Energy Efficient Steam Systems

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 force –Steam provides heat at a constant temperature lb/hr to deliver 1 mmBtu/hr of heat

Steam System

Energy Flows Energy enters a steam system as: –Fuel and combustion air –Makeup water –Pump work Energy leaves a steam system as: –Heat to the process –Exhaust air –Blowdown –Condensate loss –Flash vapor –Heat loss from the boiler, steam pipes, condensate pipes and deaerator tank.

Fuel Use Reduced Fuel use reduced by reducing: –Heat to the process –Heat in exhaust air –Blowdown –Condensate loss –Flash vapor –Heat loss from the boiler, steam pipes, condensate pipes and deaerator tank.

Energy Saving Opportunities

1) Reduce Steam Demand Insulate hot surfaces Cover uninsulated tanks

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.

3)Insulate Pipes and Tanks Insulate –steam pipes –condensate return pipes –condensate return tanks –deaerator tank –valves

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.

5)Minimize Steam Pressure Generating steam at excess pressure: –decreases boiler efficiency –increases heat loss –increases 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.

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.

Understanding Combustion Stoichiometric Combustion Air: No Safety Margin CH (O N 2 ) CO H 2 O N 2 10% Excess Combustion Air: Correct Safety Margin CH (O N 2 ) CO H 2 O N O 2 100% Excess Combustion Air: Too Much Air CH (O N 2 ) CO H 2 O N O 2

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 RATE

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.

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. O 2 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.

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 months

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 efficiency Modulation Control –No purge losses –Runs at low/mid fire with lower exhaust temperature and higher efficiency Stand by Pre-purge Ignition Run Post Purge

10) Switch to Hot Water for Low-Temperature Applications When Treturn 90% When Tinlet ~ 70 F, direct- contact water heaters capitalize on low water temperature, counter-flow design, and large surface areas for efficiency > 98%

Modeling Steam Systems SteamSim University of Dayton Industrial Assessment Center Available free at: ch/EnergySoftware.htm

Modeling Steam Systems Steam System Assessment Tool U.S. Department of Energy Available free at: ry/bestpractices/software.html