Module E2: Heating Systems

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HCB 3-Chap 15A: Furnaces and Boilers*Chapter 15A: BOILERS AND FURNACESAgami Reddy (July 2016) 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

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

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HCB 3-Chap 15A: Furnaces and Boilers*Types of Fuel and CombustionCombustion: 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)

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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)

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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) = 17.28 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) = 9.56 ft3 of air “ “

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HCB 3-Chap 15A: Furnaces and Boilers*

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Higher and Lower Heating ValuesHCB 3-Chap 15A: Furnaces and Boilers*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

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HCB 3-Chap 15A: Furnaces and Boilers*Table 15.1 Heating Values of Some Common Gaseous FuelsAll values corrected to 60o F (16o C) and sea level. *At 32o F ( 0o C) and sea level

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HCB 3-Chap 15A: Furnaces and Boilers*

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

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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 9.3 ft3 previously calculated.

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HCB 3-Chap 15A: Furnaces and Boilers*

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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

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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 efficiencyFigure 15.1 Variation of boiler flue gas losses with excess air flow

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HCB 3-Chap 15A: Furnaces and Boilers*Warm Air Furnaces Different Types Upflow, downflow, horizontal Capacity Residential house: 35 – 175 kBtu/h (10 – 50 kW) Small Commercial building: up to 1000 kBtu/h (300 kW) Efficiency (steady – state) Typical: 80 -85 % High Efficiency: 90 – 95 % Controls Operating control: regulate burner and air circulation Safety control: prevent incomplete combustion and explosion Safety is an important issue for boilers and furnaces!!

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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                                                             <>

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

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HCB 3-Chap 15A: Furnaces and Boilers*Gas-fired Furnace: Typical Sequence of OperationWhen 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

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HCB 3-Chap 15A: Furnaces and Boilers*Gas BurnersAtmospheric gas burner based on venturi principle- Power gas burner uses fan to deliver airFIGURE 15.4 Cross-section of an atmospheric gas burner

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HCB 3-Chap 15A: Furnaces and Boilers*Maintaining Proper DraftDraft 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 damperFIGURE 15.5 Control devices for combustion flue gases (a) Draft hood, (b) Damper valve

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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.  

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HCB 3-Chap 15A: Furnaces and Boilers*BoilersBoiler – 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 packagedTypes of boilers: Fire tube Water tube Electric Multi-fuel

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

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HCB 3-Chap 15A: Furnaces and Boilers*Boiler Rating and SelectionRating 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.

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HCB 3-Chap 15A: Furnaces and Boilers*Boiler Rating and SelectionSelection 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 Lo Typically, Gross output = (1 + .15) × Net Output

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HCB 3-Chap 15A: Furnaces and Boilers*Boiler Rating and SelectionExample of manufacturer table:

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Last Updated: 8th March 2018

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