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Everything listed under: Economizer

  • Steam Tip 26: Consider Installing a Condensing Economizer

    The key to a successful waste heat recovery project is optimizing the use of the recovered energy. By installing a condensing economizer, companies can improve overall heat recovery and steam system efficiency by up to 10%. Many boiler applications can benefit from this additional heat recovery, such as:

    • district heating systems
    • wallboard production facilities
    • greenhouses
    • food processing plants
    • pulp and paper mills
    • textile plants
    • hospitals

    Condensing economizers require site-specific engineering and design, and a thorough understanding of the effect they will have on the existing steam system and water chemistry.

    Use this tip sheet and its companion, Considerations When Selecting a Condensing Economizer, to learn about these efficiency improvements or contact the team at Campbell-Sevey for help.

    A conventional feedwater economizer reduces steam boiler fuel requirements by transferring heat from the flue gas to the boiler feedwater. For natural gas-fired boilers, the lowest temperature to which flue gas can be cooled is about 250°F to prevent condensation and possible stack or stack liner corrosion.

    The condensing economizer improves waste heat recovery by cooling the flue gas below its dew point, which is about 135°F for products of combustion of natural gas. The economizer reclaims both sensible heat from the flue gas and latent heat by condensing flue gas water vapor (see Table 1). All hydrocarbon fuels release significant quantities of water vapor as a combustion byproduct. The equation below shows the reactants and combustion products for the stoichiometric combustion in air of methane (CH4), the primary constituent of natural gas. When one molecule of methane is burned, it produces two molecules of water vapor. When moles are converted to pound/mole, we find that every pound of methane fuel combusted produces 2.25 lb. of water vapor, which is about 12%of the total exhaust by weight.


    Since the higher heating value of methane is 23,861 Btu per pound (Btu/lb), 41.9 lb of methane is required to provide one million Btu (MMBtu) of energy, resulting in 94.3 lb of high temperature water vapor. The latent heat of vaporization of water under atmospheric pressure is 970.3 Btu/lb. When one MMBtu of methane is combusted, 91,495 Btu of water vapor heat of evaporation (94.3 lb x 970.3 Btu/lb )is released up the boiler stack. This latent heat represents approximately 9% of the initial fuel energy content. The bulk of this latent heat can be recovered by cooling the exhaust gas below its dew point using a direct contact or indirect condensing economizer. It is possible to heat water to about 200°F with an indirect economizer or 140°F with a direct contact economizer.

    Energy Savings Potential

    The available heat in a boiler’s exhaust gases is dependent upon the hydrogen content of the fuel, the fuel firing rate, the percent of excess oxygen in the flue gases, and the stack gas temperature.

    Consider a natural gas-fired boiler that produces 100,000 lb/hr of 100-psig saturated steam. At 83% efficiency, the boiler firing rate is about 116 MMBtu/hr. At its full firing rate, the boiler consumes over 4,860 lb of natural gas each hour while exhausting 10,938 lb of high temperature water vapor each hour. The water vapor in the flue gas contains over 10.6 MMBtu/hr of latent heat. As shown in Table 2, the total heat actually available for recovery is strongly dependent upon the stack gas temperature at the condensing economizer outlet.

    Assume that an indirect contact condensing economizer is retrofitted onto this 100,000 lb/hr steam boiler to heat 50% of the makeup water from 55°F to 200°F and flue gases are cooled to 100°F. At these conditions, 12.75MMBtu/hr of total energy is available in the exhaust, of which 7.55 MMBtu/hr will be recovered to heat makeup water in the condensing economizer. More energy could be recovered if additional heat sinks are available. 

    Given 8,000 hours per year of boiler operation, and a fuel cost of $8.00/MMBtu, the annual energy recovered is valued at:

    Annual Savings = 7.55MMBtu/hr x 8,000 hrs/yr x $8.00/MMBtu/0.83 = $582,170

    Examples

    District Heating System

    A boiler plant that provides up to 500,000 lb/hr of steam for a district heating system installed a direct contact condensing economizer. This economizer saves up to 20 MMBtu/hr, depending on the boiler load. Since condensate is not returned from the district heating system, the recovered energy is used to preheat plant makeup water from 45°- 60°F up to 132°F, resulting in a steam system energy efficiency improvement of 6.3%.

    Food Processing Plant

    A food processing plant installed an indirect contact condensing economizer on a 20,000-lb/hr boiler. The condensing economizer reduced the flue gas temperature from 300°F to 120°F, while capturing 2.0 MMBtu/hr of sensible and latent heat. Energy recovered by the condensing economizer heated makeup water, reducing deaerator steam requirements from 5,000 lb/hr to 1,500 lb/hr.

    For additional information on economizers, refer to Steam Tip Sheet #3 Use Feedwater Economizers for Waste Heat Recovery. This tip is provided by the U.S. Department of Energy - Energy Efficiency and Renewable Energy. For suggested actions and resources, click to download the complete US Department of Energy Tip Sheet.  

  • Steam Tip 3: Use Feedwater Economizers for Waste Heat Recovery


    A feedwater economizer reduces steam boiler fuel requirements by transferring heat from the flue gas to incoming feedwater. Boiler flue gases are often rejected to the stack at temperatures more than 100°F to 150°F higher than the temperature of the generated steam. Generally, boiler efficiency can be increased by 1% for every 40°F reduction in flue gas temperature. By recovering waste heat, an economizer can often reduce fuel requirements by 5% to 10% and pay for itself in less than 2 years. The table provides examples of the potential for heat recovery. 

    Example 

    An 80% efficient boiler generates 45,000 pounds per hour (lb/hr) of 150-pounds-per-square-inch-gauge (psig) steam by burning natural gas. Condensate is returned to the boiler and mixed with makeup water to yield 117°F feedwater. The stack temperature is measured at 500°F. Determine the annual energy savings that will be achieved by installing an economizer given 8,400 hours per year (hr/yr) of boiler operation at a fuel cost of $8.00 per million Btu ($8.00/MMBtu). 

    From the steam tables, the following enthalpy values are available: 

    • For 150-psig saturated steam: 1,195.5 Btu/lb 
    • For 117 °F feedwater: 84.97 Btu/lb 

    Boiler heat output = 45,000 lb/hr x (1,195.5 – 84.97) Btu/lb = 50 million Btu/hr 

    The recoverable heat corresponding to a stack temperature of 500°F and a natural gas-fired boiler load of 50 MMBtu/hr is read from the table (above) as 4.6 MMBtu/hr. 

    Annual Savings = (4.6 MMBtu/hr x $8.00/MMBtu x 8,400 hr/yr)/0.80 = $386,400 

    Exhaust Gas Temperature Limits 

    The lowest temperature to which flue gases can be cooled depends on the type of fuel used: 250°F for natural gas, 300°F for coal and low sulphur content fuel oils, and 350°F for high sulphur fuel oils. These limits are set to prevent condensation and possible corrosion of the stack. 

    Potential Economizer Applications 

    A feedwater economizer is appropriate when insufficient heat transfer surface exists within the boiler to remove combustion heat. Boilers that exceed 100 boiler horsepower, operating at pressures exceeding 75 psig or above, and that are significantly loaded all year long are excellent candidates for an economizer retrofit. 

    To see if a feedwater economizer will improve your energy efficiency, contact the team at Campbell-Sevey

    This tip is provided by the U.S. Department of Energy - Energy Efficiency and Renewable Energy and originally published by the Industrial Energy Extension Service of Georgia Tech. Click to download the original DOE Steam Tip document - Steam Tip 3 - Feedwater Economizers for Waste Heat Recovery.


  • Case Studies Show Magnitude of eTech Savings

    Two cased studies from eTech with plants in the food processing industry, show some of the remarkable efficiencies that can be gained by adding the right equipment. 

    Case Study #1 - Food Processing Plant: Condensing Economizer

    Assumptions:

    • Original boiler efficiency 86%
    • Fuel cost $7.8 per mm Btu
    • Average load 8200 hrs/yr at 80%

    Application:

    • Simple condensing, preheating make-up water
    • 40,000 pph and 60,000 pph (150 psig)

    Analysis

    • Installed cost of $680,000
    • Annualized fuel savings of $640,000
    • Simple payback just under 13 months

      

    Case Study #2 - HJ Heinz Processing Plant: Circular Econopak

    Application:

    • Circular Econopak designed and manufactured for natural gas-fired 400 hp fire-tube steam boiler

    Analysis:

    • Stack gas temperature lowered from 480º F to 350º F at full-fire conditions
    • Water tempurature in economizer increased from 227º F to 275º F
    • 165º F differential translates to fuel savings of 674,123 Btu/hr at full fire
    • 4% increase in boiler efficiency

    For more info about condensing economizers, Econopaks, and other efficiency solutions, contact Campbell-Sevey.

     


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