Abstrict A method to control the performance of desiccant dryers is disclosed
that senses multiple variables and optimizes the regeneration cycle
to deliver the gas at the desired dew point. The length of the stripping
step is reduced or eliminated depending on the desired set point
and the operating conditions of the compression system. The control
system has the capability to switch to high efficiency mode of operation
should the dew point set point be changed. The savings come from
not purging as much or any gas during stripping should the system
requirements be only to meet the ISA standards for instrument air,
despite the system capability of delivering far dryer air.
Claims We claim:
1. A control system for a compressed gas delivery system to deliver
a desired dew point having a compressor and a dryer system periodically
regenerated by at least two of the steps of heating, stripping and
cooling, comprising: a dew point transmitter to sense the exit dew
point from the dryer system and to transmit a signal; a controller
to receive said dew point signal and to control the regeneration
of the dryer system in two distinct modes, an economy mode where
the delivered dew point is higher and less energy is consumed in
the regeneration and an efficiency mode where a lower dew point
is obtained while using more energy in the regeneration than in
said economy mode.
2. The control system of claim 1 wherein: said dew point in said
economy mode is within the required specification for the compressed
gas for the connected end users.
3. The control system of claim 2 wherein: said controller, in
said economy mode, adjusts the duration of the stripping step to
a smaller value than that used for the stripping step in said efficiency
mode.
4. The control system of claim 3 wherein: said controller eliminates
the stripping step in said economy mode.
5. The control system of claim 3 wherein: said controller lengthens
the heating and cooling steps when reducing the duration of the
stripping step in said efficiency mode.
6. The control system of claim 3 wherein: said controller compares
the effects of previous changes to the steps in said economy mode
as feedback for adjustment of future changes to those steps to hold
a desired dew point.
7. The control system of claim 3 wherein: said controller in a
given regeneration sequence senses the duration of the previous
step or steps in that sequence to affect the duration of a subsequent
step in that regeneration sequence.
8. The control system of claim 3 wherein: said controller senses
the temperature of the gas available for heating.
9. The control system of claim 3 wherein: said controller senses
the temperature of the absorbent material in the dryer system.
10. The control system of claim 9 wherein: said controller senses
the temperature of the absorbent material in the dryer at the end
of the heating step.
11. The control system of claim 3 wherein: said controller senses
the gas temperature at the inlet to the dryer system.
12. The control system of claim 3 wherein: said controller senses
the gas pressure at the inlet to the dryer system.
13. The control system of claim 2 wherein: said controller reduces
the duration of the heating step in said economy mode as compared
to said efficiency mode.
14. The control system of claim 13 wherein: said controller extends
the duration of the cooling step in said economy mode as compared
to said efficiency mode.
15. The control system of claim 2 wherein: said gas comprises
air and the delivered dew point in said economy mode meets the instrument
air standard of ISA.
16. The control system of claim 1 wherein: said controller automatically
switches between said modes to maintain a required dew point.
Description FIELD OF THE INVENTION
The field of this invention relates to desiccant compressed gas
dryers and techniques for regenerating them.
BACKGROUND OF THE INVENTION
Many industrial processes require the supply of air for operation
of control components. The Instrument Society of America (ISA) requires
that the dew point of instrument air be kept below the coldest anticipated
ambient air temperature so as to avoid condensation in the instrument
air lines. Many installations set dew point limits far lower than
those required by ISA for a variety of reasons. However, in many
installations the level of dryness of the delivered compressed air
is well below the actual system requirements.
To remove moisture from compressed air a plurality of towers are
used. Each tower has a desiccant material and one tower is on line
while another tower regenerates. Regeneration is periodically required
because a tower becomes saturated with moisture and the dew point
of the exiting air rises toward a preset set point. When this occurs,
the spent tower in taken off line for regeneration and another tower
that has concluded the regeneration cycle is put on line. The process
is typically controlled automatically. The regeneration of a spent
tower proceeds in three steps: heating, stripping, and cooling.
In the heating step, the exhaust gas directly from the compressor
is directed into the tower, generally in the opposite direction
as the air to be dried is normally fed in. The heat of compression
from the compressor exhaust is used to drive the moisture off the
desiccant. The exhaust air from the heating phase is run through
a cooler and a separator to knock most of the water out before the
gas is directed into the dryer that is on line for removal of the
remaining moisture to the point that the desired dew point is achieved.
The heating cycle is normally done on a time basis or by sensing
the gas outlet temperature from the tower being heated. When the
controller senses that heating is complete, it shifts the valves
so that the stripping cycle can begin. In the stripping cycle, some
of the air dried from the tower that is on line is directed to the
other tower, after the pressure is first reduced to nearly atmospheric.
This stripping flow is cooled dried air, which helps to cool the
desiccant bed and to remove any residual moisture from the tower
after the heating cycle. The stripping stream is typically vented
through a muffler. The stripping flow is typically 1-5% of the compressed
gas flow. The purging of this much gas has a related energy cost
of compression. Additionally, the compressor system may be running
close to capacity and may not be able to meet system needs if 5%
of the volume is vented for any significant time. While the stripping
helps to reduce the dew point of the gas that will flow through
the tower after regeneration is complete, it may do so well beyond
the needs of many systems. Therein lies a potential to avoid energy
waste if the regeneration performance is adapted to meet the system
needs. This energy savings is the focus of the present invention.
While past efforts to improve dryer performance have focused on
the stripping step, they have addressed the situation where the
compressor discharge temperature is low. With low compressor discharge
temperatures, the regeneration of the dryer is not as effective
and the desired dew point may not be achieved. To counter this problem,
U.S. Pat. No. 6375722 provides a booster heater to heat only the
stripping flow to compensate for the anemic heating cycle using
low temperatures at the compressor discharge. The use of the stripping
heater adds to energy cost. Again this system will produce air at
dew points well below those required for most applications for instrument
air. This reference does not address how to optimize the regeneration
of a dryer so that over-drying of the air is avoided in order to
save energy.
The last step in the regeneration sequence is the cooling cycle.
Here, a slipstream of dried air from the tower that is on line is
run into the tower being regenerated to cool it slowly. The cooling
air rejoins the main airflow at the outlet of the drier that is
on line to avoid the purging of any air from the system during the
cooling phase. Cooling the desiccant allows the regenerated tower
to go on line and produce very low dew points as desired by the
system operator.
In the past, there have been unsuccessful attempts to eliminate
the cycle with unacceptable fluctuations in the outlet dew point.
The SP design of Henderson Engineering has this feature, which includes
dew point excursions above the set point for as long as 8 minutes
until the desiccant properly cools. Another design, offered as the
MD dryer from Atlas Copco eliminates stripping by blending hot regeneration
air with the cooler dry air from the on line tower to reduce the
dew point spike. The problem with this design is that it is limited
in how low a dew point can be produced and is more costly. This
design lacks the flexibility that a dryer system that optimizes
the stripping steps to meet system demands can achieve. Finally,
heatless drying involves regeneration by a purge stream of dried
air of approximately 15% of the gas compressed and dried at the
time. It is not energy efficient due to the cost involved in compressing
the volume that is purged to get the necessary drying.
Other U.S. Pat. Nos. that relate to the area of controlling and
regenerating gas dryers are: 6171377; 6221130 and 5632802.
The present invention allows operation in an efficiency mode when
air meeting ISA instrument air specification is called for. The
system allows for high performance operation, when needed, to produce
lower discharge dew points to meet the system requirements. The
stripping cycle is reduced or eliminated depending on several parameters
such as but not limited to: dew point required, the actual dew point,
compressor discharge pressure, compressor discharge temperature,
regeneration temperature, desiccant temperature, and ambient temperature.
These and other aspects of the present invention will be readily
apparent to those skilled in the art from a review of the detailed
description of the preferred embodiment and the claims, which appear
below.
SUMMARY OF THE INVENTION
A method to control the performance of desiccant dryers is disclosed
that senses multiple variables and optimizes the regeneration cycle
to deliver the gas at the desired dew point. The length of the stripping
step is reduced or eliminated depending on the desired set point
and the operating conditions of the compression system. The control
system has the capability to adjust its mode of operation to meet
new dew point requirements should the dew point set point be changed.
The savings come from not purging as much or any gas during stripping
should the system requirements be only to meet the ISA standards
for instrument air, despite the system capability of delivering
far dryer air.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a process instrument diagram of a drying system with
one tower being heated;
FIG. 2 is the diagram of FIG. 1 with one tower in the stripping
mode; and
FIG. 3 is the view of FIG. 2 with one tower in the cooling mode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a typical system for providing dry air. In the
position of FIG. 1 the dryer 31 is in the heating cycle. Ambient
air enters the first compression stage 98. It is then cooled in
cooler 97 and the moisture that collects in separator 95 is removed
through valve 96. Gas then enters the second stage 94. Optionally,
a third stage 88 can be used, as will be described later. The present
invention can be employed with any number of stages, however. The
exiting gas from the second stage 94 passes by a temperature element
93 and pressure transmitter 92 so that pressure and temperature
can be communicated to the controller C for cycle optimization.
Gas then flows through valve 1 valve 5 and into the top of tower
31. The gas laden with moisture exits tower 31 at the bottom where
its temperature is measured by temperature element 82 and that measurement
is communicated to the controller C. Flow proceeds through valves
7 and 15 into cooler 91 and separator 89. Optionally, the gas can
be compressed again in another stage 88 then cooled in cooler 87
and moisture separated in separator 86 and removed through valve
85. The temperature is sensed at temperature element 84 and the
pressure is sensed at transmitter 83 for communication to controller
C. Flow goes into tower 30 through valve 10. Its temperature is
sensed at temperature element 79 before entry into tower 30. Thereafter,
the gas leaves the dryer assembly through valve 4 and check valve
14. A dew point transmitter 77 is connected to the dryer outlet
line to transmit the dew point of the gas to the controller C.
FIG. 2 shows a stripping cycle. Now the flow does not go through
valve 1 which is closed but instead, after second stage compression
at 94 goes through valve 2 to cooler 91. The entire flow goes through
tower 30 through valve 10. After tower 30 the bulk of the flow goes
through valve 4 and check valve 14. At this time, valve 11 is open
allowing a portion of the dried air flow to pass through orifice
21 which is piped in parallel with a check valve 100 so as to direct
the dried air flow through orifice 21. After passing valve 11 the
stripping gas flow goes through valve 5 and into the top of the
tower 31. From there its temperature is measured at thermocouple
82 and it is directed through valve 7 then valve 12 into a muffler
99 for atmospheric venting.
FIG. 3 illustrates the cooling cycle. Here valves 2 and 10 are
opened to direct gas from compression stages 98 and 94 through cooler
91 and separator 89 into tower 30. From there, the flow is through
valve 4 after which there is a split. Because valves 13 and 7 are
open, some of the flow goes into the bottom of tower 31 and out
through valves 5 and 11 then through the check valve 100 to the
dryer outlet. Valve 13 regulates the cooling flow to ensure the
system parameters continue to be met during this cooling cycle.
The present invention seeks to save energy by matching the system
capabilities to the system demand. For example, plant air systems
are frequently specified for 40 degrees Fahrenheit dew points. However
the requirements for instrument air set by the ISA is only that
the dew point not exceed ambient temperature. Running a system that
can produce very low dew points drier than the users actually require
by ISA standards results in a waste of energy. The waste is most
noticeable in the stripping operation where energy costs are expended
to compress the approximately 5% of total flow, that in the past
was vented during such a step. The present invention, using several
measure parameters and controller C, seeks to optimize energy consumption
by reducing or eliminating the stripping step if the system requirements
for the users is ISA standard of the dew point being lower than
ambient. In essence, the regeneration procedure is detuned to allow
the dew point delivered to climb within limits when conditions permit
using air that is not quite as dry as the system can optimally deliver.
The control system C has the capability to switch between an economy
mode of operation to save energy and an efficiency mode of operation
to maintain a lower dew point, when system users demand that level
of dryness.
To allow efficient operation in the economy mode where the stripping
cycle is minimized or eliminated, several parameters can be monitored
and the information relayed to the control system C. For example,
the inlet temperature to the dryers can be monitored. As that temperature
decreases, the moisture content of the air decreases. In turn, less
energy in the heating and stripping cycle needs to be consumed to
produce a given dew point. Of course, a warmer inlet temperature
has the opposite effect. Similarly, an increase in the inlet pressure
means a decrease in the moisture content of the inlet gas to the
dryers with the effect of reducing the energy required to heat and
strip the desiccant. Another measured variable is the regeneration
temperature for the heating cycle. If this temperature is higher,
more moisture is driven off and a lower dew point is obtained when
putting the regenerated tower back on line. Additionally, the desiccant
lasts longer because more moisture has been removed from it during
heating. In the same manner, the desiccant temperature at the exit
of the tower being regenerated, affects performance similarly to
higher regeneration temperature. Finally the set dew point affects
the regeneration operation. To work in the economy mode, the ambient
temperature is sensed and the dew point is reset to within the desired
range of degrees below the ambient temperature. On cold days more
opportunities arise for economy operation. Similarly on low consumption
days the inlet temperature to the dryers will be reduced and the
pressure may increase. This also promotes minimizing or eliminating
the stripping cycle and shortening the heating cycle in favor of
a longer cooling cycle. Since there is no purging in the heating
or cooling cycles varying their length has minimal, if any, energy
consumption ramifications.
The heating step length can be made a function of several parameters.
For example, the regeneration temperature and adsorbent temperature
can be used. The cycle time can also be a comparison to the previous
heat time and the relation of the actual to the set dew point. The
heating cycle can also be subject to a maximum time. The stripping
cycle and whether it is run can be a function of desiccant temperature
during the drying cycle and during heating, dryer inlet temperature
and pressure during drying, duration of the previous heating cycle
and the set point for the dew point. The cooling cycle can be directly
related to the length of the heating cycle, or the length of the
stripping cycle (if any), or a comparison between the actual and
the desired dew point. Operating data can be obtained from a specific
installation and such historical information can be used by the
control system C to optimize the drying system operation for minimization
of energy consumption. The ability to run the compressed air system
more efficiently by minimizing purging during stripping is the source
of potential energy savings. The ability of altering the delivered
dew point temperature of the dried gas allows delivery of air to
ISA specifications as a baseline of performance, with an opportunity
to improve regeneration efficiency to get lower dew points should
the end users require it. Altering between economy mode of operation
and high performance mode of operation with lower dew points can
be made manually or automatically by the control system C. The control
system C can use the previous cycle adjustments and the response
of the outlet dew point as feedback in making subsequent regeneration
cycle adjustments. Similarly, the duration of a prior step in a
given regeneration cycle can be used to affect the duration of remaining
steps in that cycle.
The foregoing disclosure and description of the invention are illustrative
and explanatory thereof, and various changes in the size, shape
and materials, as well as in the details of the illustrated construction,
may be made without departing from the spirit of the invention. |