Abstract:
A gas compressing system includes a compressor that provides a flow of compressed gas, a first desiccant tower, and a second desiccant tower separate from the first tower. The system also includes a first separator, a second separator separate from the first separator, an outlet, a first set and second set of no more than three valves each movable between an open position and a closed position. The flow flows along a flow path from the compressor to the first tower, to the first separator, to the second tower and out the outlet when the first set of valves is open and the second set of valves is closed. The flow of compressed gas flows from the compressor to the second tower, to the second separator, to the first tower and out the outlet when the first set of valves is closed and the second set of valves is opened.

Description:
RELATED APPLICATION DATA 
     This application claims priority to U.S. Provisional Application No. 61/831,458 filed Jun. 5, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to a compressed gas dryer that uses the heat of compression to dry the compressed gas, and more particularly to a compressed air dryer that includes redundant dryer towers that use the heat of compression to dry the air. 
     Heat of compression dryer systems employ a drying compound that interacts with the gas being dried to remove moisture from the flow. Periodically, the system must be taken off line and recharged. 
     SUMMARY 
     In one embodiment, the invention provides a heat of compression dryer system that includes two desiccant towers and no more than ten valves arranged to allow for the regeneration of one tower while the second tower dries a flow of compressed gas. The arrangement assures that the flow of compressed gas passes through no more than three valves during any mode of operation. 
     In one construction, the invention provides a gas compressing system that includes a compressor operable to provide a flow of compressed gas and water vapor at a first high temperature, a first desiccant tower including a first quantity of desiccant, and a second desiccant tower separate from the first desiccant tower and including a second quantity of desiccant. The system also includes a first moisture separator, a second moisture separator separate from the first moisture separator, a dry gas outlet, a first set of no more than three valves each movable between an open position and a closed position, and a second set of no more than three valves each movable between an open position and a closed position. The flow of compressed gas and water vapor flows along a flow path, in order from the compressor to the first desiccant tower, to the first moisture separator, to the second desiccant tower and out the dry gas outlet when each of the valves of the first set of valves is open and each valve of the second set of valves is closed. The flow of compressed gas and water vapor flows, in order from the compressor to the second desiccant tower, to the second moisture separator, to the first desiccant tower and out the dry gas outlet when each of the valves of the first set of valves is closed and each valve of the second set of valves is opened. 
     In another construction, the invention provides a gas compressing system that includes a compressor operable to provide a flow of compressed gas and water vapor, a first desiccant tower including a first inlet, a first outlet, and a first quantity of desiccant positioned in a first flow path between the first inlet and the first outlet, and a second desiccant tower including a second inlet, a second outlet, and a second quantity of desiccant positioned in a second flow path between the second inlet and the second outlet. The system also includes a first moisture separator, a second moisture separator, a dry gas outlet, a first set of three and only three valves each movable between an open position and a closed position, and a second set of three and only three valves each movable between an open position and a closed position. In a first arrangement, each of the valves of the first set of valves is open and each of the valves of the second set of valves is closed and the flow of compressed gas and vapor flows along a system flow path from the compressor, through the first desiccant tower to heat and regenerate the desiccant, then through the first moisture separator to remove a portion of the water vapor, then through the second desiccant tower to remove additional water vapor, then through the dry gas outlet. In a second arrangement, each of the valves of the first set of valves is closed and each of the valves of the second set of valves is open and the flow of compressed gas and vapor flows from the compressor, through the second desiccant tower to heat and regenerate the desiccant, then through the second moisture separator to remove a portion of the water vapor, then through the first desiccant tower to remove additional water vapor, then through the dry gas outlet. 
     In yet another construction, the invention provides a method of providing dry compressed gas at a dry gas outlet. The method includes providing a first desiccant tower and a second desiccant tower, each tower including an inlet and an outlet, compressing a quantity of gas to produce a flow of compressed gas and water vapor at a high temperature, and passing the flow of compressed gas and water vapor along a flow path through the first desiccant tower from the outlet to the inlet to cool the flow of compressed gas and water vapor. The invention also includes regenerating the desiccant in the first desiccant tower as the flow of compressed gas and water vapor passes therethrough, passing the flow of compressed gas and water vapor through the second desiccant tower from the inlet to the outlet, the desiccant adsorbing a portion of the water vapor from the flow of compressed gas and water vapor, and directing the flow of compressed gas and water vapor from the outlet of the second desiccant tower to the dry gas outlet. The method further includes periodically transitioning a first group of no more than three valves from an open position to a closed position and a second group of no more than three valves from a closed position to an open position to redirect the flow of compressed gas and water vapor from the compressor to the outlet of the second desiccant tower to regenerate the second desiccant tower, from the inlet of the second desiccant tower to the inlet of the first desiccant tower to remove a portion of the water vapor from the flow of compressed gas and water vapor, and from the outlet of the first desiccant tower to the dry gas outlet. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of compressor system and a heat of compression dryer system in a first mode of operation; 
         FIG. 2  is a schematic diagram of the compressor system and the heat of compression dryer system of  FIG. 1  in a second mode of operation; 
         FIG. 3  is a schematic diagram of compressor system and a second heat of compression dryer system in a first mode of operation; 
         FIG. 4  is a schematic diagram of the compressor system and the heat of compression dryer system of  FIG. 3  in a second mode of operation; 
         FIG. 5  is a schematic diagram of the compressor system and the heat of compression dryer system of  FIG. 3  in a third mode of operation; 
         FIG. 6  is a schematic diagram of the compressor system and the heat of compression dryer system of  FIG. 3  in a fourth mode of operation; 
         FIG. 7  is a schematic diagram of a compressor system and a third heat of compression dryer system in a first mode of operation; and 
         FIG. 8  is a schematic illustration of a multi-stage compression system including a heat of compression dryer system. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     It should be noted that the invention will be described as it applies to an air compression system  10 . However, one of ordinary skill in the art will realize that the invention should not be limited to air compression systems  10  alone. Rather, the system is applicable to many other systems that compress gases other than air. In addition, the system operates to deliver a flow of dry compressed gas. As one of ordinary skill in the art will realize, “dry” compressed gas does not refer to a flow of compressed gas that includes no moisture. Rather, a flow of dry compressed gas is a flow that includes a quantity of moisture at a dew point well below a desired operating temperature such that the moisture does not condense out of the flow during use. 
       FIG. 1  illustrates a heat of compression dryer system  15  that utilizes the heat of compression inherent in an adiabatic compression machine  20  to regenerate adsorptive drying media. While illustrated as an oil free compressor  20 , other compression machines could include, a reciprocating compressor, a scroll compressor, a centrifugal compressor, or any other suitable compressor. The compression machine  20 , or compressor, provides a flow of compressed gas  25  (air in the illustrated construction) that includes moisture. The dryer system  15  receives the flow of moist air from the compressor  20  and operates to discharge a flow of dry compressed air  30 . 
     The dryer system  15 , as illustrated in  FIG. 1  includes two dryer towers  35 , two separators  40 , two coolers  45 , ten valves  50 , and piping that interconnects the various components. Each of the towers  35  includes an inlet  55 , and outlet  60 , and a quantity of desiccant  65  (e.g., silica gel, activated alumina, and the like) that absorbs or adsorbs moisture as the flow of compressed air passes through the desiccant  65  at the proper temperature. The selected desiccant  65  is such that when hot moist gas (at partial relative humidity) passes through the desiccant  65 , the desiccant  65  releases moisture and is thereby recharged. When the moist compressed gas is cool, the desiccant adsorbs the moisture to dry the flow of compressed gas. The inlet  55  is the opening through which the compressed gas enters when the tower  35  is used in a drying mode and the outlet  60  is the opening through which the compressed gas exits when the tower  35  is used in a drying mode. In the illustrated construction, the inlet  55  is at the top of the tower  35  and the outlet  60  is near the bottom with other arrangements being possible. 
     Each of the coolers  45  is positioned to receive and cool a flow of compressed gas. In a preferred arrangement, the coolers  45  each include a heat exchanger that receives the flow of compressed gas as well as a flow of cooling fluid such as water, refrigerant, glycol, and the like. The cooling fluid used is selected based on the level of cooling required for the particular application. In one construction, a shell and tube heat exchanger uses a flow of cool water as the cooling fluid to cool the flow of compressed air entering the heat exchanger. 
     Each of the moisture separators  40  is positioned to receive the flow of compressed gas from one of the coolers  45  and is operable to separate a liquid portion of the moisture contained within the compressed gas stream. Several types of separators  40  could be employed including but not limited to coalescing filters, cyclonic separator, or other flow induced separators. In one form, the liquid portion is removed via a drain (not shown). In other embodiments, other means may be employed to remove the liquid portion. 
     Each of the valves  50  is selected for its particular purpose with many types of valves being suitable. For example, ball valves, butterfly valves, globe valves, gate valves, and the like could be employed. In addition, some or all of the valves  50  could be automatically controlled by an electronic or mechanical control system or could be manually actuated. Thus, one of ordinary skill in the art will realize that many different valves could be employed as desired. It should also be clear that as the number of valves  50  increases, the complexity and cost of the system increases. Therefore, it is a goal of the present system to use the minimum number of valves  50  to accomplish any desired operation. In addition, flow through a valve  50  causes a pressure drop that reduced the efficiency of the system  15 . Thus, it is also a goal to minimize the number of valves  50  that the flow must pass through during operation. 
     In operation and as illustrated in  FIG. 1 , air is compressed by a compressor  20 . During compression, moisture is compressed with the air and the compressed air is heated by the compression process. The hot, moist compressed air  25  (at partial relative humidity) enters the dryer system  15  and is directed to a first of the two towers  35 A. The compressed air enters the tower  35 A via the outlet  60  and flows through the tower  35 A to the inlet  55 . As the moist air flows through the desiccant  65 , the desiccant  65  is heated to a temperature that allows the desiccant  65  to release the moisture that had been adsorbed. The flow then passes to one of the coolers  45   a  and is cooled. As the flow cools, moisture begins to condense from the flow. The flow then passes to the moisture separator  40   a  where the condensed moisture is separated from the flow. 
     The flow exits the separator  40   a  and enters the second tower  35 B via the tower inlet  55 . The flow passes through the second tower  35 B where the desiccant  65  adsorbs additional moisture to further dry the flow of compressed air. The compressed air then exits the tower  35 B and flows to a dryer outlet  70  and ultimately to a point of use. 
     Eventually, the desiccant  65  of the second tower  35 B becomes saturated and must be regenerated. Prior to that point, the first tower  35 A is prepared to be used to adsorb moisture. During the regeneration phase, the desiccant  65  in the tower  35 A being regenerated is heated by the compressed gas. To avoid a spike in the dew point of the compressed gas  30  discharged from the dryer system  15 , the desiccant  65  in the regenerating tower  35 A should be pre-cooled.  FIG. 2  illustrates the dryer system  15  arranged to pre-cool the tower  35 A at the end of the regeneration cycle. As illustrated in  FIG. 2 , the incoming moist compressed air  25  is first directed to a cooler  45 B where the compressed air is cooled. The compressed air then flows through the moisture separator  40 B where any condensed liquid is removed before being directed to the inlet  55  of the first tower  35 A. The cool compressed gas entering the tower  35 A is heated by the hot desiccant  65  and operates to cool the desiccant  65 . Eventually, the flow of compressed gas cools the desiccant  65  to a desired temperature. The compressed gas exits the first tower  35 A via the tower outlet  60  and flows to the other cooler  45 A, separator  40 A and tower  35 B as described with regard to  FIG. 1 . Once the desiccant  65  in the first tower  35 A reaches a predetermined temperature, the valves  50  are manipulated such that the flow of compressed air is the mirror image of the arrangement illustrated in  FIG. 1 . Thus, the second tower  35 B enters a regeneration mode and the first tower  35 A operates to dry the flow of compressed gas. The transitions described herein occur automatically and seamlessly such that there is no disruption in the flow of compressed gas and there is little or no change in the dew point of the compressed gas exiting the dryer system  15 . 
       FIGS. 3-6  illustrate a dryer system  75  similar to that of  FIGS. 1-2  with the addition of a heater  80  that can be used to more precisely control the temperature of the flow of compressed gas during various phases of operation. The heater  80  can be electrically powered or can include a heat exchanger that uses a hot fluid to heat the flow of compressed gas. 
       FIGS. 3 and 4  illustrate operation of the system  75  in much the same way as is illustrated in  FIGS. 1 and 2  with the exception that the flow of hot moist compressed gas  25  passes through the heater  80  before entering the outlet  60  of the first tower  35 A for regeneration. In the operating mode illustrated in  FIG. 3 , the heater  80  is not activated but rather is simply passed through. 
       FIG. 5  illustrates operation of the system in a mode identical to that of  FIG. 3  with the exception that the heater  80  is activated to further heat the compressed gas  25 . The hotter compressed gas allows for faster more efficient regeneration of the desiccant  65  in the first tower  35 A. 
     During the normal operating mode, regeneration is performed without depressurizing the desiccant tower.  FIG. 6  illustrates a heated purge regeneration mode of operation that may be employed in some embodiments. The heated purge mode is employed as a backup mode to facilitate regeneration during low compressed air flow conditions which are not sufficient to adequately regenerate the desiccant. This mode entails de-pressurization of the regeneration tower  35 A and utilizes a supplemental heater  80  to elevate the air temperature exiting from the drying tower  35 B to purge the moisture from tower  35 A. Similarly, when tower  35 B is being regenerated, tower  35 B is depressurized, and supplemental heater  80  is used to elevate the air temperature exiting from the drying tower  35 A to purge the moisture from tower  35 B. In the heated purge regeneration mode, moist compressed gas  25  enters the dryer system  75  and is directed to a cooler  45 A where the gas is cooled. Moisture that condenses from the flow of compressed gas is separated in the moisture separator  40 A downstream of the cooler  45 A. The compressed gas is then directed to the second tower  35 B and is further dried by the desiccant  65  within the tower  35 B. The now dry compressed gas exits the tower  35 B via the outlet  60  and flows out of the dryer system  75  to a point of use. A small portion of the dry compressed gas  85  is bled from a point downstream of the tower  35 B and is directed to the heater  80 . The heater  80  heats the bleed flow before the flow enters the first tower  35 A via the tower outlet  60 . The gas flows upward through the tower  35 A to a discharge outlet  90 . After exiting the discharge outlet  90 , the flow passes through a valve  95  and a sound suppressing device  100  before being discharged to the atmosphere. In this arrangement, a small portion of the compressed gas flows through the tower  35 A to perform the desired regeneration. 
     As one of ordinary skill in the art will realize, each mode of operation has been described with one tower  35  operating as the drying tower. However, each system can be operated in the mirror image of that illustrated such that the other of the towers  35  operates to dry the compressed gas. The switch between the mirror images and the various modes can occur automatically and seamlessly such that there is no spike in the dew point of the compressed air and without a disruption or interruption in the flow of dry compressed air  30  to the point of use through the use of a controller. 
     A typical cycle could incorporate about 2.5 hours of heating and about 1.5 hours of cooling for a total duration of about 4 hours per tower  35 , and about 8 hours total. The required duration for the cycle functions and the total cycle length can be varied by changing the amount of desiccant  65  or the size of the towers  35 . A controller  110  can be used to switch the valves  50  on a time basis. A more sophisticated controller  110  could use system temperatures and compare them to ambient temperature to evaluate the adequacy of regeneration and cooling and when function switching should take place. 
     The configuration illustrated in  FIGS. 1 and 2  of two towers  35 , ten valves  50 , two coolers  45  and inter-connecting pipes permits the correct paths to be established with only three valve transits, thereby producing only a small pressure drop. Other configurations may require 4 to 6 or more valve transits to achieve the same functions. Low pressure drop is a desirable attribute for adsorption dryers as it is a significant contributor to the cost of operation. Some alternative configurations for accomplishing these functions utilize four-way valves and require 6 valve transits. Other configurations that accomplish the same flow functions require twelve two-way valves and have six or more valve transits. 
     The simplest configuration for a heat of compression dryer  105  has two towers  35 A,  35 B, eight valves  50  and a single cooler  45 . A construction similar to the eight valve configuration but including two coolers  45  is illustrated in  FIG. 7 . In this configuration, there is no possibility to pre-cool the regenerating tower  35 A prior to switching the tower  35 A into drying mode which is one reason why the 10 valve configuration of  FIG. 1  is preferred. This causes the air that is to be dried to first absorb the very high heat in the desiccant media  65  that is left from the regeneration process and to carry the heat out of the dryer  105 . The temperature spike is accompanied by a dew point spike, as extremely hot desiccant  65  cannot adsorb moisture from the air. Both the high outlet temperature and the high dew point can adversely affect downstream equipment and processes and are therefore undesirable attributes. Specifically, in  FIG. 7 , the flow enters the dryer  105  and first passes through an optional boost heater  80 . The flow then passes through the right-hand tower  35 A to regenerate the right-hand tower  35 A. Upon exiting the right-hand tower  35 A the flow passes through the left-hand cooler  45 A, the left-hand moisture separator  40 A and the left hand desiccant tower  35 B to dry the flow of compressed gas. The dry compressed gas  30  then exits the dryer system  105 . Of course, as with the other constructions, the mirror image arrangement of flow in  FIG. 7  provides for the regeneration of the left-hand tower  35 B and the use of the right-hand tower  35 A to dry the flow of compressed gas. 
     For part load operation in multistage compressors  20 , accomplished by loading and unloading the air compressor  20  in a cyclic fashion, current dryer configurations exhibit deteriorating (increasing) outlet dew point temperatures. This is partly because, without hot airflow to bring heat into the intercooler, continued cooling airflow or continued cooling water flow carries off the heat from the mass of the cooler. When flow is restored, the exit temperature from the intercooler is first very low, rising over time, causing the average temperature from the compressor  20  to be much lower than that experienced at continuous (full) flow, thereby resulting in lower regeneration heating temperature, and a consequential reduction in dew point performance. By controlling the flow of cooling media in the intercooler, or by controlled bypass of a portion of hot air from the cooler inlet to the cooler outlet, it is possible to increase the average temperature at the inlet of the final compressor stage, and thus to increase the heat and average temperature available at the compression stage outlet for regeneration of the desiccant, thereby improving the dew point performance of the dryer system. The hot air bypass system is preferable for it&#39;s faster speed of response and higher achievable average temperature. 
     A simple controller  110  could control the cooling media flow to an intercooler  115  when the compressor flow is interrupted (unloaded), then restore the cooling media flow when the compressed air flow is restored (loaded). This retains the heat contained in the mass of the cooler and cooling media, and thus does not cool the intercooler. A more sophisticated controller  110  could control the flow rate of the cooling media, in order to create a desired outlet temperature, or at least maintain the temperature above a predetermined temperature from the cooler, and thus (indirectly) a desired outlet temperature from the compressor  20  permitting outlet dew point performance to be improved at both full and part loads. The proposed control method works for continuous flow variable speed compressors  20  also, as at reduced airflow the intercooler outlet temperature more closely approaches the cooling medium temperature, and heat transfer losses in piping increase (relatively) causing the outlet dew point to be reduced. A more direct means of controlling the inlet temperature to the following compression stage is illustrated in  FIG. 8  and includes a bypass valve  113  that bypasses a fraction of the hot discharge air from a first stage compressor  120  and mixes it with air which has gone through the intercooler  115  to achieve the desired elevated inlet temperature to a following compression stage  125 , thereby elevating the discharge temperature of the compressor  20 . In an optimal configuration, both the water temperature and the air temperature could be controlled to achieve the best system response. 
     As one of ordinary skill in the art will realize, a superior system can be achieved if the compressor control  110 A and the dryer control  110 B are integrated to closely control the outlet temperature of the air exiting the compressor  20  while also controlling the operating parameters of the dryer system. 
     The device obtains superior outlet dew point performance by a simple flow path with a minimum number of components; and by incorporating control of the compressor intercooler stage to further enhance performance when the compressor  20  is operating in the load/unload, or variable speed reduced flow mode. When towers  35  are switched there is little or no temperature or dew point spike. 
     Thus, the invention provides, among other things, an absorption dryer system that is continuously operable without a temperature or dew point spike.