How Inlet Conditions Impact Centrifugal Air Compressor Performance
Compressed Air Best Practices® (CABP) Magazine recently spoke with Rick Stasyshan, Compressed Air and Gas Institute’s (CAGI) Technical Consultant, and Mr. Neil Breedlove of CAGI's Centrifugal Compressor Section and member company, Atlas Copco Compressors, about centrifugal air compressors. Specifically, the discussion outlined how various inlet conditions can impact the performance of centrifugal air compressors.
Environmental Parameters Influence Centrifugal Performance
CABP: Gentlemen, our readers are always anxious to learn more about how their compressor systems are impacted by environmental conditions. Can you shed some light on how inlet conditions can impact a centrifugal compressor system’s performance?
CAGI: Centrifugal technology is based on dynamic compression. In dynamic compressors, air is drawn between the blades of a rapidly rotating impeller and accelerates to high velocity. The air is then discharged through a diffuser, where the kinetic energy is transformed into static pressure. Most dynamic compressors are turbo compressors with an axial or radial flow pattern and are designed for larger volume flow rates.
The performance of a dynamic compressor is very much dependent on environmental conditions. We will explain the impact of the different environmental parameters and their effect on performance below. When considering an investment in a centrifugal type of compressor, it is critical to consider the yearly extremes as well as the average conditions when sizing the equipment for the application to achieve maximum performance.
The environmental parameters that influence the performance are:
- Inlet temperature
- Inlet pressure
- Relative humidity (RH)
- Cooling water temperature
CABP: Can you provide our readers with an understanding as to how each of these parameters might impact performance?
CAGI: To understand the impact of these parameters, we need to first look at the performance curves of a dynamic compressor and see how performance is impacted with changing environmental parameters.
The inlet temperature of the air has an impact on the density of the air at the intake of the compressor and will influence the kinetic energy transferred by the blades to the air. Increased density at lower intake temperatures will result in a higher free air delivery (acfm) and also higher power consumption of the compressor.
Another effect of the change in air or gas density is the available turndown of the compressor. That is the flow range where efficient regulation through use of a throttle valve or inlet guide vanes is possible. From the illustrations below, it is clear that with lower temperatures, a higher turndown range is available.
Figures 1 and 2 show the effects of inlet temperature on the performance of a turbo compressor.
Changes in inlet temperature produce large changes in performance. In cold weather, a centrifugal can deliver much more weight flow of air than in warm weather — if the drive is sized to provide the additional power required.
Lower inlet temperature:
- Increases the surge pressure.
- Increases the maximum capacity (weight flow) at a given discharge pressure.
- Increases the power consumption (horsepower).
Figure 1: Density of air increases with reduction of air temperature.
Higher inlet temperature:
- Decreases the surge pressure.
- Decreases the maximum capacity (weight flow) at a given discharge pressure.
- Decreases the power consumption (horsepower).
Figure 2: How inlet temperature affects power
The mentioned parameters have similar effects on compressor performance. The impact of these parameters can also be understood from the performamce graphs used above.
A decrease in inlet pressure will reduce the density of the air at the compressor intake. As with higher temperatures, it will result in lower free air delivery and power. Changes in inlet pressure can be caused by fouled inlet filters or changing barametric pressure. The same goes for the available turndown — lower intake pressure will result in smaller available turndown (See Figure 3).
Lower inlet pressure:
- Decreases the discharge pressure along the entire curve.
- Decreases the maximum capacity (weight flow).
- Decreases power consumption or horsepower (due to reduced weight flow).
Figure 3: How inlet pressure impacts centrifugal compressor performance
Relative Humidity (RH)
An increase in relative humidity (RH) reduces flow and power, and a decrease in RH will increase flow and power. The addition of water vapor to the air makes air humid and reduces the density of the air. This is due to the molar mass of water being less than that of air (See Figure 4).
Higher relative humidity:
- Decreases the discharge pressure at surge.
- Decreases the maximum flow capacity (weight flow).
- Decreases the flow at which surge occurs.
- Decreases power consumption (horsepower).
The higher condensate losses on high humidity days result in reduced flow delivered to the plant air system.
Figure 4: How relative humidity impacts centrifugal compressor performance
Cooling Water Temperature
The cooling water temperatures will affect the intake temperatures to the second stage and any further stages — if present. Colder water increases flow and power, and warmer water reduces flow and power.
CABP: So how do you suggest our readers take this into consideration when sizing their centrifugal compressor?
CAGI: To size a centrifugal compressor, you should consider the demand flow pattern of the user's job site, and consider the site inlet conditions (minimum/average/high) for optimum performance.
Another important point of consideration is motor sizing. If the motor/driver is selected based on performance at lower inlet temperatures, it will ensure that even during low inlet temperatures, the motor has sufficient power available to take care of flow increases. The customer can take advantage of increased flow available from their compressor (See Figure 5).
Cooling water temperature will affect the performance of the compressor stage after the first stage. The affect in performance is similar to that of inlet air temperature. This, of course, is true because cooling water temperature variations will directly affect the temperature of the air entering the second, third and subsequent stages, where there are intercoolers located between stages.
Lower cooling temperature:
- Increases discharge pressure.
- Increases maximum capacity (weight flow).
- Increases power consumption (horsepower).
Figure 5: The effect of cooling water temperature on centrifugal performance
Higher cooling water temperature:
- Decreases discharge pressure.
- Decreases maximum capacity (weight flow).
- Decreases power consumption (horsepower).
Learning More About Centrifugal Compressors
CABP: From this brief explanation, our readers will probably want to better understand these elements and how they specifically impact their operations. How can our readers get more information and assistance from CAGI and its Centrifugal Compressor Section members?
CAGI: CAGI and its Centrifugal Compressor Section Members, which include Atlas Copco, Cameron, FS Elliott and Ingersoll Rand, have trained engineers to assist and guide users through selecting the correctly sized compressor for their operation. A compressor system assessment is recommended when upgrading and/or replacing existing systems to assure that system performance is maximized.
For more detailed information about CAGI, its members, compressed air applications, or answers to any of your compressed air questions, please contact the Compressed Air and Gas Institute. CAGI educational resources include e-learning coursework on the SmartSite, selection guides, videos and the Compressed Air & Gas Handbook.
To read more about CAGI, please visit www.airbestpractices.com/standards/iso-cagi.