How Does Hose Diameter Affect Air Flow In Pneumatic Systems?

Monitoring pressure and flow in pneumatic systems is critical, as these statistics directly affect their efficiency and safety. You need to be sure that you select the correct hose diameter in order to achieve the right pressure and flow rates. This is made easier by the availability of online calculation tools.

A simple experiment can demonstrate how hose diameter affects airflow. If you try breathing through a straw, you’ll soon realise how hard it is to force a large quantity of air through such a small diameter tube. To make it through to the other end, you have to blow very gently, at a low pressure. If you try breathing hard or under greater pressure, the air won’t have sufficient space to travel to the other end. So, if you want a greater airflow, you’ll have to use a bigger straw. You can try it with different-sized straws to see how expanding the diameter makes the airflow easier. 

Pressure Vs Flow

Pressure and flow have a dynamic relationship, which is directly affected by hose diameter.

Pressure

Pressure is a physical force which is produced by compressing an air or gas into a more limited space. There are various subdivisions of pressure, including static, dynamic and total pressure, where total pressure is the sum of the first two. Static pressure refers to any gas or fluid that’s not moving, while dynamic pressure is the opposite, where the gas or air flows with some velocity.

Flow

In a pipeline or pneumatic system, flow refers to the quantity of gas or fluid passing through a specific region of the hose or pipeline in a given amount of time. While there are subdivisions such as mass or volume flow, pneumatics usually defines flow rate as the volume of air flowing through a cross-section of hose in a measured amount of time. This is typically expressed in units of measurement like cubic feet per second, minute or hour.

Hose Or Pipe Diameter

Hose or pipe diameter indicates the size, but it may refer to either the inner, outer or nominal diameter. Nominal diameter (DN) is the most frequently used in pneumatics and fluid transmission, as it helps to standardise measurements across the world. This facilitates the compatibility of components like flow meters and hose or piping products manufactured by different countries. Hose diameter directly affects airflow in pneumatic systems and the hose’s carrying capacity. 

ID And OD

Hoses often specify both the internal diameter (ID) and outer diameter (OD), but the stated size often refers to the ID. This is because that measurement defines the inner space through which the air will flow, whereas the OD can vary according to the thickness of the hose walls. This, in turn, depends on what they’re made of, whether they have insulation or reinforcing materials to make them stronger. The hose’s ID is, therefore, the critical measurement of diameter that determines its flow capacity.

Nominal Diameter

To further confuse the issue, pipes and hoses are often identified by a trade standard measurement in millimetres, known as the nominal diameter (DN). This is a dimensionless whole number, which has only a tenuous relationship to the hose’s actual size. For example, a 200mm or 8-inch nominal diameter pipe size can represent a range of hoses, pipe components or tubing of up to 220mm. The DN number relates to the physical ID of the bore or the OD of a hose’s end connections.

The original term was Diametre Nominel, referring to the metric standards established by the International Standards Organization, which is why it’s DN and not ND. You might also see the American hose specification NPS, which stands for nominal pipe size, which is measured in inches. This NPS shouldn’t be confused with the acronym referring to thread types, which stands for National Pipe Thread Straight.

The Relationship Between Flow Rate, Pressure And Hose Diameter

An equation formulated by Swiss physicist and mathematician Daniel Bernoulli in 1738 is known as Bernoulli’s principle. This demonstrates that increasing the flow rate will produce a corresponding decrease in pressure, while the opposite is also true. So, increasing the pressure in a pneumatic hose will cause a corresponding reduction in airflow. Bernoulli's principle for measuring airflow forms the basis of differential pressure flow meters. You might come across another equation that affects flow rate, called Poiseuille’s law, but this is more applicable to viscous fluids than to air.

You might also see the term ‘burst pressure’ listed among the hose specifications, which refers to the maximum pressure the hose can endure before it ruptures, as air can no longer flow through it. Many commercial hoses are manufactured with a safety margin of three to one, which means that the burst pressure is generally three times the level of the specified rating. So, if you see a hose rated at a burst pressure of 900 psi or 63 bar, it indicates that its working pressure level is 300 psi or 21 bar. 

The Importance Of Correctly Sized Hoses

Choosing the correct size of hose for your pneumatic system is critical for maintaining adequate airflow, but there are other crucial measurements that also affect the performance of your pneumatic system. Starting with the compressor, other components, such as FRL combinations and fittings, must all be correctly sized for maximum efficiency. Pneumatic hose fittings, for example, have an unexpected effect on flow rates, often causing a pressure drop where they compress the hose into the joint. 

Some fittings have a surprisingly small diameter, especially quick-connect fittings, which allow much less airflow. If you want to get the best out of your compressed air system, any restriction anywhere in the system could have a significant impact. Reduced airflow will degrade the overall performance of the application, and you’ll find you’re wasting energy and money to run it.