Abstract:
A system for variably controlling the purge cycle of a locomotive air supply system air dryer pre-filtration state. A sensor in the air inlet of the air dryer provides temperature information to a controller, which calculates an appropriate purge cycle time based on the saturation partial pressure of water vapor at the actual temperature of the air entering the air dryer and the operates the drain valve according to the particular purse cycle time.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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 
     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 MR 1  and MR 2 . 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 MR 1  and MR 2 , so that the dried air is delivered to MR 2 . The air in MR 2  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 MR 1  and MR 2 . The air in MR 1  is used for other locomotive air consumers like the windshield wipers, horn, sanders, snow-blasters, etc. When the air is consumed from either MR 1  or MR 2 , the air compressor will be operated to recharge the system. If the air pressure in MR 1  is less than MR 2 , the air compressor is operated so that air flows into MR 1  to recharge it. Air will not flow into MR 2 , however, until the pressure in MR 1  is greater than the pressure in MR 2 . 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. 
     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 MR 1  and MR 2 , 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 
     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 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. 
     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) 
       The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
         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; 
         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; 
         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 
         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 
     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 MR 1  and MR 2 , 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 MR 1  and MR 2 . 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. 
     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 . 
     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, T ref , a design reference inlet air temperature, D ref  corresponding to a predetermined minimum purge cycle time, Time(purge) min cycle . The predetermined minimum reference temperature, T ref , 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 . 
     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, T actual . 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 ×[Saturation Partial Pressure at  D   ref ]/[Saturation Partial Pressure at  T   ref ]
 
Alternatively, the maximum purge interval may be set explicitly;
 
     If T actual ≦T ref    
     Then Time(purge)=Time(purge) max    
     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 ×[Saturation Partial Pressure at  D   ref ]/[Saturation Partial Pressure at  T   actual ]
 
     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  
 
     Thus, if Time(purge) min cycle  is 2 minutes, the minimum reference temperature is −30° F. with a saturation partial pressure of 0.0062, and the design reference temperature is 100° F. with a saturation partial pressure of 0.9503, at temperatures less than or equal to −30° F., the time between purge cycles will be:
 
Time(purge)=(2 min)×(0.9503)/(0.0062)=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)×(0.9503)/(0.3633)=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)
 
     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. 
     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. 
     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 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 MR 1  and MR 2 , the air from the compressor first flows into MR 1 , allowing a significant amount of the aerosol phase water to precipitate out in MR 1 , where it is expelled by the MR 1  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.