Congrats! You’ve been given a project to upgrade your plant’s air system to properly supply a new plant expansion. Now the critical questions: How much compressed air are you using right now? What flow will your new system need? How do you size the system for maximum efficiency while providing enough flexibility for the future? You need to consider all of these when upgrading your compressed air system, but unfortunately, the answers often aren’t readily available or apparent. In addition, it’s prudent to plan for many circumstances instead of just one scenario. Here, we’ll explore some ways to do this and why you should.
Need for measurement
The first step in planning for the future is to develop a flow profile (Figure 1). But it’s tough to estimate the flow coming out of compressors just by looking at them or even by measuring the power consumption if you’re lucky enough to have submetering. Flow estimations depend on the compressor control mode, and the complexity of the control modes is enough to make your head spin.
Figure 2. Simplified power versus flow curves show system turn-down characteristics of a single compressor.
Luckily, help is available. There are many compressed air companies offering system assessment services, also called a compressed air audit. Most major compressor suppliers have national audit teams, and there are often competent local personnel to perform basic measurements. These companies know enough about compressor control to be able to estimate accurately using compressor characteristics or to measure your flow profile with a flow meter.
When an assessment is done, temporary measurement power devices would be placed on all your running compressors to determine the electrical consumption; pressure loggers would measure critical system pressure points; and flow meters might also be installed to measure the air output. These meters would log system data for a significant period of time – long enough to establish a baseline reading of average, peak, and minimum levels. This baseline, say based on a few weeks of data, might then be used to project annual operating costs, assuming the same profile repeats itself over time. But the most important outcome would be the system flow profile.
Some of the most important parts of the flow profile:
- Peak flows: This establishes the required flow capacity of your system. If the system can’t supply this flow, then low-pressure problems will occur.
- Average flows: This is the flow level where the compressors will spend the most time. For an efficient system, the compressors must be able to supply this load without wasted power. To do this, the system must have efficient “turndown.”
- Minimum flows: Minimum flows often show the level of system leakage. For shift-oriented plants with little production during nights and weekends, this is often a time where any running compressor is at its least efficient.
- Lowest pressure: The plant will have a required minimum pressure than must be achieved during the peak flow or else production will be affected.
- Highest pressure: The highest plant pressure often occurs during the lowest flows. Extreme variability in pressure is a sign of poor compressor control.
Once the system peak is established, then if a plant expansion is planned, the new load must be added to the old. The new load profile will have a characteristic shape of its own and will depend on what machines or operations will be implemented in any new area, how much air they will use, and when and how often they will operate. There might be hundreds of compressed air loads in a new part of a plant operating almost at random. There are logical ways to calculate these loads if equipment flows are known ahead of time and production levels are accurately estimated, but even with the best of spreadsheets, someone usually needs to polish up his or her crystal ball and do some guesswork.
This new load profile will overlay on top of the existing profile to create a new overall system characteristic. Sometime the peaks of both profiles will add if they are coincident, causing a new significantly higher peak. Other times, there will be no coincident flow, and the peak will change very little. This can all be very confusing and risky when facing a project of significant value.
When it comes to sizing of compressed air systems, very few people have ever been fired for oversizing a system. But it’s awfully embarrassing, and sometimes career-ending, to purchase and install a system that is too small, especially for projects costing in the high six figures. To prevent undersizing, the designers usually apply safety factors to equipment sizing.
Here’s how that usually works: End-use equipment suppliers don’t want to be blamed for undersizing, so they will quote flows that are worst-case scenarios. Engineers take these flows and estimate plant demand using worst-case, scenarios, too; plus, they add safety factors and additional factors for future growth and give the end result to suppliers for an equipment quote. The compressor suppliers will add on safety factors, too, not wanting to be blamed for an undersized system, and offer a quote for the compressor that is the next size up. All of these safety factors add up and usually result in a system that is much larger than it needs to be.
The system curve
The idea of “efficient turndown” was mentioned in talking about the flow profile. Let’s face it: No matter how carefully we measure and estimate, there will always be safety factors added to system sizing calculations. And if the compressor air system has an efficient system curve, it will be able to efficiently supply short-duration peak flow and be able to turn down to supply average flows, which occur the majority of the time. Therefore, for optimum efficiency, a system must be able to turn down the flow and power at the same time. The best system will have a turndown that is very close to the ideal curve, with a 1:1 ratio and 1% power reduction for every 1% in flow reduction, such as in the VSD curve in Figure 2. The worst systems have very little power reduction, as seen in the modulation curve, also in Figure 2.
Figure 3. System curve with three equal-size load/unload compressors.
The turndown relates to the system power vs. the flow curve. The simplest system curve might be for a single compressor; a more-complex curve might be a compound curve featuring the characteristics of many compressors stacked one on top of another (see Figure 3). Using the curve, if an accurate flow profile is known, you should be able to determine how much power would be consumed at any flow at any point in time. If you can determine how much time the system flow spends at the various load points, something a system assessment would provide, then the system energy consumption can be predicted. You can even predict the energy efficiency of the compressors using the curve.
The key to system design is to have enough compressors to handle the peak flows yet arrange the system so that the power vs. flow curve most closely resembles the ideal curve: a straight line drawn from 100% system flow to 0.
System control and storage
To achieve this efficient turndown, you must have good compressor control. The system must also have adequate levels of storage receiver capacity.
There are a number of different control modes for industrial screw compressors. The most common, in increasing order of efficiency, are inlet modulation, variable displacement, load/unload, variable-speed, and start/stop. Most modern compressors operate in a combination of these modes, often depending on the selection of the mode in the compressor control panel and the internal settings levels that have been chosen. Designing an efficient system with a good system turndown means working with compressors in their most-efficient modes. To do this with systems of multiple compressors, some sort of coordinated control system is required.
The most common control mode for industrial screw compressors is load/unload. To ensure efficient operation and good turndown, adequate system storage receiver capacity must be present to reduce the compressor load and unload cycle frequency to lower levels. More-modern systems will have storage levels of between 4 and 6 gallons per cfm trim compressor output, but the best systems have levels approaching 10 gallons per cfm.
A load/unload compressor installed with too little storage will consume more power than required, as shown by the load/unload small tank line in Figure 2. This will cause the system curve to be nonlinear, hurting system efficiency. As illustrated in the figure, a system with large storage operates much more efficiently for all points on the curve.
System storage is also required to ensure proper handoff between various levels of system flow. Perhaps system flows during the main production shift require the operation of one 200 hp compressor and two 100 hp units. When flows reduce during the evening shift, one of the compressors might be able to unload and turn off, but in a system with inadequate storage, fast-changing fluctuations might prevent this. Having adequate system storage will slow system pressure changing to allow the unneeded compressor to be automatically turned off, but will still allow enough time for the compressor to start back up for the main production shift.
Where it is possible to install a VSD compressor, this type of control usually provides the most positive effect on the system curve, providing it is sized correctly in relation to other system compressors. If size is not carefully considered, the system compressors will have some “control gaps,” within which no one compressor is the correct size for the load. In general, in a system of multiple compressors, to prevent control gap, the flow capacity of the VSD compressor should be equal to or larger than that of the base compressors with which it must work. This is a general statement; in complex systems, the rules for size relationships are sometimes more complicated, so consult the manufacturer for assistance.
Properly controlled systems with variable-speed-drive compressors (usually only one VSD is required in any system) use the VSD to supply partial loads (where only a fraction of the capacity of a compressor is required) and have the remaining compressors running either base loaded or turned off. A system with VSD compressor would run near the ideal curve shown in Figure 3.
If a system does not have a VSD then the compressors with the best part load efficiency characteristics should take partial loads. Often this would be a number of smaller compressors working with large base units. Systems set up to work this way would require a carefully set up system of coordinated control settings or a compressor master controller. A system with three identical-size load/unload compressors is shown in Figure 3. If smaller compressors are used for trim and larger storage installed, the curve would flatten and approach the ideal. Note that because three compressors are being used, instead of one as shown in Figure 2, the system curve more closely matches the ideal, even with load/unload compressors.
Sizing system compressors
What is the correct way to size compressors? Should you choose one large compressor or many smaller ones? The answers depend on what kind of system power vs. flow curve you desire. The best and most-efficient system would be made up of hundreds of well-controlled compressors, coordinated to match the load exactly, but this wouldn’t be very practical. The worst system would consist of one very large modulating compressor sized for peak conditions. Perfection lies somewhere in between.
Design philosophies vary, but system compressor sizing choices are most often incorrectly decided based on human nature and practicality. People tend to choose fewer compressors of all the same size. This looks nice and symmetrical when the compressors are all lined up together and saves on the parts inventory, but it may not be the best for system efficiency.
A nice, flexible system might consist of three or four base compressors and a variable-speed compressor that is sized about 20% to 30% larger than the base units. One of the base compressors would be backup capacity.
If system environmental conditions do not allow for VSD control, then a system of two or three large base compressors and two smaller trim compressors about half the size of the base compressors might work best.
A third scenario might consist of a system of compressors of different sizes, all controlled with a master compressor controller that works in energy mode, where it would choose the best compressor combination for the current load based on an intelligent algorithm.
In all cases, the goal is to size the system slightly larger (usually about 20%) than the highest peak demand flow yet achieve a power/flow curve that is as close as possible to the ideal.
Don't assume set flows
It is tempting to try to size the system compressors based on the measured system flows found during the audit. Say, for example, the peak flow is 1,500 cfm and the average flow is consistently 500 cfm. You might try to save money by purchasing a 500 cfm VSD trim compressor and a 1,000 cfm base unit. This might work well if the loads were exactly the same all the time, but if the load unexpectedly changes, then efficiency problems would occur.
Consider this example: A processing plant design engineer was sizing his system and determined by careful calculation that there would be two specific set of load conditions: one set level during processing and another during a special production cycle. To save money, a 75 hp VSD compressor was purchased, along with a 100 hp fixed-speed compressor. When challenged on the possibility of control gap problems, the engineer was adamant that there would be two set flow profiles.
During commissioning, it was discovered that a few compressed air requirements had been forgotten during the design stage. To make the process run correctly, some additional load had to be added. This brought the average load to a level higher than the VSD compressor’s capacity but lower than that for the larger fixed-speed compressor. The load settled exactly where it could cause the most trouble: right in the middle of the control gap. Murphy and his law had prevailed.
During system design, it should have been assumed that the load could fall anywhere in the system flow capacity window. The system, then, should have been designed to have optimum efficiency from 0% to 100% output. A system in this case will be capable of matching any load the plant can throw at it.
Other things to consider:
- System control and compressor sizing is important, but also consider the choice of air dryers and filters. Air dryers, both refrigerated and desiccant-type, also have different turndown capabilities that can affect system efficiency.
- Looking to the future is important. A well-designed system will be scalable. If added capacity is required, a block of compressor capacity can be added to a spare bay in the compressor room and connected to the system master controller without negatively affecting efficiency.
- Scalability also means having adequate piping capacity for future loads. It is always good to oversize piping capacity for a lower pressure differential; this saves compressor power. Often the incremental cost of larger sizing works out to be a very small part of the total system installation cost, but it will set up the system for future changes.
- Removing the heat of compression from the compressor room is critically important for trouble-free compressor and dryer operation – the benefits being better air quality and longer equipment life. If the heat of compression can be used to displace some other energy source, this can greatly increase overall system efficiency.
- Always choose efficient airless condensate drains to reduce compressed air waste.
- A good system of metering is recommended to enable monitoring of compressor power, flow, system pressure, and efficiency. Most modern master compressor controllers can provide this. Monitoring the outputs of your metering system can help you ensure that your compressed air system is always operating at peak efficiency.
Figure 1. Pressure/flow profile of a shift-oriented plant (power omitted for clarity).