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Measuring Work, Power and Volume Rate of Flow

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.

What is mechanical work? How do we measure it?

Mechanical work may be defined as the product of a force and the distance over which the force operates on a body. Exactly as for heat, work is energy that is transferred from one body to another. The difference is that it is now a matter of force instead of temperature. An illustration of this is gas in a cylinder being compressed by a moving piston. Compression takes place as a result of a force moving the piston. Energy is thereby transferred from the piston to the enclosed 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 energy, the kinetic energy or the thermal energy. The 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.

How do we measure power?

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

How is volume rate of flow measured?

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", i.e. 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).

What is Free Air Delivery?

FAD, or Free Air Delivery. We'll explain the term using an example. What does FAD = 39l/s for a compressor working at 13bar mean? How long does it take to fill a 390L tank at a pressure of 13bar? 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 is 39 liters in the vessel at 1 bar. After 10 seconds the pressure inside the vessel is 1 bar. After 20 seconds the pressure is 2 bar. So after 130 seconds it is filled at 13 bar. Next, the difference between reference conditions and normal conditions. Reference conditions are characterized by 1bar, 20°C, 0% Relative Humidity (RH). Normal conditions are characterized by 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.

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