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53.1 AIR-HEATING PROCESSESAir can be heated by burning fuel or by recovering waste heat from another process. In either case,the heat can be transferred to air directly or indirectly. Indirect air heaters are heat exchangers whereinthe products of combustion never contact or mix with the air to be heated. In waste heat recovery,the heat exchanger is termed a recuperator.

Direct air heaters or direct-fired air heaters heat the air by intentionally mixing the products orcombustion of waste gas with the air to be heated. They are most commonly used for ovens anddryers. It may be impractical to use them for space heating or for preheating combustion air becauseof lack of oxygen in the resulting mixture ("vitiated air"). In some cases, direct-fired air heatingmay be limited by codes and/or by presence of harmful matter of undesirable odors from the heatingstream. Direct-fired air heaters have lower first cost and lower operating (fuel) cost than indirect airheaters.Heat requirements for direct-fired air heating. Table 53.1 lists the gross Btu of fuel input requiredto heat one standard cubic foot of air from a given inlet temperature to a given outlet temperature.It is based on natural gas at 6O 0F, having 1000 gross B tu / f t3 , 910 net B tu / f t3 , and stoichiometricair/gas ratio of 9.4:1. The oxygen for combustion is supplied by the air that is being heated. Thehot outlet "air" includes combustion products obtained from burning sufficient natural gas to raisethe air to the indicated outlet temperature.

Recovered waste heat from another nearby heating process can be used for process heating, spaceheating, or for preheating combustion air (Ref. 4). If the waste stream is largely nitrogen, and if thetemperatures of both streams are between O and 80O0F, where specific heats are about 0.24, a sim-plified heat balance can be used to evaluate the mixing conditions:

heat content of the waste stream + heat content of the fresh air = heat content of the mixture orWWTW + WfTf = WmTm = (Ww + W f) T 1n

where W = weight and T = temperature of waste gas, fresh air, and mixture (subscripts w, /, andm).Example 53.1If a 60 O

0F waste gas stream flowing at 100 Ib/hr is available to mix with 1O

0F fresh air and fuel,how many pounds per hour of UO0F makeup air can be produced?

Solution:(100 x 600) + lQ W f -(100 + Wf) X (110)

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.ISBN 0-471-13007-9 1998 John Wiley & Sons, Inc.

CHAPTER 53AIR HEATINGRichard J. ReedNorth American Manufacturing CompanyCleveland, Ohio

53.1 AIR-HEATING PROCESSES 164153.2 COSTS 1643

53.3 WARNINGS 164353.4 BENEFITS 1644

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Example: Find the amount of natural gas required to heat 1000 scfm of air from 40O0F to 140O0F.o , . T ^ i 1 .1 j^o T ^ i r . /23.2 gross Btu 1000 scf air 60 mm\ 1000 gross Btu 1oSolution: From the table, read 23.2 gross Btu/scf air. Then X X + - f =\ scf air min 1 hr / ft3 gasThe conventional formula derived from the specific heat equation is: Q = we A T; so Btu/hr = weight/hr X0.24 Btu 0 . , 1 o . X nse -scfm X 1.1 X nse.

Ib FThe table above incorporates many refinements not considered in the conventional formulas: (a) % available heloss due to heat of vaporization in the water formed by combustion, (b) the specific heats of the products of coand (c) the specific heats of the combustion products change at higher temperatures.For the example above, the rule of thumb would give 1000 scfm X 1.1 X (1400 - 400) = 1 100 000 grosgross Btu/hr required. Reminder: The fuel being burned adds volume and weight to the stream being heated.

Table 53.1 Heat Requirements fo r Direct-Fired Air Heating, Gross Btu of Fuel Input per scf of Outlet "AOutlet A ir Temperature

000000000000000000Inlet A irTemperature, 0F19.919.519.018.618.217.817.415.313.211.1

8.906.714.502.26

17.517.116.716.315.915.515.113.010.9

8.766.614.432.23

15.214.814.414.013.613.212.810.7

8.636.514.362.19

13.012.612.211.811.411.010.6

8.506.414.302.16

10.810.49.999.589.188.778.366.314.232.13

8.638.237.837.437.026.626.214.172.10

6.516.115.715.314.914.514.102.06

4.434.043.643.242.842.432.03

2.392.001.601.200.8020.402

-20O

+20406080

100200300400500600700800900

1000

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Solving, we find W f = 490 Ib/hr of fresh air can be heated to UO 0F, but the 100 Ib/hr of waste gaswill be mixed with it; so the delivered stream, Wm will be 100 + 490 = 590 Ib/hr.If "indirect" air heating is necessary, a heat exchanger (recuperator or regenerator) must be used.These may take many forms such as plate-type heat exchangers, shell and tube heat exchangers,double-pipe heat exchangers, heat-pipe exchangers, heat wheels, pebble heater recuperators, and re-fractory checkerworks. The supplier of the heat exchanger should be able to predict the air preheattemperature and the final waste gas temperature. The amount of heat recovered Q is then Q = W cp(T2 - T1), where W is the weight of air heated, cp is the specific heat of air (0.24 when below 80O0F),T 2 is the delivered hot air temperature, and T 1 is the cold air temperature entering the heat exchanger.Tables and graphs later in this chapter permit estimation of fuel savings and efficiencies for casesinvolving preheating of combustion air.

If a waste gas stream is only a few hundred degrees Fahrenheit hotter than the air stream tem-perature required for heating space, an oven, or a dryer, such uses of recovered heat are highlydesirable. For higher waste gas stream temperatures, however, the second law of thermodynamicswould say that we can make better use of the energy by stepping it down in smaller temperatureincrements, and preheating combustion air usually makes more sense. This also simplifies accounting,since it returns the recovered heat to the process that generated the hot waste stream.

Preheating combustion air is a very logical method for recycling waste energy from flue gases indirect-fired industrial heating processes such as melting, forming, ceramic firing, heat treating, chem-ical and petroprocess heaters, and boilers. (It is always wise, however, to check the economics ofusing flue gases to preheat the load or to make steam in a waste heat boiler.)

53.2 COSTSIn addition to the cost of the heat exchanger for preheating the combustion air, there are many othercosts that have to be weighed. Retrofit or add-on recuperators or regenerators may have to be installedoverhead to keep the length of heat-losing duct and pipe to a minimum; therefore, extra foundationsand structural work may be needed. If the waste gas or air is hotter than about 80O 0F, carbon steelpipe and duct should be insulated on the inside. For small pipes or ducts where this would beimpractical, it is necessary to use an alloy with strength and oxidation resistance at the highertemperature, and to insulate on the outside.

High-temperature air is much less dense; therefore, the flow passages of burners, valves, and pipemust be greater for the same input rate and pressure drop. Burners, valves, and piping must beconstructed of better materials to withstand the hot air stream. The front face of the burner is exposedto more intense radiation because of the higher flame temperature resulting from preheated combus-tion air.I f the system is to be operated at a variety of firing rates, the output air temperature will vary;so temperature-compensating fue l /a i r ratio controls are essential to avoid wasting fuel. Also, toprotect the investment in the heat exchanger, it is only logical that it be protected with high-limittemperature controls.

53.3 WARNINGSChanging temperatures from end to end of high-temperature heat exchangers and from time to timeduring high-temperature furnace cycles cause great thermal stress, often resulting in leaks and short-ened heat-exchanger life. Heat-transfer surfaces fixed at both ends (welded or rolled in) can forcesomething to be overstressed. Recent developments in the form of high-temperature slip seal methods,combined with sensible location of such seals in cool air entrance sections, are opening a whole newera in recuperator reliability.

Corrosion, fouling, and condensation problems continue to limit the applications of heat-recoveryequipment of all kinds. Heat-transfer surfaces in air heaters are never as well cooled as those in waterheaters and waste heat boilers; therefore, they must exist in a more hostile environment. However,they may experience fewer problems from acid-dew-point condensation. If corrosives, particulates,or condensables are emitted by the heating process at limited times, perhaps some temporary by-passing arrangement can be instituted. High waste gas design velocities may be used to keep partic-ulates and condensed droplets in suspension until they reach an area where they can be safely droppedout.

Figure 53.1 shows recommended minimum temperatures to avoid "acid rain" in the heat ex-changer.2 Although a low final waste gas temperature is desirable from an efficiency standpoint, theshortened equipment life seldom warrants it. Acid forms from combination of water vapor with SO3,SO2, or CO2 in the flue gases.

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S U L F U R IN F UE L , PERCENTB Y W E I G H T ( A S F I R E D )Fig. 53.1 Recommended minimum temperatures to avoid "acid rain" in heat exchangers.

53.4 BENEFITSDespite all the costs and warnings listed above, combustion air preheating systems do pay. As fuelcosts rise, the payback is more rewarding, even for small installations. Figure 53.2 shows percentavailable heat3 (best possible efficiency) with various amounts of air preheat and a variety of furnaceexit (flue) temperatures. All curves for hot air are based on 10% excess air.* The percentage of fuelsaved by addition of combustion air preheating equipment can be calculated by the formula

_ _ , t . _ _ /. % available heat before\% fuel saved = 100 X 1 - _ , \ % available heat after /

Table 53.2 lists fuel savings calculated by this method.4Preheating combustion air raises the flame temperature and thereby enhances radiation heat trans-

fer in the furnace, which should lower the exit gas temperature and further improve fuel efficiency.Table 53.3 and the x-intercepts of Fig. 53.2 show adiabatic flame temperatures when operating with10% excess air,t but it is difficult to quantify the resultant saving from this effect.Preheating combustion air has some lesser benefits. Flame stability is enhanced by the faster flame

velocity and broader flammability limits. If downstream pollution control equipment is required(scrubber, baghouse), such equipment can be smaller and of less costly materials because the heatexchanger will have cooled the waste gas stream before it reaches such equipment.

*It is advisable to tune a combustion system for closer to stoichiometric air/fuel ratio before at-tempting to preheat combustion air. This is not only a quicker and less costly fuel conservationmeasure, but it then allows use of smaller heat-exchange equipment.tAlthough 0% excess air (stoichiometric air/fuel ratio) is ideal, practical considerations usually dic-tate operation with 5-10% excess air. During changes in firing rate, time lag in valve operation mayresult in smoke formation if some excess air is not available prior to the change. Heat exchangersmade of 300 series stainless steels may be damaged by alternate oxidation and reduction (particularlyin the presence of sulfur). For these reasons, it is wise to have an accurate air/fuel ratio controllerwith very limited time-delay deviation from air/fuel ratio setpoint.

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t3. Furnace gas exit temperature, FFig. 53.2 Available heat with preheated combustion air at 10% excess air. Applicable only icombustion. Corrected for dissociation. (Reproduced with permission from Combustion H

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t2, Combust ion A ir Temperature1100 1200 1300 1

24 .8 25.6 27.5 26.4 28.4 30.2 27.3 29.3 31.2 328.3 30.3 32.2 329.4 31.4 33.4 330.6 32.7 34.6 331.8 34.0 36.0 333.2 35.4 37.5 334.8 37.0 39.1 436.5 38.8 40.9 438.5 40.8 42.9 440.6 43.0 45.2 443.1 45.5 47.7 446.0 48.4 50.6 549.4 51.8 54.0 553.4 55.8 58.0 658.4 60.7 62.8 664.6 66.7 68.7 772.7 74.6 76.2 783.7 85.0 86.1 8

100022.222.923.624.425.326.227.228.329.630.932.434.136.038.140.543.446.750.855.762.170.582.2

90019.620.220.921.522.323.124.024.926.027.128.429.931.533.335.337.740.543.847.852.859.368.080.4

80017.618.218.719.420.120.821.622.523.524.625.827.228.730.432.434.737.340.644.549.556.065.078.3

70015.516.016.617.117.818.519.220.220.921.923.024.325.727.329.231.333.937.040.845.752.361.575.6

60013.413.814.314.815.316.016.617.418.219.120.121.222.524.025.727.730.133.036.741.447.957.372.2

t3, Furnace GasExitTemperature (0F)10001100120013001400150016001700180019002000210022002300240025002600270028002900300031003200

aThesefiguresare for evaluating a proposed change to preheated airnot for determining system capacity.Reproduced with permission from Combustion Handbook, Vol. I, North American Manufacturing Co.

Table 53.2 Fuel Savings (%) Resulting from Use of Preheated Air with Natural Gas and 10% Excess A

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REFERENCES1. Heat Requirements for Direct-Fired Air Heating, North American Mfg. Co., Cleveland, OH, 1981.2. SteamIts Generation and Use, Babcock and Wilcox, New York, 1978.3. R. J. Reed, Combustion Handbook, 3rd ed., Vol. 1,North American Manufacturing Co., Cleveland,OH, 1986.4. R. J. Reed, Combustion Handbook, 4th ed., Vol. 2, North American Manufacturing. Co., Cleveland,OH, 1997.

ExcessA i r (%)O10101010101010101010101010101010O

Preheated CombustionA i r Temperature (0F)

6060600700800900100011001200130014001500160017001800190020002000

W i t h 1000B t u /scfN a t u r a l Gas346833143542358136193656369237273761379438263857388739173945397340004051

W i t h 137,010B t u / g a lD i s t i l l a t eF u e l O i l353233743604364336813718375437893823385538873918394839784006403440604112

W i t h 153,120B t u / g a lR e s i d u a lF u e l O i l362734753690372737633798383138643896392739573986401440424069409541214171

Table 53.3 Effect of Combustion Air Preheat on Flame TemperatureAdiabatic Flame Temperatured (0F)