Design Selection Memorandum
“The Steam Machine”
Date: 3 November 2006
To: Dr. Ted Hubbard
Cc: Dr. Ken Wilkie
Re: Design Selection, Human-Powered Potable Water Distillation System
From: “The Steam Machine” Team 11: Dave Brushett, Oliver Cluett, Christie Kidd, Marie MacCormick
Team Acceptance:
__________________________/______________ __________________________/______________
Dave Brushett
date
Oliver Cluett
date
__________________________/______________ __________________________/______________
Christie Kidd
date
Marie MacCormick
date
Supervisor Acceptance:
Design Project Coordinator Acceptance:
__________________________/______________ __________________________/______________
Dr. Ken Wilkie
date
Dr. Ted Hubbard
date
Introduction
The American Society of Mechanical Engineers (ASME) has proposed a student design competition for
2007. The competition requires students to design and build human-powered devices that will produce
potable water from polluted water using a distillation process within a 60 period. Design team 11 considered
this problem and has recently completed the conceptual design phase of “The Steam Machine”.
The team recognizes that three major stages must take place for the distillation process to occur. These
stages are illustrated below in “The Steam Machine” process flow.
In determining a solution that will allow each of these stages to take place, power generation was considered
independently, while three different designs for evaporation and condensation were examined, all of which
were compatible with the selected power generation method.
Dalhousie University, Mechanical Engineering Department
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Power Generation
Only one team member must produce power for “The Steam Machine” at a time. Because of the limited
power available to the device, generation capability and efficiency have been the most important factors in
selecting a power generation technique. Other factors considered were: cost, reliability, size and weight, and
ease of use. A team brainstorming session resulted in six feasible power generation possibilities, which were
evaluated in the decision matrix below.
CRITERIA
ALTERNATIVES
1. Pedal Power
2. Hank Crank
3. Potential Energy
4. Closed Loop
Rope System
5. Body Heat
6. Stepper
Cost
Efficiency
Size and
Weight
Reliability
Ease
of use
Power
level
WEIGHTED
SCORE
10%
2
3
2
20%
4
4
3
10%
3
4
2
10%
5
5
4
10%
5
4
2
40%
5
2
1
4.3
3.2
2
4
4
4
4
2
2
3
5
2
1
4
5
3
5
5
5
4
0
4
2.2
3.8
As indicated above, pedaling was selected as the most suitable power generation technique. Pedal power
will be harnessed through a pedal, sprocket and chain system similar to that of a bicycle. The necessary
rotational speed output to achieve maximum power generation can be matched to each rider using a gear
train. An additional power source using upper body strength, such as a hand crank, may also be incorporated
to run any secondary components.
Water Distillation
ASME competition rules dictate that room temperature polluted water must be purified through a distillation
process. The temperature, pressure, or a combination of the two conditions must be altered to evaporate or
condense water. The figure below shows the pressure at which pure water will boil at various temperatures.
Pressure (kPa)
Pressure Versus Water Boiling Point
210
195
180
165
150
135
120
105
90
75
60
45
30
15
0
0
10
20
30
40
50
60
70
80
Temperature (deg C)
90
100
110
120
Saturated Water
Poly. (Saturated Water)
Three water distillation techniques have been examined and are described below.
Dalhousie University, Mechanical Engineering Department
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Distillation Alternative I: Electric Heating Element
Insulation
Description:
In alternative one, the pedal power
is converted to electricity through a
generator. A current flows through
Polluted
the resistive heating element as
Water
shown in the adjacent figure. A
Vent
valve restricts polluted water flow
such that only one drop is released
Valve
to the heating element at a time.
The concentrated heat of the
element causes the drop to
Distilled
Heating
Water
evaporate upon contact, and rise as
Element
vapour to the inclined roof of the
vessel, which is cooled by the ambient air in the room. The vapour will condense on the inside surface
of the roof and then “roll” (held by surface tension) down the incline into the distilled water chamber.
Advantages:
 Simple
 Cost effective
 Concentrated heat results in less heat
loss
Disadvantages:
 Losses possible through two energy conversions
(mechanical to electrical to thermal)
 Substantial thermal insulation required
 Precise drop release control action needed
Distillation Alternative II: Frictional Heating Element and Vacuum Chamber
Description:
Alternative II converts pedal power directly into
heat by means of friction. The vacuum pump is
run directly as a secondary component using a
hand crank. All of the polluted water is
contained in the conductive copper cup, which
is under low pressure. The conductive cup is
stationary while the friction cup rotates to
produce heat. The low pressure allows the water
to evaporate at a temperature lower than
ambient, rise to the top of the chamber,
condense on the upper surface due to the room
temperature conditions, and “roll” down the
incline and drop into the distilled water
collector.
Advantages:
 Only one energy conversion
(mechanical to thermal)
 Cost effective
 Good mechanical exercise
Disadvantages:
 Substantial thermal insulation required
 Significant potential for surface wear leading to
component failure
 Complex – low probability of success
Dalhousie University, Mechanical Engineering Department
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Distillation Alternative III: Vacuum Chamber
Description:
In Alternative III, a vacuum pump is powered directly by pedal power through a gear train. Air is pumped
from the polluted water reservoir through the distilled water chamber, creating low-pressure conditions in
both of the vessels. Under the low-pressure conditions, the water will boil and evaporate at room
temperature. As vapour, it travels upward through the one-way valve into the distilled water chamber, where
cooling is applied to condense the vapour before it to reaches the pump. The distilled water then collects in
the calibrated chamber. Possible cooling methods to be investigated include evaporative and thermoelectric
cooling, both of which require little energy input and could be powered by an additional energy source such
as a hand crank.
Advantages:
 No change in energy domain
 No heat = no heat loss
 Little to no insulation required
Disadvantages:
 Precise temperature-pressure control required
 Relatively expensive
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Design Selection:
Distillation Alternative III was selected as the best distillation technique for “The Steam Machine”, as
illustrated by the following process flow diagram.
To support this selection, each of the three distillation alternatives was evaluated in a decision matrix as
shown below. Efficiency, cost, reliability, and size and weight were considered in the selection criteria.
CRITERIA
ALTERNATIVES
WEIGHTED
SCORE
Cost
Efficiency
Size and Weight
Reliability
10%
60%
10%
20%
2
3
4
3
3
4
2
2
2
2.2
2
5
3
4
4.3
1. Electrical Heating
Element
2. Frictional Heating
Element and Vacuum
Chamber
3. Vacuum Chamber
Future Design Considerations:
Even under extremely low pressure conditions, water evaporation is not an instantaneous process. Because
the ASME competition allows only 60 minutes to complete the distillation process, the speed at which “The
Steam Machine” can evaporate and condense water will be an important aspect of the design. Further
experimentation with vacuum pumps and polluted water surface area will be required to ensure the design is
as fast-acting as possible.
Although heat loss will not be a concern in the chosen design, pressure loss in the vacuum chambers will
have to be considered. All tubing and vessels will have to be sealed carefully, particularly at the joints. As
well, vessels capable of withstanding extreme low pressure conditions will have to be used as polluted and
distilled water chambers.
References:
Bourgnakke & Sonntag. “Introduction to Engineering Thermodynamics” John Wiley and Sons, Toronto.
2001
Sherwin, K. “ Man Powered Flight”. Model & Allied Publications Ltd., London. 1971
Dalhousie University, Mechanical Engineering Department
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