AIR COMPRESSION DEVICE

Description

TECHNICAL FIELD

The present invention relates to an air compression device.

BACKGROUND ART

Patent Document 1 discloses “an oil-free rotary compression device characterized in that a return gas pipe provided with flow rate control means is branched off from a discharge gas pipe and connected to a suction side of a rotary compressor so that the discharge pressure can be increased while maintaining a predetermined compression ratio by returning a part of a discharge gas to the suction side of the rotary compressor to increase the suction pressure” (see paragraph 1).

CITATION LIST

Patent Document

• • Patent Document 1: JP S60-166785 A

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In the air compressor described in Patent Document 1, since a part of discharge air is directly returned to an air suction side by a pipe, a suction pressure becomes unstable.

An object of the present invention is to stabilize a suction pressure in an air compression device.

Solutions to Problems

An air compression device according to an aspect of the present invention includes: a compressor which compresses suction air sucked from a suction port via an air suction pipe; a heat exchanger which cools the compressed air; an air return pipe which returns a part of the cooled air to the air suction pipe as return air; and a buffer tank which is provided at a predetermined position of the air return pipe and temporarily stores at least the return air.

Effects of the Invention

According to an aspect of the present invention, it is possible to solve some or all of the above problems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an air compression device of a first embodiment.

FIG. 2 is a flowchart of discharge air return control of the first embodiment.

FIG. 3 is a diagram showing the pressure, flow rate, temperature, and density at each point in the air compression device of the first embodiment.

FIG. 4 is a diagram showing a configuration of an air compression device of a second embodiment.

FIG. 5 is a flowchart of discharge air return control of the second embodiment.

FIG. 6 is a diagram showing the pressure, flow rate, temperature, and density at each point in the air compression device of the second embodiment.

FIG. 7 is a diagram showing a configuration of an air compression device of a third embodiment.

FIG. 8 is a flowchart of discharge air return control of the third embodiment.

FIG. 9 is a diagram showing the pressure, flow rate, temperature, and density at each point in the air compression device of the third embodiment.

FIG. 10 is a flowchart of another discharge air return control of the first embodiment and the third embodiment.

FIG. 11 is a flowchart of another discharge air return control of the second embodiment.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to the drawings.

First Embodiment

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1, 2, and 3.

FIG. 1 is a diagram showing a configuration of an air compression device of the first embodiment, and solid lines in the figure indicate the air flow in the air compression device.

The inside of the dashed line 19 is set as the inside of the package of the air compression device. Outside air taken in from an air suction port 1 passes through an air filter 2, a check valve 3, and a buffer tank 4, and is then compressed by a compressor 7 (a first-stage machine in a main body of the air compression device). The compressed high-temperature air is cooled in an intercooler (heat exchanger) 11 and further compressed to a target pressure in a compressor 8 (a second-stage machine in the main body of the air compression device).

The compressed air is cooled in an aftercooler (heat exchanger) 12 and then discharged to the outside from a discharge port 14. A part of the discharge air returns to the buffer tank 4 via an air return pipe 10. An orifice 9 and an adjustment valve 5 are attached to the air return pipe 10.

The suction pressure is measured by a pressure gauge 6, the intermediate pressure is measured by a pressure gauge 16, the discharge pressure is measured by a pressure gauge 13, the return air is measured by a pressure gauge 20, and the values are acquired by a control panel 15 via the control line indicated by the dashed dotted line in the figure. The control of the adjustment valve 5 is performed by the control panel 15 via the control line. A check valve 17 prevents air from flowing backward from the customer side during unloading, and blow pipe 18 is an air outlet during unloading.

The air compression device is provided with each of a thermometer 21, a thermometer 22, a thermometer 23, and a thermometer 24. The thermometer 21 measures the intake air temperature. The thermometer 22 measures the return air temperature. The thermometer 23 measures the first-stage discharge temperature. The thermometer 24 measures the discharge air temperature.

The buffer tank 4 is a place where the air sucked from the outside air is mixed with a part of the discharge air, and the pressurized sucked air is stored. By temporarily storing air in the buffer tank 4, air with stable pressure can be sent to the compressor 7 (first-stage machine). Accordingly, chattering of the adjustment valve 5 is prevented. Here, chattering refers to the repeated small opening and closing of the adjustment valve 5 in an attempt to finely adjust the amount of return air due to changes in the amount of discharge air used or fluctuations in the discharge pressure. Chattering can shorten the life of the adjustment valve 5 and can cause control errors.

Further, the check valve 3 is attached to the inlet of the buffer tank 4 to prevent a part of the discharge air sent to the buffer tank 4 from flowing back from the air suction port 1.

The adjustment valve 5 is attached to the air return pipe 10 that returns a part of the discharge air, and the amount of air returned to the buffer tank 4 can be adjusted by opening and closing this adjustment valve 5. However, when the discharge air is sent directly to the adjustment valve 5, the valve will open and close while receiving high-pressure air, which can easily lead to deterioration or failure of the adjustment valve 5. In order to prevent this, the orifice 9 is attached to the air return pipe 10. Further, the orifice 9 also serves to prevent the adjustment valve 5 from opening and closing too frequently in response to small fluctuations in the discharge pressure when controlling the return flow rate.

FIG. 2 shows the control flow of the adjustment valve 5 of the first embodiment. A series of control of the adjustment valve 5 is performed by the control panel 15. The control flow will be explained below.

After the compressor is started (S201), first, normal operation is performed with the adjustment valve 5 closed (S202) The control panel 15 determines whether the discharge air pressure (P_d) measured by the pressure gauge 13 is close to a preset target pressure (target value) (for example, within ±10% of the target pressure) (S203). When Pa is close to the target pressure (S203: Yes), normal operation S202 is continued, and it is determined whether Pa is close to the target pressure S203 at predetermined time intervals.

When the result of the determination in S203 is that the pressure is not close to the target pressure (S203: No), the adjustment valve 5 is adjusted (S204). Specifically, for example, when the pressure is 90% or less of the target pressure, the adjustment valve 5 is opened by a predetermined amount to increase the amount of compressed air returned to the buffer tank 4, and when the pressure is 110% or more of the target pressure, the adjustment valve 5 is closed by a predetermined amount to decrease the amount of compressed air returned to the buffer tank 4.

Compressed air is returned to the buffer tank 4 by S204, the first-stage suction air pressure of the compressor 7 increases, and it is determined whether the air pressure (suction pressure P_s) discharged from the buffer tank 4 measured by the pressure gauge 6 is close to a preset target pressure (target value) (for example, within ±10% of the target pressure) (S205).

When the pressure is not close to the target pressure (S205: No), the adjustment valve 5 is further adjusted (S204). Specifically, for example, when the pressure is 90% or less of the target pressure, the adjustment valve 5 is opened by a predetermined amount to increase the amount of compressed air returned to the buffer tank 4, and when the pressure is 110% or more of the target pressure, the adjustment valve 5 is closed by a predetermined amount to decrease the amount of compressed air returned to the buffer tank 4. In this way, the first-stage suction air pressure of the compressor 7 is adjusted to be close to the target pressure.

When the air pressure from the buffer tank 4 measured by the pressure gauge 6 reaches a preset target value (S205: Yes), the compression ratio πi2 (ratio of intermediate pressure to discharge pressure: Pd/P1) of the compressor 8 (two-stage machine) is measured to determine whether it has exceeded a preset reference value (S206). This is because there is a risk that the compressed air will be overheated and the compressor 8 will be damaged when the compression ratio becomes too high.

When the result of the determination is that the compression ratio πi2 exceeds the reference value (S206: No), the intermediate pressure is decreased by opening the adjustment valve 5 (S204). When the result of the determination is that the compression ratio πi2 does not exceed a preset reference value (S206: Yes), the process returns to the normal operation of S202 while maintaining the open and closed state of the adjustment valve 5.

As described above, since the adjustment valve 5 is opened and closed in response to the discharge air pressure Pd, the first-stage suction air pressure P_s, and the second-stage compression ratio πi2, the suction pressure can be stabilized while protecting the compressor 8 with a simple configuration.

Next, means for speeding up the process described by FIG. 2 will be described with reference to the control flow of FIG. 10. Here, a series of control of the adjustment valve 5 is performed by the control panel 15.

This control flow is almost the same as the control flow of the adjustment valve 5 shown in FIG. 2 except that step 206 is omitted. Since the other steps are almost the same as those of the control flow shown in FIG. 2, a description thereof will be omitted.

In the process of FIG. 2, the opening and closing amount of the adjustment valve 5 is set to a predetermined value, and there is a risk that it will take time to determine whether the pressure in each part has reached the target pressure after the adjustment valve 5 is opened and closed.

In particular, when the compression ratio πi2 exceeds the reference value and reaches an alarm value, an alarm is issued to prevent damage to the compressor 8, and the air compression device must be stopped. Therefore, speeding up the process is useful. As a result, in the control flow of FIG. 10, step 206 is omitted to speed up the process.

Since it is possible to calculate the amount of opening and closing the adjustment valve 5 to bring the pressure in each part to the target pressure by adopting this means, it is possible to eliminate the need to remeasure the pressure in each part after opening and closing the adjustment valve 5 and to thereby speed up processing.

First, an example of returning a certain amount of compressed air in the discharge flow rate is shown. FIG. 3 shows the pressure P, volume flow rate Q, mass flow rate G, temperature T, and air density ρ at each position in the package 19 of the air compression device.

Since the first-stage suction flow rate is the sum of the suction flow rate and the return flow rate, it can be expressed by the following formula (1) according to Boyle's law.

Here, as the calculation conditions, the suction air pressure Ps is set to 80 [kPa](atmospheric pressure at an altitude of about 2000 m), and the first-stage suction pressure Ps+r is increased to 100 [kPa] by the return air.

[ Math . 1 ]  P s ⁢ Q s T s + P r ⁢ Q r T r = P s + r ⁢ Q s + r T s + r ( 1 )

When this formula (1) is made into a formula for the return flow rate Qr, it can be expressed as the following formula (2).

[ Math . 2 ]  P s + r P r ⁢ T r T s + r ⁢ Q s + r - P s P r ⁢ T r T s ⁢ Q s ( 2 )

Here, assuming that the suction air pressure is Qs=80 kPa, suction temperature Ts=303.15 K, density ρs=0.92 kg/m³, return air pressure is Pr=780 kPa, return temperature Tr=423.15 K, density ρr=6.43 kg/m³, first-stage suction air pressure is Ps+r=100 kPa, first-stage suction flow rate Qs+r=50 m³/min, suction temperature Ts+r=303.15 K, and density ρs+r=1.15 kg/m³, and that the suction air volume Qs and the suction air volume Qs+r of the first-stage air end are equivalent, the return flow rate Qr can be calculated as shown in the following formula (3).

[ Math . 3 ]  Q r = P s + r P r ⁢ T r T s + r ⁢ Q s + r - P s P r ⁢ T r T s ⁢ Q s = 100 [ k ⁢ Pa ] 780 [ k ⁢ Pa ] × 423.15 [ K ] 303.15 [ K ] × 50. [ m 3 / min ] - 80. [ k ⁢ Pa ] 780 [ k ⁢ Pa ] × 423.15 [ K ] 303.15 [ K ] × 50. [ m 3 / min ] = 1.789 ≈ 1.79 [ m 3 / min ] ( 3 )

Here, since the mass flow rate is constant regardless of temperature, the first-stage suction mass flow rate Gs+r is equal to the discharge mass flow rate Gd. It can be seen that the ratio of the return mass flow rate Gr to the first-stage suction mass flow rate Gs+r is the flow rate to be returned.

[ Math . 4 ]  G s + r = G s + G r = ρ s ⁢ Q s + ρ r ⁢ Q r ⁢ G r G s + r = ρ r ⁢ Q r ρ s + r ⁢ Q s + r = 6.43 [ kg / m 3 ] × 1.789 [ m 3 / min ] 1.15 [ kg / m 3 ] × 50 [ m 3 / min ] = 0.2 = 20 [ % ] ( 4 )

It can be seen from the formula (4) that the return mass flow rate Gr requires about 20% of the discharge mass flow rate Gd. Here, the control panel 15 stores a table or a formula correlating the opening and closing amount of the adjustment valve 5 and the return mass flow rate Gr in a memory section (not shown), and can obtain the opening and closing amount of the adjustment valve 5 necessary to return the required return mass flow rate Gr to the buffer tank 4.

Thus, since the adjustment valve 5 is opened and closed by the calculated opening and closing amount in S204 of FIG. 2, it is possible to stabilize the suction pressure more quickly.

Second Embodiment

Hereinafter, a second embodiment of the present invention will be described with reference to FIGS. 4, 5, and 6.

FIG. 4 is a diagram showing a configuration of an air compression device of the second embodiment, and the solid lines in the figure indicate the air flow in the air compression device.

The second embodiment has a structure in which a part of the discharge air is returned between the compressor 7 (first-stage machine) and the compressor 8 (the second-stage machine) and (between the heat exchanger 11 and the second-stage suction port).

The configuration is different from that of the air compression device of the first embodiment shown in FIG. 1 in the arrangement of the buffer tank 4 and the check valve 3. Further, in the second embodiment, a pressure sensor 25 and a thermometer 26 are newly provided. The pressure sensor 25 is a sensor for measuring the second-stage suction pressure (Pc+r) and is connected to a control panel 15 by a dashed line. The thermometer 26 is a thermometer for measuring the second-stage suction air temperature (Tc+r) and is connected to the control panel 15 by a dashed line.

The air outlet from the buffer tank 4 is provided between the compressor 7 and the compressor 8, and the buffer tank 4 temporarily stores the return air and sends the return air to the compressor 8. At this time, the check valve 3 is provided on the outlet side of the buffer tank 4 and prevents the air cooled by the heat exchanger 11 from flowing back to the buffer tank 4.

By temporarily storing air in the buffer tank 4, air with stable pressure can be sent to the compressor 8. Accordingly, chattering of the adjustment valve 5 is prevented.

Since the other configurations are almost the same as the configurations of the air compression device of the first embodiment shown in FIG. 1, a detailed description thereof will be omitted.

FIG. 5 shows the control flow of the adjustment valve 5 of the second embodiment.

This control flow is almost the same as the control flow of the adjustment valve 5 of the first embodiment shown in FIG. 2 except that the reference pressure after adjusting the adjustment valve 5 becomes the second-stage suction pressure P1+r (see S205). Since the other steps are almost the same as those of the first embodiment shown in FIG. 2, a description thereof will be omitted.

Next, means for speeding up the process described by FIG. 5 will be described with reference to the control flow of FIG. 11. Here, a series of control of the adjustment valve 5 is performed by the control panel 15.

This control flow is almost the same as the control flow of the adjustment valve 5 shown in FIG. 5 except that step 206 is omitted. Since the other steps are almost the same as those of the control flow shown in FIG. 5, a description thereof will be omitted.

In the process of FIG. 5, the opening and closing amount of the adjustment valve 5 is set to a predetermined value, and there is a risk that it will take time to determine whether the pressure in each part has reached the target pressure after the adjustment valve 5 is opened and closed.

In particular, when the compression ratio πi2 exceeds the reference value and reaches an alarm value, an alarm is issued to prevent damage to the compressor 8, and the air compression device must be stopped. Therefore, speeding up the process is useful. As a result, in the control flow of FIG. 11, step 206 is omitted to speed up the process.

Next, as in the first embodiment, an example of returning a certain amount of compressed air in the discharge flow rate is shown. FIG. 6 shows the pressure P, volume flow rate Q, mass flow rate G, temperature T, and air density ρ at each position. Since the first-stage suction flow rate is the sum of the suction flow rate and the return flow rate, it can be expressed by the following formula (5) according to Boyle's law.

Here, it is assumed that the intermediate pressure Pc+r is increased from 200 kPa to 250 kPa. From the calculation results, the return amount is 20% of the discharge. Tc is the outlet temperature of the intercooler (heat exchanger) 11.

[ Math . 5 ]  P 1 ⁢ Q 1 T 1 + P r ⁢ Q r T r = P 1 + r ⁢ Q 1 + r T 1 + r ( 5 )

As in the first embodiment, when the formula (5) is made into a formula for the return flow rate Qr, it can be expressed as the formula (6) below, and the return flow rate Qr can be calculated as the formula (7) below. It can be seen from the formula (8) that the return flow rate requires about 20% of the discharge flow rate, which can be used to determine the opening and closing amount of the adjustment valve 5 for adjusting the return flow rate in S204 as in the first embodiment.

[ Math . 6 ]  Q r = P 1 + r P r ⁢ T r T 1 + r ⁢ Q 1 + r - P 1 P r ⁢ T r T c ⁢ Q 1 ( 6 ) [ Math . 7 ]  Q r = P 1 + r P r ⁢ T r T 1 + r ⁢ Q 1 + r - P 1 P r ⁢ T r T c ⁢ Q 1 = 250 [ k ⁢ Pa ] 780 [ k ⁢ Pa ] × 423.15 [ K ] 313.15 [ K ] × 50. [ m 3 / min ] - 200 [ k ⁢ Pa ] 780 [ k ⁢ Pa ] × 423.15 [ K ] 313.15 [ K ] × 50. [ m 3 / min ] = 4.331 ≈ 4.33 [ m 3 / min ] ( 7 ) [ Math . 8 ]  G 1 + r = G c + G r = ρ c ⁢ Q 1 + ρ r ⁢ Q r ⁢ G r G s + r = ρ r ⁢ Q r ρ 1 + r ⁢ Q 1 + r = 6.43 [ kg / m 3 ] × 4.331 [ m 3 / min ] 2.782 [ kg / m 3 ] × 50 [ m 3 / min ] = 0.2 = 20 [ % ] ( 8 )

Third Embodiment

Hereinafter, a third embodiment of the present invention will be described with reference to FIGS. 7, 8, and 9.

FIG. 7 is a diagram showing a configuration of an air compression device of the third embodiment, and the solid lines indicate the air flow in the air compression device.

The main difference from the air compression device of the first embodiment shown in FIG. 1 is that the compressor 8 (second-stage machine) and the heat exchanger 12 of the air compression device body are not present. In this way, the air compression device of the third embodiment is an air compression device with a single stage configuration.

As shown in FIG. 7, outside air is taken in from the air suction port 1, is passed through the air filter 2, the check valve 3, and the buffer tank 4, and is compressed by the compressor 7. The compressed high-temperature air is cooled in the heat exchanger 11 and then discharged to the outside from the discharge port 14.

Since the other configurations are the same as the configurations of the air compression device of the first embodiment shown in FIG. 1, a detailed description thereof will be omitted.

FIG. 8 shows the control flow of the adjustment valve 5 of the third embodiment.

This control flow is almost the same as the control flow of the adjustment valve 5 of the first embodiment shown in FIG. 2 except that the reference value of the compression ratio becomes πi1 (see S206). πi1 is the compression ratio of the first-stage machine, and indicates the ratio Pd/P1 of the discharge pressure to the suction pressure. Since the other steps are almost the same as those of the first embodiment shown in FIG. 2, a description thereof will be omitted.

Next, means for speeding up the process described by FIG. 8 will be described. Since the control flow is almost the same as the control flow of FIG. 2, a description thereof will be omitted. Here, a series of control of the adjustment valve 5 is performed by the control panel 15.

In the process of FIG. 8, the opening and closing amount of the adjustment valve 5 is set to a predetermined value, and there is a risk that it will take time to determine whether the pressure in each part has reached the target pressure after the adjustment valve 5 is opened and closed.

In particular, when the compression ratio πi1 exceeds the reference value and reaches an alarm value, an alarm is issued to prevent damage to the compressor 8, and the air compression device must be stopped. Therefore, speeding up the process is useful. As a result, in the control flow of FIG. 2, step 206 is omitted to speed up the process.

Next, as in the first embodiment, an example of returning a certain amount of compressed air in the discharge flow rate is shown. FIG. 9 shows the pressure P, volume flow rate Q, mass flow rate G, temperature T, and air density ρ at each position. Since the first-stage suction flow rate is the sum of the suction flow rate and the return flow rate, it can be expressed by the following formula (9) according to Boyle's law.

Here, the calculation conditions are set such that suction air pressure P_s=80 [kPa](atmospheric pressure at an altitude of about 2000 m), and suction pressure Ps+r is increased to 100 [kPa] by return air. Then, the target discharge pressure Pd=Pr=330 [kPa].

[ Math . 9 ]  P s ⁢ Q s T s + P r ⁢ Q r T r = P s + r ⁢ Q s + r T s + r ( 9 )

As in the first embodiment, if this formula (9) is made into a formula for the return flow rate Qr, it can be expressed as the following formula (10), and the return flow rate Qr can be calculated as the following formula (11).

It can be seen from the formula (12) that the return flow rate needs to be about 20% of the discharge flow rate, which can be used to determine the opening and closing amount of the adjustment valve 5 for adjusting the return flow rate in S204 as in the first embodiment.

[ Math . 10 ]  Q r = P s + r P r ⁢ T r T s + r ⁢ Q 1 + r - P s P r ⁢ T r T s ⁢ Q s ( 10 ) [ Math . 11 ]  Q r = P s + r P r ⁢ T r T s + r ⁢ Q s + r - P s P r ⁢ T r T s ⁢ Q s = 100 [ k ⁢ Pa ] 330 [ k ⁢ Pa ] × 423.15 [ K ] 303.15 [ K ] × 50. [ m 3 / min ] - 80. [ k ⁢ Pa ] 330 [ k ⁢ Pa ] × 423.15 [ K ] 303.15 [ K ] × 50. [ m 3 / min ] = 4.2298 ≈ 4.23 [ m 3 / min ] ( 11 ) [ Math . 12 ]  G s + r = G s + G r = ρ s ⁢ Q s + ρ r ⁢ Q r ⁢ G r G s + r = ρ r ⁢ Q r ρ s + r ⁢ Q s + r = 2.7173 [ kg / m 3 ] × 4.23 [ m 3 / min ] 1.15 [ kg / m 3 ] × 50 [ m 3 / min ] = 0.1998 = 20 [ % ] ( 12 )

As shown in the first to third embodiments, since the calculated amount of discharge air is returned regardless of the first-stage machine, the second-stage machine, and the air return position, the pressure can be increased and the target discharge air pressure can be maintained.

According to the above-described embodiments, the suction pressure can be increased stably by returning a part of the discharge air to the buffer tank provided on the suction side. Furthermore, chattering of the adjustment valve 5 attached to the air return pipe can be prevented.

REFERENCE SIGNS LIST

• • 1 Air suction port
  • 2 Suction filter
  • 3 Check valve
  • 4 Buffer tank
  • 5 Flow adjustment valve
  • 6 Suction pressure sensor
  • 7 Compressor (first-stage machine)
  • 8 Compressor (second-stage machine)
  • 9 Orifice (pressure reducing valve)
  • 10 Air return pipe
  • 11 Intercooler
  • 12 Aftercooler
  • 13 Discharge pressure sensor
  • 14 Discharge port
  • 15 Control panel
  • 16 Intermediate pressure sensor
  • 17 Check valve
  • 18 Blow pipe