C1-MKIII – Compressible Flow Unit


C1-MKIII – Compressible Flow Unit

A versatile apparatus, based around a variable-speed air compressor, designed to teach the concepts of compressible flow.


The C1-MKIII Compressible Flow Unit has been developed jointly between Armfield Ltd and an experienced lecturer in mechanical engineering of Reading College of Technology.

The unit serves to introduce users to all the basic concepts of compressible flow through a number of experimental procedures with a variety of interchangeable clear acrylic test sections.

Project work: Students may devise additional experiments using the equipment as a project work exercise. Examples are provided

The C1-MkIII equipment comprises a single-stage air compressor, complete with a test section and a throttling valve, plus an electronics console containing the necessary controls and instrumentation.

The single-stage compressor is driven by an integral three-phase AC motor. The compressor speed can be varied using an advanced torque-vector frequency inverter, which gives stable and accurate speed control plus direct electronic read-out of the torque produced by the motor.

The compressor is fitted with an outlet duct incorporating a throttling valve, which allows the flow to be varied independent from compressor speed.

The equipment is supplied with a convergent-divergent test section, fitted at the compressor inlet, designed to produce Mach-1 velocity at the throat. The duct is fabricated from clear acrylic, enabling the student to see the construction and the profiles. A pressure-sensing ring tapping is provided at the inlet, at the throat and at the discharge end of the diffuser. This duct allows all the major concepts of compressible flow to be demonstrated.

The electronics console includes two high-range and two low-range differential pressure sensors plus a control for motor speed and displays for the compressor speed, the pressures and the motor torque.

Technical Specifications

  • Compressor speed: 3,300rpm (max)
  • No. stages: 1
  • Motor Power: 0.55kW
  • Sensors:

+/- 103.4 kPa    1 off
+/- 34 kPa         1 off
+/- 1744 Pa       2 off

Features & Benefits


  • Variable-speed air compressor with accurate electronic speed control
  • Electronic pressure sensors
  • Standard unit includes convergent-divergent duct designed to produce Mach 1 velocity at the throat
  • Data logging option available


The air compressor on the MkIII version comprises a single-stage, side channel blower with the following benefits:

  • Lower speed operation
  • Integral motor with direct drive
  • Reduced operating noise
  • Reduced vibration levels


  • Demonstrate the phenomenon of ‘choking’ in a convergent/divergent duct
  • Investigate the validity of the isentropic flow equations for compressible flow in a convergent duct
  • Demonstrate the effect of compressibility on flow equations for a convergent duct
  • To deduce a value of Specific Heat Ratio (γ) for air using the equation for isentropic flow in a convergent duct
  • Investigate pressure recovery along a divergent duct by measuring duct efficiency
  • Investigate the relation between friction loss & velocity for incompressible flow and to find an approximate value for the friction coefficient
  • To investigate the relation between the friction coefficient and the Reynolds number for a given pipe
  • Determine the friction coefficient for a case of compressible flow
  • Investigate the relation between the pressure recovery across a sudden enlargement and, upstream flow velocity, assuming incompressible flow
  • To determine the Coefficient of Discharge
  • Investigate the validity of the formula for the pressure rise across a sudden enlargement for compressible flow
  • To investigate, for incompressible flow, the relation between the flow rate through, and the pressure drop across, a pipeline orifice.
  • To determine the relationship between the coefficient of discharge and the ratio n for the pipeline orifice
  • Investigate the effects of compressibility on discharge coefficients
  • To investigate the variation of pressure rise, power input and isothermal efficiencies of a centrifugal compressor with mass flow rate at constant speed
  • Produce a performance characteristic using mass flow rate and pressure rise as parameters, with contours of constant speed and constant efficiency
  • Account for the energy provided by the compressor driving motor
  • To investigate the relationship between fluid velocity and pressure drop (head loss) along a 90° smooth bend
  • Investigate whether the pressure varies radially across a bend

C1-MkIII-DTA-ALITE includes two electronic thermometers and a computer interface device to allow the temperatures and pressures to be data logged on a Windows PC.

The advanced educational software provides a wide range of data logging and plotting options.

The user must have a PC with a USB port, running Windows 7 or above.

  • A benchtop unit designed to demonstrate and teach the fundamentals of compressible flow to engineering students
  • Fitted with a single-stage, side-channel blower
  • Complete with convergent-divergent duct capable of achieving Mach 1 velocity at the throat
  • Advanced torque-vector speed control of blower motor with electronic torque measurement
  • Four electronic pressure sensors
  • Test section made from clear acrylic
  • Additional test sections available (6 off) complete with benchtop stand
  • Compressor test accessory available
  • Data logging accessory as standard, complete with educational software and electronic temperature sensors

C1-MkIII-30 Additional Test Sections
C1-MkIII-35 Compressor Test Accessory

Electrical supply:

  • C1-MkIII-A: 220-240V / 1ph / 50Hz / 6A
  • C1-MkIII-G: 220-240V / 1ph / 60Hz / 6A

A 1500VA transformer is available to accommodate 120V / 1ph / 60Hz

Computer: A PC running Windows XP or above with a spare USB port (not supplied by Armfield)



Volume: 0.30m³

Gross Weight: 50Kg


Volume: 0.50m³

Gross Weight: 20Kg


Volume: 0.03m³

Gross Weight: 10Kg


Length: 0.880m

Width: 0.500m

Height: 0.527m


Length: 1.250m

Width: 0.310m

Height: 0.340m


Length: 0.290m

Width: 0.270m

Height: 0.090m


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