Lecture-1 for mecchanical design 2 for me

note

1. Mechanical Design 2
A/Professor Akbar Khatibi

2. 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)

4. Demand & Supply profiles
•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

5. Problem 1
•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 litersper 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.

6. Flywheels
•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 torqueto the generator at a relatively constant speed.

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

8. Flywheels
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.

9. Flywheels
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 Iω
max
2
-½ Iω
min
2
) I

10. Problem 2
•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 180
o
to
each other. The crankshaft resisting torque repeats the following pattern every
180
o
of revolution of the crankshaft:
Zero Nm at 0
o
then rising steadily to reach 100 Nm after 30
o
of revolution,
constant at 100 Nm for the next 30
o
, then falling steadily to zero Nm over the
remaining 120
o
.
•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 Ithat the fly wheel must
have. Include a graph of Torque versus angular position, and discussion of it, as
part of your solution.

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

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

13. Pressure and Temperature Relief Devices
•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

14. Pressure relief in the event of compressor not deactivating

15. Pressure relief in the event of compressor not deactivating

16. Pressure relief in the event of compressor not deactivating

17. Pressure relief in the event of compressor not deactivating

18. Pressure relief in the event of compressor not deactivating

19. Pressure relief where pressure rise is caused by fire

21. Over temperature protection in the event of fire
•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

25. A small solar power station is being designed. A low boiling point working fluid is to be pumped to 450m
2
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 / m
2
.
(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

26. Thermodynamic properties of the vapour are: hmQ 
Conservation of energy:

28. 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

29. Condensation in the mains

30. Condensate slug formation in
poorly drained compressed
air or steam pipe

32. Pipe internal diameter
•Compromises required in deciding internal diameter

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

34. Pressure drop through fittings

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 diameterhe 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/sec
2
; the density of water to be 1000kg/m
3
and its dynamic viscosity to
be 0.001 kg/s.m.

38. Material selection

39. Pipe thickness

40. Insulation

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