Difference in Pressure = Flow

In a previous module, we talked about how a compressor takes a large volume of air with a low density and compresses it into a small volume with a high density. This creates pressure. The compressor then takes that pressurized air and stores it in a tank. With that pressure, we can do work. We were able to prove that in the previous module by calculating F = P x A.

We will now talk about flow. Flow in a pneumatic system happens because of the difference in pressure. Air flows from areas of high pressure to areas of low pressure until the pressure is the same throughout. Look at the following diagram:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

Notice that the gauges are different when the flow is going from the high to the low from when the gauges are equalizing themselves (have the same pressure). When the flow is going from high to low, flow is occurring. When the gauges have the same pressure, there is no flow.

The rate at which the air will flow from one pneumatic component to another depends on the difference in pressure between the components and the amount of resistance in the passage. Air flows faster with higher pressure differentials and lower resistances. If the passage between two air volumes at different pressures is totally unrestricted and the pressure differential is high enough, the air will flow from the high pressure volume to the low pressure volume at a very high speed. In fact, this speed is the speed of sound (1100 ft/sec or 335 m/sec). This is why pneumatic systems are known for being able to generate high speed actuator movement.

Air Flow Restrictions

Unfortunately, air flow in a pneumatic system is not unrestricted. There are two resistances that we have to deal with:

- Loads at the actuator and air friction
  - As the resistance of either the load or air friction increases, the flow rate decreases.

- Loads at the actuator
  - The loads at the actuator while it is extending or retracting include the load to be moved, mechanical friction of the actuator, and back pressure.

The following picture demonstrates these principles:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

Air Friction

When air flows through the components in a system, it moves against the internal surfaces of the components. No matter how smooth or short the surfaces are , there is always some resistance to the air flow caused by the friction between the moving air and the internal surfaces. This resistance causes the air upstream of each component to have a higher pressure than the air downstream in order to push the air through the component. All pneumatic components cause a frictional drop in pressure. The amount of resistance drop depends on the air flow rate and the design of the components.

The following diagram demonstrates how pressure drop works:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

For troubleshooting purposes, air flow becomes a very important pneumatic principle. If you replace an old component with another component that is a different size, that component may not have the same flow characteristics and, thus, could affect the way the system operates.

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