Flooded compressor separators

A separator for a flooded liquid/gas compressor comprising a pressure vessel having an inlet for compressed liquid/gas mixture, a lower outlet for liquid and an upper outlet for clean gas, a filter element interposed between the liquid/gas mixture inlet and the clean gas outlet through which the gas flows to said outlet, and a shield member at least partially surrounding the filter element whereby liquid from the separator pressure vessel is prevented from contacting the filter element during periods of operation when the liquid volume in the pressure vessel may rapidly expand.

The present invention relates to separators commonly used with flooded 
compressor systems. The invention also envisages a compressor system 
including such an oil or other liquid separator. 
Flooded compressor systems are most commonly used with rotary compressors 
including screw and vane type compressors and although the following 
description provides particular reference to a screw type compressor, it 
will be apparent to those skilled in the art that the invention is 
applicable to other compressor systems. 
Commonly screw compressors comprise a pair of coacting screw members 
respectively having male and female helically formed parts which, upon 
rotation of the screw members, co-operate with the compressor housing to 
compress gas. To avoid complicated engineering design in relation to 
rotatably mounting the co-operating screw members there has been developed 
a system of oil flooding such compressors such that the gas being 
compressed is mixed intimately with oil as it is compressed and this oil 
must thereafter be separated from the gas in a separating vessel before 
the compressed gas is available for its intended use. The separated oil is 
then recycled through a cooler and an oil filter for re-use in the 
compressor. 
A separator for a compressor system of the aforementioned kind has 
conventionally comprised a pressure cylinder having its longitudinal axis 
in a generally vertical direction and having a generally cylindrical final 
filter element of the coalescent type surrounding the clean compressed gas 
outlet from the separator vessel. The oil/compressed gas mixture is 
introduced into the vessel above a sump or pool of oil in the base thereof 
and there may also be provided some mechanical separator means such as 
baffles to enable an initial separation of the gas from the oil with the 
oil dropping into the pool in the base of the vessel. The remainder of the 
oil is separated in the final filter which either also drops to the oil 
pool in the base of the separator vessel or is scavenged from the final 
filter element itself and returned to the compressor. 
The previously described system has operated quite satisfactorily, however, 
there are a number of disadvantages, or problems associated with the 
system. One major problem is that the coalescent type filters employed in 
the separator vessel work efficiently only when they have to deal with 
small quantities of oil in a gas mixture. They in fact become totally 
ineffective if they are contacted or submerged in bulk liquid oil. Now it 
necessarily occurs at certain periods of the cycle and when the system is 
shut down, that the pressure in the separating vessel is reduced. This 
factor results in the gas entrained in the oil in the sump of the 
separator expanding rapidly causing the mixture to expand resulting in 
foaming and a rapid rise in the oil level. It is normal for the oil volume 
to at least double in size and as a consequence the design of the 
separator vessel must be such as to permit this without the expanded oil 
volume contacting the coalescent filter. A second related problem is that 
the compression process generates substantial heat and the necessary gas 
contact results in a relatively rapid oxidation of the oil. This factor 
means that the volume of oil maintained in the system should be kept as 
great as possible to thereby keep the length of time between oil changes 
as long as possible. As the separator vessel is a pressure vessel and is 
therefore expensive to produce both in terms of the materials used and the 
manufacturing processes required, it is desired from an economic point of 
view to keep the size of the vessel as small as possible. The separator 
vessel is a major element in the overall size of the compressor system and 
this is also a reason for endeavouring to maintain the vessel as small as 
possible. 
Over recent years there has been a further development which essentially 
proposed the mounting of the compressor unit within the separator vessel 
to minimize overall size and to also minimize some of the piping 
connections between the elements in the system. This proposal necessitated 
the mounting of the separating vessel with its longitudinal axis 
substantially horizontal. The result naturally substantially aggravated 
the problem of oil contact with the final coalescent filter element which 
has conventionally been dealt with by a number of different approaches 
either separately or in combination. One approach is to try to lower the 
total oil volume within the system, however, this has the substantial 
disadvantage of requiring far more frequent down times for replacement of 
the oil. A second approach is to increase the size or volume of the 
separating vessel usually by increasing its length. Often in such 
arrangements the filter element is mounted with its longitudinal axis 
generally horizontal such that the gas discharge line passes through the 
end of the separating vessel. This is unsatisfactory as the lower regions 
of the filter element tend to become saturated with oil thereby 
substantially reducing its effectiveness and moreover the size of the 
separating vessel is a substantial disadvantage. A third alternative is to 
mount the filter element in an upwardly extending T branch from the main 
separating pressure vessel wall. This latter solution substantially 
increases the costs of producing the pressure separating vessel. 
The principal objective of the present invention is to provide an improved 
separator for use with a flooded compressor system which will enable a 
reduction in size of the separator without a reduction in the liquid 
volume capacity thereof or will enable the use of an increased volume of 
liquid in the flooded compressor system. 
Accordingly the present invention provides a liquid/gas separator for use 
with a flooded compressor system, said separator comprising a pressure 
vessel adapted to maintain a pool of liquid in a lower region thereof 
arranged to receive a mixture of liquid and compressed gas therein, said 
vessel further having a final filter element interposed between the pool 
of liquid and a clean compressed gas outlet from the vessel such that 
liquid is removed thereby from the mixture of liquid and compressed gas 
flowing through said filter element to the gas outlet, said separator 
being characterized in that shield means is provided at least partially 
surrounding said filter element enabling the compressed gas with liquid 
entrained therewith to reach said filter element with said compressed gas 
passing through said filter element to the compressed gas outlet, but 
substantially preventing liquid from said pool from flowing into contact 
with the filter element. Usually the liquid separated will be an oil or a 
synthetic liquid lubricant and the use of the terminology "oil" 
hereinafter should be interpreted as including such synthetic liquid 
lubricants. Conveniently the shield means comprises a container element 
having a base positioned beneath the filter element and an upstanding wall 
or walls sealed with or integral with the base and substantially 
surrounding the filter element whereby the filter element is maintained 
within a dry sump formed by the container element. Alternatively 
partitioning might be employed. Preferably one or more gas flow spaces are 
provided adjacent to the upper regions of the filter element whereby gas 
together with small gas borne droplets of oil pass into engagement with 
the filter element to finally remove substantially all oil therefrom. 
In accordance with a particularly preferred embodiment a shroud element is 
provided between the wall or walls of the container element whereby the 
gas and gas borne oil droplets are caused to first flow downwardly between 
the wall or walls of the container element and the shroud element and then 
upwardly and through the filter element. Such a shroud will have the added 
effect of further mechanical oil separation as well as preventing oil 
splashing on to the filter element should the oil level (in particularly 
adverse conditions) rise sufficiently to enter the container element. 
Preferably, in both the aforementioned embodiments, an oil scavenger line 
is provided for removing oil collected in the base region of the container 
element thereby preventing this oil volume rising sufficiently to contact 
the filter element.

Referring initially to FIG. 1 there is shown a schematic flow diagram for a 
conventional screw compressor system. In this system gas to be compressed 
is drawn into a screw compressor 10 through an gas filter 11 and an inlet 
flow control or throttle valve 12. The flow control valve 12 is controlled 
by a solenoid element 13 which senses that discharge pressure from the 
separator 14 has reached a predetermined level. The gas discharge line 15 
from the separator includes a minimum pressure valve 16 which ensures that 
a minimum pressure is maintained in the separator whereby oil will always 
flow therefrom to the compressor 10 because of the pressure differential 
therebetween. The oil is maintained in a sump or pool 17 in the base of 
the separator 14 and is returned therefrom to the compressor 10 via line 
18, cooler 19, and an oil filter 20 and oil stop valve 21. During 
operation of the compressor the stop valve 21 is maintained open to allow 
oil to flow to the compressor, however, during periods when the compressor 
10 is not functioning, oil is prevented from flooding the compressor, 
which would provide an unacceptably high start up loading, by closing the 
valve 21. Furthermore, the compressed gas/oil mixture exiting from the 
compressor 10 flows through a one way valve 22 along line 23 to be 
discharged into the separator 14 at a position usually above the sump or 
pool 17. The separator may include some mechanical separation means 
whereby oil will fall back into the pool and gas and gas borne oil 
droplets will flow relatively slowly upwardly to finally remove the gas 
borne oil droplets in a final filter element 24 of the coalescent type. 
Finally an oil scavenging line 25 is provided with an gas flow restriction 
26 to enable liquid oil collected in the filter element 24 to be returned 
(again by way of the pressure differential) to the compressor 10. 
Now in operation, if the rate of use of the compressed gas is less than the 
delivery rate of the compressor, the system pressure will rise and at a 
preset maximum a pressure switch 50 will sense this use and operate the 
solenoid element 13 to close the gas inlet valve 12. With the inlet gas 
supply substantially removed, the system pressure will fall until the 
pressure switch, on reaching a lower pre-set level will again operate the 
solenoid and re-open the valve 12. Now when the inlet valve 12 has been 
closed, the compressor is inducing gas at a very low inlet pressure due to 
the closed inlet valve, but the pressure in the separator is still 
relatively high, thereby causing the compressor to compress gas across a 
very high compression ratio compared to normal operating conditions. This 
would give rise to high unloaded power consumption and excessively high 
noise levels. To avoid this problem in the conventional arrangement the 
minimum pressure valve is provided with an integral non return valve and a 
pressure lowering valve 52 is also provided. This reduces the back 
pressure against which the compressor operates, however it also gives rise 
to periodic lowering of separator pressure. Moreover, a stop dump valve 51 
is provided to dump pressure from the separator on shut down of the 
compressor system. Both these situations cause the compressed gas mixed in 
with the oil in the sump 17 to expand rapidly causing the oil to foam and 
the oil level to rise rapidly. To prevent liquid oil from contacting the 
filter element 24 under such conditions, it has conventionally been 
proposed to either increase the size of the pressure tank of the separator 
14 or alternatively to reduce the volume of the oil in the system. Both 
solutions have substantial disadvantages as previously indicated. 
FIGS. 2 and 3 show preferred embodiments according to the present invention 
which provide advantageous solutions to the foregoing problem. FIG. 2 
shows a separator 14 adapted for use in a substantially conventional screw 
compression system as shown in FIG. 1. The separator comprises a pressure 
tank 30 having a compressed gas and oil inlet line 23 and a compressed gas 
discharge line 15 which may include a minimum pressure valve 16. A 
substantially conventional final filter element 24 (which may be of the 
coalescent type) is provided surrounding the gas discharge outlet line 15 
so that gas/oil mixtures must pass therethrough to separate oil from the 
gas. Mounted generally surrounding the final filter element 24 is a 
container member 31 having a base 38 and side walls 32 mounted from an end 
wall 33 of the pressure vessel 30. The container member 31 includes a 
plurality of openings or spaces 34 at its upper regions adjacent the end 
wall 33 to enable gas borne oil droplets to pass inwardly into the 
container and thereby to the filter element 24. An oil scavenging line 25 
extends from the internal regions of the filter element 24 and returns the 
oil collected to the screw compressor. In addition a second oil scavenging 
line 35 extends from the internal base region of the container member 31 
to the line 25 and also returns oil collected to the screw compressor. 
Each of the lines 25 and 35 include a restrictor 26 to minimize gas loss 
from the separator. Moreover, each of the purge lines 25,35 may include a 
float controlled valve 60 (see FIGS. 2A,2B) which are arranged to 
positively prevent gas flow through the lines 25 or 35 such that gas 
losses are avoided. The float valve 60 may be located either within or 
externally of the separator unit. Each valve 60 comprises a chamber 63 
with an access opening adapted to receive oil 61,62 from either the base 
of the container member 31 or the base of the final filter element 24. An 
outlet opening 67 is provided leading to either purge line 25 or 35 and is 
opened (FIG. 2B) or closed (FIG. 2A) by a valve element 60 operated by a 
float member 64 and lever 65. It will be appreciated that other forms of 
valving might be used to ensure against gas losses along the respective 
purge lines. In this manner a substantially dry sump 36 is formed in which 
the filter element 24 is located. A further preferred feature is the 
provision of shroud 37, also mounted from the end wall 34 and surrounding 
the filter element 24 between this element and the walls 32 of the 
container member 31. The shroud 37 performs a dual function in forcing gas 
and gas borne oil droplets to flow downwardly to the base of the filter 
element 24 before flowing upwardly therethrough to the discharge line 15. 
This provides additional mechanical oil separation and a better 
distribution of gas flow through the filter element 24 as well as 
protecting the filter element from possible oil splashing should the oil 
pool level 39 happen to rise to the level of the openings 34. 
FIG. 3 illustrates a somewhat similar arrangement to FIG. 2 and 
consequently only the differences will be described hereinafter. In this 
embodiment, the screw compressor 10 is mounted within the separator 
pressure vessel 30 and as a consequence, the vessel 30 is arranged with 
its axis substantially horizontal. This arrangement provides some savings 
in overall size of the system as well as minimizing some of the piping 
requirements. The final filter element 24 is mounted through an opening in 
a side wall of the pressure vessel 30 and closed by a cap plate 40. 
Furthermore in this embodiment the container member 31 may be formed as a 
separate unit and connected to or sitting on base support elements 41 such 
that an upper restricted gap 42 is formed between the top of the side 
walls 32 of the container member and the closure plate 40. In addition the 
oil gas mixture line 23 exiting from the compressor 10 may include nozzle 
means to direct a spray against a desired area of the system to assist in 
an initial mechanical separation of oil from the compressed gas. The 
operation of the system shown in FIG. 3 is essentially similar to that of 
FIG. 2 and it will be appreciated by those skilled in this art that 
modifications or various combinations of features disclosed herein would 
be possible within the scope of the present invention. 
It has been found that with the present invention, it is possible to reduce 
the size of the pressure vessel 30 for a given performance size of the 
compressor and/or to increase the volume of oil used in the system.