U.S. patent number 3,860,363 [Application Number 05/359,182] was granted by the patent office on 1975-01-14 for rotary compressor having improved control system.
This patent grant is currently assigned to Chicago Pneumatic Tool Company. Invention is credited to Stanley J. Minton, Ghanshyam C. Patel, David H. Silvern.
United States Patent |
3,860,363 |
Silvern , et al. |
January 14, 1975 |
ROTARY COMPRESSOR HAVING IMPROVED CONTROL SYSTEM
Abstract
A rotary air compressor driven by an electric motor and having a
control system including a control circuit which enables the
compressor to run in a modulated air flow mode of operation, shift
automatically at a certain set process pressure to an
unloading-reload mode of operation; and shift automatically when
the compressor runs unloaded beyond a timed period to a condition
in which the compressor is stopped but will automatically re-start
operating when the process pressure falls below a certain low
setting and the tank pressure drops to a safe starting value. Also
disclosed is a dual control system designed to provide automatic
transfer from the modulation mode to unloading operation.
Inventors: |
Silvern; David H. (Hollywood,
CA), Minton; Stanley J. (Woodland Hills, CA), Patel;
Ghanshyam C. (Franklin, PA) |
Assignee: |
Chicago Pneumatic Tool Company
(New York, NY)
|
Family
ID: |
23412678 |
Appl.
No.: |
05/359,182 |
Filed: |
May 11, 1973 |
Current U.S.
Class: |
417/12; 417/26;
417/295; 417/29 |
Current CPC
Class: |
F04B
49/06 (20130101); F04B 49/022 (20130101); F04B
49/225 (20130101); F04B 39/0207 (20130101) |
Current International
Class: |
F04B
49/06 (20060101); F04B 49/22 (20060101); F04B
49/02 (20060101); F04B 39/02 (20060101); F04b
049/00 () |
Field of
Search: |
;417/295,12,26,20,13,18,29,31,38,290,280,282,395,295 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Freeh; William L.
Attorney, Agent or Firm: Rudy; Stephen J.
Claims
What is claimed is:
1. A compressor system including an electric motor powered rotary
air compressor having inlet and outlet passages, a receiver tank
connected to the outlet passage having a discharge line, a demand
process line connected to the discharge line, a check valve in the
discharge line preventing backflow from the process line to the
tank, a butterfly flow control inlet valve in the inlet passage,
actuator means connected with the inlet valve responsive to a
predetermined air pressure in the tank to throttle the butterfly
valve to modulate air flow in the inlet passage to match demand
process flow; sensing means responsive to a set maximum pressure
developing in the process line beyond the check valve to close the
butterfly valve to cause the compressor to run unloaded, and
responsive to a set lower pressure developing in the process line
beyond the check valve to reopen the inlet valve to cause the
compressor to reload; means adjustable for determining the closing
of the butterfly valve substantially at the time of development of
the set maximum pressure; timing means responsive to said action of
the sensing means in sensing the maximum pressure developing in the
process line to de-energize the motor and cause the compressor to
stop running in the event said lower pressure does not develop in a
set period of time; and other sensing means responsive to the
development of both said lower pressure in the process line and a
set low pressure in the tank to reenergize the motor to restart
operation of the compressor.
2. A compressor system comprising an electric motor driven rotary
air compressor having an inlet and an outlet; a receiver connected
to the outlet having a discharge line; a first check valve between
the compressor and the receiver preventing flowback to the
compressor, a demand process line connected to the discharge line;
a second check valve in the discharge line preventing flowback from
the process line to the receiver; a piston operable butterfly inlet
valve in the inlet, the piston controlling movement of the inlet
valve relative to open and closed positions; a spring constantly
biasing the piston at one end to draw the inlet valve to open
condition; a conduit connecting the opposite end of the piston with
the receiver, the piston being responsive at its opposite end to
variations of receiver air pressure in excess of the spring bias to
move the inlet valve in a closing direction; a pressure regulator
in the conduit controlling application of receiver air pressure to
the piston below a predetermined value to effect a modulation mode
of operation of the inlet valve; a normally closed by-pass line
connected between the piston and the receiver by-passing the
pressure regulator; first pressure sensing means responsive to a
predetermined maximum rise in pressure in the process line beyond
the second check valve to open the by-pass line, the piston being
responsive to receiver pressure caused to flow through the by-pass
line to move the inlet valve to closed position to effect an
unloaded mode of operation of the compressor; bleeder valve control
means connected in the conduit between the by-pass line and the
said opposite end of the piston subject to adjustment for
predetermining the moment of operation of the piston substantially
at the time of maximum rise in pressure; the said pressure sensing
means being responsive to a predetermined drop in pressure in the
process line to effect closing of the by-pass line and restoration
of the compressor to a modulation mode of operation; and timing
means initially actuable in response to said operation of the first
pressure sensing means in response to the maximum rise in pressure
in the process line, and subsequently responsive to a failure of
the said first pressure sensing means to sense said predetermined
pressure drop within a set period of time to effect opening of the
circuit to the electric motor.
3. A compressor system as in claim 2, including an oil sump
continuously subject to pressure of the receiver having an oil feed
line connection with the compressor, a normally closed oil feed
valve in the feed line blocking pressurized flow of oil from the
sump to the compressor, a restriction in the discharge line before
the second check valve allowing rapid build-up of receiver pressure
following closing of a circuit to the motor at start-up, and means
for effecting opening of the oil feed valve at the time the circuit
to the motor is closed.
4. A compressor system as in claim 3, wherein means is provided for
causing closing of the oil feed valve upon opening the circuit to
the motor.
5. A compressor system as in claim 4, including a second pressure
sensing means responsive to a predetermined low pressure drop of
receiver pressure to condition an energizing circuit to restart the
motor; and means for completing the latter circuit in response to
the first pressure sensing means sensing a predetermined pressure
drop in the process line.
Description
BACKGROUND OF THE INVENTION
This invention relates to control systems for power driven air
compressors; and its general objective is to provide an improved
control system having desirable operating characteristics.
It is a further object to provide a control system which enables
inlet air flow to be modulated in matching relation to demand flow;
which automatically unloads the compressor when a predetermined
maximum process pressure is reached and reloads it when the process
pressure drops to a certain level; and which automatically stops
operation of the compressor when the compressor runs unloaded
beyond a set period of time, and automatically restarts its
operation when the tank pressure has dropped to a certain safe
pressure.
It is still another object to obtain the foregoing tri-mode
operation of the compressor without damage to the compressor.
A particular advantage of the tri-mode operating characteristics of
the compressor under the improved control system is a saving of
power, as well as wear and tear of the components of the
compressor.
A feature of the invention lies in the particular association of a
control system, including a control circuit, with the components of
a motor driven rotary air compressor.
A further feature of the invention lies in the nature of the
control system whereby the compressor is enabled to pass
automatically according to the demands of the compressor through
stages of flow modulation, unloading and reloading, and start and
stop operations.
Other features of the invention are various safety and warning
controls which prevent undesirable oil flooding of the compressor,
ensure adequate cooling and lubrication during start-up, signal
normal operation, guard against damage because of overtemperature
development, and stop operation entirely in the event of a power
failure.
The foregoing, as well as other objects, features and advantages
will become apparent from the detailed description which follows,
and as it is read in conjunction with the accompanying drawing
wherein an embodiment of the invention is illustrated. It is to be
expressly understood, however, that the drawing is for purposes of
illustration and description and is not to be construed as defining
the limits of the invention.
BRIEF DESCRIPTION OF DRAWING
In the accompanying drawing:
FIG. 1 is a schematic showing of an air compressor system embodying
the invention;
FIG. 2 is a diagram of the control circuit associated with the
compressor system;
FIG. 3 is a control circuit for the dual-mode system; and
FIG. 4 is a graph for selection of the unload cutin point.
DESCRIPTION OF PREFERRED EMBODIMENT
In FIG. 1 of the drawing is disclosed an air compressor system
which is controlled in its operation by the control circuit shown
in FIG. 2. The circuit may be characterized as an automatic
tri-mode control circuit in that the compressor is so controlled
that, depending upon the attendant demands of an associated process
system, it passes automatically in its operation, as needed, back
and forth from a stage of continuous modulation mode of operation
to an unload-reload mode, or from the latter to a start-stop
mode.
The compressor system, generally designated 9, includes a rotary
compressor 10, here of a screw type. It is driven by an electric
motor 11, and is provided with a cooling fan motor 12. Air enters
the compressor through a butterfly valve 13 in an inlet passage 14.
During operation of the compressor, oil is caused to be injected
into the compressor over an oil feed line 15 from an air
pressurized oil sump 16. The oil serves to lubricate, seal and cool
the compressor as the latter operates.
After leaving the compressor, the mixture of compressed air and oil
passes through a check valve 17 in an outlet line 18 to a tank 19.
The check valve prevents blow-back of air and oil from the tank to
the compressor. In the tank, the oil is separated from the air-oil
mixture by gravity and drops into the sump 16 below. A solenoid
valve S2 in the oil feed line, normally closed when the compressor
is stopped, is caused to open as the motor circuit is activated.
When the compressor is stopped, the solenoid valve closes so as to
prevent the compressor from being flooded with oil as would
otherwise happen because of the pressurized condition of the
tank.
The oil-free air flows, in order, from the tank through a venturi
or sonic choke 21 and a final check valve 22 located in the tank
line 23 to a process or high pressure air supply demand system at
24. The choke serves to restrict air flow from the tank so that
during start-up of the compressor operation the tank pressure will
build up quickly enough to timely force oil flow over the line 15
to the compressor. The final check valve 22 permits the compressor
to be unloaded or operated at low tank pressure without bleeding
off the high pressure air supply demand system 24 with which the
tank is connected.
Control means is provided for automatically modulating inlet air
flow to the compressor so as to match the demand flow while
maintaining substantially constant pressure at the demand outlet.
The control means includes a closed loop conduit 25. Conduit 25
connects at one end with the tank line 23 and connects at its
opposite end both with the piston chamber of an actuator 26 for the
inlet valve 13 and also with a restricted passage or return orifice
27. The orifice connects with the inlet passage 14 downstream of
the inlet valve.
The actuator includes a piston 28 connected with the inlet valve.
It is adapted when pressurized by air bled from the tank line over
the loop 25 to move the inlet valve in a closing direction; and is
adapted under force of a return spring 29 to return the valve in an
opening direction as pressure over the piston is relaxed.
Interconnected in the loop is a controller or back pressure
regulator valve 31. The latter is set to open in response to a
predetermined pressure developing in the tank line to bleed
pressure air from the tank to both the actuator and to the return
orifice. The air volume bleeding through the regulator varies
directly with the pressure developing in the tank line. The
actuator responds to a predetermined pressure developing over its
piston 28 to move the inlet valve in a closing direction and, as
the pressure relaxes, the return spring moves the inlet valve in an
opening direction. As the tank pressure varies above a
predetermined value, the air bleeding through the regulator to the
actuator and to the restricted orifice varies accordingly. This
opening and closing action of the inlet valve serves to modulate
inlet air flow to the compressor.
It can be seen during operation of the compressor that, as the
demand flow decreases, the tank line pressure will increase since
more air is being pumped into the tank than is leaving. As this
pressure rises above a certain value at which the regulator valve
31 has been set for response, the regulator valve opens and bleeds
off pressure air from the tank to both the actuator and the return
orifice. The bleed air passing through the orifice 27 is returned
to the compressor. The bleed air to the actuator upon overcoming
the force of the return spring 29 throttles the inlet valve to
restrict inlet air flow. Throttling of the inlet valve continues
until the inlet flow matches the demand flow. Conversely, as the
demand flow increases and the tank pressure decreases, the bleed
air through the regulator valve to the actuator and orifice
decreases thus causing the inlet valve to move in an opening
direction. This throttling continues until inlet and demand air
flow match. Through this modulation of inlet flow, the air pressure
at the demand outlet aaccordingly obtains a substantially constant
value.
The control system is designed to enable the compressor to
automatically begin an unloading operation when the pressure in the
process line 24 obtains a certain set maximum, and to automatically
reload and run in a modulated manner when the process pressure
drops to a certain lower setting. To this end, the control system
includes a differential pressure switch PS1 located in the process
system beyond the final check valve 22. It also includes in the
closed loop 25 a by-pass conduit 32 which by-passes the regulator
valve 31 and is controlled by a normally closed unloading solenoid
valve S3.
The pressure switch PS1 is adjustable in its settings to
automatically effect a desired process upper pressure point at
which the compressor will run unloaded, and a desired lower process
pressure point at which the compressor will begin to reload to
prevent dropping of the process pressure below a certain minimum.
Opening of the by-pass valve S3 allows sufficient volume flow of
tank pressure air to the actuator 26 to close the inlet valve. The
maximum or upper setting at which the operator desires this to
occur is determined by the corresponding percentage demand flow at
which the operator desires the system to begin unloading. A bleed
valve 33 to atmosphere located beyond the unloading valve S3 is
adjustable to obtain pressure actuation of the actuator
substantially at the time the process demand pressure reaches the
maximum setting and the demand flow drops to a certain percentage.
The bleed valve in effect serves to obtain a more accurate or
precise control over the time of operation of the actuator. A
suitable chart may be calculated for determining the desired upper
and lower setting of the pressure switch PS1 to meet the demands of
the process system. As an illustrated example, should it be desired
that the process pressure not exceed 110 psi nor fall below 100 psi
so as to obtain a desired percentage demand flow, the pressure
switch will be set to an upper setting of 110 psi at which it is
intended to unload. By calculation or from a previously calculated
chart, it may be determined that the lower setting should be 105
psi as a point at which the compressor will timely reload so that
the process pressure will not drop below 100 psi.
Unloading operations are desirable as they result in lower power
consumption since the most economical manner of operation of the
compressor is one in which the compressor runs unloaded as much as
possible.
To obtain a further economy of operation, the control system is
designed to provide for a third or a start-stop mode of operation.
In this mode of operation, the compressor is caused to stop should
it run unloaded beyond a set period of time. To this end, a timer
TM, and a second pressure switch PS2 controlled by the timer, are
incorporated in the control system. The pressure switch PS2 is
located in the tank line 23. Switch PS2 is normally closed at tank
pressure of 25 psi and lower. It opens in response to pressure
above 25 psi.
In this third mode of operation, the compressor is caused to stop
operating when it has run unloaded beyond a timed period and it is
caused to automatically restart operating when the tank pressure
has dropped to a predetermined safe low pressure, for example, 25
psi. The low restart pressure is desired to prevent oil flooding of
the compressor as might occur if the compressor were allowed to
restart at a higher tank pressure. Flooding of the compressor
forces the latter to work harder and thus wastes power and
increases wear upon the compressor components.
In summary of the operation of the compressor as controlled by its
control system whereby the compressor is enabled to shift
automatically from one mode of operation to another: First, the
process pressure switch PS1 is set (as desired to suit the
requirements of the process system) to, say an upper setting of 110
psi and a lower setting of 105 psi, the lower setting being
calculated so that reloading will automatically occur before the
process pressure drops to 100 psi. The off-on switch in the control
circuit (FIG. 2) is then closed. (The door interlock switch is
normally closed. It opens automatically as a safety feature when
the circuit cover is removed.) The reset contacts are momentarily
closed to energize relay R1 causing contacts R1b to close a holding
circuit to relay R1 and causing contacts R1a to close.
Closing of contacts R1a establishes a starting circuit through the
normally closed switch PS2. This circuit lights the normal
indicator light A, actuates the hour meter HR which indicates the
operating period, activates the normally open solenoid blow-off
valve S1 to closed condition, opens contacts R1c to extinguish the
red warning light R, and activates the coil of the magnetic starter
MS so as to establish an energizing circuit to the cooling fan
motor 12 and to the compressor motor 11, causing contacts MS2 to
activate solenoid oil feed valve S2 to open condition, and causing
contacts MS1 to close to maintain the circuit to the magnetic
starter since PS2 will open as the tank pressure rises above 25
psi. The function of the solenoid valve S1, which has an open
condition when the compressor has stopped, is to blow off or
relieve the tank of excess pressure, as is conventional in air
compressor systems.
As the compressor now operates, air pressure rapidly builds up in
the tank because of the choke element 21 causing oil from the sump
to be forced through the solenoid valve S2 to the compressor. The
compressor commences operation, as explained earlier, in a
continuous modulation mode during which inlet flow is modulated to
match the process demand flow.
Now, should the process demand flow fall below a predetermined
percentage as to cause the process pressure to exceed the upper 110
psi setting, the normally open pressure switch PSI will close to
cause the normally closed unloading solenoid valve S3 in the
by-pass line 32 to open. This enables back pressure air from the
tank to cause the actuator piston 28 to close the inlet valve 13.
As a result, the compressor then runs unloaded as long as the inlet
valve is held closed.
While the compressor is running unloaded, bleed air is
re-circulated over the return orifice 27 through the compressor.
This is desired to avoid undesirable vibration and consequent
damage that might otherwise occur in the compressor when the latter
runs unloaded.
Closing of pressure switch PS1 also energizes relay R2 causing
contacts R2 to open. This causes the solenoid blow-off valve S1 in
the tank line to reopen and effect a progressive reduction of the
tank pressure.
Closing of pressure switch PS1 also energizes the timer TM to
commence operating.
Now, when the process pressure eventually drops to 105 psi, the
pressure switch PSI reopens. This action recloses contacts R2 to
close the blow-off switch valve S1, stops the timer TM and recloses
the unloading solenoid valve S3 in the by-pass line allowing the
actuator spring 29 to return the inlet valve to open condition as
tank pressure over the actuating piston is relaxed through the
return orifice 27. The compressor now begins to reload and once
more obtains a modulation mode of operation.
Now, if the compressor had run unloaded for a period beyond that
for which the timer TM had been set, the normally closed timer
contacts TM would have opened. Opening of the TM contacts breaks
the circuit to the magnetic starter MS causing the motors 11 and 12
and the compressor to stop, reopens the solenoid blow-off valve S1,
reopens the MS1 contacts and also reopens the MS2 contacts allowing
the oil solenoid valve S2 to close and thereby prevent oil flooding
of the compressor.
The control system is so designed that once the compressor has been
stopped through the action of the timer, it will not restart until
the tank pressure has dropped to a safe restart pressure, such as
25 psi, at which pressure the second pressure switch PS2 is
designed to reclose. Accordingly, when the process pressure drops,
while the compressor is stopped, to the lower 105 psi setting, the
pressure switch PS1 reopens. This disconnects the timer causing the
timer contacts TM to reclose, deactivates the unloading solenoid
valve S3 to closed condition in the by-pass line, and deactivates
relay R2 causing contacts R2 to reclose. Reclosing of the contacts
R2 does not affect the open condition of the blow-off valve S1
since the MS1 and the PS2 contacts are still open. Accordingly, the
compressor will remain stopped until the pressure switch PS2 is
reclosed and the magnetic starter MS is reactivated.
When the tank pressure eventually drops to 25 psi through action of
the open blow-off valve or demand flow, switch PS2 recloses. This
reestablishes circuits to close the solenoid blow-off valve S1,
relights the normal light A, reactivates the hour meter HR and
re-energizes the magnetic starter to restart the compressor. The
system once more runs in a modulated mode of matching inlet flow to
demand flow.
The pressure switch PS2, which is closed at tank pressures of 25
psi and lower, reopens as tank pressure rises above the 25 psi
setting.
A normally closed overtemperature switch T is located in the
compressor outlet 18. It opens in response to a temperature rise
above a certain degree to break the circuit to relay R1. This would
open contacts R1a to deactivate the magnetic starter and
consequently stop operation of the motors and the compressor. In
effect, anything breaking the circuit to relay R1 will cause the
compressor to stop operating.
It is apparent from an examination of the automatic tri-mode
control system that the controls of the compressor system may, if
desired, be limited to an automatic dual mode control system by
eliminating the functions of the timer element TM and the pressure
switch PS2, as indicated by the circuit shown in FIG. 3.
The dual mode control system provides means for automatically
selecting either continuous modulation operation or unloading
operation of the compressor. No manual switching operation is
required for this action. The decision as to when to switch is left
to the control system according to the initial settings made in the
PS1 switch and the adjustments made in the bleed valve 33.
The automatic switching point at which the compressor system should
change over from continuous modulation to unloading operation is
preferably based upon minimizing the cost of operation. For a given
size plant, it can be shown that the optimum switching point is a
function of the volume of the air supply system (or, analogously,
of the time constant of the system). The optimum switching points
for plants according to their time constant may be calculated, and
a workable chart or graph for easy switch point selection may be
formed, as indicated in FIG. 4.
For example, reading from the graph, when the plant time constant
is 5 seconds and the undershoot is given as 5 psi, the switch point
for maximum economy of operation should be set at 50 per cent of
maximum flow. This means that for maximum operating economy the
compressor should operate in the continuous modulation mode for
flow greater than 50 per cent of maximum flow. And, when the demand
flow falls below 50 per cent of maximum flow, the compressor system
should be switched to the unload-reload mode where the power
consumed by the compressor system is less than that during the
continuous modulation mode.
Examination of the graph in FIG. 4 shows that there is a unique
compressor flow for each system time constant above which
continuous modulation of the inlet is optimum. It defines the set
point for unloading the compressor for any plant time constant and
allowable undershoot. It permits the compressor controls to be set
to match any operator's requirement.
Now, as a further example, assuming that the requirements are that
the process pressure not exceed 110 psi nor fall below 100 psi and
that the compressor meet all flow demands from zero to 100 percent
of the compressor capacity, should the time constant of the system,
upon being measured be found to be five seconds, and should the
allowable undershoot for this time constant be selected at 7 psi,
the system should be set to unload at 70 percent flow. Accordingly,
the lower switch setting or reload signal should be set at 107 psi.
This allows the pressure undershoot to be as large at 7 psi without
falling below the lower allowable process pressure of 100 psi. It
is to be noted that the flow in the latter instance at which the
compressor is unloaded has moved up to 70 percent of full flow for
a time constant of 5 seconds, and will provide a greater economy of
operation than when the undershoot at the same time constant is
chosen to be 5 psi, as earlier mentioned.
The pressure switch PSI here has a differential between the upper
and lower setting of between 3 and 25 psi. It is apparent from the
examples presented that if the minimum pressure switch differential
is used, the compressor will be unloaded over a larger flow range
and this will assure minimum operating costs.
Now in making initial settings of the compressor system, after the
pressure differential switch PS1 has been set to the determined
upper and lower settings for a particular flow percentage, say 70
percent of full flow, then the control bleed valve 33 is opened and
its area adjusted. The bleed valve is opened until the desired
demand flow of 70 percent is obtained at the unload cut-in point of
110 psi.
The system is now set for automatic operation. The compressor will
unload each time the process demand flow falls below 70 percent
since the pressure switch PS1 will be triggered at the upper set
point. Next, when the process pressure falls below 107 psi, the
compressor will be commanded, all as earlier explained, to reload,
which it does before the pressure can fall below 100 psi (i.e. with
an undershoot of 7 psi).
The automatic tri-mode control system earlier described is merely a
refinement of the dual mode control just explained, in that it adds
the functions of the timer TM and the second pressure switch PS2 so
as to cause automatic shut-down of the compressor (as earlier
explained) when the compressor remains unloaded for periods longer
than a characteristic time chosen by the operator. The compressor
and motor are shut down completely and no power is used; and
automatic restart occurs in the manner earlier described. Switching
in the tri-mode system through the various modes of operation is
completely automatic as determined by the initial settings made
with respect to the differential pressure switch PS1, the timer TM,
and the adjustments made to the bleed valve 33.
Selection of the mode of operation of the compressor system,
whether it be simply a continuous modulation mode, the automatic
dual mode, or the automatic tri-mode is at the option of the
operator.
While an embodiment of the invention has been illustrated and
described in detail, it is to be expressly understood that the
invention is not limited thereto. Various changes of form, design
or arrangement may be made in its parts without departing from the
spirit and scope of the invention; and it is our intention,
therefore, to claim the invention, not only as shown and described,
but also in all such forms and modifications thereof as might be
reasonably construed to be within the spirit of the invention and
the scope of the appended claims.
* * * * *