Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
IMPROVED CONTROL OF AN AIR DRYER DRAIN VALVE CYCLE
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
[0001] The present invention relates to railway air system air dryers
and, more
particularly, to a system and method for controlling the cycling of an air
dryer drain valve.
2. DESCRIPTION OF THE RELATED ART
[0002] Railway air systems generally comprise one or more air compressors
that
provide compressed air for use in connection with, among other things, the
locomotive and
railcar braking systems. For example, a typical Association of American
Railroads (AAR)
compliant locomotive air supply system has an air compressor, an air cooler,
and two main
reservoirs in series, referred to as MR1 and MR2. As the mechanical
compression of ambient
air will result in liquid and aerosolized water and oil in the compressed air
stream, a railway
air system will also include an air dryer for the removal of these
contaminants. In an AAR
system, the air dryer is usually installed between MR1 and MR2, so that the
dried air is
delivered to MR2. The air in MR2 is used as an exclusive air source for the
train braking
system and is protected by a back-flow check valve positioned in series
between MR1 and
MR2. The air in MR1 is used for other locomotive air consumers like the
windshield wipers,
horn, sanders, snow-blasters, etc. When the air is consumed from either MR1 or
MR2, the air
compressor will be operated to recharge the system. If the air pressure in MR1
is less than
MR2, the air compressor is operated so that air flows into MR1 to recharge it.
Air will not
flow into MR2, however, until the pressure in MR1 is greater than the pressure
in MR2.
Railway air systems such as this may also include a pre-filtration stage
comprised of a water
separator and/or coalescer that removes both liquid and aerosolized water and
oil from the air
stream of the air system. Pre-filtration stage may be an independent air
treatment unit or may
be combined with the air dryer. In either approach, any water and oil will be
accumulated in
the pre-filtration stage as compressed air flows through it. As a result, a
drain valve is
normally associated with the pre-filtration state for the periodic purging of
accumulated
liquid. The conventional control scheme for purging accumulated liquid is to
open and close
the drain valve according to a fixed timer that is enabled in response to
receipt of a
compressor "ON" signal from the control system of the air compressor. Thus,
whenever the
air compressor is running, the drain valve is opened and closed according to
the fixed time
cycle set by the fixed timer to purge any accumulated liquid. The drain valve
cycle consists
of a drain valve purge duration and a purge interval between drain valve
actuations.
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[0003] For example, a typical drain valve will purge (open) for 2 seconds
after every
2 minutes of the air compressor being operated. Although the conventional
approach to
purging accumulated liquids is simple and robust, it is inefficient and wastes
considerable
energy. For example, in the AAR compliant system described above, the drain
valve purge
cycle is enabled every time there is a compressor ON signal. As the air
compressor is often
operated when there no air flow between MR1 and MR2, there is no resulting
flow through
the pre-filtration stage. As a result, the drain valve is unnecessarily cycled
according to its
predetermined fixed timer despite the lack of air flow through the pre-
filtration stage and thus
lack of accumulated moisture. The fixed timing cycle is also inefficient
because it assumes
that the water content of the incoming "wet" compressed air is constant and is
therefore based
on the worst case scenario of maximum air flow and maximum wetness. In
reality, however,
the amount of water vapor in air is directly proportional to the saturation
water vapor partial
pressure, which has a highly non-linear, exponential-like, relation to air
temperature. For
example, the saturation water vapor partial pressure at 0 F is 0.01857 psia;
at 70 F it is
0.3633 psia; at 125 F it is 1.9447 psia, and at 150 F it is 3.7228 psia. Air
at 125 F can
contain 5.35 times as much water vapor as air at 70 F, and air at 150 F can
contain 10.2
times as much water vapor as air at 70 F. Air at 125 F can contain 105 times
as much water
vapor as air at 0 F, and air at 150 F can contain 200 times as much water
vapor as air at 0 F.
Thus, a fixed cycle drain valve having a purge cycle based on maximum wetness
at a high air
temperature, such as 150 F, will cycle up to 200 times more than is necessary
when the air
temperature is low, such as 0 F, and thus is very inefficient and wastes
considerable energy.
BRIEF SUMMARY OF THE INVENTION
[0004] The
present invention is a control system for a drain valve of a pre-filtration
stage in a locomotive air supply system. The system includes a sensor in
proximity to an
inlet of an air dryer that is configured to output a signal corresponding to
the actual
temperature of an air stream in the inlet and a pre-filtration stage having a
drain valve that is
opened and closed according to a purge cycle time. A controller interconnected
to the
temperature sensor and the drain valve is programmed to calculate a variable
purge cycle
time based on the saturation partial pressure of water vapor at the actual
temperature
indicated by the signal received from the temperature sensor then operates the
drain valve
according to that calculated purge cycle time. The variable purge cycle
generally consists of
a fixed drain valve open duration and a variable time interval between drain
valve actuations
(i.e., openings). The purge cycle time is generally based on the saturation
partial pressure of
water vapor at the actual temperature by adjusting a predetermined cycle time
according to
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the relationship between a reference saturation partial pressure and the
saturation partial
pressure of water vapor at the actual temperature. If the actual temperature
is above a
predetermined minimum temperature and below a predetermined maximum
temperature, the
predetermined cycle time is adjusted according to the relationship between a
reference
saturation partial pressure and the saturation partial pressure of water vapor
at the actual
temperature. If the actual temperature is below the predetermined minimum
temperature, the
predetermined cycle time is adjusted according to the relationship between a
reference
saturation partial pressure and the saturation partial pressure of water vapor
at the
predetermined minimum temperature. If the actual temperature is above the
predetermined
maximum temperature, the purge cycle time is set to be the same as the
predetermined cycle
time.
[0005] The present invention also comprises a method of controlling a
drain valve of
a pre-filtration stage in a locomotive air supply system according to a
variable cycle time that
is based on the air inlet air temperature of the air dryer. First, the
temperature of an air steam
in an inlet of an air dryer associated with the pre-filtration stage is
sensed. Next, a purge
cycle time is calculated based on the saturation partial pressure of water
vapor at the actual
temperature of the air stream in the inlet of the air dryer. Finally, the
drain valve is controlled
according to the calculated purge cycle time. If the actual temperature is
above a
predetermined minimum temperature and below a predetermined maximum
temperature, the
purge cycle time is based on the relationship between a reference saturation
partial pressure
and the saturation partial pressure of water vapor at the actual temperature.
If the actual
temperature is below the predetermined minimum temperature, the purge cycle
time is based
on the relationship between the reference saturation partial pressure and the
saturation partial
pressure of water vapor at the predetermined minimum temperature. If the
actual temperature
is above the predetermined minimum temperature, the purge cycle time is based
on the
predetermined cycle time.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0006] The present invention will be more fully understood and
appreciated by
reading the following Detailed Description in conjunction with the
accompanying drawings,
in which:
[0007] FIG. 1 is a schematic of a locomotive air supply system that
includes an air
dryer having a pre-filtration stage with a drain valve to be variably
controlled by the present
invention;
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[0008] FIG. 2 is a schematic of a control system for pre-filtration stage
and drain
valve to be variably controlled according to the present invention;
[0009] FIG. 3 is a graph water vapor partial pressure verses ambient
temperature for
use in controlling the drain valve of a pre-filtration stage according to the
present invention;
and
[0010] FIG. 4 is a flowchart of a process for controlling the drain valve
of a pre-
filtration stage according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0011] Referring now to the drawings, wherein like reference numerals
refer to like
parts throughout, there is seen in FIG. 1 a locomotive air system 10 having an
air compressor
12, aftercooler 14, first and second main reservoirs MR1 and MR2, and an air
dryer 16.
Second main reservoir is coupled to the braking system 18 and a check valve 20
is positioned
between the first and second main reservoirs MR1 and MR2. A pre-filtration
stage 22 is
associated with air dryer 16 and includes a drain valve 24 that is operated
according to a
variable drain valve purge cycle time that is dependent on actual conditions
rather than a
predetermined maximum amount of wet air.
[0012] Referring to FIGS. 1 and 2, pre-filtration stage 22 further
comprises a
controller 26 in communication with a temperature sensor 28, such as a
thermistor or
thermocouple, which is positioned in or in close proximity to the air stream
inlet 30 of air
dryer 16. Controller 26 is programmed to receive air temperature information
from sensor 28
at inlet 30 to adjust the drain valve purge cycle time, referred to as
Time(purge), so that the
purge cycle time for drain valve 24 of the water separator and/or coalescer 32
of pre-filtration
stage 22 is variably determined based on the air temperature. In most
instances, the drain
valve purge cycle time is adjusted proportionally to the saturation partial
pressure of water
vapor in air, as seen in FIG. 3, based on the actual inlet air temperature. It
should be
recognized that controller 26 and pre-filtration stage 22 may be included as
part of air dryer
16, or provided separately as a stand along unit. Controller 26 may also be
positioned
remotely from pre-filtration stage 22 provided that controller 26 is able to
communicate the
appropriate change in purge cycle time to pre-filtration stage 22.
[0013] Referring to FIG. 4, controller 26 is programmed to implement a
purge control
process 40 that adjusts the purge cycle time, Time(purge), based on actual
conditions. First,
the controller system operating parameters 42 of pre-filtration stage 22 are
established that
will be used to determine any change in the purge cycle timing. Operating
parameters may
include a predetermined minimum reference temperature, Tref, a design
reference inlet air
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temperature, Dref corresponding to a predetermined minimum purge cycle time,
Time(purge)min cycle. The predetermined minimum reference temperature, Tref,
represents the
lowest temperature at which controller 26 will adjust the purge cycle time and
results in a
maximum time interval between drain valve actuations. The predetermined design
reference
inlet air temperature is selected based on the temperature that represents the
maximum water
load, which is a function of air temperature and air flow rate, that is less
than the storage
volume of pre-filtration stage 22 and less than the amount of water which can
be discharged
through an open drain valve 24 for a predetermined purge duration, for example
2 seconds,
when system 10 is pressurized at the minimum system working pressure. The
predetermined
minimum purge cycle time represents the shortest time interval between
subsequent
actuations of drain valve 24. The minimum reference temperature, design
reference inlet air
temperature, and minimum purge cycle time, may be set as default values by the
manufacturer or user based on the specifications of a particular pre-
filtration stage 22, air
dryer 16, and/or locomotive air system 10 and then loaded into the controller
26 during first
step 42 of purge control process 40.
[0014] Once the operating parameters are loaded at step 42, the inlet air
temperature
is sensed 44, such as by sampling the output of temperature sensor 28 with
controller 26 to
determine the actual inlet air temperature, Tactual. A check 46 is then
performed to determine
if the actual inlet air temperature is less than the minimum reference
temperature. If so, the
purge cycle time is set according to the following formula 48:
Time(purge) = Time(purge)min cycle X [Saturation Partial Pressure at Dref ] /
[Saturation Partial Pressure at Tref]
Alternatively, the maximum purge interval may be set explicitly;
If Tactual < Tref
Then Time(purge) = Time(purge).
[0015] If check 46 determines that the inlet air is greater than the
minimum reference
temperature, a second check 50 is performed to determine whether the inlet
temperature is
below the design reference temperature. If so, then the purge cycle time is
set according to
the following formula 52:
Time(purge) = Time(purge)min cycle X [Saturation Partial Pressure at Dref ] /
[Saturation Partial Pressure at Tactual]
[0016] If second check 48 determines that the inlet air temperatures is
equal to or
greater than the design reference temperature, then the purge cycle time is
set as follows 54:
Time(purge) = Time(purge)min cycle
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[0017] Thus, if Time(purge)mjn cycle is 2 minutes, the minimum reference
temperature
is -30 F with a saturation partial pressure of .0062, and the design
reference temperature is
100 F with a saturation partial pressure of .9503, at temperatures less than
or equal to -30 F,
the time between purge cycles will be:
Time(purge) = (2 min) X (.9503)/(.0062) = 306 minutes
Under the same conditions with an inlet air temperature of 70 F, the time
between purge
cycles will be as follows:
Time(purge) = (2 min) X (.9503)/(.3633) = 5.2 minutes
Under the same conditions with an inlet air temperature equal to or greater
than 100 F, the
time between purge cycles will be as follows:
Time(purge) = (2 min)
[0018] It should be recognized that Time(purge) could be set as the
longest purge
cycle time allowed by system 10, and then adjusted downwardly based on the air
temperature
using an inverse approach to that described above. Similarly, first check 46
and second
check 48 may be implemented in a single or any number of computing steps so
long as
controller 26 applies the appropriate formula to adjust the purge cycle time
based on the
actual inlet air temperature provided by sensor 28 to account for the actual
amount of
moisture that may be present in the air.
[0019] Controller 26 may be programmed to receive an input representing
when air
compressor 12 is being operated to provide compressed air, e.g., an "ON"
signal. Controller
26 may be programmed to open drain valve 24 upon detecting that air compressor
12 has
been turned on, and then operate drain valve 24 as described above. Similarly,
controller 26
can open drain valve 24 when signaled that air compressor 12 has been turned
off to
completely drain any accumulated water in pre-filtration stage 22 and thus
prevent freezing in
the event that system 10 is shut down for an extended period in cold
temperatures.
[0020] In an alternative embodiment, the air dryer may use a humidity
sensor in the
outlet airstream to determine when the desiccant bed is approaching saturation
by monitoring
the instantaneous outlet humidity and temperature or other means of dew point
dependent
desiccant regeneration, such as that disclosed in application NY-1273. When
the outlet
humidity increases a pre-determined amount, the air dryer initiates a
regeneration cycle. The
air dryer may be designed so that the regeneration cycle time at some
reference operating
condition, for example 100 F and 100% inlet RH and 100 SCFM flow, is known.
For
example at the reference operating conditions the desiccant bed would become
saturated in 2
minutes. If using a humidity sensor in the outlet air stream for control of
the regeneration
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cycle, then under these conditions the outlet air stream humidity would
increase to the trigger
level in approximately 2 minutes. Using a humidity sensor the regeneration
cycle time is
proportional to the actual conditions of inlet temperature, RH, and air flow,
where the total
water volume in at those conditions is proportional to the saturation partial
pressure of water
vapor in air as previously described. Because the air dryer on a locomotive is
typically
located between MR1 and MR2, the air from the compressor first flows into MR1,
allowing a
significant amount of the aerosol phase water to precipitate out in MR1, where
it is expelled
by the MR1 spitter valve. Because the desiccant bed becomes saturated with a
fixed mass of
water reasonably independent of the ambient temperature or rate of air flow,
the total water
mass flow through the prefiltration is approximately the same as the total
water mass
removed by the desiccant. As result, the total water collected in the
prefiltration is roughly
constant with the desiccant regeneration cycle. Therefore, in an air dryer
having a
prefiltration stage followed by a desiccant stage and having a closed-loop
desiccant
regeneration cycle using a humidity sensor in the air dryer outlet, the
prefiltration drain valve
may be vented in synchronization with the desiccant regeneration cycle.
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