Energy Efficient Process Heating

1
Energy Efficient Process Heating

• Web Seminar
• By Kelly Kissock Ph.D., P.E.
• University of Dayton Industrial Assessment Center
• August 24, 2006

2
Heat Supply and Demand
Heat in Flue Gases
Furnace Heat Input
3

• Combustion

• Stoichiometric Combustion
• Flame Temperature
• Available Heat

4
Perfect (Stoichiometric) Combustion

• When fuel reacts with exactly the right amount of
  air, all of the carbon and hydrogen atoms combine
  with all of the oxygen to form carbon dioxide and
  water vapor. This is called stoiciometric
  combustion.

CH4 2O2 CO2 2H2O
5
Excess Air

• Most burners operated with more than
  stoichiometric air to guarantee that every fuel
  molecule finds an oxygen molecule.
• The quantity of air in excess to
  stoichiometeric air is called excess air.
• The quantity of excess air can be measured by
  measuring the amount of oxygen in the combustion
  gasses.

Stoichiometric CH4 2O2 CO2 2H2O
Excess Air CH4 4O2 CO2 2H2O
2O2
6
Excess Air Combustion Products Analysis for a
Typical Natural Gas
7
Excess Air

• Excess air dilutes the products of combustion,
  resulting in
• 1) a lower flame temperature
• 2) less heat transfer from the combustion gasses
  to the load
• 3) more heat carried away in the exhaust gasses.

CH4 4O2 CO2 2H2O 2O2
8
Flame (Combustion) Temperature
Flame temperature affects heat transfer and
temperature distribution within the heating
system (furnace, oven etc.).
Flame (Combustion) Temp. (F) Condition
4,450 NG, With 100 O2
3,750 NG, 900 F 0 Excess Air 3,545 NG,
900 F - 20 Excess Air 3,460 NG, 70 F - O
Excess Air 3,225 NG, 70 F - 15 Excess Air
2,750 NG, 7.01 Air/Fuel Ratio
9
Effect of Oxygen Enhancement Flame Temperature
for Natural Gas
10
Calculating Combustion Temperature

• Combustion chamber
• Stoichiometric combustion equation (for natural
  gas)
• CH4 2 (O2 3.76 N2) ? CO2 2 H2O
  7.52 N2
• Air/Fuel ratio for stoichiometric combustion
• AFs 2 (O2 3.76 N2) / CH4 2 (32
  3.76 28) / 16 17.2
• Combustion temperature (Tc) from energy balance
• Tc Tca LHV / 1 (1 EA) AFs Cpg

11
Available Heat (Combustion Efficiency)

• Available Heat
• Available Heat Fraction of energy not
• lost in exhaust
  gasses

12
Calculating Available Heat

• Process heating system
• Percent available heat (h) from energy balance on
  system
• h 1 (1 EA) AFs Cpg (Tc
  Tex) / HHV

mex Tex
combustion chamber
Qout
mng mca Tca
13
Air Flow

• - Usually the largest loss in process heat.
• - Air flow heat loss
• Boilers (250 F 350 F) 20
• Aluminum furnace (1,400 F) 50
• Glass melter (2,500 F) 70

14
Types of Air Flow

• Combustion Air needed to burn fuel
• Ventilation Air for moisture and volitile
  removal
• Infiltration Air undesirable minimized by
  proper design and maintenance

15
Managing Combustion Air Minimize Excess Air

• Optimum excess air for energy efficiency and
  pollution prevention 10 (yields 2 O2 in comb
  gasses)
• Combustion temperature increases
• Tc Tca LHV / 1 (1 ECA) AFs Cpg
• Percent available heat (combustion efficiency)
  increases
• h 1 (1 ECA) AFs Cpg (Tc Tex) / HHV
• Example
• Aluminum melt furnace
• 1,465 F exhaust gas temperature
• Operates with 95 excess combustion air
• Reducing excess air increases percent available
  heat from 39 to 60
• Energy use decreases by 35

16
Managing Combustion Air Preheating

• Recuperator schematic
• A recuperator transfers heat from exhaust gasses
  to inlet combustion air.
• Recuperator effectiveness (e) relationship
• e Q / mca Cpa (Tex1 Tca1) (Tca2 Tca1) /
  (Tex1 Tca1)

17
Managing Combustion Air Preheating

• Combustion temperature increases
• Tc Tca LHV / 1 (1 ECA) AFs Cpg
• Percent available heat (combustion efficiency)
  increases
• h 1 (1 ECA) AFs Cpg (Tc Tex) / HHV
• Example
• Aluminum melt furnace
• 2,500 F combustion temperature
• 1,465 exhaust gas temperature
• 40 effective recuperator increases comb. air
  temperature to 615 F
• Increases combustion temperature to 3,010 F
• Energy use decreases by 34

18
Managing Combustion Air Use Exhaust Air

• Exhaust gasses from ovens with high ventilation
  rates contain high O2 content, and can be
  redirected back to the burner as combustion air.
• Effective combustion temperature increases
• Tc,eff Tca LHV / 1 (1 EA) AFs Cpg
• Percent available heat (combustion efficiency)
  increases
• h 1 (1 EA) AFs Cpg (Tc,eff Tex) / HHV
  
• Example
• Curing oven at 250 F
• 18 O2 in exhaust (about 660 excess air)
• Using exhaust for combustion increases percent
  available heat from 64 to 67
• Energy use decreases by 4

19
Managing Ventilation Air Find Requirement

• Industrial ovens must never exceed 25 of lower
  explosive limit (LEL)
• National Fire Protection Agency Standard 86
• This is achieved by
• using 10,000 cu. ft. of ventilation air per
  gallon of cured paint in continuous process.
• using 380 cfm of ventilation air per gallon of
  cured substance in batch process.

20
Managing Ventilation Air Minimize

• Ventilation air can be turned down manually
  through dampers to meet process demand or can be
  controlled with LEL sensors.
• Excess air decreases
• Tc,eff Tca LHV / 1 (1 EA) AFs Cpg
• Percent available heat (combustion efficiency)
  increases
• h 1 (1 EA) AFs Cpg (Tc,eff Tex) / HHV
  
• Example
• Curing oven with 141 F exhaust gasses
• 3,700 excess air measured ? 3,470 cfm
  ventilation air
• Ventilation air could be reduced to 45 cfm
• Percent available heat would increase from 43 to
  82
• Energy use would decrease by 47

21
Managing Vent Air Using Thermal Oxidizer
Discharge Air

• Thermal oxidizers burn off volatile organic
  compounds in oven exhaust.
• Discharge air (usually around 300 F) can be
  redirected to oven.
• Effective combustion temperature increases
• Tc,eff Ta LHV / 1 (1 EA) AFs Cpg
• Percent available heat (combustion efficiency)
  increases
• h 1 (1 EA) AFs Cpg (Tc,eff Tex) / HHV
• Example
• Curing oven at 200 F internal temperature
• 75 of air entering oven is ventilation air
• Percent available heat would increase from 77 to
  90
• Energy use would decrease by 14

22
Managing Infiltration

• Ovens and furnaces are typically under negative
  pressure.
• Outside air will infiltrate through cracks, open
  doors, loose cracks, etc. through differential
  pressure and buoyancy effects.

Vertical oven opening
23
Managing Infiltration Move Opening to Floor

• Due to buoyancy effects, little cool air will
  infiltrate to a warm oven through its floor.
• Energy lost through infiltration (Qinf)
• Qinf Vinfil A ?a Cpa (Texfil Tinf)
• Qinf could be reduced by
• as much as 80.
• Example
• Second story curing oven at 435 F temperature
• Door area of 100 sq. ft.
• Infiltration was measured to be 2,900 cfm
• Energy use would decrease by 40

Horizontal oven opening
24
Managing Infiltration Lower Openings

• Lowering openings decreases buoyancy effects
  between cool and warm air.
• New infiltration velocity (Vinf2) from
  Bernoullis Equation.
• Vinf2 Vinf1
• Energy saved (Qsav)
• Qsav A Cpa Vinf1 ?a1 (Toven,1 Tinf) Vinf2
  ?a2 (Toven,2 Tinf)
• Example
• Curing oven at 450 F temperature
• Door area of 8.5 sq. ft.
• Infiltration was measured to be 2,250 cfm
• Infiltration would reduce to about 2,000 cfm
• Exfiltration temperature would decrease
• Energy use would decrease by 28

Oven opening location before and after retrofit
25
Summary of Managing Air Flow

• Savings opportunities for combustion air
• Minimize combustion air (35 savings)
• Preheat combustion air (34 savings)
• Use exhaust as combustion air (4 savings)
• Savings opportunities for ventilation air
• Minimizing ventilation air (47 savings)
• Using thermal oxidizer discharge air for
  ventilation (14 savings)
• Savings opportunities for infiltration
• Move oven opening to floor (40 savings)
• Lower oven openings (28 savings)
• Savings calculations can be assisted by
• PHAST (www1.eere.energy.gov/industry/bestpractice
  s/software.html)
• HeatSim (www.engr.udayton.edu/udiac)

26
Heat Loss

• Heat is lost through system walls by conduction,
  then convection and radiation.
• Heat is lost from heated open tanks by
  convection, radiation, and evaporation.

27
Insulating Hot Surfaces

• Heat lost (Q) from a surface
• Q h A (Ts Ta) s A e (Ts4 Ta4)
• e is 0.9 for dark surface, 0.1 for shiny
  surface
• Q A (Tf Ts) / Rshell

convection component
radiation component
conduction
28
Insulating Hot Surfaces

• Calculating convection coefficient (h)
• Laminar air if L3 DT lt 63
• Turbulent air if L3 DT gt 63
• L (length x width)1/2 for flat surfaces, L
  diameter for cylindrical objects
• Horizontal hlam 0.27 (DT / L) 0.25
  htur 0.22 (DT) 0.33
• Vertical hlam 0.29 (DT / L) 0.25
  htur 0.19 (DT) 0.33
• Horizontal hlam 0.27 (DT / L) 0.25
  htur 0.18 (DT) 0.33
• Vertical hlam 0.29 (DT / L) 0.25
  htur 0.19 (DT) 0.33
• Relations taken from ASHRAE Fundamentals

Flat Surfaces
Cylindrical Surfaces
29
Covering Heated Tanks

• Insulation floats can cover heated tanks to
  reduce convection and radiation heat transfer,
  and virtually eliminate evaporation.
• Floats cover up to 79 of
• liquid surface area.
• Energy balance on float
• hfs (Tfs Ta) e s Tfs4 Ta4
• (Tw Tfs) / Rfloat
• HeatSim iterates values of float surface
  temperature until equation is balanced.
  Convection coefficient depends on surface
  temperature.

30
Covering Heated Tanks

• Convection heat loss (Qconv)
• Qconv h A (Tw Ta) (h dependent on both
  water and air temperature)
• Radiation heat loss (Qrad)
• Qrad ? ? A (Tw4 Ta4)
• Evaporation heat loss (Qconv)
• Qevap mw hfg (both values dependent on water
  and air temperature)
• Total heat loss (Qtot)
• Qtot Qconv Qrad Qevap

31
Reducing Thermal Mass

• Continuous Systems
• Energy lost to conveyor (Qcvr) traveling at
  velocity (V)
• Qcvr V m Cpcvr (Tcvr2 Tcvr1)
• Example
• Brazing oven at 1,900 F
• Stainless steel conveyor at velocity 0.7 ft/min
• Conveyor weighs 5 lbs/ft
• Conveyor only loaded 30 of time
• Conveyor is slowed to 0.3 ft/min when unloaded
• 18,000 Btu/hr, or 40, of conveyor energy saved

32
Reducing Thermal Mass

• Batch Systems
• Thermal mass in oven must heat to temperature
  during every batch cycle.
• Thermal resistance (R1)
• R1 1 / (h A) dx / (2 k A)
• R2 R3 R4 R5 R6 R7 R8 dx / (k A)
• Finite difference equations
• Ein Eout ?Estore
• (TN T) / RN (T TS) / RS dx A ? Cp (T
  T) / dt
• T dt (TN T) / (RN dx A ? Cp)
• dt (T TS) / (RS dx A ? Cp) T

Firebrick mass
33
Reducing Thermal Mass

• Batch Systems
• Example heat treat oven
• Heat treat oven with 100 sq. ft. floor
• 8-inch thick firebrick layer
• Oven is raised to 1,700 F, remains at temperature
  for 4 hours
• Temperature decreases by 50 F every hour until
  temperature reaches 400 F
• Savings measure
• reduce firebrick thickness
• Firebricks laid flat so new thickness is 4 inches
• Energy savings is 235,000 Btu per cycle

Temperature profile for 8-inch and 4-inch
firebrick after cycle
34
Heat Loss Summary

• Shell loss depends on insulation type and
  thickness. Insulate surfaces over 150 F.
• Heat loss from heated open tanks dominated by
  evaporation. Cover tanks to minimize evaporation
• Minimize thermal mass of interior structure in
  batch processes with short cycle times
• Minimize thermal mass of conveyor in continuous
  processes.
• Savings calculations assisted by
• PHAST (www1.eere.energy.gov/industry/bestpractice
  s/software.html)
• HeatSim (www.engr.udayton.edu/udiac)

35
Summary

• Energy efficiency improved by
• Minimizing combustion air
• Minimizing ventilation air
• Minimizing infiltration air
• Insulating surfaces over 150 F
• Covering open heated tanks
• Reducing mass of structure in batch processes
• Reducing mass of conveyor in continuous processes
• More information see our web site
• www.engr.udayton.edu/udiac

36
Thank you!