U.S. patent application number 15/793075 was filed with the patent office on 2018-06-21 for product drying apparatus and methods.
This patent application is currently assigned to Wenger Manufacturing, Inc.. The applicant listed for this patent is Wenger Manufacturing, Inc.. Invention is credited to Douglas S. Clark, Keith A. Erdley, Adam S. Hinton, Adrian L. Strahm, Scott E. VanDalsem.
Application Number | 20180168202 15/793075 |
Document ID | / |
Family ID | 60674453 |
Filed Date | 2018-06-21 |
United States Patent
Application |
20180168202 |
Kind Code |
A1 |
Strahm; Adrian L. ; et
al. |
June 21, 2018 |
PRODUCT DRYING APPARATUS AND METHODS
Abstract
The present invention provides improved apparatus and methods
for the monitoring and control of apparatus designed to remove
moisture from an initially wet product, such as a continuous dryer
(14). The net rate of water removal from the wet product (16) is
determined during drying thereof, preferably on a real-time basis.
A control assembly (20) is operatively coupled with the dryer (14)
and includes sensors (24, 26, 28, 34), which are operatively
coupled with a digital controller (38). The controller (38) has a
PID controller operable to continuously determine the average net
rate of water removal from the product (16).
Inventors: |
Strahm; Adrian L.; (Sabetha,
KS) ; Erdley; Keith A.; (Hiawatha, KS) ;
VanDalsem; Scott E.; (Fairview, KS) ; Hinton; Adam
S.; (Hiawatha, KS) ; Clark; Douglas S.;
(Sabetha, KS) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wenger Manufacturing, Inc. |
Sabetha |
KS |
US |
|
|
Assignee: |
Wenger Manufacturing, Inc.
Sabetha
KS
|
Family ID: |
60674453 |
Appl. No.: |
15/793075 |
Filed: |
October 25, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15437658 |
Feb 21, 2017 |
9848628 |
|
|
15793075 |
|
|
|
|
62437124 |
Dec 21, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F26B 21/12 20130101;
F26B 17/04 20130101; F26B 25/22 20130101; F26B 23/10 20130101; F26B
21/02 20130101; F26B 21/08 20130101; A23L 3/40 20130101; A23V
2002/00 20130101 |
International
Class: |
A23L 3/40 20060101
A23L003/40; F26B 15/14 20060101 F26B015/14; F26B 21/12 20060101
F26B021/12 |
Claims
1. Apparatus operable to remove water from product passing through
the apparatus by contacting the product with input air to create
dried product and moisture-laden exit air, said apparatus including
a fan to exhaust said exit air from the apparatus, and structure
operable to determine the volumetric flow rate of said exit air
from said fan, and to use said volumetric flow rate to determine
the net rate of water removal from said product during operation of
said apparatus.
2. The apparatus of claim 1, said structure operable to determine
at least one operational parameter of said fan, and using said fan
parameter to calculate said volumetric flow rate.
3. The apparatus of claim 2, said fan parameter selected from the
group consisting of fan motor power, fan rotational speed, and fan
pressure, and combinations thereof.
4. The apparatus of claim 3, said structure operable to determine
said volumetric flow rate using two operational parameters of said
fan selected from the group consisting of: (1) fan motor power and
fan rotational speed; (2) fan pressure and fan rotational speed; or
(3) fan motor power and fan pressure.
5. The apparatus of claim 4, said structure operable to determine:
(a) the wet bulb temperature and dry bulb temperature of said exit
air; (b) the rotational speed of said exhaust fan; (c) the motor
power of said exhaust fan; (d) the wet bulb temperature and dry
bulb temperature of said input air; and operable to calculate said
net rate of water removal from said product in real time as the
product passes through said apparatus, using the determined values
(a)-(d), inclusive.
6. The apparatus of claim 2, said structure operable to calculate
said volumetric flow rate using said fan parameter in a Fan Law
equation.
7. The apparatus of claim 1, said structure operable to alter the
operation of said apparatus in response to said determined net rate
of water removal.
8. The apparatus of claim 7, said structure operable to adjust at
least one control parameter of said apparatus which will alter said
rate of water removal from said product, in response to said
determined net rate of water removal.
9. The apparatus of claim 8, said apparatus being a dryer using
heated input air to dry said product, said control parameter
selected from the group consisting of the temperature of said
heated input air, the speed of travel of the product passing
through the dryer, the contact time between said product and said
heated input air, and combinations thereof.
10. The apparatus of claim 7, said structure operable to determine
a set point rate SP which is the desired net rate of water removal
from the product during passage thereof through the apparatus, a
process variable PV which is the actual net rate of water exiting
the product during passage through the apparatus, and to control
the operation of the apparatus using said SP and said PV, to cause
said PV to approach said SP.
11. The apparatus of claim 7, said structure operable to
successively and periodically determine said SP and said PV, and to
control the operation of the apparatus using said successive SP and
PV determinations.
12. The apparatus of claim 11, said structure including a PLC with
a PID controller to carry out said successive determinations of
said SP and said PV.
13. The apparatus of claim 12, said PLC controller operable, in
each of said successive determinations, to calculate said SP, said
PV, the difference between said SP and said PV, and a control
variable CV.
14. The apparatus of claim 10, said structure operable to determine
said SP as the initial water rate (IWR) minus the final water rate
(FWR), where IWR is the rate of water delivered to the apparatus as
a part of said wet product input, and FWR is the desired rate of
water exiting from the apparatus as a part of said dried
product.
15. The apparatus of claim 10, said structure operable to determine
said PV as the rate of water leaving said apparatus as a part of
the output of said exhaust fan minus the rate of water entering
said apparatus as a part of said input air.
16. The apparatus of claim 1, said structure operable to determine
the net rate of water removal from said product in real time as the
product passes through the apparatus.
17. The apparatus of claim 16, said structure operable to determine
the average net rate of water removal from said product.
18. Apparatus operable to remove water from product passing through
the apparatus by contacting the product with input air to create
dried product and moisture-laden exit air, said apparatus including
a fan to exhaust said exit air from the apparatus, and structure
operable to calculate the net rate of water removal from said
product in real time during operation of said apparatus.
19. The apparatus of claim 18, said structure operable to determine
at least one operational parameter of said exhaust fan, and to use
said at least one determined parameter in the calculation of said
net rate of water removal from said product during operation of
said apparatus.
20. The apparatus of claim 19, said structure operable to determine
two operational parameters of said fan selected from the group
consisting of: (1) fan motor power and fan rotational speed; (2)
fan pressure and fan rotational speed; or (3) fan motor power and
fan pressure.
21. A method of monitoring the operation of apparatus operable to
remove water from product passing through the apparatus by
contacting the product with input air to create dried product and
moisture-laden exit air, said method comprising the steps of
employing a fan to exhaust said exit air from the apparatus,
determining the volumetric flow rate from said fan, and using said
volumetric flow rate to determine the net rate of water removal
from said product during operation of said apparatus.
22. The method of claim 21, determining at least one fan parameter,
and using said fan parameter to calculate said volumetric flow
rate.
23. The method of claim 22, said fan parameter selected from the
group consisting of fan motor power, fan rotational speed, and fan
pressure, and combinations thereof.
24. The method of claim 23, including the step of determining two
operational parameters of said fan selected from the group
consisting of: (1) fan motor power and fan rotational speed; (2)
fan pressure and fan rotational speed; or (3) fan motor power and
fan pressure.
25. The method of claim 24, including the steps of: (a) determining
the wet bulb temperature and dry bulb temperature of said exit air;
(b) determining the rotational speed of said exhaust fan; (c)
determining the motor power of said exhaust fan; (d) determining
the wet bulb temperature and dry bulb temperature of said input
air; and (e) determining said net rate of water removal from said
wet product in real time as the wet product passes through said
drying chamber, using the determined values (a)-(d), inclusive.
26. The method of claim 22, including the step of calculating said
volumetric flow rate using said fan parameter in a Fan Law
equation.
27. The method of claim 21, including the step of controlling the
operation of said apparatus in response to said determined net rate
of water removal.
28. The method of claim 27, including the step of adjusting at
least one control parameter of said apparatus which will alter said
rate of water removal from said product, in response to said
determined net rate of water removal.
29. The method of claim 28, said apparatus being a dryer using
heated input air to dry said product, said control parameter
selected from the group consisting of the temperature of said
heated input air, the speed of travel of the product passing
through the dryer, the contact time between said product and said
heated input air, and combinations thereof.
30. The method of claim 27, including the steps of determining a
set point rate SP which is the desired net rate of water removal
from the product during passage thereof through the apparatus,
determining a process variable PV which is the actual net rate of
water exiting the product during passage through the apparatus, and
controlling the operation of the apparatus using said SP and said
PV, to cause said PV to approach said SP.
31. The method of claim 27, including the steps of successively and
periodically determining said SP and said PV, and controlling the
operation of the apparatus using said successive SP and PV
determinations.
32. The method of claim 31, including the step of using a PLC
controller with a PID controller to carry out said successive
determinations of said SP and said PV.
33. The method of claim 32, said PLC controller operable, in each
of said successive determinations, to calculate said SP, said PV,
the difference between said SP and said PV, and a control variable
CV.
34. The method of claim 30, including the step of determining said
SP as the initial water rate (IWR) minus the final water rate
(FWR), where IWR is the rate of water delivered to the apparatus as
a part of said wet product input, and FWR is the desired rate of
water exiting from the apparatus as a part of said dried
product.
35. The method of claim 30, including the step of determining said
PV as the rate of water leaving said apparatus as a part of the
output of said exhaust fan minus the rate of water entering said
apparatus as a part of said input air.
36. The method of claim 21, including the step of determining the
net rate of water removal from said product in real time as the
product passes through the apparatus.
37. The method of claim 36, said net rate of water removal being an
average net rate of water removal.
38. A method of monitoring the operation of apparatus operable to
remove water from product passing through the apparatus by
contacting the product with input air to create dried product and
moisture-laden exit air, said method comprising the steps of
employing a fan to exhaust said exit air from the apparatus, and
calculating the net rate of water removal from said product in real
time during operation of said apparatus.
39. The method of claim 38, including the step of determining at
least one operational parameter of said exhaust fan, and using said
at least one determined parameter in the calculation of said net
rate of water removal from said product during operation of said
apparatus.
40. The method of claim 39, said parameter-determining step
including the step of determining two operational parameters of
said fan selected from the group consisting of: (1) fan motor power
and fan rotational speed; (2) fan pressure and fan rotational
speed; or (3) fan motor power and fan pressure.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of allowed identically
titled application Ser. No. 15/437,658 filed Feb. 21, 2017, which
claims the benefit of U.S. Provisional Patent Application Ser. No.
62/437,124 filed Dec. 21, 2016, entitled METHOD OF CONTROLLING
PRODUCT DRYING APPARATUS TO PROVIDE THE NET RATE OF WATER REMOVAL
FROM A PRODUCT IN REAL TIME. Both of these applications are
incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention is broadly concerned with methods and
apparatus for the operational monitoring and control of apparatus
capable of removing water from products passing therethrough, such
as dryers or coolers of the type commonly used in food or feed
manufacture. More particularly, the invention provides methods and
apparatus permitting determination of the net rate of water removal
from products during operation of said apparatus, preferably on a
real time basis.
Description of the Prior Art
[0003] During the production of certain comestible products, such
as animal feeds or human foods, an initially dry formula typically
containing protein, starch, and fats is first processed using an
extruder or other cooking device to create a continuous stream of
cooked product. The output from the extruder is normally too wet
for packaging or storage (e.g., from about 20-40% by weight
moisture), and thus must be dried. A dryer is positioned to receive
the continuous stream of cooked, wet product, and to dry the
product to a desired moisture level, such as 8-12% by weight
moisture.
[0004] A variety of dryers have been used in the past in these
contexts, such as single or multiple pass horizontal dryers, or
vertical dryers. Horizontal dryers of this type include a dryer
housing with one or more internal conveyors leading from a wet
product inlet to a dried product outlet. Similarly, vertical dryers
have a series of stacked decks where product is initially processed
in the uppermost deck and is then passed in serial order to the
lower decks, leaving to a dried product outlet. In either case,
ambient air is drawn into the dryer body and heated, either
directly or indirectly, and is then circulated for contact with the
product within the dryer body. In many instances, a cooler section
is used with product dryers, in order to cool the product for
downstream handling or packaging; such coolers do not utilize
heated air, but merely circulate air through the dried product to
lower the temperature thereof.
[0005] A longstanding problem with such equipment is that it has
been necessary to periodically take samples of the dryer output and
physically measure the moisture content thereof. Only after such
testing could the operation of the equipment be modified in an
effort to produce acceptably dried products. Thus, in the context
of dryers, during initial start-up of the dryers, or in the event
of dryer upset, 20 minutes or more may elapse before an initial
moisture reading can be taken and analyzed in a laboratory. Only
then can the dryer operation be modified, which then entails a
further wait until another sample can be taken and measured for
moisture content. As a consequence, a considerable amount of waste
product is generated until it is determined that the dryer is
operating as required to produce dry product within specifications.
Thus, the conventional practice of repeated sampling and testing is
inefficient in terms of time and costly in terms of waste product,
which has little value or utility.
[0006] There is therefore a need in the art for improved dryers and
other apparatus for water removal which can be controlled in such a
way as to minimize or eliminate periodic sampling and laboratory
moisture testing.
SUMMARY OF THE INVENTION
[0007] The present invention overcomes the problems outlined above,
and provides methods for monitoring and controlling the operation
of apparatus serving to remove water from a product via contact
between the product and input air entering the apparatus, where
such apparatus is also provided with an exhaust fan for moving
moisture-laden exit air from the apparatus. The methods generally
involve the determination of the net rate of water removal from the
product during operation of the apparatus, preferably in real
time.
[0008] In one aspect of the invention, such water removal
determinations involve the initial determination of the volumetric
flow rate of the air from the exhaust fan (usually measured as CFM
(f.sup.3/min) or m.sup.3/min), and using this flow rate to
determine the net rate of water removal from the product during
operation of the apparatus. To this end, determinations are also
made of the net rates of water entering the apparatus from both the
initially wet product to be dried and the input air, and the rate
of water removed from the apparatus as a part of the exit air.
[0009] A number of different techniques may be employed to
determine the volumetric flow rate of the exit air from the fan.
Most commonly these methods attempt to find air velocity and
thereby volumetric flow rate. For example, measurements of air
velocity (directly or indirectly), air velocity pressure and/or
static air pressure induced by the fan may be made. These methods
make use of pitot tubes, venturi, orifice plates, vortex shedders,
hot wire anemometer, or vane anemometer placed within the fan
ducting. However, the accuracy of such measurements may be
compromised by the fact that the velocity of the exit air is not
uniform throughout the cross-section of a duct, i.e., friction
slows the air moving close to the duct walls, so that the velocity
is greater in the center of the duct. In light of these
considerations, the practice of the present invention preferably
makes use of indirect methods whereby one or more operational
parameters of the exhaust fan are sensed or otherwise determined,
and these parameter(s) may then be used to calculate volumetric
flow rate, such as through the well-known Fan Law equations. Such
fan parameters include the rotational speed (rpm) of the fan, the
motor power of the fan (i.e., the energy consumed by the fan motor
to rotate the fan), fan pressure (either static, velocity, or
total), or combinations thereof.
[0010] As noted, the methods of the invention determine and make
available the net rate (usually the average net rate) of water
removal from wet product passing through the apparatus, and
preferably this net rate is determined in real time. As used
herein, "real time" refers to the fact that the net rate of water
removal is determined and available (e.g., through a visual
display) during the time that the initially wet product is passing
through the apparatus. Thus, as an amount of initially wet product
is introduced into the apparatus and passes therethrough, the net
rate of water removal from the amount of initially wet product is
determined during the time of such passage.
[0011] Apparatus in accordance with the invention include all types
of equipment designed to remove water from an initially wet product
by contacting the product with input air to create dried product
and moisture-laden exit air. For example, the apparatus may be in
the form of a horizontal or vertical hot-air convection dryers,
coolers, or any other suitable moisture removal equipment.
[0012] In preferred practice, the methods of the invention also
include the step of adjusting at least one control parameter of the
apparatus which will alter the rate of water removal from the
product, in response to the determined net rate of water removal.
In one implementation of the invention, apparatus control involves
determining a set point rate SP which is the desired net rate of
water removal from the product during passage thereof through the
apparatus, determining a process variable PV which is the actual
net rate of water exiting the product during passage through the
apparatus, and determining a control variable CV. CV is an
apparatus parameter serving to alter the rate of water removal from
the product, such as the temperature of heated input air in the
case of a dryer, the speed of product passing through the
apparatus, the contact time between the product and the input air,
and combinations thereof. Apparatus control is achieved by changing
the CV as necessary to cause the PV to approach the SP, and
ultimately to substantially equal the SP (e.g., within plus or
minus 3%, preferably plus or minus 1%, of the SP). Typically, the
SP, PV, and CV are successively and periodically determined, and
the apparatus operation is controlled using such successive
determinations.
[0013] Successive SP, PV, and CV determinations are usually carried
out using a PID (proportional-integral-derivative) controller. For
example, the PID controller may be operated so that, in each
control loop, SP and PV are calculated, along with the difference
between SP and PV; the control variable CV is then determined for
driving the PV toward the SP. SP may be determined as the initial
water rate (IWR) minus the final water rate (FWR), where IWR is the
rate of water delivered to the apparatus as a part of the wet
product input, and FWR is the desired rate of water exiting from
the apparatus as a part of the dried product output. PV may be
determined as the rate of water leaving the apparatus as a part of
the output of the exhaust fan minus the rate of water entering the
apparatus as a part of the input air.
[0014] In one embodiment, invention provides improved apparatus
comprising a drying chamber having a wet product input and a dried
product output, one or more input(s) for ambient air, and an output
for moisture-laden exit air including a motor-powered exhaust fan.
A sensor assembly is provided for determining the rotational speed
of the exhaust fan and the power of the exhaust fan motor. The
drying chamber also has apparatus for determining the wet bulb
temperatures and dry bulb temperatures of the moisture-laden exit
air and the input (usually ambient) air. A digital controller, such
as the described PLC/PID controller, is operably coupled with the
sensor assembly and the apparatus in order to control the operation
of the apparatus.
[0015] As noted above, a variety of different dryers and/or coolers
can be controlled using the invention. For example, the dryer may
employ different devices for heating ambient-derived input air,
such as an open-flame heater or firebox, or steam coils for
indirect heating of the air. Recirculation fans are normally
provided for circulating the heated air between the heating device
and the product being dried. Moreover, the exhaust fans of the
invention may be of any suitable type, such as conventional rotary
blade fans or blowers.
[0016] The improved drying methods and apparatus of the invention
may form a part of an overall system for the production of
products, such as food or feed products containing amounts of
protein (grain- or animal-derived or both), starch, fats, vitamins,
minerals, and other additives. These products are typically
formulated as raw mixtures and are processed to cook the mixtures
in order to denature the protein and gelatinize the starch. Pet
feeds, fish feeds, and certain human foods are of this character.
Systems of this type include an upstream processing assembly and a
downstream dryer. The upstream components may be conventional
single or twin screw extruders, or pellet mills, which feed a
continuous stream of wet product to the downstream dryer, which is
a dryer in accordance with the convention. In other contexts, the
invention can be used for the control of dryers for fruits,
vegetables, nuts, or other processed foods. In these instances,
different upstream processing or handling equipment is used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic block diagram illustrating the
components of an extruder/dryer system in accordance with the
invention used for the production of food or feed products;
[0018] FIG. 2 is a schematic view in vertical section of a product
dryer in accordance with the invention, shown during initial
loading of the dryer with wet product;
[0019] FIG. 3 is a graph illustrating the operation of the dryer of
FIG. 2 during initial loading thereof with wet product, depicting
the buildup of SP and PV values, but without operation of a control
variable;
[0020] FIG. 4 is a schematic view similar to that of FIG. 2, but
illustrating the dryer after full loading thereof with product;
[0021] FIG. 5 is a graph similar to FIG. 3 illustrating the
operation of the dryer of FIG. 4 after full loading of the dryer,
depicting the further buildup of SP and PV values, but without
operation of a control variable; and
[0022] FIG. 6 is a graph similar to FIG. 5 illustrating the further
operation of the dryer of FIG. 4, depicting the calculated SP and
PV values without operation of a control variable, and initiation
of the operation of a control variable CV, serving to drive PV
towards SP until PV is at least approximately equal to SP.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The following Example illustrates an implementation of the
present invention in the context of a horizontal, fuel-fired
convection dryer. It should be understood, however, that this
Example is provided by way of illustration only, and nothing
therein should be taken as a limitation on the overall scope of the
invention.
Example
[0024] The following hypothetical, computer-generated example
illustrates an implementation of the present invention during the
operation of an extrusion system 10 for the production of human
foods or animal feeds. The system 10 broadly includes extruder 12
and a single-pass, fuel-fired convection dryer 14. The extruder is
itself conventional, and is operable to produce a continuous stream
of wet, cooked product 16, which is processed in dryer 14 to yield
a dry product 18. A control assembly 20 is provided for the dryer
14 in order to determine, on a real-time basis, the net rate of
water leaving the wet product 16 during passage thereof through the
dryer.
[0025] The control assembly 20 includes an exhaust fan 22 coupled
with dryer 14, and a wet bulb/dry bulb sensor 24 designed to
measure the wet and dry bulb temperatures of the output of fan 22.
Additionally, the fan 22 is equipped with an rpm speed sensor 26
and a fan motor power sensor 28. The dryer 14 is further equipped
with a combustion air blower 30, which delivers air to a fuel-fired
air heater 32, so that the heated air is delivered to the interior
of dryer 14. An ambient air wet bulb/dry bulb sensor 34 is provided
to determine the wet and dry bulb temperature of the input air,
including the air fed to blower 30, and the make-up air 36 passing
into the dryer 14 as air is exhausted via fan 22. A temperature 37
is also operatively situated in dryer 14 for sensing the internal
temperature thereof.
[0026] A programmable logic controller (PLC) 38 controls the
operation of assembly 20 using inputs from the wet bulb/dry bulb
sensors 24, 34, the fan speed and power sensors 26, 28, and the
controller for extruder 12. The PLC 38 output controls the
operation of heater 32 by adjusting the flow of fuel thereto.
Additionally, a display 40 is operably coupled with PLC 38 so that
the net rate of water leaving the wet product 16, and other control
information, may be visually displayed to the operator of system
10.
[0027] The dryer 14 is schematically illustrated in FIGS. 2 and 4,
and includes an elongated dryer cabinet or housing 42 having an
inlet 44 for wet product 16, and an outlet 46 for delivery of dry
product 18 to a take-away conveyor 48 or similar device.
Internally, the dryer 14 includes a shiftable conveyor 50
presenting upper and lower runs 52, 54, and which is conventionally
powered in order to continuously move the wet product 16 along the
length of the dryer between inlet 44 and outlet 46. The flow of wet
product 16 is directed onto the upper run 52 of conveyor 50. This
incoming flow of wet product 16 has a determined Wet Rate, which is
the total amount of product (both solids and native and added
water) making up the wet product as it issues from the extruder 12.
Additionally, the % H2O of the wet product 16 is known, i.e., the
total % moisture of the wet product, based upon the total weight of
the wet product taken as 100% by weight. This data is determined
and stored within the extruder controller.
[0028] As illustrated, the dryer 14 also includes a combustion air
blower 30 and an associated heater unit 32, typically in the form
of an open-flame firebox. Further, the dryer has exhaust fan 22,
which exhausts moisture-laden exit air from the cabinet 42 during
operation. Make-up air 36 passes through the inlet 44 or any other
suitable location. A series of recirculation fans 56, 58
recirculate the hot air within the dryer cabinet 42 during the
course of the drying process.
[0029] The exhaust fan 22 has known operating parameters or
ratings, which are typically derived from the fan manufacturer, or
can be independently determined. The fan 22 in this example has an
operating volume of 16,000 ft.sup.3/min when operating under the
following conditions: a rotational speed of 1201 rpm, a pressure
(wg) of 12 (FSP), power of 44.8 BHP, and air density factor of
0.0598 lb/ft.sup.3. The reference data was taken from a fan
situated at an altitude of 1050 ft above sea level.
[0030] The dryer control process is implemented in PLC 38 by use of
a PID loop. The PID loop requires a SP (set point), a PV (process
variable), and it yields a CV (control variable). Successive
calculations of SP and PV are fed into a PID loop to control the
average net rate of water leaving the product 16 as it passes
through dryer 14. The SP is the desired rate of water removal from
the product 16 to achieve the desired moisture level in the dry
product 18. The PV is the net rate of water leaving the product 16
during passage through dryer 14. The CV is the temperature within
dryer 12, which is controlled by adjusting the fuel input to heater
unit 32.
[0031] In practice, the calculations are performed essentially
continuously, typically every 20 milliseconds. However, given the
relatively slow response time of dryer 14, it is not necessary that
each of these calculations be used in the control algorithm.
Rather, the conveyor 50 is divided into a series of N data
elements, here 100 of such elements. The data interval is based
upon the rate of travel and length of the conveyor 50. Calculations
are fed to the data series at equal intervals of time to represent
each data element. Accordingly, each data element represents the
desired rate of water removal from the product 16 over a
corresponding 1/100 of the length of conveyor 50. As each data
element is calculated by the PLC 38, it is stored in memory and all
100 of the data elements, including those containing zero during
dryer start-up, are averaged to obtain the average desired rate of
water removal from the dryer 14. Each of the data elements is
successively updated, and all the stored data elements are averaged
continuously. This provides a moving average of the desired rate of
water removal from the product 16 passing through the dryer 14.
[0032] The PID loop is initially tuned, which involves the process
of selecting the values of proportional, integral, and derivative
gains of the controller to achieve the desired dryer performance.
The selected values are chosen by considering a number of factors,
including the type and size of the dryer, the anticipated drying
rate, and the type of product to be processed. Such tuning is well
within the skill of the art.
[0033] SP, PV, and CV are calculated and used in PLC 38 as
follows.
[0034] Calculation of the Set Point SP
[0035] In order to calculate SP, the PLC 38 needs the following
information: [0036] Wet Rate (WR)=the total rate of material
(solids plus water, both native and added) delivered to the dryer
14 as it issues from the extruder 12, in kg/hr; [0037] % H2O=the
total moisture content of the wet product 16 issuing from the
extruder 12, in %; [0038] Dry Rate (DR)=the total rate of bone dry
material delivered to the dryer 14 as a part of the wet product 16
issuing from the extruder 12, in kg/hr; [0039] Initial Water Rate
(IWR)=the rate of water delivered to the dryer 14 as a part of the
wet product 16 issuing from the extruder 12, in kg/hr; [0040] Final
Water Rate (FWR)=is the desired rate of water delivered from the
dryer 14 as a part of the dry product 18, in kg/hr; [0041] Actual
Water Rate (AWR)=the actual rate of water delivered from the dryer
14 as a part of the dry product 18, in kg/hr; [0042] Target Dry
Moisture Content (TargetDMC)=is the desired moisture content of the
dry product 18, in %. The WR, % H2O, and AWR are values derived
from the operation of extruder 12 and are delivered to PLC 38 as
illustrated in FIG. 1. The TargetDMC is a preselected value for the
dry product 18. The remaining values are calculated as follows.
[0042] IWR (kg/hr)=WR (kg/hr)*% H2O(%)/100;
DR (kg/hr)=WR (kg/hr)-IWR(kg/hr);
FWR (kg/hr)=DR (kg/hr)/(1-TargetDMC(%)/100)-DR(kg/hr).
[0043] The final SP calculation is:
SP (kg/hr)=IWR (kg/hr)-FWR (kg/hr).
[0044] Calculation of the Process Variable PV
[0045] PV=net rate of water removed from the wet product 16 during
passage through dryer 14, in lb/hr, and is equal to (A) the rate of
water leaving the dryer 14 as a part of the output of exhaust fan
18, minus (B) the rate of water entering the dryer 14 as a part of
the output of combustion air blower 30, and minus (C) the rate of
water entering the dryer 14 as a part of the make-up air.
Therefore, in order to calculate PV, the above three different
values A, B, and C must be determined.
[0046] Determination of A=the Rate of Water Leaving the Dryer 14 as
part of the Output of Exhaust Fan 22
[0047] This calculation includes the determination of the
volumetric rate of flow of moisture-laden air from the exhaust fan
22, typically as CFM or f.sup.3/min. This determination is
preferably carried out using an appropriate Fan Law, which in turn
requires sensing of appropriate fan parameters. Such Fan Laws are
described in Fan Engineering, 6th Edition (1961), edited by Robert
Jorgensen, pp. 226-227. This reference teaches a number of ways of
calculating fan CFMs, using different fan parameters. For example,
Fan Law 10b calculates exhaust fan CFM using sensed fan horsepower
and fan rpm; Fan Law 7b calculates CFM using fan air pressure
(which may be static, velocity, or total air pressure; as used
herein, "fan pressure" refers to any of the foregoing) and fan rpm;
and Fan Law 9c calculates CFM using fan motor power and fan
pressure. In principle, any of the CFM Fan Law equations could be
employed, but from a practical point of view, 10b, 7b, and 9c are
the most useful. Therefore, in preferred forms, two operational
parameters of the fan are employed, selected from the group
consisting of: (1) fan motor power and fan rotational speed; (2)
fan pressure and fan rotational speed; or (3) fan motor power and
fan pressure.
[0048] For ease of reference, the preferred Fan Law equations are
set forth below:
CFM.sub.a=CFM.sub.b.times.(HP.sub.a/HP.sub.b).sup.3/5.times.(RPM.sub.b/R-
PM.sub.a).sup.4/5.times.(.delta..sub.b/.delta..sub.a).sup.3/5
10b:
CFM.sub.a=CFM.sub.b.times.(PRESS.sub.a/PRESS.sub.b).sup.3/2.times.(RPM.s-
ub.b/RPM.sub.a).sup.2.times.(.delta..sub.b/.delta..sub.a).sup.3/2
7b:
CFM.sub.a=CFM.sub.b.times.(HP.sub.a/HP.sub.b).sup.1.times.(PRESS.sub.b/P-
RESS.sub.a).sup.1.times.(1) 9c:
where: CFM.sub.a is the actual volumetric air flow of exhaust fan
output, in f.sup.3/min; CFM.sub.b is the fan manufacturer reference
volumetric air flow of the exhaust fan output, in f.sup.3/min;
HP.sub.a is the actual horsepower utilized by the fan motor, in
BHP; HP.sub.b is the fan manufacturer reference horsepower utilized
by the fan motor, in BHP; RPM.sub.a is the actual rotational speed
of the exhaust fan, in rpm; RPM.sub.b is the fan manufacturer
reference rotational speed of the exhaust fan, in rpm;
.delta..sub.a is the actual air density of the exhaust fan output,
in lbs/f.sup.3; .delta..sub.b is the fan manufacturer reference air
density of the exhaust fan output, in lbs/f.sup.3; PRESS.sub.a is
the actual fan pressure of the exhaust fan; and PRESS.sub.b is the
fan manufacturer reference pressure of the exhaust fan.
[0049] In order to determine the value A using Fan Law equation
10b, the PLC needs the following information: [0050] mfT(R)=dry
bulb temperature of the exhaust fan output in Rankine; [0051]
mfTS(R)=wet bulb temperature of the exhaust fan output in Rankine;
[0052] gfExtnAmbAlt=Dryer elevation (ft); [0053] mfW=Absolute
Humidity=lb water vapor/lb dry air in the exhaust fan output;
[0054] mfWS=Saturated Absolute Humidity in the exhaust fan
output=lb water vapor/lb dry air in [0055] the exhaust fan output;
[0056] mfAP=atmospheric pressure of the exhaust fan output, in
lb/in2; [0057] mfPS--Saturation Pressure of the exhaust fan output,
in lb/in2; [0058] mfV=specific volume of the exhaust fan output, in
ft.sup.3/lb; [0059] CFMa=actual air flow of the exhaust fan output,
in ft.sup.3/min; [0060] CFMb=fan manufacturer-reference air flow of
the exhaust fan output, in ft.sup.3/min; [0061] HPa=actual
horsepower of the fan motor, in BHP; [0062] HPb=fan manufacturer
reference horsepower of the fan motor, in BHP; [0063] RPMa=actual
rotational speed of the exhaust fan, in rpm; [0064] RPMb=fan
manufacturer reference rotational speed of the exhaust fan, in rpm;
[0065] AirDensitya=actual air density of the exhaust fan output;
[0066] AirDensityb=fan manufacturer reference air density of the
exhaust fan output.
[0067] The mfT(R) and the mfTS(R) values are derived from the
sensor 24. The gfExtnAmbAlt is the system elevation. The HPb, RPMb,
and AirDensityb are provided by the fan manufacturer. The remaining
values are calculated as follows.
mfPS=e.sup.(((14.61*mfTS(R))-8208.44)/(mfTS(R)-72.60))
mfWS=(0.6244*mfPS (lb/in.sup.2))/(mfAP (lb/in.sup.2)-mfPS
(lb/in.sup.2))
mfW=mfWS-((0.26*(mfT(R)-mfTS(R)))/(1359.0-(0.576*mfTS(R))))
mfAP (lb/in.sup.2)=14.696(lb/in.sup.2)*(0.01367*(gfExtnAmbAlt
(ft)/1000 (ft/inHg)) 2-1.0744*(gfExtnAmbAlt (ft)/1000
(ft/inHg))+29.92 (inHg))/29.92 (inHg)
mfV (ft.sup.3/lb)=(mfT(R)/mfAP (lb/in.sup.2)*(0.3779
(ft.sup.3/R*in.sup.2)+0.5909 (ft.sup.3/R*in.sup.2)*mfW (lb/lb))
AirDensitya (lb/ft.sup.3)=(1+mfWS (lb/lb))/mfV (ft.sup.3/lb)
[0068] Using Fan Law 10b from the previously mentioned Fan
Engineering handbook:
CFMa=CFMb*(HPa/HPb) (3/5)*(RPMb/RPMa)
(4/5)*(AirDensityb/AirDensitya) (3/5).
[0069] The value A is calculated as follows:
A=the rate of water leaving the dryer 14 in lb/hr=CFMa
(ft.sup.3/min)*AirDensitya (lb/ft.sup.3)*mfW*(lb/lb)*60 (min/hr).
[0070] Determination of B=the Rate of Water Entering Dryer 14 as a
part of the Combustion Air of Blower 30
[0071] The combustion blower 30 has a fixed fan speed with a known
CFM output. Therefore, the value B is calculated as follows:
B=the rate of water entering dryer 14 as a part of the combustion
air of blower 30 in lb/hr=CFM of combustion blower 30
(ft.sup.3/min)*ambient AirDensitya (lb/ft.sup.3)*ambient mfW
(lb/lb)*60 (min/hr). [0072] Determination of C=the Rate of Water
Entering the Dryer 14 as a Part of the Make-Up Air
[0073] The CFM of Dryer Make-up Air (ft.sup.3/min)=CFMa
(ft.sup.3/min)-CFM of combustion blower 30 (ft.sup.3/min). The
value C is calculated as follows:
C=the rate of water entering dryer 14 as part of the make-up air in
lb/hr=CFM of Dryer Make-up Air (ft.sup.3/min)*ambient AirDensitya
(lb/ft.sup.3)*ambient mfW (lb/lb)*60 (min/hr).
[0074] The final calculation of PV is:
PV=A-B-C
[0075] Determination of the Control Variable CV
[0076] CV is used to control PV. In this implementation, CV=Dryer
temperature.
CV=Dryer Temperature (.degree. F.)
[0077] FIG. 2 illustrates the system 10 during start-up as product
is being issued from extruder 12 onto conveyor run 52, and FIG. 3
depicts the displayed output from dryer control assembly 20. It is
to be understood that the dryer 14 is typically preheated to a
selected temperature before actual drying operations commence.
Accordingly, during the initial fill stage, the heater 32 will not
operate, because drying is effected owing to the preheat of the
dryer. However, as the process proceeds, the PV and SP lines moves
upwardly and the heater 32 begins to operate. Also as shown in FIG.
3, the PV curve is initially above the SP curve, meaning that more
water is being removed than is needed.
[0078] FIGS. 4 and 5 illustrate the system 10 once the dryer
conveyor 50 is fully loaded. At this point, the extruder 12 is
running at a constant rate and the PV and SP lines are becoming
straight. Again, however, the dryer is in an "overdry" condition.
FIG. 6 illustrates further progress of the dryer operations, again
initially in an over dry condition. However, towards the center of
FIG. 6, it will be observed that the CV begins to alter the
operation of the dryer 14, causing the PV plot to approach the SP
plot. Towards the right-hand end of FIG. 6, it will be seen that
the operation of the CV has brought the PV and SP plots into
essentially full convergence, meaning that appropriately dried
products are now being produced. The control assembly 20 then will
maintain this operational mode until some sort of upset occurs. For
example, if the extruder 12 produces a wetter product, such
information will be sent to controller 38 and the heater 32 will
operate to provide additional heat to dry the wetter product. At
the same time the successive calculation of SP and PV being fed
into a PID loop govern the operation of the heater 32, as
previously explained.
[0079] While the foregoing Example describes the implementation of
the present invention in the context of a horizontal, single-pass,
fuel-fired dryer 14 having an open-flame heater or firebox 32, with
a single combustion air blower 30, a single exhaust fan 22, and a
single temperature sensor 37, the invention is not so limited. For
example, multiple-pass horizontal dryers could also be used having
two or more vertically stacked conveyors. In such a case, the
multiple conveyors would each be divided into N data elements (such
as 100 data elements per conveyor) and the total number of data
elements would be stored in memory, to obtain the average net rate
of water removal from the dryer.
[0080] Further, the dryer could be steam-fired where a bank of
steam-fed finned coils is used in lieu of the heater or firebox 32,
and a plurality of recirculation fans, such as the fans 56, 58,
circulate ambient-derived drying air through the steam-heated coils
to heat the drying air for drying of the initially wet product.
Likewise, depending upon the size of the dryer, a plurality of
combustion air blowers and/or exhaust fans can be used. Normally,
in such a case, each of the exhaust fans would be equipped with one
or more fan parameter sensors (e.g., rpm and fan motor power),
multiple temperature sensors 37 would be used, and the PLC would
leverage the resultant data from the sensors to give SP and PV
values during the course of product drying.
[0081] The invention can also be used with vertical, multiple-deck
dryers or coolers. In the latter case, there would of course be no
heated drying air, but instead ambient air would be circulated
through the cooler to bring down the temperature of the product and
remove moisture therefrom. In essence, the invention may be used
with any product moisture-removal apparatus, so long as the
apparatus makes use of an exhaust fan with one or more sensors for
determining at least one fan operational parameter.
[0082] While the foregoing example sets forth a series of
calculations to determine the average net rate of water removal
from the incoming wet product in real time, it will be appreciated
that other calculations could be used towards the same end.
Although the use of a PLC with a PID controller is preferred, it
will be appreciated that other types of digital controllers may be
employed, e.g., a personal computer. Similarly, although the
invention has been exemplified through the use of hardware inputs
and outputs between the various sensors and the PLC, standard
wireless communication protocols (e.g., Ethernet IP, Modbus TCP/IP)
could also be utilized.
[0083] Although the use of an extruder 12 is preferred, it should
be understood that any suitable upstream processing unit (e.g., a
pellet mill) could be employed, so long as it is capable of
delivering a stream of set product to be dried, with known Wet Rate
and % H2O values.
* * * * *