Understanding air compressor measurements: work, power, and flow

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After learning about the basics of physics, you might want to know more about understanding air compressor measurements regarding matter.

This information is very useful when determining the appropriate size and power you need for a particular application. In this article, we will explain the basics of measuring work, power and volume rate of flow.

How is mechanical work measured

measuring instruments

Mechanical work may be defined as the product of a force and distance over which the force operates on an object. Like heat, work involves energy transferred from one body to another. The difference is that it involves force rather than temperature. An example of this is when gas becomes compressed in a cylinder with a moving piston.

Compression occurs as a result of force moving the piston. Energy, therefore, transfers from the piston to the gas. This energy transfer is work in the thermodynamic sense of the word. The result of work can have many forms, such as changes in the potential, kinetic, or thermal energy.

Mechanical work associated with changes in the volume of a gas mixture is one of the most important processes in engineering thermodynamics. The SI unit for work is the Joule: 1 J = 1 Nm = 1 Ws.

Measuring power

Power is work performed per unit of time. It is a measure of how quickly work becomes completed. The SI unit for power is the Watt: 1 W = 1 J/s. For example, power or energy flow to a compressor's drive shaft is numerically similar to system heat emissions, plus heat applied to compressed gas.

Measuring flow rate

Measuring Free Air Delivery (FAD), Formula

The volumetric flow rate of a system is a measure of the volume of fluid flowing per unit of time. It may be calculated as the product of the cross-sectional area of the flow and the average flow velocity. The SI unit for volume rate of flow is m3/s.

However, the unit liter/second (l/s) is also frequently used when referring to the volume rate of flow (also called the capacity) of a compressor. It is either stated as Normal liter/second (Nl/s) or as free air delivery (l/s). With Nl/s, the air flow rate is recalculated to "the normal state." That is, conventionally chosen as 1.013 bar(a) and 0 °C. The Normal unit Nl/s is primarily used when specifying a mass flow.

For free air delivery (FAD) the compressor's output flow rate is recalculated to a free air volume rate at the standard inlet condition (inlet pressure 1 bar(a) and inlet temperature 20 °C). The relation between the two volume rates of flow is (note that the simplified formula above does not account for humidity).

Free air delivery

The following example illustrates free air delivery (FAD). What does FAD = 39l/s for a compressor working at 13 bar mean? How long does it take to fill a 390L tank at a pressure of 13 bar? To calculate this, we need to go back to the inlet conditions. Which is 1 bar.

When we start with an empty vessel, after 1 second there are 39 liters in the vessel at 1 bar. Then, after 10 seconds the pressure inside the vessel is 1 bar. Following this, the pressure is 2 bar after 20 seconds. Therefore, after 130 seconds it becomes filled at 13 bar.

Next, the difference between reference conditions and normal conditions. Reference conditions use 1bar, 20 °C, 0% Relative Humidity (RH).

Normal conditions involve 1atm = 1,01325bar, 0 °C, 0% RH. The next definition is SER or Specific Energy Requirement. This means the amount of energy that is required to deliver 1 liter FAD at a certain pressure.

Go with the flow for air compressor measurements

Specifying your compressed air system by flow and pressure – not kW or horsepower – is the best way to match its performance to your needs. Compressor sizing should match your business requirements more precisely than just going by kW rating.

Learn more about go with the flow.

Purchasing the right size equipment

There's a lot of technical terms covered in this article pertaining to mechanical work, power, and flow. Understanding this information is important for investing in the right equipment for your application. If you purchase equipment that's either too large or too small, there's the risk of inefficiency.

What's important to consider is how much force you'll need to move an object to complete a given job in a given time frame. As mentioned above, this is expressed in flow and pressure. In addition to liters per second (l/s), flow is represented in cubic feet per minute (cfm) or cubic meters per hour (m3/h). These measurements all pertain to speed.

Pressure is both shown as bar, mentioned above, or pounds per square inch (psi). If you need to move heavy objects, you'll need more pressure. You'll also want to determine whether you need all day air delivery and if there are different requirements for your applications. This context is useful when it comes to determining size and choosing between fixed speed and variable speed drive (VSD) machines.

Fixed speed vs. variable speed drive (VSD)

When researching air compressors, you'll come across fixed speed and VSD equipment. These terms refer to how the engine operates. As suggested, fixed speed machines only operate at one speed, while VSD compressors change speed depending on demand. Each has their own advantages, depending on your workflow and needs.

A fixed speed machine is generally cheaper to purchase, while a VSD machine offers efficiency advantages. The latter provides operational energy cost savings. If you're undecided on what makes the most sense for your needs, feel free to get in touch. Our team is happy to help assess what makes the most sense.

We understand that there's not a one size fits all solution for every customer, and offer tailor made solutions.

After learning about the basics of physics here, you might want to know more concerning the physical units used to measure different aspects of matter. This can be very helpful when dealing with compressed air. In this article, we will explain the basics of measuring work, power and volume rate of flow.

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