Oil recycling in screw compressor arrangements

Abstract: The invention relates to oil recycling in screw compressor arrangements used to compress gas such as air. The screw compressor arrangement comprises a screw compressor (11), primary (13) and secondary (18) separating means for extracting oil from compressed gas discharged from the screw compressor, means (21) for returning the oil extracted by the secondary separating means to the screw compressor and valve means (22) operable to regulate the flow of oil and gas from the secondary separating means (18) back to the screw compressor.

Description: The invention relates to oil recycling in screw compressor arrangements used to compress a gas such as air. Many screw compressors require the use of oil during their operation. The oil has three major functions within the scope of compressor arrangements; it cools the compressed gas, lubricates the bearings and seals the rotors in the air end (the actual screw compressor itself). Unfortunately, as a result the discharged compressed gas contains a large quantity of oil which must be extracted before it can be used. A pressure vessel is generally used to extract the bulk of the oil from the compressed gas during a primary separation process. This is also known as a reclaimer and usually contains a filter or separator element to separate the remaining oil from the compressed gas. The oil collected by the separator element is scavenged back to the air end. In other arrangements the separator element is not fitted in the reclaimer but an alternative special filter is provided to carry out the final separation of oil still entrained in the gas after it has been through the reclaimer, and the oil is then also recycled and returned or scavenged back to the screw compressor to be re-used. In order to scavenge this oil back to the compressor a small quantity of the compressed gas is constantly bled back with the oil to effect the scavenging process. The scavenge line consists of a small open pipe connecting the base of the special filter to the air end. At the end of the pipe adjacent the air end are fitted a filter, a small orifice which limits the air flow and a non-return valve to prevent oil in the air end passing back into the scavenge pipe. Such an arrangement is substantially described in GB-A-833991, which forms the basis of the preamble of claim 1. The quantity of gas used in the scavenging process is relatively small on large compressor packages, but in small compressor units represents a significant loss in that it circulates in a closed loop requiring re-pressurisation after re-entry into the compressor to achieve the compressor final discharge pressure. It is an object of the present invention to overcome this disadvantage and to minimise the gas losses due to the scavenging process. According to the invention there is therefore provided a screw compressor arrangement comprising a screw compressor, primary and secondary separating means for extracting oil from compressed gas discharged from the screw compressor, means for returning the oil extracted by the secondary separating means to the screw compressor and valve means operable to regulate the flow of oil and gas from the secondary separating means back to the screw compressor. Preferably the valve is the solenoid or pneumatic valve which is operable to be set normally open or normally closed. The valve is preferably operable to vary the frequency in duration of valve operating. There may also be provided a control system to regulate the operation of the valve which may be electronic. In a preferred embodiment of the invention the primary oil separating means comprise a reservoir and the secondary or separating means comprise a filter. The secondary separating means may be enclosed within the primary separating means or they may be fitted downstream thereof. The invention also provides a method of recycling oil in a screw compressor arrangement comprising the steps of extracting oil from compressed gas discharged from the screw compressor in a primary separation process, extracting the remaining oil from the compressed gas in a secondary separation process, returning the oil extracted during the secondary separation process to the screw compressor by means of a small quantity of compressed gas via valve means operable to regulate the flow of oil and gas back to the screw compressor. The invention will now be described, by way of example only, with reference to the accompanying drawing Fig. 1 which is a schematic representation of a screw compressor arrangement according to the present invention. The screw compressor arrangement 10 comprises a screw compressor 11 (air end) for compressing a gas, such as air, a motor 12 which drives the air end 11 and a pressure vessel 13. The pressure vessel 13 is a reservoir for compressed gas and oil and may be made from aluminium, iron, steel or any other suitable material. Gas is taken into the air end 11 via a gas intake filter 14 which is monitored by a pressure switch 14a. The pressure switch 14a senses the differential pressure between the atmospheric pressure existing in the vicinity of the gas intake filter 14 and the pressure downstream between the gas intake filter 14 and the inlet to the air end 11. As the filter 14 progressively filters out atmospheric dirt, it slowly blocks up and the pressure switch 14a indicates when the filter 14 should be replaced. The quantity of gas intake is also controlled via a suction regulator 15 which is connected by a control line to the reclaimer 13. The discharge of pressurised compressed air from the air end 11 contains a large quantity of oil, typically 10 to 15 litres per minute for each cubic metre of free air compressed per minute. This oil has to be separated from the compressed air before it passes into use. The compressed gas and oil mixture is therefore discharged from the air end 11 to the reclaimer 13 via an appropriate duct 24. A temperature thermistor 25 monitors the temperature of the gas/oil mixture. The separation of the gas and oil is achieved in two stages; primary separation of the oil and gas is carried out within the reclaimer 13 and final separation is completed through the separator element in the reclaimer or alternatively through a special filter 18 fitted downstream of the reclaimer 13. The difference in pressure across the special filter 18 is monitored by a pressure differential switch 18a to determine when the filter 18 must be replaced. The fully cleaned gas is subsequently passed through an after cooler 19 before passing to the plant discharge 20 and into use. A pressure transducer 26 responds to the pressure in the customers gas main to energise the control system as and when required. The reclaimed oil from the special filter is recirculated from filter 18 via a small scavenge pipe 21 back to the air end 11. The scavenge pipe 21 conveys the oil and a small quantity of the compressed gas which is bled back with the oil to effect the scavenging process. Incorporated into the pipe 21 is a solenoid valve 22 which operates to introduce an intermittent, rather than constant, scavenge. The timing of the operation of this valve 22 is important in that it cannot remain closed for excessively long periods because this will lead to flooding of the bottom of the special filter 18 and reducing its effeciency. The valve 22 must also be opened long enough to ensure that all the oil is cleared from the filter 18 and passed back to the air end 11. In prior art arrangements nearly 0.08m 3 per minute of free gas is used to scavenge the oil. This can represent up to 10% of the output of a small unit, say of 5.5Kw size. To compress this gas to typically 7 bar requires approximately 0.5Kw. By using a solenoid valve 22, the power consumption to the drive motor is reduced by nearly 0.5Kw whilst the actual gas flow remains unchanged. As the rotors are virtually sealed, the intermitted introduction of scavenged gas and oil after the rotor inlet closure does not increase gas losses back to the suction and the scavenge gas and oil mixes with the charge gas to be compressed to the final discharge pressure. Thus during the greatest part of the running period additional power is not required to re-compress the scavenged gas as is the case with conventional machines. The solenoid valve 22 can also be set to be normally open or normally closed. Normally open has the benefit of providing a fail safe function in the event of valve failure. Normally closed has the benefit of preventing small quantities of oil being driven back when the plant is being blown down on stopping. A non-return valve may be necessary to prevent this action for a normally open valve.

Claims: A screw compressor arrangement (10) comprising a screw compressor (11), primary (13) and secondary (18) separating means for extracting oil from compressed gas discharged from the screw compressor (11), characterised by the provision of means (21) for returning the oil extracted by the secondary separating means (18) to the screw compressor and valve means (22) operable to regulate the flow of oil and gas from the secondary separating means (18) back to the screw compressor (11). A screw compressor arrangement (10) as claimed in claim 1 in which the valve means (22) is a solenoid or pneumatic valve. A screw compressor arrangement (10) as claimed in claim 1 or claim 2 in which the valve means (22) is operable to be set normally open or normally closed. A screw compressor arrangement (10) as claimed in any one of the preceding claims in which the valve means (22) is operable to vary the frequency and duration of valve opening. A screw compressor arrangement (10) as claimed in any one of the preceding claims in which there is provided a control system to regulate the operation of the valve means (22). A screw compressor arrangement (10) as claimed in claim 5 in which the control system is electronic. A screw compressor arrangement (10) as claimed in any one of the preceding claims in which the primary oil separating means (13) comprise a reservoir and the secondary oil separating means (18) comprise a filter. A screw compressor arrangement (10) as claimed in any one of the preceding claims in which the secondary separating means (18) are enclosed within the primary separating means (13). A screw compressor arrangement (10) as claimed in any one of the preceding claims wherein the secondary separating means (18) are fitted downstream to the primary separating means (13). A method of recycling oil in a screw compressor arrangement (10) comprising the steps of extracting oil from compressed gas discharged from the screw compressor (11) in a primary separation process, extracting the remaining oil from the compressed gas in a secondary separation process, characterised by the step of returning the oil extracted during the secondary separation process to the screw compressor (11) by means of a small quantity of compressed gas via valve means (22) operable to regulate the flow of oil and gas back to the screw compressor (11).

Inventors: HARE JOHN, HARE, JOHN
Applicants: COMPAIR BROOMWADE LTD, COMPAIR BROOMWADE LTD.
Application year: 1996
Main classification: F04C29
IPC codes: F04C29