In general, this article focuses on the operating principles of centrifugal air compressors, discussing them in simple terms to provide an understanding of application limitations and opportunities. One primary goal is to define often-confusing terminology, such as “rise to surge,” stonewall and surge,” “mass flow,” and “dynamic compression.” This article is not intended to be an engineering discussion of the various types and designs of centrifugal and other air compressors, but rather, a guideline for deciphering operating curves and understanding general performance.
Operating Basics: Positive Displacement vs. Dynamic Compression
The most common air compressor in industries today is the positive displacement type — rotary screw, rotary vane, reciprocating, and scroll — in which the inlet compressed air is mechanically trapped in the compression chamber and then mechanically reduced in volume to raise the pressure and temperature (i.e. piston cylinder).
In a positive displacement compressor, the required operating power is mostly driven by flow (cfm), and is somewhat less affected by discharge pressure, or psig (Table 1). A positive displacement performance curve will characteristically be more vertical than a centrifugal performance curve. This will produce relatively constant cfm at the available horsepower.
Table 1: Effect of discharge pressure (not including type of unloading or part load controls)
The centrifugal air compressor, on the other hand, operates over a range of flows and discharge pressures. The operating performance curve is shaped by the selected individual internal components and affected by the operating conditions, such as inlet pressure, inlet temperature, cooling water temperature, and discharge pressure.
Figure 1 shows the process of dynamic compression as applied in a centrifugal compressor operating stage, in which velocity and kinetic energy are converted to pressure and temperature as the flow is restricted. Another term for this process is mass flow — the power requirement to deliver the rated cfm of flow at the rated pressure (psig) is determined by the weight of the air (some manufacturers also use the term “density”).
|Figure 1: Dynamic compression centrifugal operating stage|
How Outside Conditions Impact Dynamic Compression (or Centrifugal Compressors)
The power requirement for the dynamic compression process, when the internal design parts are not considered, is basically dependent on the weight of the air going through the machine. Ignoring part loads controls anything else that will increase or decrease the weight of the air going through the stages to final flow, and pressure will have a direct impact on input power.
Increasing the inlet temperature will lighten the total fixed airflow and deliver less usable air, or scfm, to the user (Figure 2a), thereby reducing the input power requirement (Figure 2b). Colder inlet temperatures will produce the opposite effect.
|Figure 2a: Effect of inlet air temp on discharge pressure||
Figure 2b: Effect of inlet air temp on power
Reducing the inlet pressure (altitude, negative compressor room pressure, dirty/poorly sized inlet filter) will lighten the compressed airflow (cfm) that travels through the stages (Figure 3a). This also results in less usable air (scfm) at a reduced input power requirement. Higher inlet pressure will have the opposite effect.
Increasing the cooling water temperatures will again have the same “lightening” effect on the compressed air through the stages (Figure 4). It will also reduce the power requirements, just like the previous conditions.
|Figure 3a: Effect of inlet pressure||Figure 3b: Effect of cooling water temperature|
The actual net effect of any of these conditions is dependent on the actual performance curve and aerodynamic characteristics of the design. This is also the case of discharge pressure with a fixed-wheel, impeller, diffuser, or speed compressor stage.
Increasing the discharge pressure will normally have the effect of raising the weight of the compressed air stream through the stages, which will result in less flow of usable air (scfm) at the same input power. Lowering the pressure will often allow more flow at the same or similar power input. Actual machine-specific performance is covered later in this article.
Understanding Centrifugal Manufacturer Operating Curves
Now that we have a fundamental understanding of the differences between positive displacement and dynamic compression, we can begin deciphering the operating curves of centrifugal compressors. The data in the following sections will be discussed in these units of measurement:
- Standard cubic feet per minute (scfm) or Nm3/hr at full and part loads
- Input power in kW
- Pressures, either in psig or bar (only using psia to convert from icfm/acfm to scfm)
Any activity that lowers the inlet air weight or mass, such as higher temperature, lowers pressure after the filter and will reduce the mass flow, scfm, and input power accordingly. Figure 10 provides samples of typical centrifugal air compressor performance curves.
Figure 4: Typical centrifugal performance curves
As displayed in Figure 4, typical centrifugal performance curves bring up some new items to address, including “turndown,” “rise to surge,” and “stonewall.”
What Causes Surge?
The centrifugal compressor as used in industry is a dynamic compressor with rapidly rotating impellers accelerating the airflow. The air then passes through a diffuser section, which converts the velocity head into pressure through flow resistance.
In a dynamic or mass flow compressor like the centrifugal, the power to compress the air is basically a function of the weight of the air, the flow, the volume, the temperature, and the head or pressure.
Once the impeller is designed and a speed set, the energy that a pound of air will absorb when passing through the impeller is established. This is true despite variation in inlet temperature, pressure level, throttling, etc. A pound of air will vary in cubic feet by temperature and pressure.
A centrifugal compressor, therefore, will deliver a pound of air with a constant expenditure of energy — winter or summer. The actual volume of inlet air to be compressed will vary for a period of time with the inlet condition of pressure and temperature.
As more compressed air is produced than needed, the centrifugal compressor must unload, or deliver less air to avoid over pressure. Each centrifugal compressor has a maximum pressure it can reach for specific inlet conditions that will cause the airflow to reverse and surge, shutting off the compressor to avoid damage from the resultant vibrations.
Surge is to be avoided (including mini-surges, since these not only set up potentially damaging vibrations, but also cause a very high temperature rise at the eye clearance.)
Defining “Rise to Surge,” “Turndown,” and “Stonewall”
This is an oversimplification of the surge action, however, since each unit has a rise to surge limit or maximum pressure. Turndown is the percentage below full load flow that the compressor can run without experiencing surge. For example, 15 percent turndown means the unit can run at 85 percent flow or higher, as equipped without hitting surge. At greater turndown, it will be close to or at surge.
At some point, as the discharge pressure falls and the airflow through increases at full load, the physical limitations will not allow more air through the stages — this point is known as stonewall. Continued operation at or beyond this point can cause such high flow rates with greater pressure differential that the impellers will not totally fill the vane areas and a cavitation-like action will occur, creating another type of surge with damaging vibrations.
Operating at surge will set up high vibrations and, if not eliminated, can have a negative impact on the mechanical integrity of the unit, leading to premature failure. Most centrifugals are equipped with vibration monitors and will shut down to avoid this condition.
As the pressure falls and the compressor approaches stonewall, the increase in flow becomes very minimal, and the falling discharge pressure continues to lighten the airflow through the stages, reducing the input power. The most energy-efficient point on many centrifugals is just before stonewall.
Using Manufacturer Performance Curves to Develop Projected Operation Efficiency
With an understanding of the terminology unique to centrifugal air compressor performance curves, you can develop a predictable and probable actual projected operating efficiency based on a manufacturer’s performance curve. Figure 11 provides a representation of a sample manufacturer performance curve.
Before delving into the curve, however, a few things need to be outlined:
- Without known operating and site conditions, assume the standard CAGI operating conditions of 68°F, 14.5 ambient pressure and 0% relative humidity with site conditions of 95°F, 14.5 psia, and 60% relative humidity.
- The pressure (psig) is clear.
- The flow (cfm) is not clear — most likely it is acfm or icfm when scfm is not stated.
- Power is in BHP (compressor input shaft horsepower) and not motor input horsepower.
- The current new motor is probably a 500-hp class induction motor with a .94 ME (motor efficiency). This is not specific, but the data is necessary to accurately profile the compressor.
Figure 5: Estimated performance curve for full load compressor at 125 psig
Using Figure 5 as presented, the centrifugal compressor delivers:
- 2050 cfm at 125 psig at 430 hp (x .7457 = 321 kW) = specific power of 6.39 cfm/kW
- Turndown 1535 cfm at 125 psig at 345 hp (x .7457 = 257 kW) = 5.97 cfm/kWThe Caveat: Converting Measurements for Consistency
However, there are holes in these results, which include:
- The icfm/acfm should be scfm
- The calculated compressor shaft horsepower is accurate as converted to kW, but it doesn’t take into account any drive motor loss.
- The ME (motor efficiency) is .94 a, 6% loss at full load — a larger loss at turndown.
Table 2 addresses how to convert the icfm/acfm to scfm for comparative purposes.
Table 2: Establish a multiplier of .90 to convert icfm/acfm to scfm
Input Power to the Motor
To convert BHP (compressor shaft) to projected electric motor input kW, use:
- Input kW = (BHP)(.745) ÷ ME
- Projected motor ME = .94 at full load
- Projected motor ME = .92 at turndown
- The projected input power will be:
- Full load: (430 BHP) (.7457) ÷ .94 = 341 input kW
- Turndown: (345 BHP) (.7457) ÷ .92 = 280 input kW
After the conversions have been completed, the results in Table 3 demonstrate that the projections can be misconstrued.
Table 3: Input power to motor results comparison
When results like this are misconstrued, it may or may not be intentional. The point is to show the importance of detail when evaluating an existing compressor or reviewing several centrifugal units (in addition to other types). It is best to understand and equalize all the various OEM operating performance curves to be able to make an informed decision.
Using Centrifugal Operating Performance Curves to Optimize the Fit to Your System
Working with the OEM supplier(s) and their operating performance curves effectively will help to develop a successful application. In order for the user to provide the OEM supplier the appropriate data, the user should be familiar with the information presented to fully understand and ask for the important additional data.
Here are a couple important questions that you should ask:
- What are the operating characteristics of the impeller/diffuser with regard to surge point, turndown, specific power full load, etc.?
- What is the next set of operating characteristics for a standard impeller/diffuser for more turndown capability? (Probably a higher pressure)
- What are the cost and savings benefits of inlet guide vanes (IGV) as opposed to a standard inlet butterfly valve (IBV)?
In summation, this article was written to identify and explain the definitions behind centrifugal performance data and its importance. With this information, the user can work with their local OEM supplier and/or technical engineering groups to select and properly apply a unit to fit the specific site conditions in an optimum manner.
To read more about Air Compressors Techonology, please visit www.airbestpractices.com/technology/air-compressors.