section the accessories which are commonly used with this type of compressors will be covered.  These accessories are:

• After cooler and inter coolers.
• Check valves.
• Relief and safety valves.
• Expansion joints.
• Safety switches.

6.4.1 After Coolers and Intercoolers

are used when cooling is required.  Shell and tube type is the standard construction with air through the tubes.

An after cooler can be used to control the gas temperature discharged into the system.  It is often used with a moisture separator in 100 psig plant air system.

An intercooler is usually required in by-pass regulation in order to keep the compressor inlet temperature below the maximum allowable for low flow operation or between stages of multiple compression to maintain the gas temperature within the limitations of the compressor.

6.4.2 Silencers

Helical screw compressors usually have objectionable noise levels and are treated accordingly to bring them within allowable limits.

Inlet and discharge silencers of the sheet metal type with chambers and perorated tubes and packed spool type are two commonly used designs.

The proper selection is based on the installation and operating requirements.

6.4.3 Check Valves

Since the helical screw compressor is an expander as well as a compressor, a discharge check valve to prevent reverse rotation is recommended when there is any possibility that pressure will remain in the system after the unit is shut down.  A good example of this is parallel operation of two or more units.

6.4.4 Relief and Safety Valves

Being a positive displacement compressor, a relief or safety valve should be in the system to guard against exceeding the pressure limitations of the machine or the system whichever is lower.

If it is a closed system, relief to the inlet is permitted providing the inlet temperature to the compressor is controlled by proper mixing of the gas, or by cooling so that the maximum allowable compressor discharge temperature is not exceeded.

In a vacuum system such as a paper mill wet vacuum application, a vacuum relief valve is recommended so that the temperature limitations of the machine will not be exceeded.

6.4.5 Filters

Air filters are advisable on air service with atmospheric inlet.  Dry type with replaceable inserts or manually serviced oil bath open mesh type are available.  The former is used where oil free air is required and the latter on general service.

A lube oil filter is provided in the force feed oil system to protect the bearings, gears, and oil pump from damage due to dirt, grit, scale, and other foreign matter that might be in the oil system.

6.4.6 Expansion Joints

Since the rotary compressor operates with close tolerances, any casing distortion from thermal expansion, vibration, or misalignment can cause difficulty.  Expansion joints are recommended at the inlet and/or discharge flanges to eliminate these external forces.

6.4.7 Baseplates

A welded steel base plate under compressor and driver is often used to provide case of alignment.  If the installation requirements best suit the use of soleplates, they can be utilized with equal effectiveness.

6.4.8 Safety Switches

Safety switches, wired into the motor control system, are recommended for high discharge gas temperature and, in the case of a force feed oil system, low lube oil pressure.

SECTION – 6.5

OPERATION AND PREVENTIVE MAINTENANCE

Satisfactory performance of a stationary screw compressor requires a good preventive maintenance program.

The following information is provided as a guide for such a program.

6.5.1 Start up

Start up may be accomplished as follows:

3. Service all equipment (filters moisture separators, prime mover, etc.) following the equipment manufacturer’s instructions.
4. Drain condensate (water) from the oil reservoir. Close the drain valve securely when oil appears.
5. Check the compressor oil reservoir fluid level.

Add oil as necessary to the correct level.  Do not overfill.

7. Make certain adequate ventilation and cooling water, if required, is provided.
8. Start the unit by actuating the electrical controls or starting the engine following the equipment manufacturer’s instructions.
9. Observe the unit for leaks, unusual noise, vibration, etc. (Shut down unit and correct as required.)
10. Observe all instrumentation for proper readings.

6.5.2 Shut Down

1. Stop the prime mover following manufacturer’s instructions.

6.5.3 Preventive Maintenance

Regular testing of oil as specified in the following maintenance guide is intended to assist in establishing oil and oil filter change intervals.

Normally the oil analysis sheet will list the results of the oil test, in addition to recommendations of the analyzing laboratory, showing what maintenance is required.

6.5.3.1 Daily

3. Visually inspect the entire unit for leaks, loose hardware, etc. Correct as required.
4. Service all equipment (filters, moisture separators, etc.) following the equipment manufacturer’s instructions.
5. Drain water (condensate) from the oil reservoir. Close the drain valve securely when oil appears.

Note:

Always drain condensate after the unit has been shut down for a reasonable length of time (over night).

6. Check the compressor oil reservoir oil level. Add correct type of oil as required.  Do not overfill.
7. With the unit running, observe the oil pressure gages and compare readings. (Filter in – filter out) A difference in pressure reading of 15 PSI or more indicates that the filter element must be renewed.
8. With the unit running observe the gas pressure gages and compare readings. (Pressure drop across separator element) A difference in pressure of 15 PSI or more indicates the separator element is restricted.  If the restriction is caused by dirt or other foreign matter, renew the separator element.

Note:

Oil saturation can cause excessive restriction across a separator element.  Always check the separator drain tube and line for restriction, the check valve (installation and condition) and clean or renew the drain line filter before condemning a separator element.

6.5.3.2 Every 200 House

4. Service all equipment (filters, moisture separators, etc.) following equipment manufacturer’s instructions.
5. Repeat daily service as applicable.
6. Remove surface dirt and dust from the exterior surfaces of the oil cooler and after cooler, if used.
7. Obtain an oil sample from the compressor system and have the sample analyzed for condition. Change oil and filter, if required, based on the results of the oil analysis.

Note:

Oil samples may be obtained from the oil reservoir filler opening.  (not drain) Sample should be obtained shortly after shut down.

6.5.3.3 Every 1000 Houses

2. Repeat daily and 200 hour service as applicable.
3. Change oil filter elements.