GB2277462A

GB2277462A - Compressor lubricating oil quality maintenance

Info

Publication number: GB2277462A
Application number: GB9407516A
Authority: GB
Inventors: Alfred Neugebauer
Original assignee: Leobersdorfer Maschinenfabrik GmbH
Current assignee: Leobersdorfer Maschinenfabrik GmbH
Priority date: 1993-04-26
Filing date: 1994-04-15
Publication date: 1994-11-02
Also published as: DE4313573A1; NO941442L; NO941442D0; GB9407516D0

Description

2277462 COMPRESSOR LUBRICATING OIL QUALITY MAINTENANCE The present

invention relates to a method of reducing a water vapour condensation and expelling water vapour condensate from the lubricating oil of an oil-lubricated screw compressor and to an installation in which such a method can be carried out.

Screw compressors are included in compressor compound installations, for example an installation containing an oillubricated screw compressor for the compression of atmospheric air to a pressure up to about 15 bar and of one or more downstream piston compressors, which in accordance with the particular requirements produce a pressure between 150 and 350 bar. In operation of screw compressors for air compression it is necessary to keep the oil temperature of the screw compressor so high that moisture contained in the air cannot condenseout in the course of compression. Any precipitated condensate would mix with the lubricating oil and permanently impair the lubricating properties thereof. The oil circuit of the screw compressor must therefore be run at temperatures above the condensation point of the water vapour in the air. This is achieved, during constant load operation, by automatic temperature regulation in the oil circuit.

Problems arise during starting up of the system, if the oil is still cool, and during the shutdown of the screw compressor. During starting, water vapour condenses in the compressor oil circuit and mixes with the oil. Due to the thereby impaired lubricating quality of the oil the service life of the screw compressor can be drastically reduced, such as by damage of roller bearings in the compressor. During shutdown of the screw compressor, the risk exists that residual water vapour precipitates in the lubricating oil and leads, in the form of standstill corrosion in crevices, to formation of rust and, for example, to damage of the roller bearings.

It has hitherto been sought to counter standstill corrosion by operating the screw compressor, before shutdown, for a determined duration at reduced delivery pressure with atmospheric air. In this matter the lubricating oil is "boiled", whereby a part of the water contained in the oil could be removed with the air. This process effects only a partial drying of the lubricating oil and improves medium-term and _long-term standstill periods. In load operation of the compressor, however, there is no improvement because during starting of the cold compressor water vapour has already condensed in the lubricating oil.

There thus remains a need to minimise condensation of water vapour during compression of air by an oil-lubricated screw compressor in cold operation, to expel water vapour condensate during starting operation and to remove possible condensate residues from the lubricating oil before shutdown.

According to a first aspect of the invention there is provided a method of reducing water vapour condensation in and expelling water vapour condensate from the lubricating oil of an oil- lubricated screw compressor in an air compressing installation, comprising the steps of expanding compressed air in the installation for a determined period of time during at least one of start up and W i shutdown of the compressor and returning the expanded air to the compressor for recompression.

According to a second aspect of the invention there is provided an air comprising installation comprising an oil lubricating screw compressor arranged in a flow path of the installation, at least one piston compressor arranged in the flow path downstream of the screw compressor, cooling means for cooling air compressed by the compressors, a bypass duct connected between the intake of the screw compressor and a point of the flow path downstream of the final compressing and cooling of air therein and control means to allow compressed air to expand and flow back to the intake by way of the bypass duct for a determined period of time during at least one of start up and shutdown of the screw compressor.

In an embodiment of the installation, highly compressed air after a high-pressure booster compressor, which is connected behind the screw compressor, may have a pressure condensation point of about 50"C according to the temperature of cooling water used. If the air is expanded to atmospheric pressure, the condensation point then sinks to -100C to -200C. This extremely dry air is led to the suction side of the screw compressor in the starting and shutdown phases. Thus dry air is passed through the system and takes up water vapour quantities present in the lubricating oil circuit of the screw compressor. These condensate or water-vapour quantities are, in the course of subsequent high-pressure compression, separated off and discharged at separators arranged after coolers.

This process can be assisted by raising the temperature of the lubricating oil circuit.

An example of the method and an embodiment of the installation will now be more particularly described by way of example with reference to the accompanying drawing, the single figure of which is a schematic circuit diagram of a high-pressure compound installation, embodying the invention, for the production of highly compressed air.

Referring now to the drawing there is shown a high-pressure compound installation for the production of highly compressed air, the installation comprising an oil:-lubricated screw compressor 1 arranged in a flow path and, in the path behind the compressor 1, one or more compressor stages containing piston compressors 2. A cooler 3 and a separator 4 for condensate are arranged between the compressor stages and after the last compressor stage. A condensate discharge duct 5 is connected to each separator 4.

Arranged behind the screw compressor 1 and in the flow path of the air is an oil separator 6, an oil outlet of which is connected by way of an oil duct 7 to the screw compressor 1. A heat exchanger 8, by way of which the temperature of the lubricating oil can be adjusted to a desired value, is provided in the oil duct 7.

Atmospheric air is fed to the screw compressor 1 by way of an inlet duct 10 containing a filter 11 and a blocking or throttling flap valve 9. The air is brought to a pressure of about 15 bar in the screw compressor 1 and to a delivery pressure of 150 to 350 bar, according to requirements, in the following piston compressor. The highly compressed air is fed to a consumer by way of an outlet duct 12, in which a blocking valve 13 and a non-return flap valve 14 are arranged.

A bypass duct 15, in which one or more expansion valves 16 are arranged, branches off from the outlet duct 12 upstream of the blocking valve 13. The bypass duct 15 opens into the inlet duct 10 in front of the screw compressor 1.

When the outlet duct 12 is blocked by the valve 13, the highly compressed air is expanded in the regulated bypass duct 15 to atmospheric pressure and fed back in the circuit to the screw compressor 1. This bypass circuit is opened during starting up and shutdown of the compressor installation for a predetermined time period of about 10 minutes. Through the expansion to atmospheric pressure a very low atmospheric water vapour condensate point is achieved in the air, this point lying at -MC to -20C in correspondence with the temperature of the cooling medium used in the last cooler 3. The screw compressor 1 is operated for the stated time period with this very dry air flow during start up and shutdown. Consequently, even traces of water vapour are removed by the air from the still-cold lubricating oil during the first minutes of the start up procedure and discharged from the system by way of the separators 4. In addition, during this time period the temperature of the lubricating oil can be increased by an automatic adjustment of a temperature regulator 17 present in the oil duct 7, in order to assist the removal of water from the oil.

The same procedure with the regulated high-pressure bypass for the production of extremely dry atmospheric air and supply thereof into the suction duct of the screw compressor 1 is used before the leaving off or running down of the compressor installation, in order to ensure the drying of the lubricating -oil before the shutdown of the compressor installation. This shutdown procedure is a precautionary measure and ensures the creation of a largely waterfree system prior to shorter and longer standstill periods.

Claims

1. A method of reducing water vapour condensation in and expelling water vapour condensate from the lubricating oil of an oil-lubricated screw compressor in an air compressing installation, comprising the steps of expanding compressed air in the installation for a determined period of time during at least one of start up and shutdown of the compressor and returning the expanded air to the compressor for recompression.

2. A method as claimed in claim 1, comprising the step of heating the lubricating oil during the start up and shutdown to a temperature above a predetermined working temperature thereof.

3. A method as cl aimed i n cl aim 1 and substantially as hereinbefore described with reference to the accompanying drawing.

4. An air compressing installation comprising an oil-lubricating screw compressor arranged in a flow path of the installation, at least one piston compressor arranged in the flow path downstream of the screw compressor, cooling means for cooling air compressed by the compressors, a bypass duct connected between the intake of the screw compressor and a point of the flow path downstream of the final compressing and cooling of air therein and control means to allow compressed air to expand and flow back to the intake by way of the bypass duct for a determined period of time during at least one of start up and shutdown of the screw compressor.

5. An installation as claimed in claim 1, the control means comprising at least one expansion valve.

6. An installation substantially as hereinbefore described with reference to the accompanying drawing.