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 comes 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.
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
4. The control system of claim 3 wherein: said controller eliminates
the stripping step in said economy mode.
5. 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.
6. The control system of claim 5 wherein: said controller extends
the duration of the cooling step in said economy mode as compared
to said efficiency mode.
7. The control system of claim 1 wherein: said controller automatically
switches between said modes to maintain a required dew point
8. 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.
9. 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.
10. 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.
11. The control system of claim 3 wherein: said controller senses
the temperature of the gas available for heating.
12. The control system of claim 3 wherein: said controller senses
the temperature of the absorbent material in the dryer. system.
13. The control system of claim 12 wherein: said controller senses
the temperature of the absorbent material in the dryer at the end
of the heating step.
15. The control system of claim 3 wherein: said controller senses
the gas temperature at the inlet to the dryer system.
16. The control system of claim 3 wherein: said controller senses
the gas pressure at the inlet to the dryer system.
17. 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.
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 economy mode
when air meeting ISA instrument air specification is called for.
The system allows for efficiency operation, when needed, to produce
lower discharge due points to meet the system requirements. The
stripping cycle is reduced or eliminated depending on the dew point
required and the compressor discharge 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 switch to high efficiency
mode of operation should the dew point set point be changed. The
savings comes 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
 FIG. 3 is the view of FIG. 2 with one tower in the cooling
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 the cooled
in cooler 97 and the moisture that collects in separator 95 is roved
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
existing gas from the second stage 94 passes by a thermocouple 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 30.
The gas laden with moisture exits tower 31 at the bottom where its
temperature is measured by thermocouple 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 thermocouple 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
thermocouple 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 99 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 absorbent
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 selectively altering the output 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. The conversions between economy mode of operation and high efficiency
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
 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.