Why vacuum pipework can limit pumping performance

A vacuum pump’s quoted speed applies at its inlet. The chamber receives less of that capacity when gas passes through restrictive pipework, valves or other components first.

A larger replacement pump may offer only a modest improvement if the connection remains the main restriction.

Understanding conductance

Conductance describes how readily gas passes through a component under specified conditions. It is commonly expressed in litres per second. High conductance means relatively little resistance to gas movement.

The value depends on geometry, gas and temperature. Depending on the flow regime, it can also depend on pressure. For a pipe, internal diameter and length are important; bends, reducers, valves, filters and traps also count.

Diameter can make a large difference

For a sufficiently long, straight circular tube in molecular flow, conductance is proportional to the cube of its internal diameter and inversely proportional to length. Doubling the bore gives approximately eight times the conductance; halving the length gives approximately twice the conductance.

This relationship does not apply to every hose or pressure range. Short openings, corrugated hoses and flow at higher pressures need different treatment. It nevertheless shows why bore size matters in a high-vacuum connection.

An example at high vacuum

Consider a pump providing 100 l/s for air, connected to a chamber by a straight tube one metre long. The long-tube approximation for air at 20°C in molecular flow gives:

The figures assume the stated pump speed at the operating point and exclude other components and gas sources along the line. They illustrate a restricted connection, not a recommended installation. The pump cannot deliver anything close to 100 l/s through either tube.

For this approximation, C = 12.1d³/L, with internal diameter d and length L in centimetres and conductance C in l/s. Effective speed is SC/(S + C), where S is pump speed in l/s.

Every component counts

An isolation valve may have a smaller internal opening than the adjoining pipe. A reducer introduces a narrow section; a filter may become more restrictive as it collects material. Components in series contribute to the total resistance.

Widening one section while leaving a restrictive valve in place can deliver less improvement than expected. Traps and filters may be essential to protect equipment or the process: include their conductance in the design rather than removing them simply to gain speed.

Use component data for the relevant gas and pressure regime. Matching nominal flange sizes does not establish equal conductance.

Why two gauges may disagree

Gas flow through a restriction requires a pressure difference. During pumping, chamber pressure can therefore be higher than pressure near the pump inlet. Two correctly functioning gauges may report different values.

Before assuming a measurement fault, consider their positions and the gas load passing through the line. For process control, the measurement should represent the pressure that matters to the process.

What to review before fitting a larger pump

Record the actual internal bore and length, including reducers, bends, valves and protective components. Establish whether the restriction is part of the design or has developed in service. A system that has always been slow suggests a different investigation from one affected by a loaded filter or replacement hose.

Shorter, wider connections generally help, but the design also needs mechanical support, vibration isolation, access and compliance with the pump’s installation instructions.

Send Girovac a sketch or photographs of the chamber-to-pump route with pipe dimensions, pump model and operating pressure. This helps assess whether to change the pump, the connection or both.