The document outlines a mechanical design course focused on project-based learning, specifically the design of a compressed air supply system and various related engineering problems. It covers irrigation system requirements, flywheel design for compressor systems, safety measures for pressure vessels, and thermodynamic properties relevant to a solar power station. Additionally, it includes calculations for pipe sizing and material selection for various engineering applications.

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MIET2072: Mechanical Design 2
Dr. Fiona Zhang
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Course outline
• Project based learning
• Design of a compressed air supply system for a factory
Part A: Load definition and sizing of principal
components
Part B:Detail design of the
pressure vessel (air receiver)

3. 3

4. 4
• Determining the compressor and receiver capacities
– Plot the air flow demand versus time
– Determine the minimum compressor that can be used assuming storage is available
– Determine the storage capacity:
• must be able to supply the deficit that occurs at the peak; and
• of sufficient size to supply air without the pressure falling too much
Time
Air flow
requirement
Supply
Demand
Demand & Supply profiles

5. 5
• You work for a consulting engineering firm which has been approached by a farmer requiring advice on the sizing of
components in his pumped irrigation system.
• He wishes to irrigate a bean crop at a rate of 1000 litres per minute for 8 hours starting at 6am, then a tomato crop at a
rate of 400 litres per minute for 5 hours, then a cotton crop at a rate of 600 litres per minute for 3 hour and 20 minutes.
After irrigating the cotton crop, no further irrigation of crops takes place until 6am the next morning.
• He intends pumping the water from a river to a holding tank using an electrically driven pump; the water will flow by
gravity from the tank to the crop as required. The farmer's electricity supply is very limited in the current it can provide.
• Determine the minimum capacity of pump (litres per minute) that can be used (which will hence minimize the size of
electric motor required), and the minimum capacity tank (litres) that can be used in conjunction with this pump. Include
an appropriate graph as part of your solution.
Ans: 500 litres per minute, 240,000 litres
Problem 1

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• During peaks: engine has spare torque capacity → accelerate the flywheel slightly → storing some extra
kinetic energy.
• During troughs: the flywheel decelerates → giving up some kinetic energy → helping the engine do the
necessary rotational work to get through the trough
• Supply a relatively steady torque to the generator at a relatively constant speed.
Flywheels

7. 7
1. Draw a torque versus crank angle diagram.
Flywheels

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2. Find the equivalent steady torque that would do
the same amount of work over one complete
cycle that is done by the variable torque.
Flywheels

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3. Superimpose the graph of net steady torque onto
the graph of net variable torque
4. Identify flywheel accelerating phase (surplus energy)
→ Integrate this area → ΔE = the change in kinetic
energy: (½ Iωmax
2 - ½ Iωmin
2) → I
Flywheels

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• The design of a fly wheel for an inline, twin cylinder, single acting compressor is under consideration. The
cranks on the crankshaft are arranged at 180o to each other. The crankshaft resisting torque repeats the
following pattern every 180o of revolution of the crankshaft:
Zero Nm at 0o then rising steadily to reach 100 Nm after 30o of revolution, constant at 100 Nm for the next
30o, then falling steadily to zero Nm over the remaining 120o.
• The electric drive motor is capable of delivering adequate matching torque, however it is desired that the
flywheel have sufficient inertia such that when the compressor is running at an average speed of 400rpm the
total speed variation is not greater than +1%
• Determine the required second moment of mass I that the fly wheel must have. Include a graph of Torque
versus angular position, and discussion of it, as part of your solution.
Ans: 1.26 kg.m2
Problem 2

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• The function of an air compressor is to take air in at atmospheric pressure and deliver it at the required
pressure.
Compressors

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• The function of an air compressor is to take air in at atmospheric pressure and deliver it at the required
pressure.
Compressors

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• Pressure vessel Standards have sections that deal
with the devices that should be fitted to protect
vessels from the three abnormal and dangerous
situations.
• “AS1210-2010 Pressure Vessels” (Section 8)
published by Standards Australia
Pressure and Temperature Relief Devices

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Pressure relief in the event of compressor not deactivating

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Pressure relief in the event of compressor not deactivating

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Pressure relief in the event of compressor not deactivating

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Pressure relief in the event of compressor not deactivating

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Pressure relief in the event of compressor not deactivating

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Pressure relief where pressure rise is caused by fire

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• When the temperature has reached a value for which the allowed design tensile strength has fallen to (Z/1.21) times
what it was at the design temperature, then a temperature sensitive relief device should open.
• Relieve temperature: Tr
Over temperature protection in the event of fire

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23. 23

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A small solar power station is being designed. A low boiling point working fluid is to be pumped to 450m2
of glazed flat plate solar collectors. The vapour which is formed in the collectors and the remaining liquid
are passed to a separation vessel. Here the vapour rises to the upper part of the vessel then passes to
a turbine that drives an alternator. The liquid remaining in the vessel and liquid returning from the
turbine’s condenser are fed back through the collectors. In the event of there being no demand for
electricity a mechanism is activated that repositions the collectors such that they face away from the
sun. The collectors and drum are situated over a large concrete area for which there is no risk of fire.
The vessel and collectors have a design pressure of 1200kPa gauge. When operating at the saturation
temperature corresponding to this pressure, the solar collectors are 75% efficient in their conversion of
solar energy to heat in the liquid/vapour. The peak rate of solar energy flux (direct and reflected) at this
location is 1.3 kW / m2.
(a) In the event of their being no demand for electricity and the collector repositioning mechanism
failing, determine the required vapour discharge rate, in kg / sec,of the safety valve fitted to the
separation vessel.
(b) Select an appropriately sized and set “FIG 1541” safety valve from the catalogue data provided
on the next page (which shows gauge pressure) Give clear reasons for your choice.
Problem 3
Ans: 3.93 kg / sec

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Thermodynamic properties of the vapour are:
h
m
Q 
= 

Conservation of energy:
101kPa

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Piping
• Need to determine:
– the pipe internal diameter, thickness
– the pipe material, and associated joining methods and materials
– the appropriate location for the pipe
– thermal insulation is required and if so, what is the economic
– thermal expansion

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Condensation in the mains

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Condensate slug formation in
poorly drained compressed
air or steam pipe

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Pipe internal diameter
• Compromises required in deciding internal diameter

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Pipe internal diameter
• Calculation of fluid velocity and frictional pressure drop in pipes

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Pressure drop through fittings

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36. 36

 /
No.
Re d
c
=

37. 37
Problem 4
• A farmer has a dam in a distant mountainside gully from which he wishes to pipe water at a
rate of 70 litres per second to flood irrigate a rice paddy field. The pipe distance from the
dam to the paddy field is 550m and the elevation of the dam surface above the water level
in the field is 40m and atmospheric pressure is the same at both locations. The pipe he is
proposing to use is smooth plastic. Given that no pump is to be used in this pipeline,
determine the minimum possible pipe diameter he could install. Assume the frictional
losses of the various fittings add 50m equivalent length to the pipe run, and assume
initially that the Moody friction factor “f” is 0.013.
• For the purposes of this question take: acceleration due to gravity to be 10.0m/sec2; the
density of water to be 1000kg/m3 and its dynamic viscosity to be 0.001 kg/s.m.
Ans: 0.151 m

38. 38

 /
No.
Re d
c
=

39. 39
Material selection

40. 40
Pipe thickness

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Insulation

42. 42
Thermal expansion
Slip / Telescopic joint
Bellows joint
Expansion loops/bends