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

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

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

Review of Chemical Reactions Composition of dry air: O2 N2 By volume 20.95% 79.05% By mass 23.15% 76.85% 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

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 “ “ 1 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

HCB 3-Chap 15A: Furnaces and Boilers

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

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 51,623 188.0 142.107 118.680 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

HCB 3-Chap 15A: Furnaces and Boilers

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

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

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

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 50-100% excess air which significantly reduces boiler efficiency Fig. 15.1 Variation of boiler flue gas losses with excess air flow HCB 3-Chap 15A: Furnaces and Boilers

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

HCB 3-Chap 15A: Furnaces and Boilers Fig. 15.3 Sketch of a gas-fired furnace showing various components (downloaded from www.mybutterfly.com) HCB 3-Chap 15A: Furnaces and Boilers

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

HCB 3-Chap 15A: Furnaces and Boilers Gas Burners Atmospheric gas burner based on venturi principle Fig. 15.4 Cross-section of an atmospheric gas burner - Power gas burner uses fan to deliver air HCB 3-Chap 15A: Furnaces and Boilers

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 0.002 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

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

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

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

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

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 = (1 + .15) × Net Output HCB 3-Chap 15A: Furnaces and Boilers

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

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

HCB 3-Chap 15A: Furnaces and Boilers

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

HCB 3-Chap 15A: Furnaces and Boilers

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

HCB 3-Chap 15A: Furnaces and Boilers Approximate way of checking for proper combustion Fig. 15.8 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

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 (100-21.4 = 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

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 CH4 79.9 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

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

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

HCB 3-Chap 15A: Furnaces and Boilers

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