Steam Systems Steam systems most widely used type of process heating Advantages of steam Heat carrying capacity of steam much greater than air or water Steam provides its own locomotive ID: 561386
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Slide1
Energy Efficient Steam SystemsSlide2
Steam Systems
Steam systems most widely used type of process heating
Advantages of steam:
Heat carrying capacity of steam much greater than air or waterSteam provides its own locomotive forceSteam provides heat at a constant temperature
l
b
/
hr
to deliver 1
mmBtu
/
hr
of heatSlide3
Steam SystemSlide4
Steam SystemSlide5
Energy Flows
Energy enters a steam system as:
Fuel and combustion air
Makeup waterPump workEnergy leaves a steam system as:Heat to the processExhaust airBlowdownCondensate lossFlash vaporHeat loss from the boiler, steam pipes, condensate pipes and deaerator tank.Slide6
Fuel Use Reduced
Fuel use reduced by reducing:
Heat to the process
Heat in exhaust airBlowdownCondensate lossFlash vaporHeat loss from the boiler, steam pipes, condensate pipes and deaerator tank.Slide7
Energy Saving OpportunitiesSlide8
1) Reduce Steam Demand
Insulate hot surfaces
Cover uninsulated tanksSlide9
2) Fix Steam Traps
Steam traps are automatic valves that discharge condensate from a steam line without discharging steam.
If the trap fails open, steam escapes into the condensate return pipe without being utilized in the process.
If trap fails closed, condensate fills the heat exchanger and chokes-off heat to process. Fixing failed steam traps is highly cost-effective. Slide10
3) Insulate Pipes and Tanks
Insulate
steam pipes
condensate return pipescondensate return tanksdeaerator tankvalvesSlide11
4) Preheat Boiler Feed-water Using Exhaust Air
Economizer is heat exchanger that preheats feed-water to the boiler using heat from the exhaust gasses.
Economizers are most cost effective in process boilers that operate all year. Slide12
Minimize Steam Pressure
Generating steam at excess pressure:
decreases boiler efficiency
increases heat lossincreases flash loss. Reducing boiler pressure to match the highest required process temperature decreases these losses. Reducing steam pressure to match local required process temperature reduces flash loss. Thus, always produce and supply steam at the minimum pressure required to meet the process temperature requirement. Slide13
6) Install Automatic Blow Down Controls
Blow down is the practice of expelling steam to reduce contaminant build ups.
Typical blowdown rates range from 4% to 8% of boiler feed-water.
Manual blowdown relies on intuition or periodic testing. Always results in excess blow down that wastes energy or insufficient blow down that creates excess scale on heat transfer surfaces and reduces efficiency. Automatic blow down reduces energy, water and water treatment costs.Slide14
Understanding Combustion
Stoichiometric Combustion
Air
: No Safety Margin
CH
4
+
2
(O
2
+ 3.8 N
2
)
CO
2
+ 2
H
2
O +
7.6
N
2
10% Excess
Combustion Air
: Correct Safety Margin
CH
4
+
2.2
(O
2
+ 3.8 N2) CO2 + 2 H2O + 8.4 N2 + 0.4 O2100% Excess Combustion Air: Too Much AirCH4 + 4 (O2 + 3.8 N2) CO2 + 2 H2O + 15.2 N2 + 2 O2 Slide15
Understanding Combustion Efficiency
Efficiency highest at LOW EXCESS AIR and LOW STACK TEMPERTURES
STACK TEMPERATURE increases with FIRING RATE
EFFICIENCY HIGHEST at LOW EXCESS AIR and LOW FIRING RATESlide16
7) Reduce Excess Air by Adjusting
Air/Fuel
Most boilers use linkages that connect natural gas supply valves with combustion air inlet dampers.
Unfortunately, the linkages do not function perfectly, and the air/fuel ratio is seldom held constant over the firing range. The linkages should be adjusted to maintain 10% excess air at high fire.Slide17
8) Reduce Excess Air with “O
2
Trim” Control
Most boilers linkages do not function perfectly, and the air/fuel ratio is seldom held constant over the firing range. O2 trim combustion controls regulate combustion intake air to maintain 10% excess air across the entire firing range. O
2 trim costs about $20,000, hence most cost-effective for boilers that operate all year long.Slide18
8)
Reduce Excess Air with “O
2
Trim” Control
Annual Savings:
1,200
mmBtu
, $
12,000
Cost
of O
2
Trim System:
$20,000
Payback:
20
monthsSlide19
9) Switch from
On/Off to Modulation Control
On/Off Control
Each time a boiler cycles on and off, it purges combustion gasses and loses heat.When on, boiler runs at high-fire with high exhaust temperature and low efficiencyModulation Control
No purge lossesRuns at low/mid fire with lower exhaust temperature and higher efficiencyStand byPre-purgeIgnitionRunPost PurgeSlide20
10) Switch to Hot Water for
Low-Temperature
Applications
When Treturn < 120 F, condensing boiler efficiency > 90%When Tinlet ~ 70 F, direct- contact water heaters capitalize on low water temperature, counter-flow design, and large surface areas for efficiency > 98%Slide21
Modeling Steam Systems
SteamSim
University of Dayton Industrial Assessment Center
Available free at: http
://
academic.udayton.edu/kissock/http/research/EnergySoftware.htmSlide22
Modeling Steam Systems
Steam System Assessment Tool
U.S. Department of Energy
Available
free at
: http
://www1.eere.energy.gov/industry/bestpractices/software.html