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FLUE/EXHAUST GAS RECUPERATION



    Manufacturing processes involving the use of boilers, furnaces, ovens, and dryers are widespread throughout industry.  When possible the combustion air provided to these energy users should be preheated. The application of a system to preheat the combustion air will increase the efficiency of the user thereby decreasing its fuel use. Based on data obtained from IAC audits, depending on the size of the boiler, oven, dryer or furnace simple annual pay backs will vary. Retrofits on boilers ranging from 350 to 750 HP yielded on average a two year pay back. Retrofits on (typically gas fired) furnaces, dryers, and ovens ranging from 0.2 to 8.52 MMBtu/hr yielded an average a pay back of 3.2 years.



General Rules of Thumb
  • The average cost of electricity is $0.05/kWh ($15/MMBtu)
  • The average cost of natural gas $0.35/CCF
  • The average cost of #2 fuel oil is $4/MMBtu
  • There are 2000 hours per year per shift (based on the assumption that one shift is 8 hours per day, 5 days per week, 50 weeks per year).
    1. A typical boiler or furnace has a combustion efficiency of 80%
    2. 90% of the heat loss from a hot, uninsulated surface can be economically eliminated by installing insulation.
    3. Switching from electric heat to natural gas or #2 fuel oil can reduce heating costs by 78%
    4. A furnace, oven, or boiler will consume less energy when supplied with preheated air. Boiler efficiency typically improves 1% with each 40°F rise in intake air temperature.

        Note:
        Before choosing the following targeted recommendations READ THE FOLLOWING:
    Pay back estimates for the following recommendations will use the equation below.  They will vary depending on the, application, type of installation, and purchase quantity of material and labor associated with each recommendation.  It will be up to the person doing the analyses to use the URL references below each equation to help estimate an implementation cost.


The data correlating to the variables below each equation will be prompted for in order to execute a calculation.  Frequently the fuel cost (FC) associated with the specific recommendation will be prompted for in order to calculate the annual cost savings (ACS). Unless otherwise specific to a particular recommendation the ACS will be calculated as follows:




Preheat combustion air of oven, furnace or dryer
        Installation of an air-to-air heat exchanger package will be required for each oven, furnace, or dryer. Also insulation of the surfaces of the oven or furnace will aid in the optimization of energy use. The heat exchanger will preheat the outside air supplied to the oven or furnace. This can be accomplished by ducting the exhaust gases and combustion air through opposite sides of a heat exchanger so that the combustion air will be preheated.
        The implementation cost has several components. The primary expense is a heat exchanger which must be coated against corrosion, since chemical solvents can be present in the exhaust air. One possible commercial coating is Heresite Coating® which will protect the heat exchanger exhaust side against all of the existing fumes. Duct work is required for inter-installing the heat exchanger to the dryer stage. The dimensions for the duct work will have to be obtained to yield an estimated cost. Grainger or MacMaster-Carr are possible references for costing analysis. Engineering costs are estimated at 30 % of material costs.

    Power Engineering Books
    ASHRAE
    Thomas Register

    Q = required intake air flow rate (CFM) according to the measured exhaust flow rate in the stack, ft3/min;
    Tse = supply entering temperature (annual average daily outdoor temperature during operation hours, ºF (can be obtained from Engineering Weather Data)
    TMY weather files

    Tsl = exit air temperature currently leaving oven or furnace, ºF (from supplier);
    (rCp) = specific heat capacity at the average temperature ºF, Btu/ft3 ºF (will be calculated);
    H = annual number of operating hours, (obtained from plant engineer);
    h = estimated efficiency of the burners based on their fuel type (oil, 0.90), (gas, 0.85);

    Data Conversions?
    RPN Calculator?
    Press the calculate button to execute an estimation. 

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Preheat Boiler Combustion Air
       The Installation of concentric ducting about the exhausting stack to the ceiling of a boiler directed to the intake fan will draw the warmer stratified air, from the top of the boiler room, and will be further heated as it passes down around the exhausting stack. As stated above, the increased temperature of the combustion air will decrease fuel consumption by the boiler.
        The implementation on a boiler involves the installation of sheet metal ducting to deliver the warm ceiling air to the boiler combustion air intake. An access door for intake fan accessibility is also recommended.  Pay backs will vary depending on the size of the boiler, furnace, oven, or dryer, therefore it will be up to the person doing the analyses to use the URL references below and suggestions above to estimate an implementation cost  The following equation illustrates the savings potential.


    Power Engineering Books
    ASHRAE
    Thomas Register

    EUA = Energy Used Annually by the Boiler; MMBtu/yr (assuming gas purchase quantity of CCF (see energy management for conversion table) or MMBtu and oil purchase quantity of barrels (see conversion table) or MMBtu)
    hcf = Estimated Current Boiler Efficiency; (boiler test by maintenance personnel)
    hpf = Proposed Boiler Efficiency; (will be calculated)

    Note:

      Fuel cost = Average Weighted Fuel Cost (the cost of the amount of fuel used excluding process or domestic use) of Fuel for Boiler; $/MMBtu (from utility bills)

    Data Conversions?
    RPN Calculator?
    Press the calculate button to execute an estimation. 

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