Centrifugal Compressor

Centrifugal Compressors

A centrifugal compressor is a radial flow rotodynamic fluid machine that uses mostly air as the working fluid and utilizes the mechanical energy imparted to the machine from outside to increase the total internal energy of the fluid mainly in the form of increased static pressure head.

During the second world war most of the gas turbine units used centrifugal compressors. Attention was focused on the simple turbojet units where low power-plant weight was of great importance. Since the war, however, the axial compressors have been developed to the point where it has an appreciably higher isentropic efficiency. Though centrifugal compressors are not that popular today, there is renewed interest in the centrifugal stage, used in conjunction with one or more axial stages, for small turbofan and turboprop aircraft engines.

Read more;压缩机的应用是什么

Characteristics Features of Centrifugal Compressors

)它占据一个更小的长度比同等axial flow compressor.
b）它对异物损害具有更好的抵抗力。
c) Because of the relatively short passage length, loss of performance due to buildup deposits on blade surfaces will not be as great as the axial flow compressors.
d) It can work reasonably well in a contaminated atmosphere as compared to axial flow compressor.
e) It has the ability to operate over a wide range of mass flow rates at any particular rotational speed.
f) Its efficiency under the most favorable circumstances, are less than those of axial compressors designed for the same duty, by as much as 3 or 4 %.
g) However, at very low mass flow the axial flow compressor efficiency drops, blading is small and the advantage appears to lie with the centrifugal compressor in its relative simplicity and cost.

Centrifugal Compressor Parts :

Parts of centrifugal compressor and its function

离心压缩机的主要成分如图所示。下面给出了每个部分的细节。
1.带有加速（收敛）喷嘴的入口套管
2。Inlet guide vanes (IGV)
3。Impeller
4.扩散器
5.滚动或焊接
6.诱导者部分

Centrifugal Compressor Diagram :

1。Inlet casing with accelerating nozzle

• 这functionof the inlet casing is to accelerate the fluid from its initial condition to the entry of inlet guide vanes and to provide uniform velocity at the eye.
• 这inlet flange is axisymmetric and the inlet duct takes the form of a simple converging nozzle.
• 这outlet of the inlet casing is known as the impeller eye.

2。Inlet guide vanes (IGV)

• 这functionof inlet guide vanes is to direct the flow in the desired direction at the entry of the impeller.
• 这inlet guide vanes should be chosen so as to obtain a minimum relative Mach number at the eye tip.

3。Impeller

• 这functionof the impeller is to increase the energy level of fluid by whirling it outwards by increasing the angular momentum of the fluid. (Refer Fig.)
• Both static pressure and velocity of fluid are increased in the impeller.
• 这impeller vanes help to transfer the energy from the impeller to the fluid.
• 这hub is the curved surface of revolution of the impeller A-B.
• 这shroud is the curved surface C-D forming the outer boundary to the flow of fluid.
  (Shrouding an impeller eliminate tip leakage losses but at the same time increases friction losses.)
• 叶轮可以通过将裹尸布连接到叶片末端（称为浮雕的叶轮）或在叶片末端和固定壁之间没有较小的间隙缝隙来封闭。

4。A diffuser由许多固定分歧的通道组成，其中空气减速，随之而来的静压升高。

5. Scroll or Volute

• 这air leaving the diffuser is collected in a spiral passage known as volute or scroll and the volute discharges the air through the delivery pipe.
• 用于静脉传递的不同横截面是矩形，圆形和梯形的。

6.诱导者部分

• At the entry to the impeller the relative flow has a velocity Vr1, at angle α1 to the axis of rotation as shown in Fig.
• This relative flow is turned into the axial direction by the inducer section or rotating guide vanes.
• 诱导剂始于眼睛，通常在流动开始转向径向方向的区域中结束。

Working Principle of Centrifugal Compressor :

Air is sucked into the impeller eye and whirled outwards at high speed by the impeller disk. At any point in the flow of air through the impeller the centripetal acceleration is obtained by a pressure head so that the static pressure of the air increases from the eye to the tip of the impeller. The remainder of the static pressure rise is obtained in the diffuser, where the very high velocity of air leaving the impeller tip is reduced to almost the velocity with which the air enters the impeller eye.

通常,大约一半的总压强occ上升urs in the impeller and the other half in the diffuser. Owing to the action of the vanes in carrying the air around with the impeller, there is a slightly higher static pressure on the forward side of the vane than on the trailing face. The air will thus tend to flow around the edge of the vanes in the clearing space between the impeller and the casing. This results in a loss of efficiency and the clearance must be kept as small as possible. Sometimes, a shroud attached to the blades as shown in Figure (d) may eliminate such a loss, but it is avoided because of increased disc friction loss and of manufacturing difficulties.

通常采用直叶片和径向叶片，以避免在叶片中设置任何不良弯曲应力。径向叶片的选择还确定总压力升高在叶轮和扩散器之间平均分配。

跨离心压缩机的压力和速度变化：

• Air enters the compressor at a mean radius with a low-velocity V1, and atmospheric pressure P1 as shown in Fig.
• 然后将其加速至高速V2，并根据叶轮的离心作用，并加速P2。
• 这air now enters the diffuser where its velocity is reduced to some value V3, and pressure increases to P3.
• In practice, about half of the total pressure rise per stage is achieved in the impeller and the remaining half in the diffuser.

Velocity Diagram of Centrifugal Compressor

• 这velocity diagrams at the inlet and outlet of the impeller of a centrifugal compressor are shown in Fig. (a) and (b).
• 在分析离心压缩机中，做出了以下假设：
  (i) The flow phenomenon is steady and uniform throughout.
  (ii) There is no separation of flow.
  (iii) The flow through the impeller is frictionless.
  (iv) There are no shock waves occurring anywhere.

这following are the notations used in the analysis of a centrifugal compressor.
让我们来;
α1=从入口处的导板从导板出口=在入口处的绝对角度
β1 = Inlet angle to the rotor or impeller
β2 = Outlet angle from the rotor or impeller
α2 = Inlet angle to the diffuser or the stator
u1 = Mean blade velocity at inlet
u2 = Mean blade velocity at exit
V1 = Absolute velocity of air at inlet to the rotor
V2 =在转子出口处的空气的绝对速度
Vr1 = Relative velocity of air at inlet to the rotor blade l
Vr2 = Relative velocity of air at the exit to the rotor blade
Vw1 = Velocity of whirl at inlet (tangential component of absolute velocity 1 V )
VW2 =出口处旋转的速度（绝对速度2 V的切向成分）
Vf1 = Velocity of flow at the inlet (Component of 1 V perpendicular to the plane of rotation)
VF2 =出口处流量的速度（2 V的分量垂直于旋转平面）。
m = Mass flow rate, kg/sec

(i) If no pre-whirl, the air enters the impeller eye in an axial direction,α1 = 90度,Vf1 = V1, lea Vw1 = 0和空气ving the impeller in the radial directionβ2= 90 degree, Vf2=Vr2 and Vw2 = Vu2

(ii) If the air enters the impeller eye in an axial direction α1= 90 degree but air will not leaving the impeller in radial direction β2 < 90 degrees, Vr2 ≠ Vf2, and Vw2

Centrifugal Compressor Efficiency :

Isentropic Efficiency:

• 这compression process in a reciprocating compressor may approach isothermal compression due to slow speed, cooling of the cylinder, and interstage cooling.
• But in centrifugal compressor; running at high speed, there is a lot of friction between molecules of air, between air and blade passages, eddies formation, and shocks at inlet and exit.
• 这些因素导致内部热量产生，因此达到的最高温度将大于绝热压缩。
• 因此，未冷压缩压缩压缩的索引可能高达1.7，压缩指数的高价值需要大量的压缩工作，并且可以通过冷水夹克和使用围绕空气通道来降低此值中冷器。
• 但是，通常不可能提供足够的冷却来将压缩曲线带到等粒线的左侧。
• 正是由于这个原因，往复式压缩机的热力学效率的标准是等温的，而旋转压缩机的标准是等激素压缩。
• “Isentropic efficiency is defined as the ratio of isentropic work required to compress the air from P01, to P02 to the actual work required for the same pressure ratio.”

Isentropic Efficiency = Isentropic Compression Work /Actual Compression Work

离心压缩机的优势：

a.) When compared to other compressors, it is relatively agile and easy to manufacture.

b.) As this compressor does not require any special foundation it is highly energy-efficient and reliable.

c.) They consist of a small number of rubbing parts and are absolutely oil-free in nature.

d。）它比轴向流动压缩机的每个阶段产生更高的压力比。

Disadvantages of Centrifugal Compressor.

a.) They produce a limited amount of pressure and are not suitable for very high compression.

b.) As they work at relatively high speed an enlightened or worldly mounting is required.

c。）他们对拖延和窒息等问题非常敏感。

离心压缩机的应用：

1. In gas turbines and auxiliary power units.
2. 在汽车发动机和柴油发动机涡轮增压器和增压器中。
3. In pipeline compressors of natural gas to move the gas from the production site to the consumer.
4. 在炼油厂，天然气加工，石化和化学植物中。
5. Air-conditioning and refrigeration and HVAC: Centrifugal compressors quite often supply the compression in water chillers cycles.
6. In air separation plants to manufacture purified end product gases.
7. 在油田重新注入高压天然气以改善石油回收率。
8. 在中央植物冷却系统中，带有超过400吨制冷的冷却载荷的大型建筑物通常使用水冷冷却器或带离心压缩机或涡轮增压器的水冷冷却器。

Difference between Centrifugal Compressor and Axial Flow Compressor :

Comparison of Centrifugal and Axial flow compressor are as follows :

高级。	Centrifugal compressor	Axial Flow Compressor
1。	Flow is perpendicular to the axis of the compressor.	这flow of air is parallel to the axis of compressor.
2。	Low manufacturing and running costs.	高制造和运行成本。
3。	需要低启动扭矩。	Requires high starting torque.
4。	Not suitable for multi-staging.	适用于多阶段。
5.	Requires a large frontal area for a given rate of flow.	Requires less frontal area for a given rate of flow.
6.	Pressure ratio per stage is4:1.	这pressure ratio is 1.1 to 1.2.
7.	Isentropic efficiency is 70%	Isentropic efficiency is 80%.
8.	Used in supercharging I.C. engine and for refrigerants and industrial gases.	Used universally with large gas turbines.

Difference between Centrifugal Compressor and Reciprocating Compressor :

高级	参数	往复式压缩机	Centrifugal compressor
1。	Balancing	A poorly balanced and vibration problem occurs.	更好的平衡；因为没有回报部分。
2。	机械效率	Less efficiency due to more sliding and bearing members	由于轴承成员较少。
3。	Pressure ratio	这pressure ratio per stage is high about 5 to 8.	每个阶段的压力比高约3至4.5。
4。	Initial cost	High	Less
5.	递送压力	Capable to deliver high pressure.	能够提供中等压力。
6.	Capacity	Handles small volume	处理大容量
7.	Flexibility	More flexible with capacity and pressure range.	No flexibility in capacity and pressure range.
8.	Compression efficiency	Higher at compression ratio above 2.	Higher at compression ratio below 2.
9.	Speed	对低速驱动的适应性	Adaptability to high-speed low maintenance cost drivers such as a turbine.
10.	Maintenance cost	High	Low
11。	Suitability	对于低压，高压以及低压和中等气体。	For low and medium pressure and large gas volumes.

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