Transcript Understanding Steam - Grain Processing Experts

Economics of Steam
Improving overall steam plant
efficiencies in the Feed Mill
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Basic Steam Principles Review
• 1 pound mass of steam addition is equal
to 1 pound of water addition.
• Water at 32 deg F has 0 BTU/lb of
energy.
• Water at 212 deg F has 180.2 BTU/lb of
energy.
• Steam at 212 deg and 0 psig has 1150.5
BTU/lb of energy.
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Basic Steam Principles Review
• Sensible Heat:
Heat which results in a
temperature increase.
Water at 212 deg F has 180.2
BTU/lb of sensible heat.
• Latent Heat:
Heat which causes a
change of state (Water to
vapor).
Steam at 212 deg F and 0
psig, has a total energy of
1150.5 BTU/lb. 970.3
BTU of latent heat and
180 .2 BTU/lb of sensible
heat
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Basic Steam Generation Graph
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Steam Generation Graph
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Basic HP Steam design
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Calculating Energy Requirements
(Optimum)
Heating Requirements
• Q=cm▲T
Q = Energy
c = Specific heat
m = Mass
▲T= Temperature
Increase.
Calculation
Q = (.504) * 2000*112
C = .477 btu/lb F(14%)
C = .531 btu/lb F(20%)
C avg = .504 btu/lb F
Q = 112,896 btu
Q = 1.128 therms/ton
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Factors effecting Energy Requirements
• Boiler Efficiency = Output / Input
Output = Input – Losses
Boiler Efficiency = 100% - % losses
• Losses:
Burner – Tube Transfer (Scale, Sludge)
Radiation – Stack
• Steam Quality
• Distribution System
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Factors effecting Energy Requirements
(Losses)
Combustion Efficiency
(Natural Gas)
• Curves relate %CO2
net stack temperature.
• 10% CO2 and 400 deg
net stack temp equals
a combustion
efficiency of 81%.
Tube Transfer
(Chemistry)
• 1/50th of an inch of
scale build up on the
boiler tubes can
increase fuel cost as
much as $20,000 yr at
a fuel cost of
$0.65/therm.
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Factors effecting Energy Requirements
(Losses)
Radiation Loss
• A comprehensive term
covering conduction,
radiation and
convection losses to
the ambient air.
• Anything that can
loose heat, should be
insulated.
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Factors effecting Energy Requirements
(Steam Quality)
• Steam Quality is defined as the percentage
of steam contained in a mixture.
• “Wet Steam” – A term commonly used for
steam qualities of less than 100%
• “Dry Steam” – A term commonly used for
steam qualities equal to 100%
• A number of factors can affect steam
quality
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Steam Quality
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Steam Quality
•
•
•
•
Other problems with Low quality Steam:
Surging Boiler water levels or Nuisance low
water Cut-Out operations.
Difficulty in maintaining boiler chemical
levels or over use of boiler chemicals.
Apparent decrease in Boiler Capacity.
Increase in Overall Maintenance – Steam
trap and control component failure, and
increased corrosion problems.
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Factors effecting Energy Requirements
(Distribution System)
• Overall length from steam source to end use.
• Use and operation of steam separators and
traps.
• Proper insulation on steam lines and headers.
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Factors effecting Energy Requirements
(Distribution System)
Steam leaks in the steam lines or Traps can be
a large source of energy loss.
• A 1/16th inch leak in a 100 psi steam line
results in 13.2 lb/hr loss.
• A 1/8th in leak in a 100 psi steam line results
in a 52.8 lb/hr loss.
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Calculating Energy Requirements
• Optimum energy requirement = 112.896 btu
Some assumptions for this calculation:
Combustion Efficiency = 85%
Tube Transfer = 90%
Radiation Loss = 10%
Steam Quality = 90%
Steam leaks = 2 @ 1/16th and 1 @ 1/8th inch.
Production rate of 60 ton/hr
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Calculations
We need to Calculate from the Steam Chest Back to the boiler
and then add any raw steam losses.
• Optimum Energy Required = 112,896 Btu/ton
• Steam Quality of 90% causes a loss in energy of 8.4%:
(112,896 btu / .916 = 123,249 btu)
• Radiation loss of 10%:
(123,249 btu / .9 = 136,943 btu)
• Tube Transfer of 90%:
(136,943 btu / .9 = 152,159 btu)
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Calculations
• Combustion Efficiency of 85%:
(152,159 btu / .85 = 179,011 btu)
• This calculations shows that while our
optimum energy requirement is only
112,896 btu/ton, after adding the system
losses, the required energy changes to
179,011 btu/ton.
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Calculations
Adding in the effect of steam leaks:
• 2 1/16th inch steam leaks with a header pressure of 100 psi
• 1 1/8th inch steam leak with a header pressure of 100 psi.
•
•
•
•
2 @13.2 lbs/hr = 26.4 lbs/hr
1 @ 52.8 lbs/hr = 52.8 lbs/hr
Total steam loss = 79.2 lbs/hr
79.2 lbs of steam at 100 psi:
(79.2 lbs @ 1187 btu/lb = 94,010 btu’s)
• 94,010 btu’s / 60 ton/hr = 1,567 btu’s/ton
• 179,011 btu/ton + 1,567 btu/ton = 180,578 btu/ton
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Calculations
Feed water temperature:
• All Steam tables are calculated with the
assumption that feed water temperature is 212 deg.
• When feed water temperature drops below 212
deg. The amount of energy input to the boiler
increases dramatically.
• British Thermal Unit (Btu): The heat required to
raise one pound of water one degree F between 32
and 212 degrees F.
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Calculations
• If we assume that we are adding 4 points of
moisture with steam in the chest then we are
adding approx. 128 lbs or water per ton of grain.
• If the feed water temperature is at 190 deg., we
need to add an additional 22 btu’s per pound of
water.
(128 lbs x 22 btu/lb = 2,816 btu)
• We still need to calculate the efficiency losses to
get the total energy requirement for this added
energy.
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Calculations
• 2,816 btu / 85% = 3,313 btu (Combustion)
• 3,313 btu / .9 = 3,681 btu (Tube x-fer)
Adding all together.
• Boiler system requirement = 179,011 btu
• Steam leaks and traps = 1,567 btu
• Feedwater temperature = 3,681 btu
• Total Btu required = 184,259 btu / ton
• Total Therm requirement = 1.84 therms/ton
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Calculations
• Optimum energy requirement = 112,896 btu
• Calculated energy req’d = 184,259 btu
• As we can see, due to losses in the steam
system, distribution system, and the boiler
make-up system we are actually required to
add 63% more energy input to provide us
with our energy need.
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Calculating energy requirement.
To increase the temperature of 2000 lbs of grain from 100 F to 212 F the following
calculations is used.
Q=CM? T
C = .504 Btu/lb F (Average of grain moistures from 14 to 20%)
M = 2,000 lbs
? T = 112 deg F (212 – 100)
This calculation shows an optimum steam requirement of 112,896 btu’s per ton of grain.
Calculating your individual energy needs with system losses.
1.
Steam Quality : ______________ Your steam quality.
112,896 btu’s / (100% - _________ ) = ____________________ Btu’s
Your steam qlty
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2.
Radiation Loss: _____________ Your radiation loss. (Minimum of 10%)
__________ btu’s / (100% - __________ ) = ________________ Btu’s
Btu’s from step 1
3.
Your radiation loss
Tube Transfer : ______________ Your tube transfer loss (Minimum of 5%)
__________ btu’s / (100% - __________ ) = ________________ Btu’s
Btu’s from step 2
4.
Your tube x-fer loss
Combustion Efficiency: ____________ Your Combustion Efficiency.
___________ btu’s / _____________ = __________________ Btu’s
Btu’s from step 3
Your burner efficiency
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5.
Steam Leaks:
Consult the steam flow thru steam trap orifice table to calculate flow rate.
Total lbs/hr of steam leakage = _______________ lbs
Total tons production per hour = ______________ tons
Btu’s hr loss from steam = ________ lbs/steam x ________ btu/lb mass (from
steam table) = ______________ Btu/hr energy loss due to leaks.
___________ Btu/hr loss / ____________ tons/hr = _____________ btu loss/ton
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6.
Feed water temperature:
Temperature of boiler feedwater tank = ___________ deg F.
212 Deg F - ___________ = __________ deg below 212.
Feedwater temp
Moisture added in the steam chest = _______________ %
2000 lbs / (100% - __________ ) = ____________ total weight with moisture.
Moisture in chest
_________ Total weight – 2000 lbs = _________ Lbs of water added in chest
___________ x ____________ = ____________ Additional Btu’s needed.
Deg below 212
Lbs of water added
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Total energy required:
Total from item 4 (Steam quality, radiation, Tube and Combustions) ____________ btu
Total from item 5 (Steam leaks and faulty traps)
____________ btu
Total from item 6 (Low boiler water make-up temperature
____________ btu
Total calculation for your system
____________ btu
Overall calculated system efficiency:
__________________ btu - 112,896 btu’s = _________________ btu’s
Your total system calculation
Additional btu’s required
__________________ btu / 112,896 btu’s = _______ % energy required over optimal.
Additional btu’s required
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Example of energy requirement
• Commercial feed mill processing 75 ton/hr
• Gas meter shows a usage of 164 therms/hr
• Therm per ton usage of 2.19
• Optimum calculation shows a need of 1.13
therms/ton.
• Energy usage is 93% higher than optimal.
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Example of energy requirement
Some possible reasons for this difference:
• Adding more than 4 pt’s moisture with
steam in the chest.
• Boiler make-up water temperature.
• Steam leaks
• Auxiliary steam loads not in the calculation.
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Improving Energy Requirements
• Combustion Efficiency:
Work with your boiler company to ensure
that your burner efficiency is optimal for the
load and cycles for your needs.
• Tube heat transfer efficiency:
Maintain proper chemistry, have qualified
operators responsible for maintaining your
boiler chemistry and make-up water
chemistry.
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Improving Energy Requirements
• Radiation Loss:
If you can feel heat, then is should be
insulated. Remember that insulation
thickness needs to be based on your coldest
expected ambient temperatures.
• Steam Quality: Again, Proper chemistry is
important. Also, make sure all of the boiler
trim system is maintained and operating
properly
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Improving Energy Requirements
• Steam Leaks:
Maintain proper operation on all steam
traps. Have a good preventative
maintenance program to take care of leaks.
• Feed Water Temperature:
Make sure you are using some kind of heat
recovery or heat input to your boiler make
up water. Condensate returns will also
lower chemical requirements.
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Managing Energy Usage
• Steam Flow Meters:
These meters can be used to monitor the
actual pounds mass of steam being applied
to the steam chest or total system.
• Natural Gas Meters:
It is possible to work with your gas
company and install gas meters with analog
outputs to monitor therm per ton usage.
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Managing Energy Usage
• Automatic Steam valve controls:
Have the ability to more effectively control
steam application to the steam chest. Must
insure that the PID loop control is
continually limiting.
• Ensure you have quality well trained
operators:
This could be the most important
management item of all. Do not expect
untrained operators to understand the
importance of what they do.
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Conclusion
• There are a multitude of factors in steam
production that effect your overall steam
system efficiency.
• Monitoring your energy usage per ton is
important.
• Make sure your operators know what they
are doing and the impact that they can have
on the total cost of production.
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