Page 375 - Mechanical Engineers' Handbook (Volume 4)

364 Air Heating

Solution:
(100 600) 10W (100 W ) (110)
ƒ
ƒ
Solving, we find W 490 lb/hr of fresh air can be heated to 110 F, but the 100 lb/hr of
ƒ
waste gas will be mixed with it; so the delivered stream, W will be 100 490 590
m
lb/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 refractory 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 Wc (T T ), where W is the weight of air heated, c is
p 2 1 p
the specific heat of air (0.24 when below 800 F), T is the delivered hot air temperature, and
2
T is the cold air temperature entering the heat exchanger. Tables and graphs later in this
1
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
temperature 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 gen-
erated 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, chemical 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.)

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 800 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 combustion 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.