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Understanding Differential Pressure in Your Compressors

Understanding Differential Pressure in Your Compressors

What is Differential Pressure?

Differential pressure (DP) describes the difference in pressure between two points. Understanding the two points based on your compressor type can help you monitor the health of your compressor and reveal telltale signs that your compressor needs maintenance. Changing air filters, discharge check valves, and performing regularly scheduled maintenance is important to maintaining good health and pressure across your system. 

Pressure Dropping: Why It’s Important to Monitor

Pressure drop is described as the depletion of air pressure in the compressor within a fluid-carrying network. Excess pressure drop can result due to the below-average performance of your compressor from improperly scheduled maintenance and lead to unnecessary repairs and increased energy costs. 

On the other hand, certain pressure drops do occur during normal operations. Inlet air filters, inlet valves, coolers, separators, discharge coalescing/particulate filters, and discharge check valves all contribute to normal differential pressure drops. Because differential pressure drops can vary across different types of air compressors, it is important to understand each type and know the needs of your plant. 

Differential Pressure Across Different Compressor Types

Oil-flooded rotary compressors are the best choice if your plant requires a single or two-stage compressor, which are typically known to operate between 5-350 HP. Used in a wide range of applications, rotary compressors can be found playing a vital role in industries ranging from automotive to light industrial. Depending on the size of the compressor applied, the differential pressure could range from anywhere between 9-16 psi. Understanding the differential pressure for a rotary oil-flood compressor will let you know when it’s time to schedule a repair or maintenance if the pressure is higher than 16psi.    

Similarly, oil-free rotary compressors are available with single or two-stage options that also range from 5-350 HP. However, as the name suggests, these compressors offer oil-free air which can be used in a variety of other applications such as food and beverage manufacturing. Compared to oil-flooded, oil-free rotary compressors have less of a pressure drop typically around 5-10 psi. However, with an oil-free rotary compressor, the coating on the rotors can begin to wear off over the course of a few years, which will increase interstage pressure, reduce flow, and cause a larger pressure drop across the machine over time.    

Centrifugal air compressors, unlike the rotary screws, are the best option if your plant needs anywhere from 250 HP-6,000 HP and up to four stages with ISO 8573-1 Class 0, 100% oil-free air. FS-Elliott’s centrifugal air compressors are ideal for industries such as food and beverage, pharmaceuticals, electronics, and other manufacturers that require oil-free air. As you can see from the chart below, the centrifugal air compressor has the least amount of pressure drop across the entire machine compared to rotary compressors.

Centrifugals also do not require the separator and discharge coalescing/particulate filters like the rotary machines, which helps eliminate the DP drop of these items. Having four stages of compression makes it the most efficient compressor on the market. After reviewing the table and data below, we can see that a centrifugal compressor is the most streamlined when it comes to differential pressure drop. 

 Compressor Technology  Oil-Flooded Rotary  Oil-Free Rotary  Centrifugal
 Inlet Air Filter  .5 psi  .5 psi  .5 psi
 Inlet Valve   1 psi  1 psi  .5-1 psi
 Cooler(s)  1-3 psi   1-3 psi  1-2 psi
 Separator(s)  4-6 psi  N/A  N/A
 Discharge Coalescing or
 Particulate Filter(s)
 2-5 psi  2-5 psi  N/A
 Discharge Check Valve  .5 psi  .5 psi  .5 psi
 Total Pressure Drop  9-16 psi  5-10 psi  2.5-3.5 psi



Every psi you lose on a compressor can be directly related to cost in electricity, maintenance, and more. Understanding the different pressure drops across multiple types of compressors can help you: detect problems faster and assist you in making the right decision when purchasing a compressor for your plant.

Contact your authorized FS-Elliott channel partner or sales representative today to see how we can help you look over your compressors to better gain efficiencies and save on hidden costs of compressed air.



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