1. Chapter 15A: BOILERS AND FURNACES
Agami Reddy (rev- May 2017)
Types of fuels
Review of combustion reactions
Stoichiometric and excess air
Residential warm air furnaces, efficiency, rating
Boiler, efficiency and rating and selection
Improving performance by monitoring flue gas
Seasonal bin method calculation
HCB 3-Chap 15A: Furnaces and Boilers

2. Combustion Based Heating Systems
Two major heating sources:
- warm air furnace
- boiler with hydronic system
Equipment that transfer chemical energy contained in fuels to heat air or water
HCB 3-Chap 15A: Furnaces and Boilers

3. Types of Fuel and Combustion
Rapid chemical reaction of the
combustible substance in a fuel with
oxygen in air which produces heat
Types of fuel
Natural gas (methane- CH4 and ethane- C2H6)
Oil (different grade No. 1 to 6) lighter – denser
Coal (different grade)
Complete combustion - Incomplete combustion
CO2, H2O, and SO2(pollutant) CO (very toxic)
Smoke (inadequate oxygen)
Ash (noncombustible solids)
Soot (carbon – ash particles)
HCB 3-Chap 15A: Furnaces and Boilers

4. Review of Chemical Reactions
Composition of dry air:
O2 N2
By volume % %
By mass % %
Mass of dry air ~ 4.32 times mass of O2
Volume of dry air ~ 4.78 times volume of O2
(3.78 for NO2 and 1.0 for O2)
100/23.15 = 4.32
HCB 3-Chap 15A: Furnaces and Boilers

5. Review of Chemical Reactions
Theoretical or stoichiometric air required for combustion of methane:
CH4 + 2 O2 = CO2 + 2 H2O
By mass:
(12+4) (2 x 32) using molecular weights
64 lbs of O2 are needed to burn 16 lb of CH4
i.e., 4 lbs “ “ lb “
(4 x 4.32) = 17.3 lb of air “ “
By volume: using moles
2 ft3 of O2 are needed to burn 1 ft3 of CH4
i.e., (2 x 4.78) = 9.54 ft3 of air “ “
HCB 3-Chap 15A: Furnaces and Boilers

6. HCB 3-Chap 15A: Furnaces and Boilers

7. Higher and Lower Heating Values
Higher Heating Value (HHV): also called the gross
heating value, includes heat of vaporization of water
vapor formed during combustion
Lower Heating Value (LLV): also called net heating
value, assumes that all the products of the combustion
remain gaseous
HCB 3-Chap 15A: Furnaces and Boilers

8. HCB 3-Chap 15A: Furnaces and Boilers
Table 15.1 Heating Values of Some Common Gaseous Fuels
Substance
Molec.
Symbol
Molec. Weight
Higher HV Btu/lbm
Lower HV
Btu/lbm
Specific Volume ft3/lbm*
Higher HV MJ/kg
MJ/kg
Density
kg/m3 *
Carbon (to CO2) C 12
14,093 -
32.788
32.780
Hydrogen H2 2
61,095
51,623
188.0
0.085
Carbon monoxide CO 28
4,347
13.5
10.111
1.187
Methane
CH4 16
23,875
21,495
23.6
55.533
49.997
0.679
Ethane
C2H6 30
22,323
20,418
12.5
51.922
47.492
1.28
Propane
C3H8 44
21,669
19,937
8.36
50.402
46.373
1.92
All values corrected to 60o F (16o C) and sea level.
*At 32o F ( 0o C) and sea level
HCB 3-Chap 15A: Furnaces and Boilers

9. HCB 3-Chap 15A: Furnaces and Boilers

10. HCB 3-Chap 15A: Furnaces and Boilers
monoxide
HCB 3-Chap 15A: Furnaces and Boilers

11. HCB 3-Chap 15A: Furnaces and Boilers
A rule of thumb to check the preceding calculation has been proposed: 0.9 ft3 of air is required for 100 Btu of fuel heating value (about 0.25 m3 of air per 1 MJ of heating value).
For example, the heating value of natural gas is about 1000 Btu/ft3, requiring 9 ft3 of air according to the above rule.
This compares well with the value of 9.5 ft3 previously calculated.
HCB 3-Chap 15A: Furnaces and Boilers

12. HCB 3-Chap 15A: Furnaces and Boilers
Excess Air
Theoretical air/fuel ratio or stoichiometric ratio: amount of air required for complete combustion per unit of fuel.
Because perfect mixing is difficult to obtain to ensure complete combustion, excess air that is more than the theoretical amount is supplied.
But if excess air is too much, then unnecessary amount of air will be heated and energy will be wasted.
The minimum amount of the excess air that is required to ensure complete combustion is what we need
Depends on type of fuel, construction of the device and control
Varies from 5 up to 100 % above theoretical air
Large device requires less excess air – better mixing
Recommended by manufacturer
HCB 3-Chap 15A: Furnaces and Boilers

13. HCB 3-Chap 15A: Furnaces and Boilers
The amount of excess air provided is critical to the efficiency of a combustion process.
Excessive air both reduces combustion temperature (reducing the heat transfer rate to the working fluid) and results in excessive heat loss through the flue gases (see Figure).
Insufficient excess air results in incomplete combustion and loss of chemical energy in the flue gases.
Recommendations of the manufacturer should be followed. The optimum excess air fraction is usually between 10 and 50 %. Unfortunately, it is common practice to provide % excess air which significantly reduces boiler efficiency
Fig Variation of boiler flue gas losses with excess air flow
HCB 3-Chap 15A: Furnaces and Boilers

14. HCB 3-Chap 15A: Furnaces and Boilers
Warm Air Furnaces
Directly supply heat to the supply air stream
Used for residential house and small commercial
Cheaper than hydronic systems
Warms up building faster when a night temperature setback is used
Components:
Heat Exchanger
Fuel Burner
Air Blower
Control system
Housing Cabinet
Filter & Humidifier - optional
                                                            <>
Fig. 15.2
HCB 3-Chap 15A: Furnaces and Boilers

15. HCB 3-Chap 15A: Furnaces and Boilers
Fig Sketch of a gas-fired furnace showing various components (downloaded from
HCB 3-Chap 15A: Furnaces and Boilers

16. Gas-fired Furnace: Typical Sequence of Operation
When heating is needed, a safety sensor checks whether pilot flame is on
If on, main gain valve opens, and ignites the main gas stream
Pilot flame safety continuously checks for blame burning. If no flame, valve closes.
Fan control thermostat located in the fan plenum automatically starts the fan when air is heated to a comfortable level. (An alternate arrangement is to use a timer)
A limit switch thermostat will shut off the gas valve if air temperature is too high (2000 F). This can be caused by dirty or clogged air filters
HCB 3-Chap 15A: Furnaces and Boilers

17. HCB 3-Chap 15A: Furnaces and Boilers
Gas Burners
Atmospheric gas burner
based on venturi principle
Fig Cross-section of an atmospheric gas burner
- Power gas burner uses fan to deliver air
HCB 3-Chap 15A: Furnaces and Boilers

18. HCB 3-Chap 15A: Furnaces and Boilers
Maintaining Proper Draft
Draft includes maintaining proper pressure to enable combustion
Can use natural means or small fans
Draft in residential furnaces about WG (0.5 Pa)
Momentary busts of outdoor wind and other fluctuations
can be controlled by a draft hood or by a damper
Fig. 15.5
Control devices for combustion flue gases (a) Draft hood,
(b) Damper valve
HCB 3-Chap 15A: Furnaces and Boilers

19. HCB 3-Chap 15A: Furnaces and Boilers
Furnace design and selection
Selection of a furnace is straightforward once the fuel source and design heat loads of the space are determined. The following factors must be accounted for in furnace sizing and type selection:
- Design heat loss of area to be heated, in Btu/h or kW
- Morning recovery capacity from night setback
- Constant internal gains or waste heat recovery that reduces the needed heat rating of a furnace
- Humidification load
- Fan and housing size sufficient to accommodate air conditioning system, if any
- Duct heat losses if heat so lost is external to the heated space
- Available space for furnace location.
HCB 3-Chap 15A: Furnaces and Boilers

20. HCB 3-Chap 15A: Furnaces and Boilers
Types of boilers:
Fire tube
Water tube
Electric
Multi-fuel
Boiler – boiling does not necessarily occur
Hot water boiler (hot water generator < 250°F & 30 psig ) or
steam boiler (generate steam, up to 15 psig)
For all types of buildings with hydronic systems
Generate hot water or steam
Need other device (heat exchanger) to transfer heat to air
Components
Combustion chamber
Burner
Heat exchanger
Controls
Enclosure
Classification
Pressure and temperature
Material of construction
Water tube or fire tube
Type of fuel
Build up or packaged
HCB 3-Chap 15A: Furnaces and Boilers

21. Large Industrial Water Tube Boiler
HCB 3-Chap 15A: Furnaces and Boilers

22. Boiler Rating and Selection
Operational standards developed to prevent “unhealthy” operation of boiler that might decrease safety and shorten the boiler
The efficiency and other characters for a boiler that is operated under standard tests are called ratings and are supplied by manufacturers as the basis for boiler selection
A.G.A, D.O.E, I-B-R are all organizations that supply testing standards.
HCB 3-Chap 15A: Furnaces and Boilers

23. Boiler Rating and Selection
Net Output:
Amount of heat that is needed to make up the building heating load and other hot water load
Piping and Pickup Loss:
Certain amount of extra heating capability
Gross Output:
= Net Output + Piping loss + Pickup loss
Typically, Gross output = ( ) × Net Output
HCB 3-Chap 15A: Furnaces and Boilers

24. Boiler Rating and Selection
Use manufacturer table:
Heat Input under A.G.A standard test
Gross Output under D.O.E standard test
Net Output under I-B-R test
HCB 3-Chap 15A: Furnaces and Boilers

25. Different Efficiency Measures
Annual Fuel Utilization Efficiency
Varies with working conditions
HCB 3-Chap 15A: Furnaces and Boilers

26. HCB 3-Chap 15A: Furnaces and Boilers

27. Improving and Monitoring Performance
HCB 3-Chap 15A: Furnaces and Boilers

28. HCB 3-Chap 15A: Furnaces and Boilers

29. HCB 3-Chap 15A: Furnaces and Boilers
Indirect Method- Flue Gas Analysis
Flue gas analysis is a method of determining the amount of excess air in a combustion process. This information can be used to find an approximate value of boiler efficiency
HCB 3-Chap 15A: Furnaces and Boilers

30. HCB 3-Chap 15A: Furnaces and Boilers
Approximate way of checking
for proper combustion
Fig How excess air affects the composition of CO2 and O2 in the flue gas of different types of fuel
HCB 3-Chap 15A: Furnaces and Boilers

31. Combustion Efficiency from Tables
Along with a figure such as Fig. 15.8, boiler manufacturers also provides tables which can be conveniently used to deduce boiler combustion efficiency from two simple measurements of the flue gas:
-flue gas temperature and
- excess air percentage
Table 15.6 is a typical example of how these two quantities affect the boiler fractional heat loss for a specific type of natural gas composition.
For example, if for this boiler, the flue gas composition of CO2 concentration was 10%, while the difference between the flue gas temperature and the boiler room was 500o F, then
- fractional boiler loss is 21.4%, i.e., the boiler has a combustion efficiency of ( = 88.6%).
Inspection of this table clearly reveals that combustion efficiency increases with decreasing flue gas temperature and increasing CO2 concentration.
HCB 3-Chap 15A: Furnaces and Boilers

32. Table 15.6 Deducing Thermal Efficiency
Natural Gas
% CO2
Difference Between Flue Gas and Room Temperature (oF)
300
350
400
450
500
550
600
650
700
750
800
Fuel Analysis:
1120 Btu/ft3
% by Volume CH
C2H6 17.3
CO2 0.3
N2 2.5
4.0
25.1
27.7
30.4
33.1
35.8
38.3
40.9
43.5
46.2
48.8
4.5
23.6
25.9
28.3
30.7
33.0
35.4
37.8
40.1
42.6
44.8
47.2
5.0
22.2
24.4
26.8
28.7
30.9
35.7
37.3
39.7
41.8
43.8
5.5
21.2
23.4
25.2
27.3
29.2
31.3
33.2
35.3
39.2
41.0
6.0
20.4
22.3
24.1
25.8
27.8
29.6
31.5
33.3
35.2
36.8
38.8
6.5
19.8
21.4
23.2
24.8
26.5
30.0
31.7
33.5
34.6
7.0
19.1
20.7
23.9
25.5
27.1
28.8
32.0
33.8
7.5
18.5
20.0
21.5
23.0
24.6
26.1
29.1
30.8
32.2
8.0
18.0
19.5
20.9
22.4
23.8
26.7
28.1
29.5
31.0
32.4
8.5
17.6
19.0
21.7
23.1
24.5
28.6
29.9
9.0
17.2
19.9
20.1
22.5
26.4
29.0
30.3
9.5
16.9
18.1
21.9
25.6
26.9
28.2
29.4
10.0
16.6
17.8
20.2
22.6
25.0
26.2
27.4
Flue gas loss percentage = Flue gas loss/ Heat Input
= 1 – Combustion Efficiency
HCB 3-Chap 15A: Furnaces and Boilers

33. HCB 3-Chap 15A: Furnaces and Boilers
Use of Bin Method
HCB 3-Chap 15A: Furnaces and Boilers

34. HCB 3-Chap 15A: Furnaces and Boilers
Table 15.5 Summary Solution for Example 15.5
22,439
10,525
overall annual boiler efficiency = 10,525/22,439 = 47 %
(compared to 80% at rated condition)
HCB 3-Chap 15A: Furnaces and Boilers

35. HCB 3-Chap 15A: Furnaces and Boilers

36. HCB 3-Chap 15A: Furnaces and Boilers
Outcomes
Knowledge of the different types of fuels
Be able to solve problems involving chemical reactions
Understanding of higher and lower heating values of fuels
Understanding of different types of warm air furnaces and the functioning of their components
Be able to solve problems involving combustion of fuels in actual furnaces
Understanding of the importance of safety and operational controls in furnaces
Understanding of different types of boilers and the functioning of their components
Be able to use tables to determine boiler rating and selection
Be able to perform simple boiler analysis from operational information
Be able to use tables and graphs to analyze flue gas in order to deduce excess air and combustion efficiency
Be able to perform seasonal boiler energy analysis using the bin method
HCB 3-Chap 15A: Furnaces and Boilers