Abstract:
A heat of compression gas drying apparatus includes an air inlet fed by a source of compressed moisture laden air, an air outlet and first and second desiccant-containing towers for processing air flowing therethrough. A portion of cooled dried air delivered to the air outlet is selectively diverted to either a pathway having a heater so that heated dried air may be delivered to a tower having its desiccant regenerated, or to a separate pathway so that cooled dried air is delivered to the regenerating tower. In each case, a purge isolation valve is controlled to deliver and hold pressurized dried air in the regenerating tower after which the dried air in the regenerating tower is purged to atmosphere by pulsing an exhaust valve arrangement connected to the regenerating tower.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to a twin tower gas drying apparatus and, more particularly, pertains to an externally heated twin tower gas drying apparatus which uses an enhanced heat of compression pulse purge regeneration (PPR) design which is more energy efficient than previous designs. 
   BACKGROUND OF THE INVENTION 
   The presence of moisture and gases leads to difficulties in many industries and operations. With a slight drop in temperature, condensation can occur in pipelines and reservoirs which can lead to corrosion, scales, freeze-ups, dirt, etc. which may damage instruments and controls and cause blockages in airlines, produce excessive pressure drops, increase downtime and reduce the life of tools. Similarly, in chemical, food and metalworking industries, the presence of moisture in the air and gases produces undesired oxidation. It has also been found that the robotics field requires extremely dry air for the operation of its pneumatic systems. 
   In order to produce extremely dry air i.e., dew points of −40° Fahrenheit or lower, it is often desirable to use a heat of compression dryer system. In such a system, hot moisture laden air from a compressor is fed through a regenerating tower and then passed to an aftercooler and drain system so that cooled dry air is transferred into and through a drying tower for delivery to an air outlet. In a system such as described, if the compressor is operated at less than capacity, it does not generate enough heat (approximately 250° Fahrenheit) to adequately facilitate regeneration of a tower. 
   It is desirable to provide an improved twin tower heat of compression drying system that rectifies the drawbacks of the prior system by enhancing the previous dryer design. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to provide a heat of compression dryer which utilizes a heater and pulse purge regeneration (PPR) characteristics to improve dryer performance. 
   In one aspect of the invention, a heat of compression gas drying apparatus includes a dryer inlet for receiving compressed moisture laden air, and a dryer outlet for delivering compressed dry air. The apparatus includes first and second desiccant-containing towers and an aftercooler, separator and drain system. Both towers have exhaust valve arrangements. A conduit structure is provided for interconnecting the first and second towers in the aftercooler, separation and drain system and enabling passage of the moisture laden air from the dryer inlet to be processed by the first and second towers in the aftercooler, separator and drain system for delivering cooled dry air to the dryer outlet. A control arrangement is connected to the conduit structure for controlling the flow of air through the first and second towers and the aftercooler, separator and drain system, the control arrangement including a purge isolation valve. A heater is interconnected to the first and second towers by the conduit structure and is selectively energized by the control arrangement to receive a portion of dry air from one of the towers in response to periodic actuation of the purge isolation valve so as to deliver heated dry air to the other of the towers with its exhaust valve arrangement open. The control arrangement allows de-energization of the heater and bypassing of the dry air outside the de-energized heater through the conduit structure for periodically delivering cooled dry air to the other of the towers. Closing of the exhaust valve permits building of a predetermined pressure in the conduit structure, the heater and the other of the towers and momentarily holding of the pressure in the other of the towers for a predetermined interval. When the predetermined pressure is reached, the isolation valve is closed to hold the pressure in the heater shell and the other off the tower. After the interval, the exhaust valve arrangement of the other of the towers is pulsed open to purge the moisture laden air therein to atmosphere. 
   A pulse purge regeneration valve may be positioned downstream of the heater in the conduit structure and may be pulsed on/off upon actuation of the purge isolation valve. 
   The invention further contemplates a method for drying gas in an apparatus having an air inlet fed by a source of compressed moisture laden air, an air outlet and first and second desiccant-containing towers for processing air delivered therethrough. Each tower has an exhaust valve arrangement. The method includes the steps of providing a purge isolation valve and a heater in communication with the air inlet, the air outlet and first and second desiccant-containing towers, the heater being controllably energized and de-energized; introducing compressed moisture laden air from the air inlet into one of the towers such that the desiccant therein is regenerated when the air flows therethrough, the air exiting the one tower being delivered to an aftercooler, separator and drain system where the air is cooled and partially dried and transferred to the other tower to be dried as the air passes through the other tower with the dried air flowing to the air outlet; selectively stopping the flow of air from the air inlet to the one tower and delivering the air from the air inlet directly to the aftercooler, separator and drain system for passage to the other tower and the air outlet; selectively energizing the heater; controllably diverting a portion of the cooled dry air passing to the air outlet through the isolation valve and the energized heater such the heated dry air is delivered to the one tower having its desiccant regenerated, the heated dried air passing through the one tower and being purged to the atmosphere by briefly pulsing the exhaust valve arrangement for the one tower following a predetermined interval in which the exhaust valve is momentarily closed to build up and hold pressure in the heater and the one tower. When the predetermined pressure is reached, the isolation valve is closed to hold the pressure in the heater shell and the other off the tower; and selectively de-energizing the heater and bypassing the portion of cooled dried air along a separate pathway connected to the one tower such that cooled dried air is delivered to the one tower, the cooled dry air passing through the one tower and being purged to atmosphere by opening the exhaust valve arrangement. 
   Various other objects, features and advantages of the invention will be made apparent in the following description taken together with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic view of the heat of compression gas drying apparatus embodying the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , the heat of compression gas drying apparatus  10  embodying the present invention includes a pair of towers  12  and  14 . These towers contain a desiccant such as activated alumina. The twin tower system allows one of the towers to be used for drying, while the other tower is having its desiccant regenerated. Each tower  12 ,  14  is provided with a pressure relief valve  16 , a pressure gauge  18  and a temperature gauge  20 . 
   A compressor  22  which may be a multi-stage compressor is connected to a source of air, and has an output of hot moisture laden, superheated air which is delivered to an air inlet  24  for the drying apparatus  10 . A conduit  26  is provided with a temperature indicator  28 , a pressure indicator  30  and a temperature transmitter  32  and connects the air inlet  24  with air inlet valves  34  and  36 . 
   The opening and closing of valves  34  and  36  as well as the opening and closing of other valves and components in the system is controlled by a control arrangement  37  which by mechanical and/or electrical/electromechanical/computer operation responds to various pressures, temperatures and dew point readings, and causes solenoids and actuators to time and actuate the various valves. 
   Conduit  38  connects the air inlet valve  34  with the top of the tower  14  while conduit  40  joins the air inlet valve  36  with the top of the other tower  12 . Conduit  42  interconnects conduits  38  and  40  and purge flow check valves  44  and  46 . Conduit  48  further interconnects conduits  38  and  40  and is provided with a re-pressurizing valve  50 . 
   Conduit  38  is connected to air outlet valve  52  and the air outlet  54  for the drying apparatus  10 . Conduit  40  is connected to an outlet valve  56  and the air outlet  54  which is connected to an afterfilter  58  and a drain valve  60  monitored by a differential pressure indicator  62 . Air delivered to the outlet  54  also is monitored by a temperature transmitter  64 , a temperature indicator  66  and a pressure indicator  68 . Conduit  70  is connected to the air outlet  54  as well as an isolation valve  72 , a pilot air filter  74 , a humidity sensor  76 , a bleed valve  78  and a digital dew point monitor  80 . Conduit  82  joins air outlet  54  with conduit  26  and is provided with a differential pressure indicator  84 . Conduit  86  includes a purge isolation valve  88 , a purge adjusting valve  90 , a pressure indicator  92 , a purge orifice assembly  94 , a heater isolation valve  96 , and a heater  98  and may further include a pulse purge regeneration valve  99 . Conduit  100  is in communication with conduit  86  and is connected to a heater bypass valve  102 . 
   Conduit  104  is connected to conduit  26  and to a pressure vessel bypass valve  106 , a temperature indicator  108 , a tower water aftercooler  110 , a chilled water aftercooler  112 , a separator  114 , a manual drain valve  116 , a pair of drain isolation valves  118 , a pair of drain outlets  120 , a pair of drain vent isolation valves  122 , a pre-filter automatic drain  124  with a differential pressure indicator  126 , a drain isolation valve  128  an auxiliary drain valve  130  and a pre-filter automatic drain  132  in communication with a drain outlet  134 . The aftercoolers  110  and  112  have temperature transmitters  111  and temperature indicators  113 . 
   Conduit  136  is connected between conduit  104  and the bottom of tower  12  and is further connected to an aftercooler outlet valve  138 , a depressurizing valve  140  having a muffler  142  and a purge outlet valve  144  having mufflers  146 . Conduit  148  connects conduits  104  and  136  and an aftercooler inlet valve  150 . Conduit  152  is connected to an aftercooler inlet valve  154  and a conduit  156  which connects the bottom of tower  14  with conduit  104 . The conduit  156  is provided with an aftercooler outlet valve  158 , a depressurizing valve  160  with muffler  162  and a purge outlet valve  164  having mufflers  166 . The mufflers  142 ,  146 ,  162 ,  166  exhaust air to the atmosphere. 
   In describing operation of the gas drying apparatus  10 , it will be assumed that tower  12  is in a drying phase and that tower  14  is in a regeneration phase. A typical cycle for two consecutive drying and regenerating phases is eight hours. At the four hour mark, the process reverses i.e., tower  12  which was in the drying phase and now goes into a regeneration phase, and tower  14  which was in a regeneration phase now enters a drying phase. In operation with tower  12  in a drying phase, the other tower  14  is readied for desiccant regeneration in a heat of compression (PHC) mode by receiving the hot moisture laden superheated air from the compressor discharge. The control arrangement  37  is utilized to close valves  52 ,  140 ,  144 ,  158 ,  160  and  164  and open valves  34  and  154 . Valves  36 ,  50  and  150  are closed. Bypass valve  106  is closed so that hot compressed moisture laden air from inlet  24  passes through conduit  26 , valve  34  and conduit  38  into the top of tower  14 . The hot moisture laden superheated air passes through the desiccant bed in tower  14  to release the moisture absorbed during the previous cycle. The moisture is carried by the hot air out of the bottom of tower  14  through conduit  156 , valve  154  and conduits  152  and  104  to the first aftercooler  110 . The hot air flowing through aftercooler  110  is partially cooled and condensed, and flows through the second aftercooler  112  where the moisture in the air is condensed by chilled water. Next, the air passes through the separator  114  where remaining moisture is released and drained using components  116 - 134 . The cooled mostly dried air now flows through conduit  104 , opened valve  138  and conduit  136  into the bottom of tower  12 . The desiccant bed in tower  12  adsorbs any moisture as the air passes upwardly through tower  12 . Compressed dry air exiting tower  12  passes through conduit  40 , opened isolation valve  56  and afterfilter  58  to air outlet  54 . 
   After a predetermined time in the PHC mode, if the temperature of the desiccant bed in tower  14  has not reached a predetermined temperature as sensed by thermostat  20 , regeneration of desiccant of bed in tower  14  from incoming air is stopped by closing valves  34  and  154 . All incoming air is bypassed from inlet  24  to the aftercoolers  110 ,  112  via conduit  104  and bypass valve  106  which is opened. Tower  14  is depressurized by opening valve  160 . Purge outlet valve  164  is also opened to commence a purge heating and cooling (PPR) mode. When a pressure switch associated with pressure indicator  18  on tower  14  closes indicating depressurization is complete, the external heater  98  is energized. At the same time, the purge isolation valve  88  is opened so that about 3-7% of the dry air flowing to the air outlet  54  will be directed through conduit  86  depending on the setting of purge adjusting valve  90 . Purged dry air flows through flow orifice  94 , open heater isolation valve  96 , heater  98 , open regeneration valve  99  (if included) and check valve  44  and into tower  14  to further heat and dry the desiccant bed therein. Periodically, exhaust valves  160  and  164  are closed for a set time interval, during which the line pressure in conduit  86 , heater  98  and tower  14  builds up to the predetermined pressure. When the predetermined pressure is reached, the isolation valve is closed to hold the pressure in the heater shell and the other off the tower. At the end of the set time interval, the valves  160  and  164  are briefly pulsed open so that hot air passing from the bottom of tower  14  is exhausted to atmosphere through valves  160  and  164  and mufflers  162  and  166 , respectively. Purge isolation valve  88  again opens for a predetermined time. 
   The normal duration of the pulse purge external heating mode is normally about 1 hour. During this interval, the temperature of the desiccant bed in tower  14  is monitored by thermostat  20 . Upon reading the desired preset temperature, the heater  98  is de-energized and locked out, heater isolation valve  96  and regeneration valve  99  are closed, and heater bypass valve  102  is opened to commence a cooling cycle. Cooling dried air exiting tower  14  continues to be exhausted to atmosphere during the cooling cycle. Near the end of the four hour period, the valves  160  and  164  are closed and repressurizing valve  50  is opened and then closed to deliver dry air from tower  12  through conduits  38 ,  40  and  48  to pressurize tower  14 . 
   If the dew point at the air outlet  54  is better than the desired dew point as sensed by dew point monitor  80 , the operation will stop and the sequence will be monitored by dew point demand. The drying cycle will continue as long as the dew point remains better than the desired set point. If the dew point is worse than the preset level, the apparatus  10  continues to operate through the normal sequence. 
   After a four hour interval, the functions of the towers  12  and  14  switch. Valves  52  and  158  are opened to start the drying cycle through regenerated tower  14 . At the same time, valves  56 ,  138 ,  140  and  144  are closed on tower  12  valves  160 ,  164  are closed on tower  14 , and purge exhaust valve  88  is also closed. Valves  44 ,  50  and  154  are closed. Air inlet valve  36  and aftercooler inlet valve  150  are opened on tower  12  to start the regeneration cycle. The hot air from compressor  22  flows into air inlet  24  and through conduit  26 , valve  36  in conduit  27  and conduit  40  into the top of regenerating tower  12 . Air flow exiting regenerating tower  12  passes through conduits  136  and  148 , valve  150  and conduit  104  for delivery to the aftercoolers  110 ,  112  and components  114 - 134 . Air continues to flow along conduit  104  through valve  158  and conduit  156  and into the bottom of drying tower  14 . Dry air passes out of tower  14 , through conduit  38  and valve  52  to the air outlet  54 . 
   After a predetermined time in the PHC mode, if the temperature of the desiccant bed in tower  12  has not reached a predetermined temperature as sensed by thermostat  18 , regeneration of desiccant of bed in tower  12  from incoming air is stopped by closing valves  36  and  150 . All incoming air is bypassed from inlet  24  to the aftercoolers  110 ,  112  via conduit  104  and bypass valve  106  which is opened. Tower  12  is depressurized by opening valve  140 . Purge outlet valve  144  is also opened to commence a purge heating and cooling (PPR) mode. When a pressure switch associated with pressure indicator  18  on tower  12  closes indicating depressurization is complete, the external heater  98  is energized. At the same time, the purge isolation valve  88  is opened so that about 3-7% of the dry air flowing to the air outlet  54  will be directed through conduit  86  depending on the setting of purge adjusting valve  90 . Purged dry air flows through flow orifice  94 , open heater isolation valve  96 , heater  98 , open regeneration valve  99  (if included) and check valve  46  and into tower  12  to further heat and dry the desiccant bed therein. Periodically, exhaust valves  140  and  144  are closed for a set time interval, during which the line pressure in conduit  86 , heater  98  and tower  12  builds up to the predetermined pressure, when the predetermined pressure is reached, the isolation valve is closed to hold the pressure in the heater shell and the other off the tower. At the end of the set time interval, the valves  140  and  144  are briefly pulsed open so that hot air passing from the bottom of tower  12  is exhausted to atmosphere through valves  140  and  144  and mufflers  142  and  146 , respectively. Purge isolation valve  88  again opens for a predetermined time. 
   The normal duration of the pulse purge external heating mode is normally about 1 hour. During this interval, the temperature of the desiccant bed in tower  14  is monitored by thermostat  20 . Upon reading the desired preset temperature, the heater  98  is de-energized and locked out, heater isolation valve  96  and regeneration valve  99  are closed, and heater bypass valve  102  is opened to commence a cooling cycle. Cooling air exiting tower  12  continues to be exhausted to atmosphere during the cooling cycle. Near the end of the four hour period, the valves  140  and  144  are closed and repressurizing valve  50  is opened and then closed to deliver dry air from tower  14  through conduits  38 ,  40  and  48  to pressurize tower  12 . 
   If the dew point at the air outlet  54  is better than the desired dew point as sensed by dew point monitor  80 , the operation will stop and the sequence will be monitored by dew point demand. The drying cycle will continue as long as the dew point remains better than the desired set point. If the dew point is worse than the preset level, the apparatus  10  continues to operate through the normal sequence. 
   In the normal sequence just before the end of the four hour interval, the purge exhaust valve  88  is closed. Once the entire 8 hour cycle has finished, the towers  12 ,  14  again switch and the entire process repeats. 
   The invention further contemplates use where it is desirable during the external heating mode to pulse the purge regeneration valve  99  downstream of heater  98  on/off to transmit spurts of pressurized heated dry air to the applicable tower  12 ,  14 . In such use, the purge isolation valve  88  is open during the entire external heating mode as are the appropriate exhaust valves  160 ,  164  or  140 ,  144  on the towers  12 ,  14 . 
   While the invention has been described with reference to a preferred embodiment, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made without department from the spirit thereof. Accordingly, the foregoing description is meant to be exemplary only and should not be deemed limitative on the scope of the invention set forth with the following claims.