Note: Descriptions are shown in the official language in which they were submitted.
I I J 6803~ .
l Field of Invention
I
The present invention relates generally to the
production of concxete articles, and more particularly to a
method of recovering and utilizing heat energy used during the
curing of the concrete articles so as to reduce the total energy !
consumption required to cure other concrete articles.
. Background of Invention
In the production o~ concrete articles, concrete
. aggregates and mixing water are mixed togethex to form concrete
from which is manufactured concrete articles~ After manufacture,l
the concrete articles must be cured and as well known in the art, !
the term curing refers to the ~ime during which hydration or
hardening of the cement takes place through chemical reaction
between the cement and water and during which the newly formed
(green) concrete articles acquire their strength. Many types
. of curing techniques have been developed and at present, in the
concrete block industry, the most widespread technique is low
pressure steam curingO Xn the low pressure steam system, the
green concrete articles are loaded into a kiln after which a
sufficient quantity of steam is injected directly into the kiln
.to heat the internal kiln atmosphexe to the desired temperature
while, at the same time, maintaining a high degree of saturation
thereby providing the combination of heat and moisture which
accelerates the hy~ration and hardening of the.cement.
One drawback of the low pressure steam system is that
. considerable energy is required to generate the steam which, in
most low pressure steam systems, is maintained at a temperature
...,
I .1 6803 1
,-
on the order of 160-200F. It has been found that considerable
heat energy is consumed in heating up the kiln surfaces (walls,
roofs, floors, etc.), in heating up the steel pallets on which
the blocX is stacked, and in heating up the steel racks which, 1,
if used, support the pallets in the kiln. In addition, heat
energy is lost to the outside air due to conduction through the
kiln walls and roof. Also, heat energy is required to heat up
the volume of air within the kiln itself. In view of the
continually escalating cost of fuel, careful consideration is
being given to ways in which to conserve energy in not only the
curing phase but in the entire process of concrete block
production.
.
Summary of Invention
It is, therefore, a primary object of the present
invention to provide a method of production of concrete articles
which requires less energy consumption than the methods presently¦
being used.
It is another object of the invention to provide a
heat-reclaiming system for reclaiming heat energy used during the
curing of concrete articles and utilizing the reclaimed heat
energy in the production of other concrete articles thereby
lowèring the total energy consumption required to produce
concrete articles.
Another object of the invention is to provide a method
of producing concrete articles in which, during the cool-down
phase of curing, the high temperature kiln atmosphere is -
~recirculated through a heat exchanger to remove therefrom heat
energy which is utilized in the subsequent production of other
1 ~ 68~3~
concrete articles.
A still further object of the invention is to provide
a method of producing concrete articles utilizing two or more
kilns for curing and in whlch the high temperature kiln
atmosphere from one kiln is recirculated and intermixed with
the low temperature kiln atmosphere of another kiln thereby
reclaiming heat energy from the kiln being cooled down and
utiliæing the reclaimed heat energy in the kiln being heated up.
The above and other objects are achieved in accordance
with the invention by reclaiming a portion of the heat energy
from the kiln atmosphere during the curing of the concrete
articles, and then utilizing the reclaimed heat energy to pre
heat mixing water used to form other concrete articles or to
add to boiler Eeed water used to generate low pressure steam, or
both. In the case where two or more kilns are operated
simultaneously at staggered curing cycles, the high temperature
kiln atmosphere from the kiln undergoing cool down is intermixed
with the low temperature kiln atmosphere from the kiln undergoing
heat up thereby reclaiming heat energy from one kiln and using it
in ~he other kiln thereby reducing the total energy consumption
required for curing.
Having in mind the foregoing and other objects,
features and advantages of the invention which will be evident
from an understanding of this disclosure, the present invention
comprises the metho,d of producing concrete articles as
illustrated in the presently preferred embodiments whi~h are
hereinafter set forth in sufficient detail to enable those
persons skilled in the art to clearly understand the function,
~ J ~31
operation and advantages of the invention.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a schematic side elevational view, partly
in section, of a kiln equipped with the heat-reclaiming
system of the invention;
Fig. 2A, 3A, 4A and 5A are schematic top plan views,
partly in section, of the heat-reclaiming system used in con-
junction with plural kilns and Figs. 2B, 2C, 3B, 3C, 4B, 4C,
5B and 5C are time-temperature curves of different conditions
within the ~lural kilns; and
Fig. 6 is an explanatory time-temperature curve use-
ful in explaining the energy savings achieved by the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Two embodiments of the invention will be described
with reference to the application drawings and for ease of
description, the invention will be described with reference
to systems having one or two kilns. The principle of the
invention can, of course, be carried out with any number of
kilns as will be apparent from the following description.
The invention will also be described with reference to curing
concrete blocks and it is understood that the invention is
not limited to block curing and is applicable to curing all
types of concrete articles. In the United States, the majo
rity of concrete block curing is done by low pressure steam
so that the invention will be described with respect to this
curing technique.
t 1 S~03 1
As shown in Fig. 1, a kiln X of conventional
construction has a wall portion 10 covered by a roof portion 12
and is dimensio~ed to receive vertical stacks of pallets
containing green blocks to be cured. The front of the kiln is
provided with a kiln door l4 which, in the embodiment shown,
comprises a roll-up canvas door. For sake of clarity, the kiln
X has been shown primarily in outline form as the particular
kiln construction is immaterial to the present invention.
In order to supply low pressure steam to the kiln,
a steam header 20 is connected at one end to a boiler ~not
shown). One or more steam lines 21 branch ofE of the steam
header 20 and extend along the length of the kiln. The steam
lines 21 have a series of spaced apart nozzles for injecting
steam into the kiln to heat the internal kiln atmosphere to the
desired temperature and to provide the necessary moisture to
accelerate hydration and hardening of the cement. The steam is
preferably injected into the kiln by bubbling it through a trough
of water thereby enabling the steam to pick up additional
molsture content from the water to help maintain the internal
kiln atmosphere at a relative humidity of between 90~-100%. The
bubbling of the steam through water also avoids ~reation of a
superheated steam condition which is disadvantageous in that
superheated steam could cause a moisture or hydration reduction
state within the green blocks. In order to regulate the
injection of steam-into the kiln, a valve 22 is inserted in the
steam llne 21.
In accordance with the invention, a heat-reclaiming
system is provided for reclaiming some of the heat energy
--6--
r
I l 68031
expended during curing of the block and the reclaimed heat
energy is then us~d in the ne~t curing cycle, either in the saSme
~ ~J~ h Ot~g
kiln or in ~ ~ kiln, thereby lowering the total
consumption of energy needed for curing the block. The heat-
reclaiming system comprises an exhaust duct 30 connected to a
series of branch exhaust ducts 31 which extend through the kiln
roof and open into the kiln interior. The exhaust duct 30 is
connected to a header 33 which is connected to the inlet side of
a recirculating fan 35. The outlet side of the fan is connected
by ductwork to both a fan outlet header 36 and a heat exchanger
37. The header 36 is connected through ducts 38 and 39 to the
interior of the kiln to enable recycling of the kiln atmosphere
through the loop forrned by the branch exhaust ducts 31, the
exhaust duct 30, the recycling fan 35, the header 36 and the
ductwork 38 and 39. The heat exchanger 37 is of conventional
type and includes tubes 40 for circulating water from a water
storage tank (not shown) in heat-exchange relationship with the
kiln atmosphere exhausted by the fan 35. The inlet side of the
heat exchanger 37 is connected through ductwork to the outlet
side of the fan 35 and the outlet side of the heat exchanger is
connected through a heat exchanger header 42 to a duct 43 which
communicates with the duct 39. By such a construction, the
heated and moisture-laden kiln atmosphere which is exhausted by
the recirculating fan 35 can pass throùgh the heat e~changer 37
wherein heat energy is given up to the water flowing through the !
tubing 40 and the cooled kiln atmosphere then flows through the
header 42 and duct 43 into the duct 39 fQr reentry into the kiln.
In addition, some of the moisture conta~ned in the kiln
atmosphere condenses on the tubing 40 and is collected and fed
to the boiler feed water tan~ as hot water for use as boiler
water make-up. 7
.. .
.,.~ , , !
~'''''. . '.
3 6~031
A set of dampers are provided within the ductwork of
the heat-reclaiming system for controlling the manner of flow of
the kiln atmosphere during operation of the kiln. A damper 45
is disposed in the exhaust duct 30 and a similar damper 47 is
mounted in the duct 39, and both dampers are adjustably movable
between op~n and closed positions to regulate flow through their
respective ducts. Another damper 48 is pivotably mounted at the
intersection of the duct 43 with the duct 39 for regulating the
flow of the exhausted kiln atmosphere through the heat-reclaiming
system and back into the kiln thxough the duct 39. The damper 48
is movable between one position wherein the damper closes the
duct ~3 thereby communicating the ducts 38 and 39 and another
position wherein the damper closes the duct 38 thereby
communicating the ducts 39 and 43; Though not shown in the
drawings, the set of dampers can be mechanically interconnected
for common operation.
A description will now be given of the mode of ~
operation o~ the heat-reclaiming system with respect to one
curing cycle carried out in the kiln X. Initially, the kiln is
empty, and green concrete blocks to be cured are charged into the
; kiln. In the event cured blocks were previously unloaded from
the kiln, the kiln interior surfaces may be at a temperature in
the range o~ 120-140F and when empty, the kiln would normally
have a low humidity condition on the order of 50%-60% relative
humidity. After loading of the green blocks into the kiln,
which would take from 45 minutes to two hours, the kiln door 14
is closed thereby sealing the kiln interior. - -
As well known in the art, different types of cement
1 1 68031
require different pre-set periods before high temperature can be
applied to accelerate the hydration or curing of the cement.
During the pre-set period, high humidity low pressure steam is
injected lnto the kiln by opening the steam valve 22 and
permitting steam to flow through the steam line 21 from which
it l S bubbled through a water trough into the kiln. During this
period, the dampers 45 and 47 are closed and the recirculating
~an 35 is off. Throughout the pre-set period, the humidity
condition throughout the kiln is maintained in the range of
90~-100% relative humidity. After the pre-set period, the
steam input is controlled to raise the kiln at~osphere to an
accelerated temperature of about 160-200F at a rate of about
30F/hour and again, the steam is bubbled through water to
assure that the kiln atmosphere maintains 90%-100% relative
humidity during this portion of the curin~ cycle. After the
final accelerated curing temperature is reached, the supply of
steam is terminated to allow the kiln to soak for a period of
some six-ten hours.
Following the soaking period, the recirculating fan
35 is turned on, the dampers 45 and 47 are opened, and the
damper 48 is positioned to close the duct 38 thereby permitting
withdrawal of the kiln atmosphere through the exhaust ducts 30,
31,and the withdrawn kiln atmosphere passes through the heat
exchanger 37 where it undergoes a reduction in temperature and
humidity level and then the cooler and dryer kiln atmosphere is
re-introduced back into the kiln through the ducts 43 and 39.
The heat and water which are removed from the kiln atmosphere
are stored in the watex for use as boiler feed water for the next
.'
. _9_
(~ ~
t 3 ~3~31
curing cycle and as mix~gwater for themanufacture of the next
batch of concrete blocks. During this heat-reclaiming period,
the total kiln temperature (block, pallets, kiln surfaces, etc.j
is reduced to a level of 120-140F and the kiln atmosphere is
reduced to a low humidity whereby the kiln door 14 can be opened
and the kiln unloaded by conventional manual means or automated
fork lift truck. The reclaimed heat energy is used in the next
curing cycle resulting in a lower total consumption of energy
for curing concrete block. -
Thus the total curing process allows for controlledpre-setting of the concrete block before accelerating the
temperature to the ~inal curing temperature. The curing process
utilizes a final curing temperature on the order of 160-200F
which is advantageous from the standpolnt of achieving early
hig~ compressive strengths as well as allowing for the inclusion
of fly ash (Pozzolan) as part of the cementacious material
resulting in a lower per unit material cost. Further, the
process reclaims much of the energy required for an accelerated
ultimate high temperature level and supplies this reclaimed hea~t
energy to the subsequent concrete block to be cured as either hot
mixing water or to the boiler feed water supply to reduce the
energy needed to ge~erate the steam. Thus the heat-reclaiming
system achieves energy conservation since the subsequent concrete
blocks are preheated by the reclaimed heat energy extracted by
the heat exchanger~37. In this embodiment, the extracted heat
energy is in the nature of heated water which can be used as
hot mixing water for the next batch of concrete block or as
boiler feed water, or both, thereby reducing the amount of heat
--10--
` i . l'
I l B8031
energy needed for the next curing cycle. Alternatively, the
extracted heat energy can be used to furnish heat to the
manufacturing building.
The heat-reclaiming system of the invention can also
be used with two or more kilns to obtain even greater energy
conservation. Figs. 2-S show the system being used in connection
with two kilns X and Y and in each figure, the corresponding
temperature-time curve is shown alongside its respective kiln.
The representations in Figs, 2A, 3A, 4A and 5A are diagrammatic
and are in the nature of top plan views as opposed to Fig. 1
which is in the nature of a side elevational view.
With reference to Fig. 2, and assuming the initial
conditions that kiln X is in hour 12 of its curing cycle and
kiln Y is just beginning its curing cycle (hour 0), a
description will be given as to the manner of operating the
he~t-reclaiming system using t~o ~ilns. I~ kiln X, the kiln
atmosphere is maintained at 190F and a relative humidity between
90~-100% by modulating on and off the steam valve 22 and with thei
dampers 45 and 47 closed and the damper 48 in the position shown.
In ~iln Y, the green b~ock has just been charged into the kiln
which has a kiln atmosphere of about 70F and a low relative
humidity on the order of 50%-60%. The dampers 45a and 47a are
closed and the damper 48a is in the position shown. After the
block has been loaded in kiln Y, the relative humidity of the ki~
atmosphere is quickly raised to 90%-100% by injecting a controlled
amount of steam through the water trough. Figs. 2B and 2C show
j/ - ~esp ~c~y
~ ~ ~ the temperature conditions within the ~cp~ccti ~ kilns at this
point in time.
Fig. 3 shows ~he conditions two hours later when the
soaking period in kiln X has ~een completed and when the pre-set
period of the block in kiln Y has been completed. At this time, ¦
the heat-reclaiming system is activated. The recycling fan 35
is turned on in conjunction with the opening of the dampers 45
,. 1~ . ~.
1 3 ~S~031
and 47 leading to kiln X and the opening of the dampers 45a and
47a leading to kiln Y. The fan withdraws the high temperature
and high humidity atmosphere from kiln X and withdraws the low
temperature and lo~ humidity atmosphere from kiln Y and the two
kiln atmospheres are intermixed in the fan outlet header 36 and
recycled back to both kilns. In this fashion, some of the heat
energy from the atmosphere in kiln X is transferred to the
atmosphere in kiln Y.
The heat-reclaiming system continues this mode of
operation for several hours during which time the kiln X undergoec
a cooling down as its kiln atmosphere is mixed with the cooler
kiln atmosphere from kiln Y. Kiln Y, on the other hand, undergoes
a heating up as its relatively cooler atmosphere is mixed with
the hotter atmosphere from kiln X. This condition is shown in
Fig. 4 which shows in Figs. 4B and 4C the respective temperatures
in kilns X and Y at a time two and one-half hours later than that~
shown in Fig. 3. This mode of operation is continued until the
two kilns substantially reach temperature equilibrium, or as
close thereto as possible, after which no further heat
conservation can be obtained in this mode of operation.
As shown in Fig. 5, when the kiln atmosphere in both
kilns X and Y reaches approximately 130F,the heat-reclaiming
system undergoes a second mode of operation. In this second
mode, the dampers 45 and 47 remain open; however, the damper 48
is moved into the p~sition shown so as to close the duct 38
thereby communicating the duct 43 with the duct 39 so that the
atmosphere exhausted from the kiln X passes through the heat
exchanger 37 and is then re-introduced into kiln X. In this
I 1 ~031
manner, the atmosphere exhausted from kiln X will be cooled and
dried in the heat exchanger 37 before being returned to kiln X 1`
thereby lowering the temperature and humidity within kiln X
before the kiln door is open to unload the kiln. At the same
time, the dampers 45a and 47a are closed while the damper 48a
remains in the position shown,and the steam valve 22a is opened
to permit low pressure steam to bubble through the water trough ir._-
the kiln Y so that the kiln reaches its final accelerated
curing temperature of 190F. Thus, when kiln Y has gained all
of the possible heat recovery available from klln X, additional
heat energy is supplied by the low pressure steam. In the
meantime, the heated water obtained from the heat exchanger 37
is conserved and used as pre-heated mixing~ater for subsequent
concrete block to be manufactured or as boiler feed water for
subsequent steam generation.
The heat energy saving achieved by the heat-reclaiming
system can be seen from the time-temperature curve of Fig 6.
During operation of kilns X and Y, the heat energy added in zone
l is reclaimed in zone 2 and added to the adjacent kiln ,during
its heat-up in zone 3. It is understood that more than two
kilns can be operated in this manner and the showing of two
kilns was done for ease of description.
While the invention has been described with reference
to two preferred embodiments, it is understood that many
modifications thereof and changes thereto will become apparent
to those ordinarily skilled in the art and the present invention
is intended to cover all such obvious modifications ~nd changes
which fall within the spirit and scope of the invention as
defined in the appended claims.
. -13-
. ~ I
