1. Estimating the Available Amount of Waste Heat
From the Asphalt Dispenser Machine of a
Dry Cell Manufacturing Plant
Harland C. Machacon, M. Eng.
Department of Mechanical-Industrial Engineering
College of Engineering, University of San Carlos,
Cebu City
The Asphalt Dispenser Machine melts and dispenses asphalt into Le-
clanche type dry cells thus producing a leak proof seal between the carbon electrode
and the zinc can.
Asphalt chunks are melted to a temperature of 180° C using electric strip heaters.
The molten asphalt is then dispensed into a batch of dry cells indexed beneath the
dispensing tank. This batch of cells is then led into a torching conveyor where three
large LPG burners spread out the dispensed asphalt evenly to form a meniscus at
the electrode and at the can lip. These burners also ensure that the asphalt seal is free of
air bubbles that might eventually form into pin-sized holes. This piece of equipment
has a capacity of over half a million Leclanche dry cells for each operating day.
The hot gases resulting from the combustion process of LPG in these large
burners are collected in an exhaust hood-and-chimney set-up. The flue gas is then
exhausted into the environment via a direct-drive exhaust blower. Huge amounts of
energy are dissipated into the environment. This is indeed a major economic loss aside
from being a source of thermal pollution.
Within the last four years the cost of fuel has more than doubled. Prices of
petroleum products are always affected by price increases in the world market and also
by the devaluation of our local currency. In the manufacture of dry cells, energy cost
contribute approximately 5 percent of the total manufacturing cost. Hence the battery
company has launched vigorous cost-reduction efforts. This study is about estimating
the recoverable heat from the flue gas that would otherwise be considered lost energy
and a cause of thermal pollution.
2. Figure 1 The Asphalt Dispenser Machine
Source of Waste Heat
The major source of waste heat from the Asphalt Dispenser Machine is the hot
gases resulting from the combustion process of Liquefied Petroleum Gas in the large
post-torch burners. The flue gases are then collected in an exhaust hood-and-chimney set-
up and eventually exhausted into the environment via a direct-drive exhaust blower.
The chemical formula of the fuel used in the burners, Liquefied Petroleum Gas is,
C 3 H 8 , since LPG is composed primarily of propane[ 1 ]. The chemical reaction is
described as follows;
C 3 H 8 + 5(O2 + 3.76 N 2 ) → 3CO2 + 4 H 2 O + 5(3.76 N 2 ) (1)
The products of combustion are carbon dioxide, water vapor, and nitrogen[ 2 ].
3. CHIMNEY
EXHAUST HOOD
FLUE
GAS
LPG BURNER 1 LPG BURNER 2 LPG BURNER 3
TORCHING CONVEYOR
Figure 2 Schematic Illustration Showing the Heat Source of the
Asphalt Dispenser Machine
Physical Properties of the Flue Gas
The specific heat of the flue gas, in kJ/kg-°K is the summation of the products of
the respective mass fractions and the corresponding specific heat values of each
individual component of the flue gas[ 3 ].
n
c pg = ∑ yi c pi (2)
i
Hence,
c pg = yc pCO2 + yc p H 2O + yc p N 2
Table 1 Determining the Specific Heat of the Flue Gas
Products Number Mole Molecular Mass cpi
of of Fraction Weight Fraction
Combustion Moles xi Mi M i xi yi BTU/lb°R yicpi
CO2 3 0.1163 44 5.1172 0.1808 0.187055 0.033820
H2O 4 0.1550 18 2.7900 0.0985 0.491999 0.048462
N2 18.8 0.7287 28 20.4036 0.7207 0.249080 0.179512
TOTAL 25.8 1.0000 28.3108 1.0000 0.261794
4. BTU kJ
kJ
c pg = 0.261794 × 4.187 kg ° K = 1.096131
lb° R BTU
lb ° R kg ° K
The density of the flue gas is the summation of the densities of each individual
component of the flue gas[ 4 ].
n
ρ g = ∑ ρi (3)
i
The density values of the individual components of the flue gas at atmospheric
pressure, can be taken from Table A-6, pages 647 – 648, Heat Transfer by J. P. Holman,
8th edition. The flue gas temperature of 120°C ( 393°K ) will be the reference
temperature. Hence,
ρg = ρ for CO2 + ρ for water vapor + ρ for N 2 (4)
kg kg kg kg
ρ ≡1.3695
g 3
+0.5654 3 +0.8740 3 =2.8089 3
m m m m
The flue gas pressure at the burners is atmospheric because the burners exhaust
directly to the atmosphere. The static pressure at the exit of the chimney right before the
exhaust blower was supposed to be measured by attaching a pressure gage to a
piezometer opening in the side of the conduit normal to and flush with the conduit
surface.
Similarly, the velocity of the flue gas at the above-mentioned point would have
been measured by attaching a pressure gage to a pitot tube at the same level where the
piezometer opening is located[ 5 ]. The difference of the readings of the two pressure
gages represent the velocity head of the flue gas at such point. Hence the velocity of the
flue gas could have been determined by computation. The steady flow energy equation
applied at the stagnation point “0” could have been used to measure the velocity of the
flue gas, vg . Simplifying the equation, we have the following result;
2
( v g − 0 ) sec
m
lbs in 2 1kg (3.28 ft ) 2 1
( p 0 − p g ) 2 × 144 2 × × × =
in ft 2.2lbs m 2
kg m
2.7504 3 2 9.8 2
m sec
Given the values of p0 and p g from the pressure gauge readings, the velocity of
the flue gas, vg can be solved.
The partial pressure of the water vapor, pH 2 O is the product of the mole fraction
of the water vapor multiplied by the pressure of the flue gas at the exit of the chimney.
5. Table 2 Determining the Mole Fraction of the Products of Combustion
PRODUCTS OF NUMBER OF MOLE FRACTION
COMBUSTION MOLES xi
CO2 3 0.1163
H 2O 4 0.1550
N2 18.8 0.7287
TOTAL 25.8 1.0000
Therefore,
pH 2 O = xH 2 O p g = ( 0.1550) p g
The corresponding dew point temperature is then read out from the Steam Tables
( Properties of Saturated Water : Pressure ).
pg PIEZOMETER
TUBE
p0
( p0 - pg )
0
PITOT TUBE
FLUE GAS
CHIMNEY MANOMETER
Figure 3 Installation of Pitot Tube and Piezometer Tube at the Chimney
6. Available Amount of Sensible Heat from the Flue Gas
The temperature of the flue gas can be lowered from its initial temperature of
120°C down to point “a” which is 10°C above the dew point temperature of the water
vapor. The safe level of 10°C above the dew point temperature of the water vapor
ensures that the water vapor in the flue gas will not start to condense.
The sensible heat from the flue gas was calculated using the equation
Qs = mg c pg (Tgi − Tgf ) (5)
but
mg = ρgAcvg (6)
and
Tgf = 10 C + Tdp (7)
and then substituting equations ( 6 ) and ( 7 ) in equation ( 5 ), the resulting equation for
the sensible heat is;
[ (
Qs = ρ g Ac vg c pg Tgi − 10 C + Tdp )] (8)
T p=C
pH 2 O
SAFE LEVEL
a
120°C
p=C 10°C
DEW POINT
S
Figure 4 T-s Diagram of the Process of Lowering the Temperature of the Flue Gas
From 120°C to Point “ a ”
7. Results
Both the piezometer and the pitot tube methods for determining the velocity and
pressure values at the chimney exit was not realized. This is one difficulty encountered
by the undersigned in this study. The management of Energizer Philippines did not allow
the undersigned to set-up a piezometer opening at the side of the stack. Instead, they
requested that the static pressure of the flue gas be estimated based on the distance
between the chimney exit and the elevation where the burners are located. This distance
is measured as 6 meters. From the energy equation,
p1 v12 p 2 v22
z1 + + = z2 + +
γ 2g γ 2g
Since v1 = v 2 , and p1 = 101,325 N / m 2 ( flue gas pressure at the burners is
atmospheric), the energy equation can be simplified into;
p1 − p2 p − p2
= 1 = z 2 − z1 = 6m
γ ρg
Substituting values, we have;
101,325 N / m 2 − p 2
= 6m
(2.8089kg / m 3 )(9.8m / sec 2 )
Where p 2 = 101,159.8 N / m 2
14.696 psi
( )
p 2 = p g = 101,159.8 N / m 2
101,325 N / m 2 = 14.672 psi
And ∆p = p 2 − p1 = 14.672 psi − 14.696 psi = −0.024 psi
1" Hg
∆p = ( − 0.024 psi )
0.4898 psi = −0.049" Hg
Neither did management allow the setting up of a pitot tube to measure the flue
gas velocity. Instead, their engineers provided the specifications of the exhaust blower.
Direct Drive Exhaust Blower, 1 Horsepower
1” static 2” static 3” static 4” static 5” static
pressure pressure pressure pressure pressure
800 cfm 740 cfm 680 cfm 600 cfm 500 cfm
The delivery volume of 800 cfm was chosen because 1 inch static pressure is closest to
the computed static pressure of 0.049 inch. Figure 5 below shows the actual set-up of the
post-torcher chimney on the right and the melting tank chimney on the left.
8. 800 cfm
TO EXHAUST BLOWER
7.5”
400 cfm
MELTING TANK
CHIMNEY POST TORCHER
400 cfm CHIMNEY
7.5”
POST TORCHER
Fig 5 Diagram Showing Post Torcher Chimney and Exhaust Blower
1m 3
(
400 ft / min
3
)
35.31 ft 3 (1 min/ 60 sec )
vg = = 6.624m / sec
π ( ( 7.5inch )(1 ft / 12inches )(1m / 3.28 ft ) )
2
4
The partial pressure of the water vapor, pH 2 O is the product of the mole fraction
of the water vapor multiplied by the pressure of the flue gas at the exit of the chimney.
pH 2 O = xH 2 O p g = ( 0.1550) p g
Substituting the value of p g , obtained from 6.4, we have,
p = (0.1550)(14.672 psi ) = 2.274 psi
The corresponding dew point temperature was then read out from the Steam
Tables ( Properties of Saturated Water : Pressure ).
Tdp = 54.588°C
And the exit temperature of the flue gas “ Tgf ” was computed by adding a safety
margin of 10ºC as;
Tgf = 54.588°C + 10°C = 64.588°C say 65°C
The sensible heat from the flue gas was calculated using the equation as follows
KJ
Q g = m g 1.096131 (120°C − 65°C )
kg °C
9. but
kg π (0.1905m) 2
m g = 2.8089 3 (6.624m / sec) = 0.5303kg / sec
m 4
KJ
Therefore Qs = ( 0.5303kg / sec ) 1.096131 (120°C − 65°C )
kg °C
kJ
Q = 31.97
sec
Conclusion
It was found out in this study that for an 8-hour per day operation the available
amount of waste heat is 920.736 MJ. Considering only half of this energy will be
recovered for plant process requirements, and basing on the cost of electricity at 6.253
pesos per kilowatt-hour, an energy cost savings of 800 pesos per day can be realized.
A heat exchanger with water in the inside can be designed, fabricated , and then
mounted into the space inside the hood located directly above the post torcher burners to
absorb the waste heat. The hot water coming from this heat exchanger can then be used
for internal process requirements.
REFERENCES
The World Book Encyclopedia, Volume 2, World Book International Inc., 1995,
p. 676
Borman G. L. and Ragland K. W., Combustion Engineering, International
Edition, Mc Graw-Hill, 1998, p. 67
Alkhamis T. M., Alhusein M. A., and Kablan M. M., (1996) “Utilization of
Waste Heat From the Kitchen Furnace of an Enclosed Campus” Energy
Conservation and Management Journal, Vol. 39, No. 10, 1998, p 1115
Borman G. L. and Ragland K. W., Combustion Engineering, International
Edition, Mc Graw-Hill, 1998, p. 64
Daugherty R. L. and Franzini J. B., Fluid Mechanics With Engineering
Applications, 7th Edition, McGraw Hill, 1977, pp. 374 – 377