Environmental Engineering Reference
In-Depth Information
The macroscopic mass balance and the heat balance of the system can be
expressed by Equation 2.1:
˙
˙
˙
˙
mm m m
+=+
in
rec
exh
rec
(2.1)
QQQQQQQQQ
++ +
α
= + + + +
ev
rec
m
ev
rec
cold
f
loss
exh
where = mass flow rate, Q = heat flow rate, and α = ratio of the recirculated heat
to the total heat contained in the recirculated gas flow. The subscripts are in = input,
rec = recirculation, exh = exhaust, cold = initial state, f = fuel, ev = excess value,
m = heating materials, and loss = loss. Based on this equation, the characteristics
of the improved heating method with heat and combustion product recirculation,
particularly in the thermal field where radiation controls heat transfer, were studied
by changing major parameters affecting furnace temperatures and thermal efficien-
cies of furnaces.
m ˙
2.2.4.2
Heat Balance in the System
2.2.4.2.1 Gross Heat Input
The quantities of heat transferred from the hot flue gases to the fuel and air by a
counter type of heat exchanger can be given by Equation 2.2:
) (
)
(
Q
=
ξ
Cm
˙
T
-
T
ev
h
in
out
cold
(2.2)
(
)
(
)
(
)
(
)
˙
˙
ξ
=
LK
Cm
1
+
LK
Cm
h
in
in
where
T cold
= initial temperature of fuel and air
T out
= flue gas temperature at the furnace outlet
( C
m ˙
) in
= sum of heat capacity of fuel and air
ξ h
= heat exchange coefficient
L
= length of heat exchanger
K
= gross heat transfer coefficient of heat exchanger
The temperature at the exit of the heat exchanger, T in
1 , and the temperature after
the mixer, T in
2 , are given in Equations 2.3 and 2.4, respectively. The ratio of mass
recirculation to the inlet mass is defined in Equation 2.5.
Q
Cm
ev
TT
=
(2.3)
(
)
in
1
cold
˙
in
(
)
T
+
α
Q m
R
˙
in
1
rec
im
T
=
(2.4)
in
2
1
+
 
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