JPS62688A - Capacity adjusting method for double-stage screw compressor - Google Patents

Abstract

PURPOSE:To enable economic partial load run by performing full capacity run by a high-stage screw compressor only until pressure reaches a specified intermediate value which depends on temperature or dew point of gas discharge from the high-stage compress and thereafter performing capacity adjusting run by both the high-state and the low-stage screw compressors to maintain each specified intermediate pressure respectively. CONSTITUTION:Until pressure reaches a specified intermediate value, that is, a set value by a pressure switch PSL which depends on temperature or dew point of the gas from the discharge side of a high-stage screw compressor 2, a low-stage screw compressor 1 performs capacity adjusting run by a slide valve 3 while until the intermediate pressure loweres to the set value, a high- stage screw compressor 2 performs full capacity run. After pressure reaches the specified intermediate pressure, the high-stage screw compressor 2 also performs capacity adjusting run by virtue of a slide valve 4 which is controlled by a controller 6. Since loading at the time of capacity adjusting run is thereby reduced to improve the unloading characteristics, power generating ratio may be reduced so that economic partial load run may be enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a capacity adjustment method for a two-stage screw compressor equipped with a slide valve type capacity adjustment device at least on the higher stage side.

(Prior Art) Conventionally, there are the following two methods for adjusting the capacity of this type of two-stage screw compressor.

The first method, as shown in FIG.
By making the opening degree of the slide valve 4 adjustable via the positioner 5 and the control device 6 based on the signal from the slide valve 3, the slide valve 3 is controlled so that the intermediate pressure becomes a constant pressure.

The second method, as shown in FIG.
．． 4 is provided with a control device 6, and by simultaneously applying control signals, the opening degrees of each slide valve 3.4 are controlled to be the same. Also in this method, the change in shaft power with respect to the change in suction capacity (hereinafter referred to as unloading characteristic) during capacity adjustment operation of low stage screw compressor I and high stage screw compressor 2 is the same, so the intermediate pressure is It remains almost constant.

This intermediate pressure is determined so that the shaft power W of the two-stage screw compressor is minimized in order to reduce the negative part.

In general, the shaft power W when the slide valve is at full capacity is determined by setting the suction capacity to V and the required torque to Tr, and setting the required torque Tr as a function of the suction pressure Ps and the discharge pressure Pd (Tr(Ps, Pd).
), W= VXTr(PS, Pd) −・
・Represented by ■. In this case, as shown in FIG. 7, as the suction pressure Ps decreases and as the discharge pressure Pd decreases, the required torque Tr decreases and the shaft power W decreases. Therefore, in the case of a two-stage screw compressor, as shown in FIG. 2 as PIV 2 , T t, and that of the discharge gas of the high stage screw compressor 2 as P 3. V
'y, T 3, the shaft motion of the low stage and high stage screw compressors) W, , VTI, are respectively W + = V IX T r (P +, P t
)W2-V2XTr(P,,P3), so W=W, +W.

...■ becomes. Here, P1. T+, Tt,
Assuming that P s remains unchanged, C = (T v / T + )
Let it be X P l .

FIG. 8 shows the relationship of equation (2), and it can be seen that there exists a suction pressure of the high-stage screw compressor 2, that is, an intermediate pressure P, .

(Problem to be solved by the invention) When adjusting the capacity, it is important to reduce the load and perform economical partial load operation, and the unloading characteristic is such that the shaft movement horn decreases in proportion to the suction capacity. That's preferable.

However, in any of the above methods, the intermediate pressure is constant, and the unloading characteristics of the low stage and high stage are relatively gentle curves as shown by the solid lines in Figures 3 and 4, respectively, and the suction capacity is The rate of decrease in shaft power is smaller than the rate of decrease in .

Therefore, the present invention provides the following advantages by improving the unloading characteristics.
It is an object of the present invention to provide a method for adjusting the capacity of a two-stage screw compressor, which reduces the load during capacity adjustment, reduces power costs, and allows economical partial load operation.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a capacity adjustment method for a two-stage screw compressor in which capacity is adjusted at least on the high stage side by operating a slide valve. Until the predetermined intermediate pressure determined by the temperature or dew point of the discharged gas is reached, only the high stage side is operated at full capacity, and after reaching the predetermined intermediate pressure, both the low stage side and the high stage side are operated at their predetermined levels. In order to maintain an intermediate pressure, operation is performed at a capacity of 1 m.

The method for determining the predetermined intermediate pressure is different depending on whether the high stage side is an oil-free screw compressor or an oil-cooled screw compressor.

In the case of an oil-free screw compressor, the pressure (intermediate pressure) and temperature of the suction gas are Pz and T7, respectively, and the pressure and temperature of the discharge gas are P3. When T3 and the specific heat ratio are k, the discharge gas temperature T3 is expressed as follows.If the discharge gas pressure P3 remains unchanged, lowering the intermediate pressure P increases the compression ratio (p3/P2), The discharge gas temperature T3 increases. However, in this case, when the discharge gas temperature T3 reaches, for example, 250 to 300°C, the screw rotors come into contact with each other or with the casing, making it impossible to lower the intermediate pressure P2 below a certain pressure. Therefore, in the case of an oil-free screw compressor, the predetermined intermediate pressure is limited by the discharge gas temperature T3.

On the other hand, in the case of an oil-cooled screw compressor, the predetermined intermediate pressure is limited by the dew point of water vapor contained in the discharged gas on the higher stage side. If the suction gas contains water vapor, the dew point on the discharge side will rise due to compression, but if the discharge gas temperature exceeds T3, this water vapor will condense and water will be added to the lubricating oil removed by the oil recovery device. will be mixed in.

When this lubricating oil mixed with water is returned to the compressor body,
Since this adversely affects the lubrication effect on bearings and the like, the discharge gas temperature T3 must always be maintained above the dew point.

(Example) Next, one embodiment of the present invention will be described with reference to the drawings.

In FIG. 1, two pressure switches pSL and ps are provided between the low-stage screw compressor 1 and the high-stage screw compressor 2.
Based on the signal from H, the opening degree of the slide valve 3 of the high-stage screw compressor 2 can be adjusted via the control device 6. Then, the pressure switch PSL is set to a predetermined intermediate pressure (Pm
), and the pressure switch PS is set to Pm+α, and when the intermediate pressure is below Pm, the capacity adjustment operation starts, and the pressure switch PS is set to Pm+α.
With the above steps, the system is configured to operate at full capacity. Here, the reason why the control point is provided with a shift of +α is to prevent the slide valve 3 from causing hunting.

The value of the predetermined intermediate pressure (Pm) is determined based on the dew point on the discharge side of the high-stage screw compressor 2. First, the suction gas temperature T of the high-stage screw compressor 2 [°C], the suction gas humidity ψ [%] and the compression ratio P3/P2 (P3: discharge gas pressure,
P2: suction gas pressure) is detected, and the dew point Tdw['C] of the discharged gas is determined by the following equation.

Let vapor pressure P be a function of temperature T, P-φ(T) = 225.65xexp[-(7,21379+(
1,l52x10-'+4.787xlO-9T)X(
T-483, 16) Month X (647, 31-T)/Tl
...■, then Tdw-φ-・(φ(Tt) xyu×yu)...■to
o p.

As is clear from Equation 0, when the suction gas pressure P2, that is, the intermediate pressure, decreases, the dew point Tdw of the discharged gas increases, but as described above, the discharged gas temperature T3 also increases at the same time. Since the rate of increase in the dew point Tdw of the discharged gas and the temperature T,3 of the discharged gas does not necessarily match, there is a point where the dew point Tdw of the discharged gas becomes higher, and the intermediate pressure P2 at this time is set as Pm. In this case, the dew point Tdw of the discharged gas is calculated using the formula 0, for example,
It is also possible to set Tdw=Tdw+10°C with a margin of about IO°C.

After setting the predetermined intermediate pressure Pm as described above, in order to perform a capacity adjustment operation, the low stage screw compressor 1 performs a capacity adjustment operation using the slide valve 3, and the high stage screw compressor 2 performs a capacity adjustment operation when the intermediate pressure Pm Operate at full capacity until it drops to .

Hereinafter, the unloading characteristics during capacity adjustment operation using the method according to the present invention will be explained in comparison with the conventional method for each of a low stage screw compressor and a high stage screw compressor.

Assuming that the suction capacity of the low stage screw compressor 1 becomes αV (0 < α × 1), the intermediate pressure P2 is P, = αXPo
p, and the shaft power W of the low-stage screw compressor I is as shown by curve 2 (broken line) in FIG.

On the other hand, in the conventional capacity adjustment method, the intermediate pressure is constant, so it becomes as shown by curve 1. Therefore, as is clear from the figure, curve 1 can be defined as W, = f(α)2 curve 2o W
Since -=ct(α) and X-tlf-σ(n)<f(α), the shaft power is reduced compared to the conventional capacity adjustment method.

Furthermore, as shown in FIG. 4, when the suction capacity of the low stage screw compressor 1 becomes αV, as mentioned above, the intermediate pressure, that is, the suction pressure of the high stage screw compressor 2 becomes Pz=αXP.
The suction capacity is maintained at V2, and the shaft power W of the high stage screw compressor 2 is maintained at full capacity operation until it becomes OP or a predetermined intermediate pressure is reached.

is the value of point B on the curve of P, -αXPop. On the other hand, in the conventional capacity adjustment method, the intermediate pressure is constant, so when the suction capacity of the low-stage screw compressor 1 becomes αvl, the suction capacity of the high-stage screw compressor 2 becomes α■, and the high-stage screw compressor The shaft power W of machine 2 is P! =Pop
This is the value at point A on the (constant) curve. Although there is no significant difference in calculation between the axial power W of the high-stage screw compressor 2 shown at this point A and the above-mentioned point B, due to the decrease in the axial power W of the low-stage screw compressor 1, the 2-stage screw This means that the shaft power W for the compressor is reduced, and the unloading characteristics are improved.

Next, a method for controlling the discharge gas temperature T3 to be higher than the dew point Tdw of the discharge gas will be explained with reference to FIG. 2.

That is, a lubricating oil circulation line runs from a lubricating oil reservoir of an oil recovery device 7 provided on the discharge side of the high-stage screw compressor 2 to the high-stage screw compressor 2 via an oil cooler 8, an oil filter 9, and an oil pump 10. , a three-way valve 11 is provided between the oil cooler 8 and the oil recovery device 7, and one outlet of the three-way valve 11 is connected to the inlet side of the oil cooler 8 and the other outlet is connected to the outlet side of the oil cooler 8 to connect the bypass line of the oil cooler 8. Form. And high stage screw compressor 2
A temperature indicating controller TIC is provided on the discharge side of the oil cooler 8, and the three-way valve it is switched according to the temperature T3 of the discharged gas, thereby making it possible to change the flow rate ratio between the line passing through the oil cooler 8 and its bypass line. Furthermore, on the suction side of the high-stage screw compressor 2, the pressure Pt, temperature T, and humidity ψ of the suction gas are
Detector XD detects the discharge gas pressure P3 on the discharge side.
pressure detector PD that detects the
Based on the detected value from D, the dew point T of the discharged gas is determined by the above formula 0.
A control device that calculates dw and makes it possible to set the set temperature of the temperature indicating controller Tic to Tset=Tdw+l 0°C! 2 will be provided. With the above configuration, the dew point is determined according to the state of the suction gas of the high-stage screw compressor 2, and the set value of the temperature indicating controller Tic is always set above the dew point. Therefore, for example, when the temperature indicator controller TTC detects a discharge gas temperature T3 that is lower than the set value T set,
Since the three-way valve 11 operates to reduce the flow rate passing through the oil cooler 8 and increase the supply temperature of lubricating oil, it becomes possible to always control the discharge gas temperature T3 to be equal to or higher than the dew point Tdw.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, the load during volume agitation adjustment operation is reduced and the unloading characteristics are improved, so the power ratio is reduced and the economical part is achieved. Load operation becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of a two-stage screw compressor to which the capacity adjustment method according to the present invention is applied, Fig. 2 is a system diagram showing a method of controlling discharge gas temperature, and Fig. 3 is a system diagram of a low-stage screw compressor. Figure 4 is a diagram showing the unloading characteristics of a high-stage screw compressor, and Figures 5 and 6 are diagrams showing the system of a two-stage screw compressor to which the conventional capacity adjustment method is applied. Figure 7 shows the relationship between suction pressure, discharge pressure, and required torque, Figure 8 shows the relationship between intermediate pressure and shaft power, and Figure 9 shows the state quantity of suction gas. It is an explanatory diagram of symbols. ■...Low stage screw compressor, 2...High stage screw compressor, 3.4...Slide valve, 6...Control device. Patent applicant Kobe Steel Co., Ltd. Agent Patent attorney Aohaku Ao and 2 others Figure 1 Figure 2 Figure 3 00.6xlAOJlxvIV+ Low Ribbon Suction Customer Volume Figure 4 0 αXV2 V2 High ffl
Suction amount figure 6

Claims (1)

[Claims] (1) In a capacity adjustment method for a two-stage screw compressor in which the capacity is adjusted at least on the high stage side by operating a slide valve, until the predetermined intermediate pressure determined by the temperature or dew point of the discharge gas on the high stage side is reached. , a two-stage system characterized in that full capacity operation is performed only on the high stage side, and after reaching the predetermined intermediate pressure, capacity adjustment operation is performed on both the low stage side and the high stage side in order to maintain the predetermined intermediate pressure. How to adjust the capacity of a screw compressor.