The reasonable selection of centrifugal compressor should make the compressor run near the design working condition, because the compressor efficiency is high at this time. However, due to the inevitable change of process parameters in production, it is often necessary to manually or automatically adjust the compressor to meet the production requirements.

There are generally two kinds of adjustment of centrifugal compressor: one is equal pressure adjustment, that is, the flow is adjusted on the premise that the compressor discharge pressure is unchanged; The other is equal flow regulation, that is, to regulate the discharge pressure of the compressor under the condition of ensuring constant flow. The former is more commonly used. The essence of compressor regulation is to change the operating point of the compressor. In principle, the method used is to try to change the characteristic curve of the compressor or the characteristic curve of the pipe network. There are mainly the following adjustment methods.

1. Outlet throttling regulation

Install a regulating valve at the outlet of the machine (Fig. 3 - 28). If the pressure of the vessel in the pipe network remains unchanged, the flow is required to be reduced from Gs to Gs. At this time, just turn down the outlet regulating valve to make the pipe network characteristic curve steeper, from Curve 2 to Curve 3, the compressor operating point moves from S point to S "point, and the flow at S" point is Gs, so as to achieve the purpose of regulation. Both the regulating mechanism and the operation method of this method are very simple. The disadvantage of this method is that it wastes a lot of power, and the differential pressure (ps: - ps) is completely consumed on the throttling loss of the valve. If the pressure in the vessel changes, but the flow is required to remain unchanged, the outlet throttling method can also be used to adjust the pressure.

It can be seen that the outlet throttling regulation is not economical, especially when the compressor performance curve is steep and the range of regulated flow is large, its shortcomings are more prominent. Therefore, except for fans and small blowers, compressors are rarely used.

2. Inlet throttling regulation

As shown in Fig. 3-29, install the regulating valve on the air inlet pipe of the compressor. When the control valve is fully open, the gas pressure P behind the valve is equal to the gas pressure ps in front of the valve. At this time, the compressor characteristic curve is shown in Figure 3-29 (b) Curve 1. Turn down the regulating valve, and the compressor inlet pressure will drop to Ps<ps, as shown in Fig. 3 - 290) Curve 2. The reduction of inlet pressure will directly affect the medium pressure pa of the compressor, moving the compressor characteristic curve from curve 1 to curve 3. If you continue to close the inlet regulating valve, the inlet pressure will

The relationship of air flow changes from curve 2 to curve 4, and the compressor characteristics change from curve 3 to curve 5.

If the compressor used to work at point S, the vessel pressure pr would not change, and now the flow would be required from Gs to Gs. When the inlet throttling method is used for regulation, just turn down the inlet regulating valve to make the compressor characteristic curve move down to curve 3. At this time, the operating point of the compressor is S ", and the flow is Gs", so as to achieve the purpose of regulation. Similarly, if the discharge pressure of the compressor is required to be reduced and the flow rate is expected to remain unchanged, the inlet throttling regulation can also be used to achieve that the compressor is in the S "position. Compared with the outlet throttling method, the inlet throttling method is more economical, so it is a relatively simple and commonly used regulation method.

3. Change the speed regulation

For compressors driven by steam turbine, gas turbine and variable frequency motor, speed regulation is adopted for convenience. When the compressor changes its speed, its characteristic curve will also change accordingly, so this method can be used to change the operating point to meet the production requirements.

As shown in Fig. 3-30, if a compressor delivers gas to a vessel with pressure of pr, the design flow is Gs, the design speed is n, and the operating point is S. Now, if pr is required to remain unchanged while the flow is reduced to Gs (or increased to Gs), as long as the speed of the compressor is reduced to 1n15 (or increased to ns), the compressor characteristic curve can be changed to a new operating point S "(or S), so that the flow can reach Gs (or Gs). Similarly, when the pressure of the container is reduced to p, while the flow is required to remain unchanged, as long as the speed is reduced to ns to work

Move the point from S to S ".

Since the energy head Ht of the centrifugal compressor is proportional to n2, a considerable adjustment range can be obtained by using the variable speed adjustment method. Figure 3-31 shows the characteristic curve of speed regulation, that is, the curve of compressor pressure ratio and efficiency changing with flow. It can be seen from the figure that changing the speed regulation method does not cause other additional losses, so it is the regulation method often used by large compressors at present.

4. Adjust the inlet guide vane

A rotatable inlet guide vane is set in front of the inlet of the compressor impeller, and each guide vane can rotate around its own axis by a special organization, so as to change the installation angle of the guide vane, so that the air flow entering the impeller can generate pre rotation, so as to change the compressor characteristic curve and realize the compressor condition adjustment. After the inlet guide vane rotates, if the air flow at the inlet of the impeller gets a rotation that is consistent with the rotation direction of the impeller, it is called positive rotation; If the rotation direction of the impeller is opposite to that of the impeller, it is called negative rotation, so this method is also called intake pre rotation adjustment. According to Euler equation Hr=u2C2u - unCu, the compressor head H τ Change with Cu. When C1o>0, Hr will decrease and the compressor performance curve will move downward; When cu<0, Hr will increase and the performance curve will move up. Figure 3-32 shows the curves for different positive turns. The larger the pre turn angle, the steeper the performance curve, and the smaller the surge flow. At the same time, after rotating the inlet guide vane, there is an attack angle at the inlet of the impeller, which increases the impact loss, so the efficiency decreases, but the energy loss is smaller than that of the inlet throttling.

The regulating mechanism of the rotatable guide vane itself is relatively complex, especially for multi-stage compressors, if the rotatable guide vane is used in front of each stage, the whole machine will be very complex, or even impossible to achieve. Generally, they are used on ventilators or blowers because their D1/D2 is larger than that of compressors. It can be seen that the regulation effect is obvious at this time.

Figure 3-33 shows the economic comparison of three adjustment methods: inlet throttling, rotating inlet guide vane adjustment and changing speed. AP is the power saved based on inlet throttling. Curve 1 shows the power saved by rotating inlet guide vane compared with inlet throttling; Curve 2 shows the power curve saved by changing the speed compared with the inlet throttling, obviously, the economy of changing the speed is the best.

4、 Series and Parallel Connection of Centrifugal Compressors

Compressors can operate in series or parallel. If the pressure of one compressor fails to meet the required requirements when it is working, the two compressors can be considered to work in series. Similarly, if the flow of one compressor cannot meet the required requirements when it is working, two or more compressors can also be operated in parallel.

Taking the joint operation of two compressors as an example, the working characteristics of parallel and series compressors are introduced respectively.

1. Parallel connection characteristics

The parallel connection of compressors is often used in the following situations: first, the gas transmission capacity must be increased without the need to rebuild the existing compressor; Second, the gas consumption is large, and one compressor may be too large or difficult to manufacture. In this case, two small compressors should be considered to supply gas in parallel; Third, the user's gas consumption often changes. When the required gas transmission capacity is large, two compressors operate at the same time. When the gas transmission capacity is small, only one compressor is opened. For example, the gas transmission capacity of long distance natural gas pipeline fluctuates greatly in winter and summer. At this time, the peak shaving of gas transmission capacity can be achieved by using two compressors in parallel. Figure 3-34 shows the flow chart when two crimping machines are used in parallel in the compressor station of the long-distance natural gas pipeline.

The main purpose of parallel operation of centrifugal compressors is to increase the flow. Its performance curve is shown in Figure 3-35. The performance curves of the two compressors are I and II respectively. According to the principle of equal pressure ratio and flow addition in parallel, the performance curve (I+lI) of the two compressors after parallel operation is obtained.