JPH0125907B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a capacity adjustment method for a positive displacement multistage compressor that enables economical operation by automatically adjusting the maximum value of the intermediate stage bypass capacity according to the pressure used. . The capacity adjustment method for a positive displacement multi-stage compressor is as follows:
Generally, a rotation speed control method, a suction valve throttling method, a suction valve ON-OFF method, an intermediate stage bypass method, etc. are adopted. In oil-free screw multistage compressors, an intermediate stage bypass system is often adopted. In a multi-stage compressor, this intermediate stage bypass system bypasses a portion of the discharged gas from the discharge side of the first stage compressor and the remaining amount from the discharge side of the final stage compressor using a flow rate adjustment valve. This method adjusts the final discharge capacity by returning it to the suction side of the compressor or releasing it to the atmosphere. According to this method,
In a two-stage compressor, the discharge pressure of the first-stage compressor is reduced by the bypass, and as a result, the power of the first-stage compressor is reduced, and this power is recovered. Also,
In a three-stage compressor, the discharge pressure of the first and second stage compressors decreases, thereby reducing the power, and this power is recovered. On the other hand, the power distribution in a multi-stage compressor varies depending on the conditions of use, but it is designed to be approximately equal, so it is estimated that a two-stage compressor will have 30% more capacity than a first-stage compressor. If a bypass is used, the power recovery will be 30% x 1/2 = 15 (%), and in a three-stage compressor it will be 30% x 2/3 = 20 (%). This will be explained below using the case of a screw compressor as an example. The theoretical gas power of the compressor is expressed by the following equation. Nth=98/3600×(Vi ^K-1 - K/K-1×PS +1/Vi×PD)×Vth (Kw) where PS: Suction pressure (Kg/cm ² A) PD: Discharge pressure (Kg/ cm ² A) K: Specific heat ratio of gas (specific heat at constant pressure/specific heat at constant volume) Vth: Stroke volume (m ³ /hr) Vi: Internal volume ratio (In a screw compressor, the gas is compressed inside the casing to a point close to the discharge pressure. (This is the ratio of the suction volume to the discharge volume.) On the other hand, the actual power is Nth/ηad・ηm, where ηad: adiabatic efficiency ηm: mechanical efficiency. Here, if K=1.4 (Air, etc.), the result will be as follows. Nth=98/3600×(C1・PS+C2・PD)×Vth/ηad・ηm Furthermore, C1 and C2 for each value of Vi are as follows.

[Table] Note that Vi is generally used in the range of 1.6 to 2.5, but [C1・PS] is a very small value compared to [C2・PD], so it can be roughly set to 0. , and ηad・ηm remain almost constant even if the operating conditions change. Therefore, it can be considered that the power Nth is proportional to the discharge pressure PD. In an ideal gas, the formula P.V=G.R.T holds true, so the discharge pressure P of each stage is proportional to G, that is, the discharge capacity (capacity without bypass - bypass capacity). Since V is a volume type, it is almost constant, and P is a constant. However, it is assumed that T is constant. When adjusting the capacity of a three-stage type, the discharge pressure and shaft power of each stage are as follows, assuming the power distribution of each stage is 1:1:1.
The results will be as shown in Table 2 and Table 3 below.

【table】

[Table] In other words, the power recovery is 20 (%). that's all,
Although we considered a screw compressor, the same tendency applies to other positive displacement compressors. Therefore, when adjusting the capacity using the intermediate stage bypass method, it is found that it is economical from the point of view of power recovery to increase the bypass capacity from the first stage compressor as much as possible. However, if the bypass capacity from the first stage compressor is increased, there will be no problem with the first stage compressor, but
The differential pressure between the suction pressure and the discharge pressure in the final stage compressor increases, and as a result, the pressure ratio increases and the discharge temperature becomes excessive. When setting, the differential pressure limit and the discharge temperature limit are imposed. That is, for a three-stage compressor,
This will be explained according to the diagram. In the figure, 1A, 1B, and 1C indicate the compressors of each stage, and the discharge side of the first stage compressor 1A (intermediate stage)
A bypass passage 4 is provided, and the bypassed compressed gas is returned to the suction side pipe line 2 of the first-stage compressor 1A via the bypass passage 5. The bypass passage 4 is provided with an orifice 6 to ensure that the discharge temperature of the final (3) stage compressor 1C or the pressure difference between the suction side and the discharge side of the final stage is within a predetermined value. The maximum bypass capacity is set. Further, the bypass line 4 is provided with a pressure regulating valve 7, and the opening degree is adjusted within the maximum bypass capacity according to the bypass capacity signal 9 from the pressure detector 8 provided in the discharge side pipe line 3, and the bypass capacity is adjusted. It has been decided.
In addition, in the drawing, a bypass passage 10 communicating with the bypass passage 5 is provided before the pressure detector 8, and a pressure regulating valve 11 that opens and closes in response to a signal from the pressure detector 8 is provided in the bypass passage 10. Although the bypass passage 10 and the pressure regulating valve 11 are not necessarily necessary, and the final stage can be adjusted by other means. A bypass may also be used. Additionally, a differential pressure switch 12 is installed on the suction side and the discharge side of the three-stage compressor 1C, and a temperature switch 13 is installed on the discharge side.
is provided so that if the differential pressure or discharge temperature of the three-stage compressor 1C accidentally reaches a dangerous value, the drive power for the compressor 1 is turned off. 14 is a filter;
15 is a cooler. By the way, conventionally, the maximum bypass capacity has been set with a large margin in consideration of safety, for example, the jet pressure of the safety valve of the final stage compressor, and in reality, it is possible to further increase the maximum bypass capacity. There are many cases. In other words, if the design discharge pressure is selected higher than the operating pressure, there may be a process in which the discharge pressure falls below the design discharge pressure when the capacity is adjusted, and in these cases, the maximum bypass capacity must be Although it is possible to further increase the number, no countermeasures have been taken in the past. In other words, that much power recovery was wasted. For example, the suction pressure is atmospheric pressure and the discharge pressure is 7Kg/
When designing a cm ² G two-stage compressor, the capacity adjustment width by the first stage bypass is 100 to approximately 70%, and the remaining capacity is approximately
70% to 0% will be achieved by a two-stage bypass. This 70% is regulated from the viewpoint of preventing an excessive rise in the discharge temperature due to an increase in the pressure ratio of the two-stage compressor. On the other hand, at a discharge pressure of 5Kg/cm ² G, from the above point of view, approximately
Up to 50% is possible. In this way, if the discharge pressure changes, the amount that can be bypassed from the first stage will change, but with a conventional device, if it is designed at 7 kg/cm ² G, it will be 5.
Even when used at Kg/cm ² G, the capacity adjustment width by the single-stage bypass remains at 100 to 70%, and that amount of power is wasted. The present invention has been made in view of the above-mentioned conventional circumstances, and its purpose is to make the maximum capacity of the intermediate stage bypass variable in accordance with the actual operation of the compressor, recover power as much as possible, and achieve economic efficiency. It is an object of the present invention to provide a method for adjusting the capacity of a positive displacement multi-stage compressor, which allows a positive displacement compressor to be operated in a consistent manner. Next, an explanation will be given with reference to FIG. 2, which is an embodiment of the present invention. As is clear from a comparison with FIG. 1, the present invention has pressure transmitters 15a on the suction side and discharge side of the final (3) stage compressor 1C, which detect and transmit pressure, respectively.
15b is attached, and each pressure transmitter 15a, 1
5b is connected to the arithmetic unit 16. Further, this calculator 16 includes a differential pressure calculator 16a, a pressure ratio calculator 1
6b. As mentioned above, the differential pressure calculator 16a transmits an output according to the differential pressure (P2-P1) between the suction and discharge pressures of the final stage compressor 1C,
Since the discharge temperature is determined by the pressure ratio which is the primary factor, the pressure ratio calculator 16b transmits an output in accordance with the pressure ratio. Note that 17 is a first selector that connects the differential pressure calculator 16a and the pressure ratio calculator 16.
Among the signals b, the one that reduces the opening degree of the pressure regulating valve 7 is selected and transmitted to the second selector 18. That is, the discharge temperature TD of the compressor is generally determined by the following formula. Here, TS: Suction temperature ηc: Cooling coefficient Note that since the adiabatic efficiency ηad and the cooling coefficient ηc are approximately constant, the discharge temperature depends on the pressure ratio PD/PS. In particular, since the discharge temperature of a screw compressor is limited, the pressure ratio that affects the discharge temperature is limited. The detection signals from both arithmetic units 16a and 16b are input to a first selector 17, and when either one reaches a limit value, the limit value signal is input to a second selector 18. Here, the main factors that determine the mechanical usage restrictions of the compressor are the strength of parts such as the shaft and the securing of clearances at necessary locations; the former may be limited by differential pressure, and the latter may be limited by discharge temperature. Among these, the discharge temperature depends on the pressure ratio as described above, so it is limited by the pressure ratio. Although the discharge temperature switch may be used as a limiter, there is a delay in temperature detection and the limiter does not function well against sudden pressure fluctuations, so a pressure ratio method is more practical. however,
For compressors with low discharge temperatures, a discharge temperature switch can also be used. The second selector 18 is provided in front of the pressure regulating valve 7, which opens and closes in response to a signal from the pressure detector 8.
The signal from the first selector 17 and the pressure detector 8
The signal that reduces the opening degree of the pressure regulating valve 7 is selected from among the signals from the pressure regulating valve 7 and transmitted to the pressure regulating valve 7. Thus, the pressure regulating valve 7 has a differential pressure calculator 16a,
Among the signals from the pressure ratio calculator 16b and the pressure detector, the operation is performed by the signal that causes the smallest opening degree. In other words, if the differential pressure and pressure ratio are within predetermined limits, the pressure regulating valve 7 opens based on the signal from the pressure detector 8, increasing the bypass capacity from the bypass path 6. Then, when a predetermined value is reached, the differential pressure or pressure ratio is adjusted to a predetermined value. In other words, a positive displacement compressor always pumps a constant amount even if the discharge pressure changes, so if the process side on the discharge side does not consume gas or there is a shortage of process gas on the suction side, a bypass is used to reduce the flow rate. It is necessary to strike a balance. For example, if the capacity of the compressor is 100% and the amount of gas consumed on the process side or the amount of gas generated on the suction side is 40%, it is necessary to bypass 60% of the gas as a capacity adjustment. Here, increasing the amount of first-stage bypass leads to energy saving. The amount that cannot be bypassed in the first stage is bypassed in the final stage, but the final stage bypass does not lead to power reduction. If you want to bypass the entire amount, use 1 to the extent possible.
It is advisable to perform a stage bypass, but if a single stage bypass is performed, the differential pressure and pressure ratio (discharge temperature) at the final stage will increase, so the amount of bypass is limited by these set values (tolerable limits). In the case of a compressor with a low discharge temperature, a discharge temperature transmitter may be used instead of the pressure ratio calculator 16b as described above, and the differential pressure switch 12, temperature switch 13, etc. may be protected as before. A device may also be provided. Other parts that are the same as those in FIG. 1 are given the same reference numerals and their explanations will be omitted. Further, in the above embodiment, the entire bypass capacity was returned to the suction side of the first-stage compressor 1A, but the same applies to a type in which the bypass capacity is discharged to the atmosphere. As is clear from the above description, according to the present invention, the maximum permissible bypass capacity from the intermediate stage changes automatically depending on the actual operating conditions of the compressor.
In other words, the bypass capacity can always be kept at its maximum limit, and as opposed to the conventional system where it was uniformly regulated regardless of the operating conditions, the intermediate stage pressure can be further reduced, which increases the power output. can be recovered and the compressor can be operated economically. That is, as mentioned above, how to lower the intermediate stage pressure when adjusting the capacity is important for energy saving. Further, since the intermediate stage pressure is proportional to the pumping amount during the first stage bypass, if the discharge pressure is lowered, the intermediate stage pressure can be lowered accordingly. (If the discharge pressure decreases, both the differential pressure and the pressure ratio decrease, so the intermediate stage pressure is lowered.) If the intermediate stage pressure can be lowered, the range of capacity adjustment by the first stage bypass will be increased.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing a conventional method for adjusting the capacity of a positive displacement multi-stage compressor, and FIG. 2 is a system diagram showing the method of the present invention. 1...Compressor, 1A, 1B, 1C...Each stage compressor,
4, 5... Bypass path, 7... Pressure regulating valve, 8... Pressure detector, 15a, 15b... Pressure transmitter, 16a...
Differential pressure calculator, 16b...Pressure ratio calculator, 17, 18
...Selector.

Claims (1)

[Claims] 1. Bypassing an allowable arbitrary volume of compressed gas from the discharge side of the first stage compressor in a positive displacement multistage compressor,
In the capacity adjustment method of the intermediate stage bypass method, the capacity is adjusted by bypassing the remaining amount from the discharge side of the final stage compressor and returning it to the suction side of the first stage compressor as necessary. The maximum bypass capacity for bypassing from the discharge side to the suction side is adjusted by a bypass capacity signal outputted to increase or decrease the bypass amount from the discharge side of the first stage compressor according to pressure fluctuations in the process line, and the bypass capacity is A capacity adjustment method for a positive displacement multi-stage compressor, characterized in that the maximum value of a capacity signal is regulated by a limit value signal determined by a predetermined differential pressure between the suction and discharge sides of the final stage compressor or a predetermined discharge temperature. . 2 Bypassing an allowable arbitrary volume of compressed gas from the discharge side of the first stage compressor in a positive displacement multistage compressor,
In a capacity adjustment method using an intermediate stage bypass method in which the capacity is adjusted by bypassing the remaining amount from the discharge side of the final stage compressor and releasing it to the atmosphere as necessary, the remaining amount is discharged to the atmosphere from the discharge side of the first stage compressor. The maximum bypass capacity to be discharged is adjusted by a bypass capacity signal outputted to increase or decrease the bypass amount from the discharge side of the first stage compressor according to pressure fluctuations in the process line, and the maximum value of the bypass capacity signal is adjusted. A capacity adjustment method for a positive displacement multi-stage compressor, characterized in that the capacity is regulated by a limit value signal determined by a predetermined differential pressure or a predetermined discharge temperature between the suction and discharge sides of the final stage compressor.