Compressor Choke or Stonewall
Compressor choke is an abnormal operating condition for centrifugal compressor. Choking of centrifugal compressor occurs when the compressor is operating at low discharge pressure and very high flowrates. These high flowrates at compressor choke point are actually the maximum that the compressor can push through. Any further decrease in the outlet resistance will not lead to increase in compressor output. This operating condition is also known as stonewalling of a centrifugal compressor.

Figure 1 - compressor map showing the choke line which represents the maximum flow limit for a compressor
How does a compressor choke?
Stonewall or choke point for a centrifugal compressor occurs when the resistance to flow in the compressor discharge line drops significantly below the normal levels. Due to low resistance, compressor discharge sees very low back pressure. As suggested by the compressor maps for a fixed RPM value, compressor output increases as the backpressure at compressor discharge drops down. This leads to increased gas velocity in the centrifugal compressor. The increase in gas velocity can occur until it can reach its maximum at sonic velocity. When the gas velocity in any of the compressor parts reaches close to sonic velocity (MACH1), this is said to be the choke point or stonewall for compressor operation. The gas velocity and gas flow rate cannot go beyond this value at the choke point.
Damages due to compressor choking
Prolonged operation of a compressor at its choke point can lead to damaging the compressor parts. Compressor choking is not particularly damaging to single-stage centrifugal compressors but can cause serious damage to the rotors and blades of multistage centrifugal and axial compressors.
How to prevent compressor choking
To prevent the compressor choke or stonewall from happening it is needed to maintain a certain level of flow resistance in the compressor outlet line. Anti-choke valves can be used for this purpose in the compressor outlet line. Anti-choke valves close to restrict the flow to keep compressor from stonewalling. When flow resistance in compressor outlet falls and flow begins to increase, the anti-choke valves close to develop resistance to the increasing flow.


 
Centrifugal Compressor Surge

Compressor Surge
Centrifugal compressor surge is seen as a very dangerous and detrimental phenomenon in compressed air systems, dangerous because it causes the compressor to vibrate and detrimental because it causes damage to the compressor parts. Compressor surge only occurs in dynamic compressors (Axial and Centrifugal) due to their nature. To understand compressor surge, a solid understanding of how a compressor actually works is needed.
A centrifugal compressor is a machine that imparts energy to the gas flowing through it. This energy is in the form of velocity and is imparted from compressor impeller to the gas. Kinetic energy of the gas is then converted to pressure head when gas is diffused, slowed down.
Figure-1 indicates a schematic of a centrifugal compressor system which demonstrates this phenomenon. On the left is the suction side and the right is the discharge side. The centrifugal compressor is in the centre. Compressor draws in gas near the centre of the compressor impeller with a low energy and imparts kinetic energy as the gas gets hurled in radial direction by the rotating impeller. This kinetic energy is converted to pressure head near the impeller periphery when the gas slows it down in the diffuser and it is forced into the discharge line.

Figure 1: centrifugal compressor schematic
To understand compressor surge phenomenon, imagine the following situation. A compressor is running at 80% of its maximum pressure output for 100% rpm producing 8 bar. Then the compressor starts to produce 100% of its possible pressure output at 10 bar. This now puts the operating point on the surge line as shown in the figure-2. As soon as the system pressure in the discharge line reaches 10 bar the compressor begins to surge.

Figure 2: Sample compressor map
Compressor surge can be understood by the help of schematic of a compressor impeller in figure-3. At point (1) on figure-3, suction end of a centrifugal compressor impeller, fluid has the lowest energy. As the gas moves to point (2) of figure-3, energy of the fluid increases due to kinetic energy imparted by impeller. Energy reaches a maximum at point (3) on figure-3. When the compressor is imparting as much energy as it possibly can, i.e. 100% pressure at 100% rpm and backpressure at pump discharge is too high to be overcome, the fluid flow stalls near point (3). This means the pressure at point (3) increases because the kinetic energy changes to pressure as it slows to a stop. Thus the energy at point (3) is greater than at points (2) and (1) so the flow reverses and flows backward through the impeller.

Figure 3: Schematic of centrifugal compressor impeller
When the flow reverses, energy and pressure at point (3) is relieved and drops down. Now the compressor can compress fluid, which it does and with the increase of pressure the backflow occurs again and this is the reason why compressor surge is a cyclic phenomenon. Compressor surge puts strain on many of the compressors parts such as the bearings, seals and the impeller itself and thus damage can occur if the compressor is left to surge. The vibration caused by the surging can severely damage the motor compressor coupling and also the baseplate.
There are many methods of surge control in industry today and choosing the right one is difficult and very subjective, so choose wisely.



Compressor Map
Compressor maps are developed by the manufacturer of dynamic compressors. They are compressor equivalents of the pump performance curves. It is the performance chart of a specific compressor which manufacturer calculates and draws up for the unique design characteristics of that compressor. An example of an air compressor from a car turbocharger is shown in figure – 1.

Figure 1: Sample air compressor map
A compressor map is two dimensional and has all the information an engineer needs for design purposes. In the sample compressor map represented in figure-1, the blue curved lines represent compressor curves for different impeller speed values with the uppermost line being the maximum speed that the compressor can reach. The skewed ellipses are the efficiency “islands” or the efficiency areas. The Y axis is the pressure ratio which is explained below and the X axis is the “air / gas flow before the turbo”. To read of a point from the compressor map is straight forward. For example the red circled point on the map, represents compressor output of 520 CFM (corrected air flow) at a pressure ratio of 2.1. At this point the compressor is spinning at 144000 rpm and has an efficiency of only 61% (indicated by the corresponding efficiency area).
The Y axis (Vertical Axis)
Y axis of a compressor map indicates pressure ratio. Pressure ratio is the ratio of the compressor discharge pressure to the compressor suction pressure. If the compressor suction pressure is known, pressure ratio can be decided to achieve required output. For example if the inlet pressure is 1bar (Atmospheric pressure) and it is required to boost the gas to a pressure of 2 bar, then the pressure ratio needs to be two. The same principal applies to industrial compressors except that they have higher pressure ratios, especially multi-stage compressors. The following is a formula of the calculation that has just been described:
Outlet pressure = Inlet pressure × pressure ratio
Thus if at sea level a certain compressor gives an outlet pressure of 2 bar and then you take the compressor to a much higher elevation the outlet pressure will be lower because the inlet pressure has dropped.
Efficiency Islands / Efficiency Areas
The “ellipses” can be used just like contours on geographical maps, except that here they show a range of efficiencies. Usually the efficiency islands converge to the centre of the compressor map as shown in figure-1, where the efficiency is at its maximum. This line where the efficiency islands on compressor maps converge is known as “Peak Efficiency Line”. Usually operating the compressor near “Peak Efficiency Line” is always the most desirable as the most possible work output can be obtained using same or less work input.
Surge and Choke lines (Orange)
The orange line on the left hand side is the Surge line and the Orange line on the right hand side is the choke line. If the compressor operates on the left of the surge line, this can result in compressor surge. Compressor surge is a pulsating back flow of gas through the device.Compressor surge is a highly undesirable phenomenon as it can mechanically damage compressor parts and must be avoided.
If the compressor operates on the right of the choke line then the compressor will experience choked flow. Choked flow is when the flow reaches the speed of sound and this is a problem because it limits the maximum flow rate through the compressor. Thus the choke line on a compressor map signifies its maximum flow rate limit. When designing a compressor system careful consideration needs to be taken to make sure that the designed operating point does not fall outside the surge and choke lines.
Max Flow and Max Pressure
The maximum flow that a compressor can handle is easily found on the sample compressors map, in figure-1. The point of maximum flow is the extreme right point on the map. The red circled point in figure-1 happens to be the point of maximum flow. If more flow needs to be pushed through then a different compressor is required. At the maximum flow point, compressor efficiency is at its lowest so it is highly desirable to use a different compressor.
The maximum discharge pressure that a compressor can achieve is found using the uppermost point on the map. At this point the pressure ratio can be found and using the formula above the outlet pressure can be calculated.
Compressor maps are very important in design of systems because they give you vital information on surge, choke and compressor speeds. They also let you know if your compressor is going to be efficient enough for your application.