Compromise in Design
Peter Fransham, PhD
VP Technology
ABRI-Tech Inc.
Namur, Quebec
TERM 7 DESIGN CLASS
Theory VS Reality
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Rule #1 there is never enough time to do
the design you want
Rule #2 There are a number of very high
powered programs -eg finite element that
would be great to use, but the cost is high
and client enthusiasm is low - budget.
Rule #3 Experience and personality play a
large role in design.
Competing Forces
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Human factors including team profile
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Unnecessary complexity because complexity is
associated with knowledge
Inability to retreat and start over
We have done it this way for 20 years and we
are not changing now
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Client requirements – course requirments
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Budget – time balancing with other course.
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Risk – factor of safety – what are the
consequences of failure
First Steps in Design
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What is the outcome you want
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Write down a few simple statements
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Producing a concrete kayak
Has to float
Egonomic factors – has to fit a human body
comfortably
You need a fixed point on the horizon but
there are many paths to that point
Unlikely there will be an optimum path as
the competing forces will select or force
Design Creep
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Almost all of us have been taught that
failure is bad.
We will alway tend to err on the
conservative side.
For small projects this is not generally too
much of a problem because the number of
components is too small.
Complex project this is a real issue.
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Designing an aircraft – large team with multidiciplines
Design Creek Corollery
With design creep comes cost creep.
The project ends up over budget and may not
perform according to specifications.
Now you have a major problem – you have
no more time, no budget and a hostile
customer.
Another rule – keep it simple even if you want
to look intellegent by a complex design.
Case History – Two Pyrolysis
Systems
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Pyrolysis is a thermo-chemical process to
convert biomass into liquid fuels.
Wood is a polymer and we are cracking
that polymer into smaller molecules, some
of which are liquid.
Objectives: maximize liquid yield and
quality for use as a fuel. For now we will
assume there is a market for the fuels.
Heat transfer from heat carrier to biomass
What are the Primary Constraints?
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Selling gigajoules – a commodity with a
price that varies internationally. The amount
of revenue is largely out of our hands.
Capital and operating costs – The first rule
of business is “don't loose money”!! You
don't necessarily have to make money, just
don't loose it.
DESIGN #1 CIRCULATING FLUID
BED
BLOWER
CIRCULATING BED
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Circulating hot sand
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Verical tube mixing sand and biomass
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For commercial scale tube is 10 – 15 m high
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Residence Time about 1 second
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Velocity 10 – 20 m/sec
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Circulating gas has to be continuously
heated and cooled
All biochar and NCG gas is burned to
As Built
DESIGN #2 AUGER WITH STEEL
SHOT
AUGER PYROLYSIS
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CIRCULATES STEEL SHOT AROUND A
LOOP
BIOMASS IS INCORPORATED INTO HOT
SHOT AND PYROLYZED
CHAR STRIPPED FROM SHOT WITH
CIRCULATING FAN AND CYCLONE
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SIMPLE TUBE AND SHELL CONDENSERS
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SYSTEM OPERATES AT +1 CM H2O
As Built
OBVIOUS COMPARISONS
AUGER
CFB
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Mecanical mixing
Pneumatic mixing
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Intimate contact
Disperse contact
Dense high
thermoconductivity heat
carrier
Low density low
thermoconductivity heat
carrier
Reactor temp 470 C
Reactor temp 515 C
No fluidization blower
and no carrier gas to
heat and cool
Energy consumptive
blower and carrier gas
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HEAT TRANSFER CARRIER
AUGER
CFB
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Q= cp*Delta T* M
Q= cp*Delta T* M
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cp shot 0.5 kJ/kg-K
cp sand 0.8 kJ/kg-K
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Thermconductivity 43 W(m-k)
Thermconductivity 0.15 W(m-k)
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Density 4.5 kg/l
Density 0.5 kg/l
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Delta T 20 C
Delta T 20 C
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45 kJ/l
8 kJ/l
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Pyrolysis heat 1000 kJ/kg
Pyrolysis heat 1000 kJ/kg
22 litres of shot per kg of
biomass
125 litres of sand per kg of
biomass
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Plus the inert gas moved
Basic Ecoomics
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What is important:
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Capital cost
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Operating cost
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Revenue from products sold
Capital Cost
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Need to minimize capital cost
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Interesting problem
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Use cheaper parts to minimize capital cost
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Push maintenance onto the owner
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Risk having reputation as a poor product
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No simple answer.
Operating Cost
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Labour – Output per manhour
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OSHA and Labour Legislation
Feedstock cost – need to minimize yet get
quality
Maintenance – this is a big one
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For many industries maintenance is between
3% and 5% of capital cost.
Back to the design and pushing the capital
cost down at the expense of maintenance.
Since maintenance is a percent of capital it is
Back to Case History
100 tonne per day
Parameter
Auger
CFB
Capital Cost
$5 million
$20 million
Electricity
.25 mW
1.3 mW
Operating hours
7500
7500
Electricity Cost
$175,000
$975,000
Maintenance 5% Capital
$250,000
$1,000,000
Total
$425,000
$1,975,000
Production T/yr
21,450
23,100
Cost per tonne
$19.80
$85.50
Sale price Bunker C
$175/tonne
$175/tonne
Biomass Cost $/dry tonne
$60.00
$60.00
Minimum Profit 10%
$17.50
$17.50
Summary
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Always look for the simplest solution
Avoid the trap of trying to look smart by a
complex design
Continuously evaluate the factor of safety
to make certain the design is safe but not
excessively so.
Don't be afraid to back up and redesign – it
is better than the concrete kayak that
sinks.