Screw Compressor with Oil Feed System

Abstract

A rotary screw compressor with an oil feed system that has a plurality of oil orifice nozzles to feed oil to the compressor rotors and the compressor bearings. One of the oil orifice nozzles is generally always provided with a regulated flow of oil. At least one and preferably two on/off oil valves can be selectively opened and closed. Each of the on/off oil valves are of a different size and are selectively opened and closed to step regulate the amount of oil being fed to the compression chamber according to a predetermined compression chamber discharge temperature.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

FIELD OF THE INVENTION

The present invention relates to the oil feed for a twin screw compressor and to the system of feeding oil to a twin screw compressor to control the temperature of the compressor and the temperature of the discharge air.

BACKGROUND ART

Oil in an oil flooded rotary twin screw compressor is used to seal, cool and lubricate the process of compression. Oil is injected to absorb the heat of compression. The air/oil exiting the compression chamber is typically at a temperature of around 180 to 200® F. The oil is cooled in a heat exchanger and then re-injected into the compression chamber at a temperature of typically around 150° F.

In a conventional system the oil is injected into the compression chamber through a single hole or port, or a series of holes or ports that continuously supply oil onto the rotors. The temperature of the oil is adjusted to maintain optimal operating temperatures. An air/oil mixture is discharged from the compression chamber and the oil is separated from the air/oil mixture in an air/oil separator. Some of the oil passes through an oil cooler and some of the oil is bypassed around the cooler to adjust the temperature of the oil in the compression chamber. The oil is generally filtered prior to being reinjected into the compression chamber. If the compressor is operating colder than the mixing valve setting, all of the oil bypasses the cooler. When the compressor is operating hotter than the thermal mixing valve setting, all of the oil flows through the cooler. Most of the time the mixing valve moves some of the oil through the cooler and some of the oil around the cooler to maintain the proper operating temperature.

DISCLOSURE OF THE INVENTION

The present invention provides a rotary screw compressor that has a plurality of oil orifice nozzles to feed oil to the compressor rotors and the compressor bearings. One oil orifice nozzle is an on oil orifice nozzle that is generally always provided with a regulated flow of oil and at least one other is an on/off oil orifice nozzle that can be selectively opened and closed. Each on/off oil orifice nozzle(s) is controlled by an on-off valve that controls the amount of oil flowing into the compression chamber. The on-off valves are selectively opened and closed to regulate the amount of oil being fed to the compression chamber. The temperature of the air/oil mixture discharged from the compression chamber is measured and compared to a predetermined desired temperature for the discharge. Depending on the temperature reading, selected on/off oil valves are opened or closed.

In a preferred embodiment, we provide the twin screw rotary housing with an on oil orifice nozzle and three on/off oil orifice nozzles, with each of the orifice nozzles, opening to the outer periphery of at least one of the rotors. The oil orifice nozzles are connected to separate oil feed lines. An oil supply feeds oil to each of the oil lines. The on oil line and its orifice nozzle do not have a valve to close the flow of oil to the compression chamber. However the on oil line can have an oil regulating valve which regulates the flow of oil to the on oil orifice nozzle. Each of the three on/off oil orifice nozzle valves are selectively opened or closed to regulate the oil volume flow to the compressor to obtain a desired temperature of the air/oil discharged from the compression chamber. The three valves are connected to a temperature sensor that indicates the discharge temperature of the air/oil mixture being discharged from the compressor. The sensor affects the sending of an appropriate signal to the valves to selectively open or close them based on the air/oil discharge temperature.

Another feature of the present invention is to provide a compressor assembly having an air inlet, an oil supply, an air/oil separator, an oil cooler and four oil feed lines feeding oil from the oil supply and/or air/oil separator to twin screw rotors to lubricate these twin screws and twin screw compressor bearings. Air is sucked into the air inlet; the outlet end of the compression chamber is connected to an air/oil separator that separates oil from the air/oil discharged from the compression chamber. The air/oil separator has an air outlet and an oil outlet. A first air line which can return air from the air/oil separator to the compressor inlet when the normally closed blow down valve is opened and a second air line sends the remaining air to an air discharge. The oil outlet of the air/oil separator supplies oil to each of the four oil feed lines. One of the oil feed lines is always open when the compressor is in operation and the second, third and fourth oil lines each have an on/off oil control valve that is selectively opened or closed to control the temperature of the air and oil exiting the compressor.

Although the invention is described as being applicable to air compressors it is understood that any type of gas is applicable where oil is used as one of means to regulate the gas temperature.

The following description sets forth specific embodiments of our invention and is not intended to limit the scope of our invention to the specific embodiments described and shown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of our invention.

FIG. 2 is a front elevation perspective view of our compressor.

FIG. 3 is a rear elevation perspective view of our compressor.

FIG. 4 is a bottom perspective view of FIG. 3.

FIG. 5 is a partial cross sectional view taken along line 5-5 of FIG. 3.

FIG. 6 is a partial cross sectional view taken along line 6-6 of FIG. 3.

FIG. 7 is a front elevation perspective view of our manifold used to connect the two compressors of our two stage compressor.

DETAILED DESCRIPTION OF THE INVENTION

In our two stage rotary screw compressor we provide a stepped system that adjusts the oil volume flow rate to achieve the desired discharge temperature from a compression chamber. We do this by a series of independent oil injection paths each having a shut off valve. One path is left on all the time to insure the compression chamber never operates without injection of oil. As additional cooling is needed the valves on the additional paths are turned on. By adjusting the volume flow rate of oil, the temperature of the compressor discharge flow stream can be changed very quickly. It is possible to achieve optimal discharge temperatures even with cold injection of oil and cold oil system components. With a conventional oil injection system the compressor will operate well below the optimal temperature until there has been enough heat put into the system to bring the oil mass and oil system components up to a hot enough temperature to inject hot oil into the compression chamber. This can be a long time if the compressor is in a cold environment, operating at a low load, a low speed, operating intermittently, or a combination of these.

Operating below the optimal discharge temperature results in the compressor potentially condensing water out in the oil system. This water in the oil system causes premature failure of the compressor. We maintain our discharge temperature at least above the dew point and preferably 10° F. above the dew point.

By having the optimal amount of oil injected into the compression process, we improve the operating efficiency of the compressor. Too much oil results in wasted power losses through viscous drag of the oil. Too little oil results in leakage of the gas being compressed due to poor sealing.

Injecting less oil through oil volume control results in less oil being circulated through the compressor and thus less oil needs to be separated from the discharge air stream, allowing better separation efficiencies and reduced oil carryover.

Injecting cooler oil can result in improved oil life due to reduced average oil operating temperatures. This can extend the time between oil changes. Injecting cooler oil into the bearings increases the viscosity of the bearing oil and extends bearing life. The stepped injection system allows cold oil to be fed into the compressor by control of the volume flow rate of oil injected into the rotors.

Our system is illustrated in FIG. 1 which shows the flow diagram of a two stage variable speed rotary compressor. However our system as hereinafter described is also applicable to a single twin screw compressor.

In FIG. 1 air or gas is indicated as “______”; oil is indicated as “______” and water is indicated as “______”.

The two stage system shown in FIG. 1 has a first stage low pressure compressor compression chamber, or air end C connected to a second stage high pressure compressor compression chamber or air end D. Oil is fed to the rotors and bearings in the first stage air end C by oil lines U(a), U(b), U(c) and U(d) with oil line U(d) feeding oil to the rotors via on oil orifice nozzles N4, and N5 and bearing oil orifice nozzle N10. However, it is also possible to make the bearing oil feed independent of the oil injected into the first stage air end cylinder. This would allow us to separate the effect of bearing oil reheating on the air end performance. We could thus hold a fixed flow rate to the bearings and change the amount of oil sent through the cylinder.

Oil lines U(a), U(b), U(c) are connected to on/off oil orifice nozzles N1, N3, and N2 respectively by respective on/off valves 26, 27 and 28. When U(a) and U(b) are opened, a portion of the oil therein is fed via oil lines 39 and 40 through check valves X to the on/off oil orifice nozzles N6, N7, N8 and N9 located in the manifold 24 that joins first and second stage air ends C and D. These are used to regulate the discharge temperature from second stage air end D. N1-N5 are internal orifice nozzles in the first stage air end C castings; N6-N9 are externally piped nozzle assemblies; N10-N11 are externally piped orifice nozzles that are fed by oil line 38 to supply oil to the compressor bearings; and N12 is an oil scavenge external orifice nozzle.

PT 1 is an Oil Pressure Transducer;

PT 2 is an Interstage Pressure Transducer;

PT 3 is a Reservoir Pressure Transducer;

PT 4 is a Separator Pressure Transducer;

PT 5 is a Plant Pressure transducer;

T1 is an Ambient Temperature Thermistor;

T2 is an Interstate Temperature Thermistor;

T3 is a Second Stage Discharge Temperature Thermistor;

T4 is a Separator Temperature Thermistor;

T5 is a Package Discharge Temperature Thermistor,

A is an Air Filter;

B is an Inlet Butterfly Valve;

C is a Low Pressure Compressor;

D is a High Pressure Compressor;

E is an Oil Separation Vessel;

F is an Oil Coalescer(s);

G is a Minimum Pressure Valve;

H is an Aftercooler;

I is a Condensate Separator;

J is a Condensate Drain Device;

K is an Oil Cooler,

L is an Oil Mixing Valve;

M is an Oil Filter;

N is an Orifice nozzle (N1-N12);

O is a Relief Valve-High Pressure;

P is a check Valve-Interstage Pressure;

Q is a Pressure Regulator-Inlet Valve Control Air,

R is a 3-Way Solenoid Valve-Inlet Valve Control Air,

S is a 2-Way Solenoid Valve-Blowdown;

T is a Strainer;

U is a Two-Way Solenoid Valve-Oil Flow Control;

V is a Check Valve-Inlet Butterfly Valve bypass;

W is a Ball Valve-Oil Drain(W1-W3);

X is a Check Valve-Oil Flow Control;

Y is a Vacuum Switch;

Z is a Thermostatic Flow Regulator for Water Temperature Control

AA is a Water Flow Stop Solenoid Valve

55 (FIG. 4) is a Fill Plug

As shown in FIG. 1 air is fed to the low pressure first stage air end C via air line 21 and air line 22. The temperature of the air intake side of the first stage air end C, line 21, is recorded by ambient temperature thermistor T1. Generally, the air is at ambient temperature, i.e., 60° to 90° F. However, in cooler environments, the temperature can be as low as 32° F. or even lower. The air is fed to line 21 through an air filter A and vacuum switch Y. In FIG. 1, P is a spring operated check valve. This allows air to flow only in the direction from air end of C, the low pressure air end, to the discharge of air end of D. Under most circumstances the valve is closed to prevent the high pressure discharge of the second stage from leaking back into the discharge of the first stage. In the event that the first stage discharge pressure is higher than the second stage discharge pressure check valve P will open allowing the first stage to bypass the second stage. Line 22 has an on/off valve S (for instance a 2-way solenoid valve) between the air/oil separator E and the first stage air end C. The air from the air/oil separator E to control the Butterfly Valve B, first passes through a pressure regulator inlet feed valve Q, then another feed valve R (for instance a three-way solenoid valve), and another valve B(for instance, a butterfly valve). The first stage or end C can be referred to as first stage compressor C. The second stage air end D can be referred to as second stage compressor D.

Line 22 is the blow down line. It functions to (1) allow the compressors to start without having to overcome pressure in the separator E. Starting against pressure takes more power. (2) It is also opened in the event that we are making more air at minimum speed then the system demands. This allows some of the compressed air from the reservoir to circulate back to inlet dropping the capacity we make at our minimum operating speed. This wastes power and air but can avoid stopping the compressor.

Under normal running conditions the valve S is closed. This prevents air from the separator E from getting to the inlet pipe 21. When the compressor is stopped or shutting down the valve S is opened. This allows air from the separator E to vent to the pipe on the inlet filter side of the butterfly valve B.

The Purge Check Valve V that is shown connecting line 22 to C is needed to allow the compressor to make some air when there is no pressure available to open the butterfly valve B. Since the valve logic for B is normally closed air to open, when there is no air in item E you can not open valve B. Item V solves this problem by allowing a small bypass leg so the compressor can suck some filtered air from the filter side of the butterfly valve into the first stage compressor inlet. Once it has sucked enough air through the check valve to build up pressure in the reservoir E then pressure builds in line 23 and opens the butterfly valve. The FILL PLUG (indicated as 55 in FIG. 4) is connected to lines 21 and 22. The fill plug 55 can be removed and oil dumped in this large hole to insure that there is oil in the compressor for an initial start up. Filtered air from 21 can bypass the inlet butterfly valve B and get into the first stage compressor C through a one-way check valve allowing air to enter at this point. This is necessary so that a system with no air pressure can have a way to build pressure to open the normally closed inlet valve (air to open logic spring close). Also when valve S opens, air from the reservoir can exit the reservoir and enter the inlet of the first stage if it is spinning and drawing a vacuum, through this connection.”

Line 23 is control air for the inlet butterfly valve. When item R the three way valve connects the regulated output of Q to the operating piston on the butterfly valve B the pressure overcomes the spring pressure on the back side of the piston and the inlet valve opens. There is no mixing of this control air in line 23 with the inlet air from the filter. When the three-way valve R shifts the other way the air on the operating piston of butterfly valve B is vented to atmosphere. The spring on the back of the piston causes the butterfly valve to close.

Oil, generally at a temperature of about 150° F. is injected into the first stage compressor C and the inter-stage manifold 24 and is mixed internally with air. The oil being cooler than the air being compressed, cools the air in the first stage compressor C and the air/oil mixture being fed to the second stage compressor D. It is desirable to keep the injected oil as cool as possible, within limits. If the oil is too cold it may not flow enough volume through the small passages. The thermal mixing valve L is a cooler bypass valve. We want the oil to heat up quickly so that we avoid condensation of moisture in the system. Once the oil is up to about 150° F. we want to keep the oil as close to this temperature as possible so that we can inject the minimum amount of oil we need to keep the discharge temperature acceptable and keep the viscous drag losses low.

Only a small amount of oil is fed into the second stage. This is for bearing lubrication (N11). There is also a small flow of oil/air mixture from the air oil separator element. Line 34 passing through check valve “X”, strainer “T” and nozzle N12 functions to extract the oil that the separator element collects. This flow is very small and the oil is returned to an intermediate pressure zone in the second stage air end. This is done to reduce the pressure drop across the orifice and allow less flow with a larger (less prone to clog) orifice.

The compression process generates heat. The function of the oil in this case is to absorb the heat of compression. The positions of the injection holes in the first stage are selected so that the oil is put into the compressor C after the compression has begun. Using the oil to heat the air would loose efficiency. Likewise oil is injected into the inter-stage pipe through a series of nozzles to atomize the oil into a fine mist of droplets to maximize the heat transfer from the hot air into the oil. This mixture is ingested by the second stage where additional compression work results in additional heating of the mixture.

The volume flow of the cool oil should be matched to the operating load of the compressor to absorb the excess heat of compression. Ideally the ail/oil mixture being discharged from the second stage compressor D is at a temperature above the dew point and below the temperature at which the lubricant breaks down too rapidly.

Our system provides a plurality of oil orifice nozzles for the first stage compressor C. As stated above, we generally have one inlet nozzle always having an oil flow. Although one nozzle can be sufficient for this, in this embodiment we divided this into to streams of oil to feed oil to the two oil orifice nozzles N4 and N5 that always have a flow of oil. The other orifice nozzle or orifice nozzles are on/off oil orifice nozzles N1, N2 and N3 that each have on/off valves 26, 28 and 27 respectively. The on/off valves are selectively opened to control the flow of oil to the compressor C. The number of oil orifice nozzles shown is for our preferred embodiment and there can be more or less oil orifice nozzles. However, there needs to be at least one on oil line connected to one on orifice nozzles and one on/off oil line connected to one on/off orifice nozzle. In our system as shown in FIG. 1, we decided oil line U(d) would be the on oil line and would be connected to two interconnected nozzles N4 and N5. Oil line U(b) would be the at least one on/off oil line and would be connected to two interconnected nozzles N1 and N3 as shown in FIGS. 4 and 5 or connected to is a separate nozzle N3 as shown in FIG. 1 with the separate nozzle N1 connected to an on/off oil line U(a) which is our alternative embodiment. When there are more than one on/off oil line and orifice nozzle, the compressor on/off oil lines and orifice nozzles are sized to allow the volume of oil being fed to the compressor to be progressively larger or smaller to control the temperature of the compressor to be within acceptable limits.

In our preferred compressor system and compressor C, we do not have a oil line U(a) nor an individual nozzle N1. We have the interconnected nozzles N1 which is indicated as nozzle 54 and N3 which is indicated as nozzle 56 and are shown in FIGS. 4 and 5. Either 54 or 56 could be Ni with the other being N3.

As shown in FIG. 1, the air/oil discharge from compressor C is delivered via pipe or manifold 24 to high pressure second stage air end D. Compressors C and D each have oil drain valves W1 and W3 and manifold 24 has an oil drain valve W2 to drain oil there from. The drain valves would be left closed under operation. Only in the event that the compressor needed to be drained would the valves be opened. This would be done for example when the lubricant was changed in the compressor to get as much of the old oil out of the system as possible. It would also be done prior to a major repair of either compressor. Compressors C and D can be referred to collectively as a compression chamber. The system could also throttle the oil to control the flow.

Referring to FIG. 1, oil supply line 20 supplies oil to the compressor C oil lines or paths U(a), U(b), U(c) and U(d). The U(d) oil flow path is always on. Oil line 20 is connected to on/off valves 26, 27 and 28 which connects oil line 20 to the respective oil lines or paths U(a), U(b) and U(c). In this embodiment oil orifice nozzles are separate nozzles and are not interconnected by a common line. When on/off valve 26 is opened oil line U(a) delivers oil to first stage compressor oil orifice nozzle N1 and via oil line 39 to inter-stage oil orifice nozzles N8 and N9 located in manifold 24. When on/off valve 27 is opened, oil line U(b) delivers oil to compressor orifice nozzle N3 and via oil line 40 to inter-stage oil orifice nozzles N6 and N7 located in manifold 24. When on/off valve 28 is opened, oil line U(c) delivers oil to compressor oil orifice nozzle N2. Oil line U(d) is always open and delivers oil to compressor oil orifice nozzles N4 and N5. In this embodiment, the N1 orifice nozzle is almost half the size of the N3 orifice nozzle and the N3 orifice nozzle is almost ½ the size of the N2 orifice nozzle.

When the on/off valve 26 is opened, oil is also delivered to oil line 39 which delivers oil to the manifold 24 on/off oil orifice nozzles N8 and N9. When the on/off valve 27 is opened oil is also delivered to oil line 40 which delivers oil to the manifold 24 on/off oil orifice nozzles N6 and N7.

As stated above, when the two stage compressor assembly is turned on, air is sucked into the compressor C at air inlet 200 through air line 21. Oil is injected into the compressor C from orifice nozzles N4 and N5 via oil line U(d). Compressor C has an OIL DRAIN VALVE to drain oil from the first stage air end housing. The air/oil discharge from compressor C is delivered to compressor D via the inter-stage manifold 24 which connects the air/oil discharge of compressor C to the air/oil inlet of compressor D. The inter-stage manifold 24 has an oil drain W2. Compressor D has an oil drain W3 to drain oil from its compressor housing. The air/oil discharge from compressor D is delivered to the air/oil separator E which has an oil drain valve to drain oil. The separator E contains a supply of oil 25a that is used to feed oil to compressor C via oil line 20.

When valve 26 is opened, the oil pressure as noted by P2 is maintained so that the volume of oil into compressor C is increased accordingly. The pressure of the oil at PT1 is at a substantially constant desired value in order that the volume of oil will increase or decrease according to the amount of oil desired to be fed to the compressors.

Our control system that we illustrate in FIG. 1 has 4 normal modes of oil control. They are listed below in order of flow from the minimum.

(1) Base—oil is fed to line U(d)/nozzles N4 and N5 and line 38/nozzles N10 and N11

(2) Base+A—in addition to the above base flow valve 26 is opened putting oil into line U(a)/nozzle N1 and line 39/nozzles N8 and N9.

(3) Base +B—in addition to (1) above valve 27 is opened {valve 26 is shut}. This feeds oil lines U(b)/nozzle N3 and line 40/nozzles N6 and N7.

(4) Base+A+B—Both valves 26 and 27 are open.

In this embodiment, we elected to make the valve 28 and circuit U(c) a safety start up circuit. It is used to supply additional oil in the event that the normal A and B circuits did not supply enough oil to keep the temperature T3 below 215° F. On starting it is used to insure a good supply of oil even with little system pressure.

The U(d) line supplying nozzles N4 and N5 in the first stage compressor is always on. The 38 line is also always on supplying N10 and N11 the bearing oil supplies. Together the bearing oil and the N4+N5 oil forms the base oil supply. When we start the system we open the three valves 26, 27 and 28. This supplies a large amount of oil on starting while the system builds pressure. Once the unit has run a short time (15 seconds or so) provided the discharge temperature T3 is below 185° F. we close valve 28 shutting off the flow to line U(c).

The valve 28/line U(c) is what we refer to as the start up valve or our C circuit. The N2 nozzle on the compressor C air end is at atmospheric or near atmospheric pressure. Dumping oil in through a large hole early in the compression process is a waste of efficiency, so normally we don't want to do this. On start up it is justified.

So now the compressor is up to pressure and running along with valves 26 and 27 open. If T3 is below 185° F. the control decides that the discharge temperature is too cold and drops back to the next lower oil flow level. It would close the valve 26 stopping the flow of oil to lines 39 and U(a). With the valve 26 shut and the valve 27 open the compressor is running in base+B mode.

If the T3 temperature is still below 185° F. with the valve 28 and 26 shut, the controller will step down the oil flow trying to get the temperature at T3 up to 185° F. Running in Base+B mode it needs to switch to Base+A mode. So it will close the valve 27 and open the valve 26.

If it is still running below 185° F. it steps from base+A to base mode by shutting valve 26. In this mode all the valves are shut.

The B circuit consists of valve 27, lines U(b) and 40, first stage injection nozzle N3, and inter-stage pipe injection nozzles N6 and N7. This circuit B supplies a large amount of oil to both the first stage compressor C and the inter-stage pipe 24.

The A circuit consists of valve 26, lines U(a) and 39, first stage injection nozzle N1 and inter-stage pipe injection nozzles N8 and N9. This circuit A supplies about half the oil of the B circuit.

The control logic is if T3>215° F. then the system needs to step up the oil flow. If T3<185° F. the system needs to step down the oil flow. If T3 is between 185° F. and 215° F. it stays in that state. If not, it moves up or down one step. The response of temperature T3 is within seconds, Stepping down the oil flow rapidly increases T3. Stepping up the oil flow rapidly drops T3. It only takes a few seconds for the control to match the oil flow to the power.

The following is an illustration to explain of our method when we suppose we need a total oil flow of 40 gallons per minute at full load with the valves 26 and 27 both open and the valve 28 closed.

The base/bearing oil flow circuits are both always on. Using only valves 26 and 27, we have 4 steps of control possible. Step 1 minimum flow=valve 26 and valve 27 shut. Say this is about 10 gallons per minute of oil. Step 2, open valve 26 with valve 27 closed. This adds the A circuit capacity to the base oil flow. Say that the oil flow rate of the A circuit was 10 gpm added to the 10 gpm of base oil flow, now the Base+A oil flow is 10+10=20 gpm.

The next step up is to turn on valve 27 with valve 26 off. The circuit B would be sized for 20 gpm. The Base flow is 10 gpm and thus the total injection flow would be 30 gpm. The next step is to turn on valve 26 again so that the Base, A and B circuits all flow oil at the same time. Now the injected oil flow is 10 gpm (Base)+10 gpm (A)+20 gpm (B) for a total of 40 gpm.

With two valves we can approximately control the oil flow in 4 steps of 10 gpm from 10 to 40 gpm.

With additional valves sized appropriately it is possible to have much tighter control of the oil flow. Tighter control of the oil flow=tighter control of the discharge temperature. A third valve gets a total of 8 steps of control (2³). So our operating range could be 200+/−7.5° F. with a third valve. A fourth valve gets 16 steps (2⁴) and so on. Each additional valve/circuit adds cost so a balance is struck.

The ideal orifice sizes are selected so that: the difference between the maximum and base flow is divided by the number of steps less one.

Using the above illustration the maximum flow=40. The minimum flow=10 so the difference is 30. If we used 3 valves to control the oil flow we can have 8 steps so 8−1=7. Thirty divided by seven=4.28=minimum step size=A circuit size. Circuit B is double circuit A and circuit C is 4× circuit A.

Given the 10 gpm base flow and a system with 3 valves and a 40 gpm maximum flow. Valve 26=4.28 gpm, valve 27=8.57 gpm, valve 28=17.14 gpm.

First step (all valves off/Base flow on)=10 gpm.

Second step Base+A=14.28 gpm

Third step=Base+B=18.57 gpm

Fourth step=Base+A+B=22.85 gpm

Fifth step Base+C=27.14 gpm

Sixth step=Base+A+C=31.42 gpm

Seventh step=Base+B+C=35.71 gpm

Eighth step=Base+A+B+C=40 gpm

In practice what happens is that as the flow is increased the pressure drop between the oil reservoir and the point of injection increases slightly so the A circuit supplies a little more oil when it is open with just the base circuit. And a little more oil when it is opened with base+B circuit (fourth step) turned on.

The system of course can contain more or less than the five oil orifice nozzles and vary the steps of injection if desired.

In our preferred embodiment, the U(a) oil line is eliminated from FIG. 1 and the N1 and N3 nozzles are interconnected with a common line as shown in FIG. 4. The common oil line shown in FIG. 4 is connected to oil line U(b).

Our preferred control system also has 4 normal modes of oil control. They are listed below in order of flow from the minimum.

(a) Base—oil is fed to line U(d)/nozzles N4 and N5 and line 38/nozzles N10 and N11

(b) Base+A—in addition to (a) above valve 26 is opened. This feeds oil line 39/nozzles N8 and N9.

(c) Base+B—in addition to (a) above valve 27 is opened. This feeds oil line U(b)/nozzles N1 and N3 and oil line 40/nozzles N6 and N7

(d) Base+A+B—Both valves 26 and 27 are open.

In this embodiment, we elected to make the valve 28 and circuit U(c) a safety start up circuit. It is used to supply additional oil in the event that the normal A and B circuits did not supply enough oil to keep the temperature T3 below 215° F. On starting it is used to insure a good supply of oil even with little system pressure.

As above, the U(d) line supplying nozzles N4 and N5 in the first stage compressor is always on. The 38 line is also always on supplying N10 and N11 the bearing oil supplies. Together the bearing oil and the N4+N5 oil forms the base oil supply. When we start the system we open the three valves 26, 27 and 28 with 26 supplying oil to nozzles through line 39. This supplies a large amount of oil on starting while the system builds pressure. Once the unit has run a short time (15 seconds or so) provided the discharge temperature T3 is below 185° F. we close valve 28 shutting off the flow to line U(c).

So now the compressor is up to pressure and running along with valves 26 and 27 open. If T3 is below 185° F. the control decides that the discharge temperature is too cold and drops back to the next lower oil flow level. It would close the valve 26 stopping the flow of oil to line 39.

With the valve 27 open and valve 26 shut the compressor is running in base+B mode. If T3 is below 185° F. the control decides that the discharge temperature is too cold and drops back to the next lower oil flow level. It would open valve 26 resuming the flow of oil to line 39, and close valve 27, stopping the oil flow to lines 40 and U(b).

With the valve 26 open and valve 27 shut the compressor is running in base+A mode. If T3 is below 185° F. the control decides that the discharge temperature is too cold and drops back to the next lower oil flow level. It would close valve 26 stopping the flow of oil to line 39/nozzles N8 and N9.

With valves 26, 27 and 28 shut the compressor is running in base only mode. This is the minimum oil flow.

If the T3 temperature rises above 215° F. with the valves 26, 27 and 28 shut, the controller will step up the oil flow trying to get the temperature at T3 below 215° F. Running in Base mode it needs to switch to Base+A mode. So it will open the valve 26.

If it is still running above 215° F. it steps up from base+A to base+B mode by shutting valve 26 and opening valve 27.

The B circuit consists of valve 27, lines U(b) and 40, first stage injection nozzles N1 and N3, and inter-stage pipe injection nozzles N6 and N7. This circuit B supplies a large amount of oil to both the first stage compressor C and the inter-stage pipe 24.

The C circuit consists of valve 28, lines U(c), and first stage injection nozzle N2. This circuit C supplies about twice the oil of the B circuit.

The control logic is if T3>215° F. then the system needs to step up the oil flow. If T3<185° F. the system needs to step down the oil flow. If T3 is between 185° F. and 215° F. it stays in that state. If not, it moves up or down one step. The response of temperature T3 is within seconds. Stepping down the oil flow rapidly increases T3. Stepping up the oil flow rapidly drops T3. It only takes a few seconds for the control to match the oil flow to the power.

The following is an illustration to explain of our method when we suppose we need a total oil flow of 40 gallons per minute at full load with the valves 27 and 28 both open and the valve 26 closed.

The base/bearing oil flow circuits are both always on. Using only valves 26 and 27, we have 4 steps of control possible. Step 1 minimum flow valve 26 and valve 27 shut. Say this is about 10 gallons per minute of oil. Step 2, open valve 26 with valve 27 closed. This adds the A circuit capacity to the base oil flow. Say that the oil flow rate of the A circuit was 10 gpm added to the 10 gpm of base oil flow, now the Base+A oil flow is 10+10=20 gpm.

The next step up is to turn on valve 27 with valve 26 off. The circuit B would be sized for 20 gpm. The Base flow is 10 gpm and thus the total injection flow would be 30 gpm. The next step is to turn on valve 27 again so that the Base, A+B circuits all flow oil at the same time. Now the injected oil flow is 10 gpm (Base)+10 gpm (A)+20 gpm (B) for a total of 40 gpm.

With two valves we can approximately control the oil flow in 4 steps of 10 gpm from 10 to 40 gpm.

As previously stated, the ideal orifice sizes are selected so that: the difference between the maximum and base flow is divided by the number of steps less one.

Referring to FIG. 1, oil is fed from oil separator E to compressor C via oil line 29. Oil line 29 is divided into oil lines 31 and 32. Oil line 32 passes through water cooler K and enters the cool oil connection f of mixing valve L. Oil line 31 carries hot oil and enters the hot oil connection e of mixing valve L. The hot and cool oil are mixed and exit the mixing valve outlet d. The mixed oil passes through the oil filter M and is then fed to oil line 20. Although FIG. 1 depicts a unit with a thermal mixing circuit, the stepped injection should be equally applicable to a system without a thermal mixing valve, where all oil would pass through the oil cooler and this cold oil would be injected at the appropriate mass flow rate to achieve the desired discharge temperature.

The separator E contains an oil coalescer F, oil supply 25a, scavenge oil 25b, and air 25c. The coalesced oil from 25b is fed to the scavenge connection of the high-pressure compressor D via line 34. The air exits separator E via air line 37. The air that is discharged from our system is discharged via air line 37 which preferably first passes through a water-cooled heat exchanger H. The air/oil discharge from compressor D is fed via air/oil line 36 to Separator E.

The oil injection temperature is adjusted by thermostatic mixing valve L mixing hot oil that has bypassed the oil cooler with cooler oil that has passed through the cooler. The thermostatic valve L when used with stepped injection can supply cooler oil at a lower flow rate to achieve the desired discharge temperature at T3.

The oil system is a common supply 20 that connects to the oil lines U(a) if present, 39, 40, U(b) and U(c) through on/off valves 26, 27 and 28 and oil lines 38 and U(d). As noted above, these oil lines supply the oil to the oil orifice nozzles N1, N2, N3, N4, N5, N6, N7, N8, N9 and N10. The suction end bearing oil is preferably returned to a low-pressure zone of compressor C. The discharge bearing oil is preferably returned to a low-pressure zone at the discharge end of compressor C.

It should be noted that increasing the oil injection pressure by reducing the size of the cylinder injection orifice nozzles at a constant oil flow into the oil gallery results in a re-distribution of the oil within the air end of the compressor. Thus, more oil would pass through the bearings and return to the suction area of the compressor rotors, thus creating pre-heating of the air at the suction end of the rotors. The size of the injection oil orifice nozzles can be changed for a particular application by using a plug of a different (smaller) size in the injection orifice nozzles with the plug being connected to the selected oil line U(a), U(b), U(c) and/or U(d).

The temperature and pressure of compressor C are monitored by interstage temperature thermistor T2 and interstage pressure transducer PT2. Generally, after operating for a period of time at near full load, the temperature of the air/oil entering separator E increases over a desired temperature i.e., 215° F. As stated above, when that occurs, appropriate valves are opened so that additional oil is injected into compressor C and manifold 24 via the appropriate oil lines and on/off valves. This additional oil that is injected into the compressor C and the inter-stage connecting manifold 24 cools the compressors and the air/oil discharge temperature T3 to below 215° F.

The position of the temperature thermistor T1 is above the compressor inlet valve. The position of the temperature thermistor T2 is near the discharge port of compressor C. The temperature readings that T2 sees are the discharge temperature of compressor C. This temperature will be higher than the temperature of the injected oil as the heat of compression is added to the mass of oil and air passing through compressor C. The position of the temperature thermistor T3 is prior to the air/oil separator E. The temperature thermistor T4 is at the air discharge of the vessel E and the temperature thermistor T5 is located at the air discharge outlet. The readings of the thermistors are done by any appropriate temperature thermistor reader. It is important that the air and air/oil exiting the compressor D and that within the separator E is above the pressure dew point to prevent condensation of water which is detrimental to the compressor assembly. The thermistors T1-T5 and the pressure transducers are connected to appropriate controls and processors to regulate the oil orifice nozzle valves and oil feed to provide the desired volume and temperature of oil being fed to the compressor C and the inter-stage manifold 24. In addition to using thermistors, it would be possible to measure the temperature using RTD's, thermocouples or various other temperature sensor technologies.

The general construction of compressor C and D are well known. To the known general structure, we have provided compressor C with our improved structure of having a plurality of oil orifice nozzles to deliver oil to compressor C, a plurality of oil orifice nozzles to deliver oil to the manifold connecting compressor C and compressor D, and having one oil orifice nozzle to compressor C always open. As shown in FIG. 2, our twin-screw compressor C has a twin screw rotor housing 50 with an air intake or suction inlet 51. The housing 50 has therein compressor screws. FIG. 5 shows at 52 and 53 where the two parallel screw rotors would be located. The twin screw rotor housing of has five oil orifice nozzles N1, N2, N3, N4 and N5 that are controlled by the respective on/off valves. In FIG. 5, the oil orifice nozzle N1 is nozzles 54 and the oil orifice nozzle N3 is nozzle 56 joined by a common conduit. As stated previously, there would be no common conduit joining nozzles 54(N1) and 56(N3) if this was the alternative embodiment of FIG. 1. Nozzles 54 and 56 are positioned to simultaneously inject oil into the gaps of the rotors. The on/off oil line U(b) connects to the pair of oil orifice nozzles N1 and N3; and the on/off oil line U(c) connects to the orifice nozzle N2; and the on oil line U(c) connects to the pair of oil orifice nozzles N4 and N5 (nozzles 57 and 58 respectively) which are positioned to simultaneously inject oil into the gaps of the rotors. The combined volume of oil path A for our preferred embodiment is about equal to the volume of our base oil path and our oil path B is about twice the volume of our oil path A.

Our oil valves are connected to their respective nozzles by an appropriate connection i.e. a threaded connection. The on/off valves are preferably operated automatically to sequentially open and close depending on the temperature T3 and are appropriately connected to controllers which are commercially available and are not a part of our invention.

The housing 50 has motor adapter 61 on one and at the opposite end the air/oil exhaust or discharge 62.

Housing 50 is provided with oil orifice nozzle N10 that delivers oil to the compressor C bearings. The housing 50 has an oil drain outlet 59 that is to be connected to oil drain valve W1.

Referring to FIG. 7 there is illustration of one type of manifold 24 that is used to connect compressor C to compressor D. The manifold has an inlet end 71 that attaches to the exhaust 62 of compressor C. The arrow 72 shows the direction of flow of the air/oil mixture that flows through the manifold from compressor C to compressor D. The manifold has a pair of mounting extensions 73. The mounting extensions 73 are on one side and are shown as two axially spaced rounded end rectangular extensions. On the side opposite the extensions 73 are the pair of on/off oil orifice nozzles N6 and N7 and at the elbow entrance of the manifold, the pair of on/off oil orifice nozzles N8 and N9. The oil lines 39 and 40 are connected to the orifice nozzles by an appropriate connection i.e. nozzle adaptor. The discharge end of the manifold 24 can have an oil orifice nozzle 74, if desired, that is connected to valve 26, 27 or 28 depending on the desired oil flow for a particular step.

Although the above description is directed to twin stage compressors, it is of course, understood that our invention is applicable to a single stage twin screw compressor such as compressor C used alone without compressor D and the components necessary for forming the twin stage.

The particular controls that we use are not part of our invention and would be within the ordinary skills of the artisan knowing our disclosure. The particular temperatures and pressures that are used are known to those in the field and will vary depending on the size of the compressor or compressors and on the operating conditions.

Claims

1. An improved rotary screw compressor that has compression chamber, said compression chamber including a pair of parallel cooperating compressor screws within a housing, said compression chamber having an air inlet and an air/oil outlet, the improvement comprising; a plurality of oil orifice nozzles, provided in said housingone of said plurality of oil orifice nozzles being an on oil orifice nozzle that is always open or always regulating the flow of oil,another of said oil orifice nozzles being connected to an on/off valve, andsaid on/off valve being selectively opened and closed to regulate an oil volume flow into said compression chamber to regulate a discharge temperature, said discharge temperature being a temperature of a discharge from said compression chamber.

2. The improved rotary screw compressor of claim 1 wherein the housing has a plurality of oil orifice nozzles that are connected to at least two on/off valves, and each on/off valve being selectively opened and closed to regulate the oil volume flow to regulate the discharge temperature.

3. The improved rotary screw compressor of claim 2 wherein the oil orifice nozzles are sized to provide a different volume of oil to be fed to the compression chamber when by each on/off valve.

4. The improved rotary screw compressor of claim 3 wherein said on/off orifice nozzles are selectively opened and closed to maintain the discharge temperature slightly above the pressure dew point of the discharge mixture.

5. The improved rotary screw compressor of claim 3 wherein there are two on/off valves are selectively opened and closed when the discharge temperature is at a predetermined temperature.

6. The improved rotary screw compressor of claim 5 wherein of said two on/off valves, the second on/off valve and oil nozzles connected to the second on/off valve provide a larger volume of oil to the compressor than said first on/off valve and oil orifice nozzles connected to the first on/off valve.

7. The improved rotary screw compressor of claim 5 wherein after start up, said second on/off valve is closed and the first on/off valve is opened. if the temperature is below the predetermined temperature with the first on/off valve opened and the second on/off valve closed, the oil flow is stepped down by closing the first on/off valve; andif the temperature rises above a second predetermined temperature with the first and second on/off valves closed, the first and second on/off valves are selectively opened and closed to step up the oil flow to reduce the temperature to below the second predetermine temperature.

8. The improved rotary screw compressor of claim 4 wherein there are three said on/off valves, the second on/off valve and oil nozzles connected to the second on/off valve provide a larger volume of oil to the compressor than said first on/off valve and oil orifice nozzles connected to the first on/off valve; and the third on/off valve and oil nozzles connected to the third on/off valve provide a larger volume of oil to the compressor than said second on/off valve and oil orifice nozzles connected to the second on/off valve.

9. The improved rotary screw compressor of claim 7 wherein at least part of the oil from the open oil line is directed to compressor bearings.

10. The improved rotary screw compressor of claim 8 wherein at least part of the oil from the open oil line is directed to compressor bearings.

11. A compressor system comprising: a compression chamber;an air inlet valve;an air inlet in fluid communication with said air inlet valve, said air inlet opening into said compression chamber;a discharge outlet in fluid communication with an air/oil separator, said air/oil separator connected to said discharge outlet, said air/oil separator having a separator air outlet and a separator oil outlet, said discharge outlet downstream of said air inlet and in fluid communication therewith,an air line connecting said oil separator outlet to an air discharge;an oil supply,three oil lines connected to the oil supply,each of said oil lines connected to an oil nozzle with each of said oil nozzles connected to deliver oil to the compression chamber,two of said oil lines each having an on/off valve connected between the oil supply and the compression chamber and the third oil line always being opened when said compressor is in operation, andsaid first and second on/off valves being selectively opened and closed to control a compression discharge temperature.

12. The compressor system of claim 11, wherein the compressor is a two stage variable speed compressor wherein said compression chamber comprisesa first pair of rotary screws within a housing to form a first stage;a second pair of rotary screws within a housing to form a second stage;an air/oil orifice nozzle in an inter-stage connecting pipe injecting additional oil into the discharge air oil stream of said first stage and supplying air/oil mixture with supplemental injected oil to the inlet of said second stage;said second stage having the discharge outlet connected to said air/oil separator, andsaid air/oil separator having an oil line connected to the second stage to feed oil thereto.

13. The compressor system of claim 12, wherein the second on/off valve and the oil nozzles connected to the second on/off valve has a larger volume capacity than the first on/off valve and the nozzles connected to the first on/off valve, and the two on/off valves are selectively opened and closed to control the compression chamber discharge temperature.

14. The compressor system of claim 12, wherein four oil lines are connected to the oil supply and the compression chamber of said first stage compressor,each of said four oil lines is connected to an oil nozzle with each of said oil nozzles connected to deliver oil to the compression chamber,three of said oil lines each having an on/off valve connected between the oil supply and the compression chamber and the fourth oil line always being opened when said compressor is in operation, andsaid first and second and third on/off valves being selectively opened and closed to control a compression discharge temperature.

15. The compressor system of claim 14, wherein the second on/off valve and the oil nozzles connected to the second on/off valve has a larger volume capacity than the first on/off valve and the nozzles connected to the first on/off valve, the third on/off valve and the oil nozzles connected to the third on/off valve has a larger volume capacity than the second on/off valve and the nozzles connected to the second on/off valve, and the three on/off valves are selectively opened and closed to control the compression chamber discharge temperature.