30079_53.pdf

CHAPTER 53

AIR HEATING

Richard J. Reed

North American Manufacturing Company

Cleveland, Ohio

53.1

AIR-HEATING PROCESSES

1641

53.3 WARNINGS

1643

53.2

COSTS

1643

53.4

BENEFITS

1644

53.1 AIR-HEATING PROCESSES

Air 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 wherein

the 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 or

combustion of waste gas with the air to be heated. They are most commonly used for ovens and

dryers. It may be impractical to use them for space heating or for preheating combustion air because

of lack of oxygen in the resulting mixture ("vitiated air"). In some cases, direct-fired air heating

may be limited by codes and/or by presence of harmful matter of undesirable odors from the heating

stream. Direct-fired air heaters have lower first cost and lower operating (fuel) cost than indirect air

heaters.

Heat requirements for direct-fired air heating. Table 53.1 lists the gross Btu of fuel input required

to 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 0 F, having 1000 gross Btu/ft 3 , 910 net Btu/ft 3 , and stoichiometric

air/gas ratio of 9.4:1. The oxygen for combustion is supplied by the air that is being heated. The

hot outlet "air" includes combustion products obtained from burning sufficient natural gas to raise

the air to the indicated outlet temperature.

Recovered waste heat from another nearby heating process can be used for process heating, space

heating, or for preheating combustion air (Ref. 4). If the waste stream is largely nitrogen, and if the

temperatures of both streams are between O and 80O 0 F, 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 or

W W T W + W f T f = W m T m = (W w + W f ) T 1n

where W = weight and T = temperature of waste gas, fresh air, and mixture (subscripts w, /, and

m).

Example 53.1

If a 60O 0 F waste gas stream flowing at 100 Ib/hr is available to mix with 1O 0 F fresh air and fuel,

how many pounds per hour of UO 0 F makeup air can be produced?

Solution:

(100 x 600) + lQW f - (100 + Wf) X (110)

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Table 53.1 Heat Requirements for Direct-Fired Air Heating, Gross Btu of Fuel Input per scf of Outlet "Air."

Outlet Air Temperature, 0 F

Inlet Air

Temperature, 0 F

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

-20

+20

100

200

300

400

500

600

700

800

900

1000

2.39

2.00

1.60

1.20

0.802

0.402

4.43

4.04

3.64

3.24

2.84

2.43

2.03

6.51

6.11

5.71

5.31

4.91

4.51

4.10

2.06

8.63

8.23

7.83

7.43

7.02

6.62

6.21

4.17

2.10

10.8

10.4

9.99

9.58

9.18

8.77

8.36

6.31

4.23

2.13

13.0

12.6

12.2

11.8

11.4

11.0

10.6

8.50

6.41

4.30

2.16

15.2

14.8

14.4

14.0

13.6

13.2

12.8

10.7

8.63

6.51

4.36

2.19

17.5

17.1

16.7

16.3

15.9

15.5

15.1

13.0

10.9

8.76

6.61

4.43

2.23

19.9

19.5

19.0

18.6

18.2

17.8

17.4

15.3

13.2

11.1

8.90

6.71

4.50

2.26

22.2

21.8

21.4

21.0

20.6

20.2

19.8

17.7

15.5

13.4

11.2

9.03

6.81

4.56

2.29

24.7

24.3

23.8

23.4

23.0

22.6

22.2

20.1

17.9

15.8

13.6

11.4

9.16

6.91

4.63

2.32

27.1

26.7

26.3

25.9

25.5

25.1

24.6

22.5

20.4

18.2

16.0

13.8

11.6

9.30

7.01

4.69

29.7

29.3

28.8

28.4

28.0

27.6

27.2

25.0

22.9

20.7

18.5

16.3

14.0

11.7

9.43

7.11

32.2

31.8

31.4

31.0

30.6

30.1

29.7

27.6

25.4

23.2

21.0

18.8

16.5

14.2

11.9

9.57

34.9

34.4

34.0

33.6

33.2

32.7

32.3

30.2

28.0

25.8

23.6

21.3

19.0

16.7

14.4

12.1

Example: Find the amount of natural gas required to heat 1000 scfm of air from 40O 0 F to 140O 0 F.

o, . T^

i 1.1

j^o

T^ i r • ™.

/23.2 gross Btu

1000 scf air

60 mm\

1000 gross Btu

1on o .,

Solution: From the table, read 23.2 gross Btu/scf air. Then

= 1392 cfh gas.

\ scf air min 1 hr / ft 3 gas

The conventional formula derived from the specific heat equation is: Q = we AT; so Btu/hr = weight/hr X specific heat X temp rise = — — X

X '

— X

mm hr

It*

0.24 Btu 0 .

, 1 o .

———— X °nse - scfm X 1.1 X °nse.

Ib F

The table above incorporates many refinements not considered in the conventional formulas: (a) % available heat which corrects for heat loss to dry flue gases and the heat

loss due to heat of vaporization in the water formed by combustion, (b) the specific heats of the products of combustion (N 2 , CO 2 , and H 2 O) are not the same as that of air,

and (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 gross Btu/hr: whereas the example finds 1392 X 1000 = 1 392 000

gross Btu/hr required. Reminder: The fuel being burned adds volume and weight to the stream being heated.

Solving, we find W f = 490 Ib/hr of fresh air can be heated to UO 0 F, but the 100 Ib/hr of waste gas

will be mixed with it; so the delivered stream, W m 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 preheat

temperature and the final waste gas temperature. The amount of heat recovered Q is then Q = W c p

(T 2 - T 1 ), where W is the weight of air heated, c p is the specific heat of air (0.24 when below 80O 0 F),

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 cases

involving 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 highly

desirable. For higher waste gas stream temperatures, however, the second law of thermodynamics

would say that we can make better use of the energy by stepping it down in smaller temperature

increments, 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 in

direct-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 of

using flue gases to preheat the load or to make steam in a waste heat boiler.)

53.2 COSTS

In addition to the cost of the heat exchanger for preheating the combustion air, there are many other

costs that have to be weighed. Retrofit or add-on recuperators or regenerators may have to be installed

overhead to keep the length of heat-losing duct and pipe to a minimum; therefore, extra foundations

and structural work may be needed. If the waste gas or air is hotter than about 80O 0 F, carbon steel

pipe and duct should be insulated on the inside. For small pipes or ducts where this would be

impractical, it is necessary to use an alloy with strength and oxidation resistance at the higher

temperature, and to insulate on the outside.

High-temperature air is much less dense; therefore, the flow passages of burners, valves, and pipe

must be greater for the same input rate and pressure drop. Burners, valves, and piping must be

constructed of better materials to withstand the hot air stream. The front face of the burner is exposed

to more intense radiation because of the higher flame temperature resulting from preheated combus-

tion air.

If the system is to be operated at a variety of firing rates, the output air temperature will vary;

so temperature-compensating fuel/air ratio controls are essential to avoid wasting fuel. Also, to

protect the investment in the heat exchanger, it is only logical that it be protected with high-limit

temperature controls.

53.3 WARNINGS

Changing temperatures from end to end of high-temperature heat exchangers and from time to time

during 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 force

something 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 new

era in recuperator reliability.

Corrosion, fouling, and condensation problems continue to limit the applications of heat-recovery

equipment of all kinds. Heat-transfer surfaces in air heaters are never as well cooled as those in water

heaters 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 dropped

out.

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, the

shortened equipment life seldom warrants it. Acid forms from combination of water vapor with SO 3 ,

SO 2 , or CO 2 in the flue gases.

SULFUR IN FUEL, PERCENT

BYWEIGHT(ASFIRED)

Fig. 53.1 Recommended minimum temperatures to avoid "acid rain" in heat exchangers.

53.4 BENEFITS

Despite all the costs and warnings listed above, combustion air preheating systems do pay. As fuel

costs rise, the payback is more rewarding, even for small installations. Figure 53.2 shows percent

available heat 3 (best possible efficiency) with various amounts of air preheat and a variety of furnace

exit (flue) temperatures. All curves for hot air are based on 10% excess air.* The percentage of fuel

saved 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. 4

Preheating 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 with

10% 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 heat

exchanger 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 conservation

measure, but it then allows use of smaller heat-exchange equipment.

t Although 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 may

result in smoke formation if some excess air is not available prior to the change. Heat exchangers

made of 300 series stainless steels may be damaged by alternate oxidation and reduction (particularly

in the presence of sulfur). For these reasons, it is wise to have an accurate air/fuel ratio controller

with very limited time-delay deviation from air/fuel ratio setpoint.

t 3 . Furnace gas exit temperature, F

Fig. 53.2 Available heat with preheated combustion air at 10% excess air. Applicable only if there is no unburned fuel in the products of

combustion. Corrected for dissociation. (Reproduced with permission from Combustion Handbook. 3 ) See also Figs. 44.3 and 44.4.

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