It is becoming a “best practice” to install a variable frequency drive (VFD) air compressor whenever one is replacing an old air compressor. As a result, real systems have fixed-speed and VFD air compressors, mixed. I have observed several VFD compressor sizing methods. In my last article, I referred to a common method: size one VFD compressor for the whole system. This can work. However, if it doesn’t meet a higher peak demand, one or more of the old compressors will be started, and a mixed system results. Another method is to replace a compressor with the same size, but with a VFD. If the compressor that was replaced is large, a big VFD is installed. If small, a small one.
When selecting an air compressor for your manufacturing operation, the common choice is the industry-standard rotary screw compressor. Known as the work horse of compressed air machinery, the rotary screw compressor comes in a multitude of sizes and power levels. However, centrifugal compressors have seen some exciting technological progress in recent years and offer a wide range of pressures, flow and turndown. Long known for their longevity and durability, they offer higher efficiency, even qualifying for energy rebate programs offered by local utilities and all, notably, produce Class 0 oil free air.
As an end user, have you ever heard the message to put in the biggest VFD air compressor, and the system will always be reliable and efficient. Why do an audit? Just add up the compressors on site and put one VFD for that size or larger. Why have the complexity of multiple compressors, storage, sequencing, etc? Even better, put in two of them, one for the whole system, and one for back-up. If you could wave a wand, wouldn’t that be what every system should look like? Perfect peace and efficiency, with 100% confidence of reliability.
A complete compressed air system assessment should provide detailed information on both the supply and demand sides of the system. The supply-side refers to the equipment supplying compressed air – the air compressor, dryers, filters, piping and storage tanks. The purpose of this article is to illustrate what information we believe a factory should receive from a supply-side system assessment and more importantly – what information a plant should always know about their compressed air system.
This article will talk about testing. I will assume a “typical” system, a screw air compressor mix with regenerative dryers. Testing has to be done at several stages and locations, due to the cobbled-together nature of a compressed air system.
A pharmaceutical product manufacturer spends an estimated \$137,443 annually on electricity to operate the oil-free air compressors in its compressed air system. The compressed air system operates well and is providing the level of purification required. Our team visited the plant and identified a group of projects which could reduce compressed air demand and reduce energy costs by \$42,248 – or 31% of current use.
The Lafarge Cement Distribution terminal located in Winnipeg, Canada has significantly reduced the site electrical demand and energy charges by changing the way they transport their cement. Two new low-pressure rotary screw air compressors have replaced two large high-pressure air compressors that previously powered their dense phase transport system. The resulting power reduction has saved the company 46 percent in transport operating costs.
A centrifugal air compressor operates over a range of flows and discharge pressures. The operating performance curve is shaped by the selected individual internal components and affected by operating conditions such as inlet pressure, inlet temperature, and cooling water temperature. The process of dynamic compression, as applied in a centrifugal compressor operating stage, is velocity and kinetic energy 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.
Quite often the typical variability in compressed air flow demand does not proportionately translate into power reductions at the air compressors. This can be a result of numerous problems with the compressed air supply system. It is important to understand the supply-side’s ability to respond to the demand-side of the compressed air system. If the air compressors, on the supply-side, are not able to translate flow reductions into energy savings, implementation of demand reduction projects should be re-evaluated.
IWF is largest woodworking technology trade show in North America and is held every other year at the Georgia World Congress Center in Atlanta, Ga. It is ranked among the largest trade shows in the world.
In general, this article focuses on the definitions of terms often used to understand centrifugal air compressor performance. Comments are also made on how to measure power consumption. This article is not intended to be an engineering discussion of the various types and designs of centrifugal and other air compressors.