Air compressors require a dedicated lubricant designed specifically to answer the operational challenges and equipment requirements of this type of equipment. While this product is known simply as an air compressor oil different viscosities are available. In this blog, we’ll study air compressor oil and look at its viscosity in greater detail.
What makes compressor oil different from engine oil
Air compressor oils have specific additive packages that make them perfect for work inside air compressors. In contrast to standard engine oil, which is more widely known, air compressor oil does not contain any detergent and usually has far lower levels of carbon and sulphur.
Different levels of viscosity
Air compressor oil viscosity is sometimes referred to by different terms like “air compressor oil type” and “air compressor oil grade”. Since the emergence of modern lubrication viscosity levels in today’s oils tend to range between SAE 20 and SAE 30 range, however oils with more extreme viscosity remain available, but are seldom used in machine applications.
The same trend applies to air compressor oils. Both SAE 20 and SAE 30 oils are ideally suited for use with an air compressor. The difference between SAE 20 oils and SAE 30 oils for air compressors is not huge. In simple terms, SAE 20 is not quite as viscous. This means that at colder temperatures the compressor oil causes less drag on the air compressor allowing it to run slightly more effectively.
SAE 30 air compressor oil is more viscous. As a result, when operating at colder temperatures, its viscosity can be slightly higher than desired. With the impact being that the compressor will run a little less efficiently when running at colder ambient temperatures.
Compressor oil behaviour
It’s worth noting that as a compressor runs, heat generated during operation will effectively reduce the viscosity of SAE 20 and SAE 30 air compressor oils. However, in sustained use at lower temperatures, SAE 20 oil can outperform SAE 30 when it comes to efficiency.
In sustained use in operating temperatures which are higher or moderate, SAE 30 compressor oil will keep more of its lubrication properties than a number SAE 20 compressor oil. Under such conditions, SAE 20 compressor oil will possess sub-optimal viscosity, becoming thinner at a lower operating temperature than a SAE 30 air compressor oil would.
This distinction is important but only for equipment users who are operating their air compressor continuously or daily.
Understanding SAE Grades
The acronym SAE before the viscosity grade number on compressor oil products (from manufacturers like CPI Fluid Engineering, among others) is short for the Society of Automotive Engineers. A professional organisation, SAE develops and then publishes engineering standards – including oil viscosity – via documents like SAE J300.
The SAE viscosity grades marked on oil compressor containers show how well an oil flows at a range of temperatures, helping users select the correct compressor oil for their application and equipment.
SAE focuses primarily on classifying lubricants based on viscosity (a measure of how easily an oil flows). However, it also considers temperature dependence, as the viscosity of oil is impacted by temperature changes. As a result, SAE grades can also help determine how well an oil performs in both hot and cold conditions.
Measuring kinematic viscosity
In compressor oil, centistokes (or cSt, for short) is a unit utilised to accurately measure kinematic viscosity to find out how easily an oil flows at a set temperature. More specifically, kinematic viscosity refers to how long it takes a certain volume of oil to flow under gravity through a capillary tube at a controlled temperature (typically at 40 °C or 100 °C).
ISO grades on compressor oils specifically refer to the kinematic viscosity in cSt at 40°C. The ISO Viscosity Grade system was established by the International Organisation for Standardisation, and it employs a number after the letters ISO VG to display the oil’s thickness.
The higher a compressor oil’s cSt value, the thicker the oil is, making it flow more slowly, and the lower the value, the thinner and freer flowing the oil is. Typical ISO grades for compressor oils can range between ISO VG 32 and ISO VG 100 – or even be as high as ISO VG 220 in some cases.
Manufacturer recommendations
Understandably, compressor oil viscosity advice varies depending on the compressor type and its manufacturer. Rotary screw compressors commonly require oils of ISO VG 46 (up to ISO VG 68), while reciprocating compressors can use ISO VG 100 and sometimes higher. Before purchasing, always consult your manufacturer’s specifications to find the most correct viscosity for your specific compressor model.
At Oil Store, we stock a range of air compressor oils, available from top brands including CPI Fluid Engineering, Millers and Mobil. Shop the range today.
Curious to know more? Find out how to select compressor oil for your system.
