The Part Load Race is ON!

Part load capacity controls: Variable Displacement versus Variable Speed – What’s right for you?

Compressed air systems are typically the most inefficient, energy consuming monsters in a facility. How these systems are controlled can play a big factor on how efficient these systems are. Typically, there are two ways you want the compressors to operate; fully loaded or off. Unfortunately for all of us, facilities typically do not have demands that are exactly at the capacity of their compressors, and it is rare that the need for air will be completely eliminated, even when production halts.

So what are your options? Let’s take a moment to review the different types available to you:

Load / Unload (aka Online / Offline):

This part load control scheme utilizes a mechanical pressure switch or microprocessor to run the compressor in two different states, 100% loaded and fully unloaded. This causes a pressure fluctuation in the plant of typically 10 PSIG and requires a substantial amount of storage capacity to be efficient (5 to 10 gallons per CFM rating of the compressor). The compressor loads when the pressure falls to the “Load” setting of the compressor. It runs 100% fully loaded until it can bring the pressure at the discharge of the compressor to the “Unload” setting. Too little storage at low demands could cause load/unload controlled compressors to cycle dramatically. Efficiencies match that of Inlet Modulation at 3 gallons per CFM, so for a 100 HP compressor at 500 per CFM Capacity, you would need 1500 gallons of storage minimum. Typically 5 to 10 gallons are recommended (2500 gallons to 5000 gallons).

Percent power at 50% load:

• 1 Gallon per CFM = 94% Power
• 3 Gallons per CFM = 81% Power
• 5 Gallons per CFM = 74% Power
• 10 Gallons per CFM = 64% Power

Inlet Modulation:

Inlet Modulation reduces the amount of capacity produced by the compressor by throttling the inlet valve to hold a constant discharge pressure down to 40% of full load capacity. By doing this, it creates a vacuum under the inlet valve, thus increasing the compression ratio across the compressor air end. This makes this unit very inefficient at part load conditions. Storage does not affect energy cost for this part load function, but you also cannot gain efficiency by adding storage like you do with Load / Unload control. At 50% load, you consume approximately 83% power

Variable Speed Drive – VSD (aka Variable Frequency Drive – VFD):

Variable speed drive has been the buzzword on compressors for years. They are truly the most energy efficient means of part loading a compressor available today. Energy savings are not contingent on storage and it keeps the inlet valve wide open throughout the entire range of capacity which does not increase the compression ratio of the compressor. The motor simply speeds up and slows down to try and match the demands of the facility. Savings at part load are significant. At 50% load you consume approximately 53% power. There are downfalls to this technology, and the most common are:

? First, it takes power to invert power. At full load, this compressor will consume 2-3% more power than a fixed speed compressor from simply inverting the power.

? Second, there is a significant cost. The cost to purchase a compressor with a VSD can be as much as $10 to $15 per Horsepower.

? Lastly, the environment can play a large factor on the reliability and longevity of this technology. Ideally, VSDs require a clean and cool environment.

So what option do you have if you have limited capital funds, a dirty environment, or perhaps reliability and uptime outweighs significant energy savings? There is another part load control that can get you very close to the savings of a VSD without a significant investment and where environment will not play havoc on reliability. It is called Variable Displacement.

Variable Displacement (aka Variable Capacity Control – VCC)

Variable Displacement controlled compressors utilize a valve within the compression chamber of the air end to reduce the capacity of the compressor down to 50% without closing off the inlet valve, similar to the VSD, keeping the compression ratio the same. Also like the VSD, it tries to maintain a stable plant pressure, reducing flow gradually as pressure increases or increasing flow as pressure decreases. As pressure increases (demand reduces), a signal is sent to the valve’s control letting it know to begin to open. As it opens, ports allow uncompressed air to circulate back to the inlet. It only compresses the air downstream of the open ports. By keeping the compression ratio the same, it dramatically increases the efficiency of the compressor down to 50% load. At 50% load you consume approximately 62% power. The benefits to this technology are:

? No sophisticated control components that rely on a clean, cool environment

? From 50% to 100% load, savings are very close to VSD efficiencies

? Maintains full load efficiencies if required to run full load

? No incoming power line filtration required

? Does not require inverter duty rated motor

? Up front cost is very low, paying off the additional investment in a matter of months

In summary, there are three ways to affect the capacity of a compressor: the Diameter of the rotors, the length of the rotors and the speed of the rotors. VSD will control the speed, whereas Variable Displacement controls the length. Consult your IAC compressed air professional to help you determine what is best for your facility. https://www.iacserv.com