SUPPRESSION OF THE STEAM PLUME
FROM INCINERATOR STACKS
F. W. ROHR
P & W Engineers, Inc.
Chicago, Illinois
nation of the atmosphere.The plume reveals the presence
of an incinerator which may be capable of inoffensive
operation in an urban area, but its continuously identi­
fiable presence also can become a standard by which
property value is graded.
Air pollution control equipment which does not utilize
water in contact with the combustion gases is of course .
available. The electrostatic precipitator preceded by a
spray chamber for the purpose of cooling the gases to a
temperature acceptable to the construction of the precipi­
tator may also be used.In this case, however, the water
injected into the combustion gases for cooling purposes
may contribute to the generation of a steam plume. The
electrostatic precipitator, combined with the waste heat
boiler plant for the cooling of combustion gases, is a more
desirable system from the steam plume standpoint, as
water does not come into contact with the combustion
gases and a plume is not formed from this cause. This
equipment operates most efficiently, however, in plants
designed for 24-hour-per-day operation and a "sink" for
steam generation is necessary.
The cyclone collector may be used in place of the
electrostatic precipitator with some sacrifice in efficiency,
but a similar requirement remains for the use of the waste
heat.The precipitator and waste heat boiler combination
appears to be the most practical air pollution control sys­
tem for incinerator plants where steam can be utilized on
a continuous basis. A large number of smaller-sized in­
cinerator plants will, however, continue to be located in
areas having no provision to utilize steam and will not be
INTRODUCTION
The rapid increase in volume of refuse generated by our
expanding population has resulted in a corresponding de­
crease in available landfill space for disposal of refuse near
many urban areas.As a means of conservation of this de­
creasing landfill space, various methods of volume reduc­
tion of refuse have been used.The method in growing use
now that will possibly remain so in the foreseeable future
is incineration.Incineration, however, has not always been
received with favor, as some plants in the past have not
been designed or operated with sufficient regard to air pol­
lution control.
It has become axiomatic that as a populated area in­
creases in size the municipal incinerator becomes more
closely integrated with the center for reasons of availa­
bility of space and hauling distance. This close association
raises the necessity of increasing the level of plant clean­
liness and to limit the amount of stack particulate emis­
SlOns.
Air pollution control equipment for municipal inciner­
ators as well as industrial and commercial incinerators fre­
quently utilizes scrubber systems in which water for cool­
ing purposes comes in contact with the combustion gases.
The use of the water scrubber system results in the gener­
ation of a super saturated water vapor which leaves the
stack in the form of a visible white plume.Although the
resulting steam plume may be objectionable only from the
aesthetic standpoint, public concern about air pollution
has made any form of visible emission suspect to contami216
A. Construction costs include induced draft fans,
heat exchangers and scrubbers. In each case, the
cost of the furnace, secondary chambers and
stack is intended to remain constant and has
been omitted from the estimate.
B. Operating costs are computed for electric power
at $0.02/KWHand water at $0.30/1000gallons.
C. A maintenance allowance is included for minor
repair and replacement of small components.
D. Amortization is based on 4%percent for a
20-year life.
The above costs are based on 1 2 hours per day opera­
tion, 5 days per week and 5 2 weeks per year. A compari­
son of the owning and operating costs for the various
methods is included in Tables 1 and 2.
designed for a 24-hour-per-day operation. Under these
circumstances the high-pressure-drop type of water
scrubber appears to be a practical air pollution control
device and can be designed to operate within the effi­
ciency requirements of even the most strict code limita­
tions. If the formation of the steam plume from this type
of equipment is objectionable, various methods for sup­
pression by thermodynamic means can be used which are
described in this paper.
COST PARAMETERS
In order to evaluate the cost of various methods for
the suppression of the steam plume, a standard basis for
comparison has been assumed. This standard is a scrubber
system utilizing the components shown in Fig. 1 following
a refractory-lined continuous-feed municipal incinerator
of 250-tons-per-day capacity. The scrubber system in this
.
TABLE I
case is desgined to remove particulate matter from the
combustion gases to the level at which an optically clean
appearance, or approximately 0.10 grains per SC FMcon­
verted to 50 percent excess air, would be expected except
for the steam plume.
For comparison purposes, all other air pollution con­
trol methods described for the purpose of suppression of
the steam plume would have comparable dust emission
control. The cost of these systems is compared to the cost
of the standard scrubber systems in Tables 1 and 2. These
costs are expressed per ton of refuse burned as the owning
and operating charges imposed by the equipment require­
ment to suppress the steam plume and include the followmg.
COST COMPARISON OF SYSTEMS FOR
SUPPRESSION OF STEAM PLUME AT 20 F
Estimated
System
Construction
Estimated Owning
and Operating Cost
Cost, S
Basic System (Fig. 1)
•
per ton of refuse
burned, Siton
79,000
0.91
System No. 1 (Fig. 6)
162,000
1.15
System No. 2 (Fig. 8)
117,000
1.09
System No. 3 (Fig. 10)
139,000
0.87*
*Operating cost does not include cooling water. If cooling water
is purchased at $0.30/1,000 gallons, the operating cost per ton
of refuse burned for water alone would be $4.50.
TABLE II
COST COMPARISON OF SYSTEMS FOR
COOl INC
TOWER
SCRlAIBER
/I. D.
fAN
SUPPRESSION OF STEAM PLUME AT 40 F
STACK
./
System
Estimated
Construction
Cost, S
INCINERATOR
Estimated Owning
and Operating Cost
per ton of refuse
burned, Siton
Basic System
79,000
0.91
System No. 1
212,000
1.32
System No. 2
108,000
0.78
System No. 3
139,000
0.80*
*Operating cost does not include cooling water. If cooling water is
purchased at $0.30/1,000 gallons, the operating cost per ton of
FIG. 1 BASIC SCRUBBER SYSTEM FOR 250 TON PER DAY
refuse burned for water alone would be S3.10.
MUNICIPAL INCINERA TOR
217
FORMATION OF THE STEAM PLUME
background of the sky.The formation of the droplets
occurs approximately at the dew point temperature since
it is known that actual formation is influenced by dust
nuclei.The actual analysis of the conditions under which
water vapor becomes visible as droplets of water of ice is
outside the scope of this paper.
Design conditions can be assumed, however, regarding
the formation of visible water droplets which produce
fogged air.These design conditions include the following:
1) The state point of the mixture of combustion gases
and ambient air, formed when the stack gases are dis­
charged into the atmosphere, can be represented on a
psychrometric chart. 2) If the state point of the mixture
of combustion gases and ambient air exists in the fogged
field of the psychrometric chart, a visible steam plume
will be formed.The results of psychrometric computations
can be expressed graphically by means of a chart whose
geometry is shown in Fig.2.
When an air-vapor mixture is unsaturated, the tempera­
ture of the superheated water vapor is the same as the
temperature of the gas with which it is mixed.The actual
temperature of the mixture is therefore higher than the
dew point and may be shown by any state point to the
right of the saturation curve.If the air-vapor mixture is
cooled, the temperature at which the steam becomes
saturated and moisture begins to condense is called the
dew point temperature.The dew point temperature is the
saturation temperature corresponding to the pressure of
the water vapor mixed with the air.The saturation curve
is the location of the state points at which the saturation
temperature or dew point of the water vapor is equal to
the dry bulb temperature of the air-vapor mixture.
Introduction
The suppression of the steam plume to an invisible air
vapor mixture, or clear stack condition, implies that the
white visible portion of the plume is composed of water
droplets and invisible gases which can be treated by
thermodynamic methods.The elimination of either white
or black smoke and particulate matter which otherwise
detract from a clear stack condition are combustion and
collection problems which must be solved separately.
All atmospheric air contains water vapor.However, the
water in the air is visible only under certain conditions,
producing what is commonly termed "fogged air." Fogged
air by definition is a mixture of air, saturated steam, and
liquid in the form of fine water droplets held in suspension
in the air.Fog forms when air is cooled below its dew
point temperature. The formation of the water vapor
occurs around nuclei which may be dust particles in the
air.The conditions under which the water droplets pro­
ducing fogged air become visible, however, are complex.
The degree of opacity of the fogged air or steam plume,
is dependent upon variables which include the number or
concentration of the water vapor droplets and their size,
the depth of field or plume thickness, and the lighting
SATURATION CUltVE AT
BAROMETRIC PRESSUltE
COMBUSTION GAS
LEAVING SCRUBBER
'"
...
FOGGED
FJm)
CLEAR FIELD
...
-,
!!!
«
�
o
..
AMBIENT AIR
ENTERING FURNACE
�..
- --
- - -
-
For purposes of design, therefore, state points of mix­
tures on the chart to the right of the saturation curve are
located in the clear field. State points of mixtures to the
left of the saturation are located in the fogged field and
will contain visible droplets of condensed water.The
degree to which these droplets will be visible can only be
determined by field conditions.By using the saturation
curve on the psychrometric chart as the design condition
at which the air-vapor mixture becomes visible, methods
can be utilized to process the combustion gases so that the
state point of the resulting mixture with ambient air will
remain in the clear field.
Determination of the Moisture Content of Combustion Gas
DRY BUlB TEMPERA TUItE, of.
In order to evaluate the possible methods for suppres­
sion of the steam plume, the design and operation condi­
tions of a hypothetical municipal incinerator are assumed.
FIG. 2 PSYCHROMETRIC CHART SHOWING STATE POINTS
OF THE AIR VAPOR MIXTURE IN THE INCINERATOR
PROCESS.
21 8
100 F or more may take place under constant moisture
content conditions. For the purpose of this discussion,
however, it is assumed that no heat loss takes place be­
tween the furnace chamber combustion gas outlet and
the entrance to the spray chamber or water scrubber.
For a spray chamber or scrubber in which water is
sprayed into the air at the same temperature as the wet­
bulb temperature of the air and the make-up water to re­
place the loss due to evaporation is supplied at the same
temperature the path of the mixing process will follow a
constant wet-bulb line (adiabatic saturation line) as shown
between state points 2 and 3 in Fig. 2.
The final temperature of the gases leaving the water­
type air pollution control device will normally depend
upon the effectiveness of the mixing process between the
water and gases. The usual spray chamber will reduce the
combustion gases to approximately 600-800 F while a
high-pressure-drop venturi or flooded-disk scrubber may
cool the gases to a condition approaching saturation as at
state point 3.
For the purpose of this study, the gases leaving the
water scrubber will be assumed to be saturated. In an
actual process, the air would not become completely
saturated although the temperature difference may not
be detectable.
These conditions included a continuous-fee 250-ton per­
day refractory-lined unit burning municipal refuse with an
average calorific value of 4500 Btu/lb. Computations
based on methods used in [1] and [ 2] were carried out
for different conditions of excess combustion air. The
furnace gas temperature variation in terms of excess air
for the given refuse fuel input is shown plotted in Fig. 3
from which a desired operating temperature condition can
be selected. In this case, a temperature of 1 6 25 F requiring
1 20percent excess air was chosen as being within the
range of good practice.
The results of the computations for the design oper­
ating conditions are shown plotted on the psychrometric
chart in Fig. 2. As noted on this chart, air for combustion
and 1 20percent excess air for cooling the combustion
gases enter the furnace at the assumed ambient conditions
of 80 F and 60percent relative humidity as indicated by
state point No. 1.
During the incineration of the refuse water is added to
the gases from the following sources: (1) the combustion
process, ( 2) from the moisture in the refuse,and (3) from
vapor released by the quenching of residue in the ash
conveyor trough.
State point No. 2 indicates the increased temperature
and moisture content of the combustion gases leaving the
furnace chamber where the residue is burned. After leaving
the furnace chamber at the maximum temperature of
1 6 25 F, the gases may pass through a combustion or set­
tling chamber where a cooling effect of approximately
Conditions Under Which the Steam Plume is Formed
A steam plume is formed at the incinerator stack when
the combustion gases, including a quantity of water vapor,
are mixed with cooler ambient air. The diffusion of the
combustion gas with ambient air can be represented
graphically on the psychrometric chart. For this analysis,
the combustion gases are assumed to have properties
similar to air. The presence of particulate matter and
some change in the percentage of the constituents of the
products of combustion are not considered to have a
marked effect on its specific heat. The presence of 1 20
percent excess air also tends to dilute the products of
combustion and their possible effects on the properties of
2200
•
�
o
2000
•
...
�
�
� 1800
DESIGN CONDITION
>-
I 62SOF
...
.
120%
EXCESS AIR
...
>­
•
au .
...
A schematic diagram of a portion of a psychrometric
chart indicating the conditions under which a steam plume
may be formed is shown in Fig. 4. In this diagram,the am­
bient air entering the furnace is shown at point 1. The
state point of the combustion gases leaving the furnace
chamber are shown at point 2 and the condition of the
gases leaving the scrubber near the saturation point are
shown at point 3. The locus of points representing all mix­
tures of air and combustion gas is a straight line between
points 3 and 1 when the gases leave the stack.
':t
z
'"
OJ
� 1600
1400
so
100
ISO
200
EXCESS AIR. PER CENT
FIG. 3 FURNACE TEMPERATURE VARIATION WITH
EXCESS AIR
219
The steam plume forms when these points representing
the mixtures pass through the super-saturated or fogged
field section of the psychrometric chart.The maximum
moisture content which the gases leaving the stack can
contain, without resulting in a mixture with the ambient
air in the fogged field of the chart, lies on a line tangent
to the saturation curve shown passing through point 4 in
Fig.4.The maximum allowable moisture content of the
stack gases that will not produce a steam plume is a func­
tion of the ambient temperature and relative humidity
and the temperature of the discharged gases.
Fig. 5 is a psychrometric chart on which are plotted
the maximum moisture concentrations which the stack
gas and ambient air can have for different ambient air
temperatures without producing moisture conditions
which lie in the fogged field.The state points of the
combustion gas conditions plotted on Fig. 5, as deter­
mined for Fig. 2, indicate the relationship between the
condition of the gases leaving the furnace and stack, and
the moisture content at various ambient temperatures
which may produce a steam plume.The location of point
2, which is the state of the gases leaving the furnace
chamber, indicates that the steam plume may be formed
at an ambient air temperature slightly above zero degrees
if no additional moisture is added to the combustion
gases.If the gases then pass through the scrubber, indi­
cated by the path between points 2 and 3, sufficient
moisture is added which will result in a steam plume even
if the ambient air temperature is in excess of 80 F.
Suppression of the steam plume formed, when air pol­
lution control equipment is used which adds moisture to
the products of combustion, can be accomplished by
dehydrating the gases to some point below the maximum
allowable moisture concentration for a particular ambient
air temperature to those shown in Fig. 5.
CLEAR FIELD
---
'"
i:;
FOGGED FIELD
'"
0
<
i:;
0
�
(3)
0
0
MAXIMUM MOISTURE CONTEN
�
'"
�
WHICH GAS LEAVING STACK
\
�
CAN CONTAIN WITHOUT
�RODUCING
A STEAM PLUME
�
!d
�
I
�.
--
-
-
-
�
-..:{,
(21
(I)
DRY BULB TEMPERATURE, of.
FIG. 4 SCHEMA TIC DIAGRAM SHOWING CONDITIONS
UNDER WHICH A STEAM PLUME MA Y BE FORMED
LINES OF CONSTANT AMBIENT
TEMPERATURE TANGENT TO TH
SATURA TlON CURVE'
SCRUBBER EXHAUST GAS
'"
<
i:;
o
o
METHODS FOR THE SUPPRESSION O F THE
STEAM PLUME
Ta ::
O"F.
--
- -- ---�-GAS LEAVING
AIR ENTERING FURNACE
80
FIG.
FURNACE CHAMBER
DRY BULB TEMPERATURE, OF.
1625
5 STATE POINTS OF COMBUSTION GAS AND ALLOW·
ABLE MAXIMUM MOISTURE CONCENTRATlONS AT
VARIOUS AMBIENT AIR TEMPERATURES
220
C
e
I
�.
Various methods are possible for the reduction of the
moisture content of gases, which are:
A. Electrostatic precipitation of water droplets
from fogged air.
B. Mechanical separation of water droplets from
fogged air.
C. Absorption or adsorption of water vapor.
D. Mixing of the moist gases with relatively dry
heated air.
E. Condensation of the moisture by direct contact
with water or on cold surfaces.
F. Reheat of scrubber exhaust gases.
�
Methods A and B require that ambient air be mixed
with the scrubber exhaust gases to condense moisture
prior to entering a precipitator or centrifuge where water
droplets would be removed. Since it would not normally
be economically feasible to use a water scrubber and elec­
trostatic precipitator in the same system, method A will
AMBIENT
not be considered. Methods Band C, although also techni­
cally feasible, will not be considered since this discussion
is intended to include thermodynamic methods only.
Method D.
with heated relatively dry air.
•
/ '\
./
STACr.
HEAT
EXCHANGER
r·
/I. o.'I
SCR�BER
FAN
'-.. ./
I
INCINERATOI
FIG. 6 (SYSTEM No. 1) FLOW DIAGRAM FOR PROCESS TD
SUPPRESS THE STEAM PLUME BY TRANSFER OF
HEAT FROM COMBUSTION GAS AND MIXING WITH
HEATED AMBIENT AIR
�
o
"
..
�
•
�
OMBUSTION GAS LEAVING
HloAT EXCHANGER
Cl
-
i
�
o
�
J::
U
GAS
CO MB UST ION GAS
ENTERING HEAT EXC
�
-'
�
Z
GAS
GAS
LEAVIN
FURNACE
�
---- �....-
... - ­
-
,.. -
-
---
-
­
.-
(System No.1)
Fig. 6 shows the flow diagram by which low moisture
content ambient air may be reheated and mixed with the
saturated scrubber exhaust. The state points of the proc­
ess are shown on the psychrometric chart in Fig. 7. In this
process, combustion gases leaving the furnace at point 2
are first cooled by mixing with ambient air at 20 F. This
tempering process is necessary in order to limit the tem­
perature of the combustion gases entering the heat ex­
changer to prevent damage to the heat exchanger metal.
Heat is then removed from the combustion gases be­
tween points 3 and 4 in Fig. 7. This heat is transferred to
the ambient air which is raised in temperature between
points 1 and 1, when passing through the heat exchanger.
The combustion gases are next decreased in temperature
along a constant wet-bulb line between points 4 and 5 in
the scrubber leaving at a saturated condition at point 5.
The maximum design moisture content of the mixture of
combustion gases and heated ambient air leaving the sys­
tem is dependent upon the ambient air temperature at
which the steam plume is to be suppressed. Since the
moisture content of the ambient air is fixed for a given
temperature condition, the wet-bulb temperature of the
exhaust gases leaving the scrubber determines the moisture
content of the exhaust gases and also the design tempera­
ture at which the steam plume will be suppressed.
For a given wet-bulb temperature of the exhaust gases
leaving the scrubber, various mixture ratios of heated
ambient air and exhaust gases will result in a locus of state
points which fall on the maximum allowable moisture
concentration line for a given ambient air temperature.
Mixtures of heated ambient air and scrubber exhaust gases
at 130 F will coincide with the maximum moisture con­
centration line for 20 F. In the example in Fig. 7, a ratio
of 2.75 to 1 ambient air to combustion gas by weight was
used which will result in a heated ambient air temperature
of 4 25 F and a mixture temperature at the induced draft
fan of 330 F. A ratio of 1 to 1 may also be used, however,
the ambient air leaving the heat exchanger would be ap­
proximately 1100 F and the mixture state point at 600 F.
.1
'-.
Cooling of the moist gases followed by mixing
-
TED AM BIE NT AIR
20
DRY BULB TEMPERATURE. of.
Method E.
Condensation of Moisture by Direct Contact
With Water
FIG. 7 (SYSTEM No. 1) PSYCHROMETRIC CHART FOR
In the practical case of a high-pressure-drop scrubber
in which an excess of cold water is used for cooling pur-
PROCESS TO SUPPRESS THE STEAM PLUME BY
TRANSFER OF HEAT FROM COMBUSTION GAS
AND MIXING WITH HEATED AMBIENT AIR
2 21
with heated ambient air or (2) reheating the scrubber ex­
haust gas without mixing with heated ambient air.
(1) Mixing With Heated Ambient Air (System No.2).
Fig.8 shows the flow diagram by which the moist furnace
gases are dehydrated by cooling in the scrubber by direct
contact with water, followed by mixing with heated am­
bient air for suppression of the steam plume.The state
points of the combustion gases in the incineration process
and through a two-stage scrubber are shown in Fig. 9. The
combustion gases are cooled in the first scrubber stage
from points 2 to 3 along a constant wet-bulb temperature
line of 174 F.Cooling of the combustion gases takes place
in the second stage of the scrubber from point 3 to 110 F
wet-bulb at point 4. The heat gained by the cooling water
in the second stage of the scrubber is used in this case to
heat relatively dry ambient air at 20 F from points 1 to 1,
for mixing with the scrubber exhaust gases. For the case
shown in Fig.9, the ambient air must be heated to a mini­
mum of 80 F and a ratio of 5 parts of this air must be
mixed with one part of scrubber exhaust gas in order to
achieve the desired results.
The system shown in Fig.8 utilizes a separate fan for
the supply of ambient air to be heated and mixed with the
combustion gases. The use of this separate fan permits
flexibility in the ratio of the heated ambient air to com­
bustion gas.The mixture ratio of scrubber exhaust gas to
ambient is determined by the design condition or temper­
ature at which the steam plume is to be suppressed. For
example, less heated ambient air would be required for a
higher ambient air design temperature for suppression of
the steam plume.
(2) Reheat of Scrubber Exhaust Gases, It is apparent
from Fig.5 that if the saturated scrubber exhaust gases at
a wet-bulb ,temperature of 174 F are reheated, the state
points of the process will follow a horizontal line at con­
stant specific humidity which will cross the ambient air
design temperature line at some elevated dry-bulb tem­
perature.For example, if the scrubber exhaust gases at
state point 3 in Fig.5 are reheated, it will require a dry­
bulb temperature of approximately 1500 F to suppress the
formation of a steam plume at plus 40 F ambient air.Sub­
cooling of the scrubber exhaust gases to a lower wet-bulb
temperature will greatly reduce the reheat temperature
necessary to suppress the steam plume at moderately low
ambient air conditions. Fig.10 shows the flow diagram by
which the moist furnace gases are dehydrated in the scrub­
ber by direct contact with cooling water followed by re­
poses, the wet-bulb temperature of the scrubber exhaust
gases can be reduced by direct contact with the water.
The second stage of the process to suppress the steam
plume may consist of either (1) mixing the scrubber gases
HEAT EXCHANGER
HEAT TRANSFERRED TO
AMIIENT AIR FI:OM
SECOND STAGE
SCRuelER WATER
COOLING
TOWER
TWO STAGE
SCRUtlOIR
INCINERATOR
FIG. 8 (SYSTEM No. 2) FLOW DIAGRAM FOR PROCESS TO
SUPPRESS THE STEAM PLUME BY DEHYDRATION
OF COMBUSTION GAS IN SCRUBBER FOLLOWED
BY MIXING WITH HEATED AMBIENT AIR
"
'-
......
<
10
"" GAS
'lOof,
LEAVING
FURNACE
"
(2)
-- -
1625
20
DRY BULB TEMPERATURE, of,
FIG. 9 (SYSTEM No. 2) PSYCHROMETRIC CHART FOR
heating to suppress the steam plume.The state points of
the combustion gases through the process are shown in
Fig. 11.The gases leaving the furnace at point 2 are cooled
PROCESS TO SUPPRESS THE STEAM PLUME BY
DEHYDRATlON OF COMBUSTION GAS IN
SCRUBBER FOLLOWED BY MIXING WITH
HEATED AMBIENT AIR
222
in a conditioning spray chamber from point 2 to 3 in order
to reduce and permit control of the temperature of the gas
entering the heat exchanger. The gases are then scrubbed
and cooled in the two stages of the scrubber through points
4, 5 and 6. The gases leaving the scrubber at point 6 are
reheated by the heat exchanger through points 6 and 7 by
the heat transferred from the gases during cooling from
points 3 and 4. For the design ambient air condition of
20 F shown the combustion gases must be reheated to a
temperature of 575 F in order to suppress the steam
plume.
STACK
CONCLUSION
OOLING
TOWER
This paper discussed various methods by which the air
pollution control system of a municipal incinerator can be
designed to suppress the steam plume due to moisture in
combustion gases.
For a comparison of the technical feasibility of the
various methods, a design ambient air temperature of 20 F
was chosen for the flow diagrams and charts.This temper­
ature is a more severe test of system design than may be
warranted in certain locations having moderate climates.
The relative costs for systems designed for 40 F ambient
are therefore included in Table 2. The costs for Systems
No.1 and 2 are reduced by design for the higher ambient
temperature because of the reduction in heat exchanger
cost and power cost for mixing air.The costs for System
No.3 remain relatively constant because flow quantities
remain relatively unchanged. Less cooling water would be
required however as noted in Table 2.
Although the construction costs for any of the systems
to suppress the steam plume are higher than a basic scrub­
ber system, the operating costs are not proportionally
higher.The operating costs for the basic scrubber include
the cost of water which is lost to evaporation.Systems
No.1 and 2 do not require significant amounts of water
although they do have somewhat larger power costs. As
noted in Tables 1 and 2, System No.3 requires large
amounts of cooling water for the conditions shown. The
cost of this water would be prohibitive unless it can be
obtained or reprocessed in a cooling tower at low cost.
A cost estimate has indicated that System No.2 can be
operated at less cost than the basic scrubber system with­
out means for vapor suppression because of reduced water
costs. Both Systems No.1 and 2 may also be provided
with means to reduce the amount of mixing air during
temperature conditions which are warmer than the design
ambient air temperature. This reduction of mixing air
quantity would result in reduced operating costs during
warm weather. System No.3 does not have this degree of
flexibility since the flow through the system would remain
relatively constant for all ambient air temperatures unless
TWO STAGE
SCRUBBER
HEAT
EXCHANGER
INC IN ERATOR
FIG. 10
(SYSTEM No.3) FLOW DIAGRAM FOR PROCESS
TO SUPPRESS THE STEAM PLUME BY DEHY·
ORA TlON OF COMBUSTION GAS IN SCRUBBER
FOLLOWED BY REHEATING
[ -I;
GAS LEAVING HEAT
EXCHANGER
(5)
'"
"
3)
.)
GAS LEAVING S CON
STAGE OF SCRUBBER AND
ENTERING HEAT EXCHANGER
�
GAS LEAVING
SPRAY CHAMBER
�
"-
\2\
-- -- -
20
FIG. 11
1625
DRY BULB TEMPERATURE. of.
PSYCHROMETRIC CHART FOR PROCESS TO
SUPPRESS THE STEAM PLUME BY DEHYDRA TlON
OF COMBUSTION GAS IN SCRUBBER FOLLOWED
BY REHEATING
223
ACKNOWLEDGMENT
the heat exchanger is bypassed through a separate breech­
ing.
This investigation of the methods of steam plume sup­
pression and an evaluation of systems utilizing these
methods indicates that the cost of scrubber systems with
the means for suppression is related to the lowest design
ambient air temperatures at which the system is to be
effective and to the method of dehydration of the flue
gases.This preliminary analysis has indicated that the
costs of these systems for use at temperatures above 20 F
may be comparable to wet scrubber systems in which no
provision is made for suppression.
The author wishes to recognize with thanks the infor­
mation concerning scrubbing equipment contributed
during the preparation of this paper by Mr.E.B.Henby
of The National Dust Collector Corporation.
REFERENCES
[1]
[2]
Steam, The Babcock & Wilcox Co. 37th Edition.
Kaiser, Elmer R., "Combustion and Heat Calculations for
Incinerators," Proceedings of 1964 National Incinerator Confer­
ence.
224