Flow is the general movement of fluid.
Flow has two components to consider: flow rate and flow velocity.
Flow rate is the movement of a specific volume of fluid in a set amout of time. Flow rate is typically measured in U.S. gallons per minute (gpm) or litres per minute (lpm), using a flow meter.
Flow velocity is the distance a specific volume of fluid travels in a set amount of time.
The flow velocity is not measured directly, but is instead calculated using the flow rate and the cross section area of the hose.
Flow velocity is directly dependant on flow rate and hose size.
If we change the flow rate of the pump but leave the hose size unchanged, we can change the flow velocity of the fluid.
If instead we keep the pump size unchanged, but change the hose size we have the same effect.
As flow velocity increases, heat also inceases.
This is due to friction.
Friction is caused by the fluid molecules rubbing against the inside surface of hoses and pipes.
We imagine that fluid flows as a single mass but in reality that is not the case.
At low velocities, fluid flows in distinct separate parallel layers.
Each of these layers is moving at a slightly different rate.
This state is known as laminar flow.
As the velocity of a fluid increases, tiny imperfections in the surface of the flow conductor (hose or pipe) disturb the flow path.
This creates a chaotic state rather than the organized layers of laminar flow.
This turbulent flow (due to friction) causes an increase in heat.
Turbulent flow is evident anywhere in a hydraulic system where bends and restrictions occur.
Keeping hoses and fittings large helps to minimize this effect.
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