heating equip

Upload: lindi-newman

Post on 03-Apr-2018

233 views

Category:

Documents


0 download

TRANSCRIPT

  • 7/28/2019 Heating Equip

    1/8

  • 7/28/2019 Heating Equip

    2/8

    T13 HeatingEquip2012

    1

    ME 414/514 HVAC Systems Topic 13 Heating Equipment

    Combustion

    Fuel +air carbon dioxide +water +excess oxygen +nitrogen +pollutants (NO, CO,unburned HCs, PM, SO2, Hg, dioxins )

    If we model natural gas as methane, the reaction for complete combustion is as follows:

    CH4 +2/ (O2 +3.78 N2) CO2 +2 H2O +(2/ - 2) O2 +2/ 3.78 N2

    where is the equivalence ratio (the actual fuel-to-air ratio divided by the stoichiometricfuel-to-air ratio; stoichiometric combustion means that there is exactly enough fuel andair). However, practical combustion is never complete. Carbon monoxide (CO),nitrogen oxide (NO), and unburned hydrocarbons (HCs) including soot are alwaysreleased. (Take ME529 Combustion and Air Pollution if you want to learn more.)

    Furnaces

    Types of hot-air furnaces used in large buildingsDesign and selection (vendor data)

    Heat load known including duct lossesMorning recovery from setbackDe-rating issuesHumidification loadResidential fan and housing shared for AC, space

    Efficiency and energy calculations

    Boilers

    Steam or hot water production for heatingA furnace may power a boilerLarge heat of vaporization of water makes steam an efficient heat transport fluid

    Low-pressureHigh pressure

    Heat rate expressed as pound-mass of steam per hour (or kW)ASME Boiler and Pressure Vessel Code followed for construction of boilers.Design and selection (vendor data)

    Typical output ratings of furnaces, boilers, and heat pumpsResidential Commercial

    Furnace 35,000 - 175,000 Btuh (10 51 kW) 1,000,000 Btuh (300 kW)Steam Boiler 50 50,000 lbm/hr (15 15,000 kW)Hot Water Boiler 50 50,000 lbm/hr (15 15,000 kW)Heat Pump 10 kW 15,000 kW

  • 7/28/2019 Heating Equip

    3/8

    T13 HeatingEquip2012

    2

    FuelOutput ratingPressure and working fluidEfficiency and part-load performanceSpace, control, combustion air, safety, ASME code compliance, water softening

    Add maximum heat load of each zone during peak demand to size the boiler.Adjust natural gas boilers for atmospheric pressure (fans are constant volume devices;forced draft burners needed to bring in more combustion air)

    The rule of thumb for sizing boilers in section 9.2.2.1 of the text says that the input ratingfor the boiler per heated square foot of space is 1/3 to 1/5 the design temperaturedifference (the difference between the indoor and outdoor 99% design temperature).Verify this by calculating the heating load for a building and choosing a boiler to meetthis need.

    Steam traps for separating out condensate; calculate condensate rejection rate

    Pressure relief valve

    Expansion tanks

    In class we will take a closer look at flue gas analysis, the rules of thumb in section9.2.2.3 of the text and values calculated in Table 9.4

    The chemical reaction for stoichiometric combustion of methane, when =1, is

    CH4 +2/ (O2 +3.78 N2)

    CO2 +2 H2O +(2/ - 2) O2 +2/ 3.78 N2

    EA =excess air =(V of air supplied V of air at=1)/( V of air at=1)

    Excess air is required for boilers to meet their exhaust emissions permit. Smaller powerplants do not have to provide continuous emission monitoring equipment (CEMS).During their permitting process, the fraction of oxygen in the stack is recorded along withthe criteria pollutants (SO2, NOx, CO, PM). The plant must always be operated with thatfraction of O2 or install CEMS equipment.

    Higher and Lower Heating Values

    The chemical composition of many practical fuels is not known. Also, the enthalpy datamust be known for ALL the reactants and products to calculate the enthalpy ofcombustion. Otherwise, the enthalpy of the combustion reaction must be obtainedexperimentally in a calorimeter. Both flow calorimeters and constant-volume (bomb)calorimeters usually measure the higher heating value (HHV) of a fuel. Engineers needto distinguish between HHVs and the lower heating value (LHV) of fuels.

  • 7/28/2019 Heating Equip

    4/8

    T13 HeatingEquip2012

    3

    The HHV includes the heat of vaporization of water vapor formed during combustionbecause it assumes that all of the water in the combustion products has condensed toliquid. The LHV assumes that all of the products of combustion remain gaseous.

    When experiments are done at room temperature in the calorimeter (that is, reactants start

    out at 298 K and reactants are cooled to 298 K), water vapor formed during combustionwill condense. This increases the apparent heat release due to the latent heat ofvaporization.

    Since exhaust temperatures are usually high enough to prevent condensation, the LHV ismore relevant. Hence, when the HHV is given, it often needs to be converted into aLHV.

    LHV =HHV - n hfg,H2O

    ________________________________________________________________Example on HHV and LHV

    The HHV of a fuel oil with the molecular formula CH2.186 is measured as -44,135 J/g.Calculate the LHV in kJ/mol and J/g.

    Solution: The latent heat of vaporization at 298 K for water is -43,961 kJ/kmol.

    LHV =-44,135 J/g x (12 +2.186) g/mol- 2.186/2 mol/mol x ( -43,961) J /mol=-626,099 J /mol +48,049 J/mol =- 578 kJ/mol-578,000 J/mol x [1/(12 +2.186) g/mol] =-40,744 J /g

    ________________________________________________________________

    The boiler efficiency is defined by the Boiler Efficiency Institute as:

    HHVm

    Q

    steam

    steamboil

    =

    Where Qsteam is the steam output rate (BTU/hr);mfuel is the fuel supply rate (lbm/hr); and

  • 7/28/2019 Heating Equip

    5/8

    T13 HeatingEquip2012

    HHV is the higher heating value of the fuel

    HHV is measured in standardized laboratory tests (there is an ASTM standard for this),often in a bomb calorimeter. The method allows all the water formed in combustion tocondense. In practice, usually exhaust products are kept above their dewpoint to prevent

    condensation in the stack. Hence, not all of the energy is extracted from the fuel andcombustion products. The HHV is corrected for the condensation of water analyticallyand the result is the lower heating value (LHV). The LHV is more useful for mostengineers and power production processes. Why the Boiler Efficiency Institute chose toignore this is puzzling.

    The best way to get boiler efficiency for sizing a boiler is from a manufacturer, whoshould have available results from standardized testing including efficiency at full loadand either data or an expression for efficiency at part-loads.

    The Part Load Ratio (PLR) is quantified as:

    PLR =Qo/Qo,full Find the PLR here.

    Where Qo is the boiler heat output at part load, and Qo,full is the rated output at full load.From manufacturers data (or from a curve fit provided) you can fit an nth orderpolynomial:

    +++=2

    ,

    )()( PLRCPLRBAQ

    Q

    fulli

    i Use the PLR here to calculate the fuel input.

    Here, Q is the fuel input (energy input to the furnace) needed to meet the part-load

    requirement.All this gets simplified by a bin analysis using the same temperature and bin data fromTopic 9 Energy Calculations.

    ( ) ( )( )

    =jboil

    jjjoyeari

    T

    TnTQQ

    ,

    Qo is the heat load (boiler load) that depends on outdoor temperature; n is the number ofhours every year thatthis location experiences the outdoor temperature, and is theboiler efficiency. Example 9.3 illustrates the mechanics of the calculation.

    Electrical Resistance Heating (Section 9.3) can be performed by an electric furnace, anelectric baseboard heater, or electric portable space heaters. The heating for single phase

    power supply is, EiQ = , wherei is the current andE is the voltage of the electrical

    power.

  • 7/28/2019 Heating Equip

    6/8

  • 7/28/2019 Heating Equip

    7/8

  • 7/28/2019 Heating Equip

    8/8