Abstract:
An air dryer system for a locomotive having optimized purge air control over a twin tower desiccant-type air dryer. The computer controlled locomotive brake system is used to determine when the air dryer is being used and approximately how much air flow has actually passed through the active tower of the dryer using the air consumption state, the change of pressure in the second main reservoir, the air compressor on/off state, the total accumulated time of air flow since last purge cycle, and/or the calculated air flow through the air dryer since the last purge cycle. When the actual air flow reaches the capacity of the active tower of the air dryer, the computer controlled locomotive brake system commands the air dryer to perform a purge cycle. The system thus maximizes the use of each tower in the air dryer rather than switching according to a preset time period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to locomotive air dryers and, more specifically, to an approach for optimizing purge cycling of an air dryer. 
     2. Description of the Related Art 
     Twin tower desiccant air dryers are used removing water vapor from compressed air in locomotive braking systems. In a twin tower system, one tower comprises a column of desiccant material and is used to absorb the water vapor in the inlet air stream by flowing wet air through the desiccant column. The desiccant absorbs the water vapor from the air stream so that the air discharged from the outlet of the air dryer is nominally dryer by an amount proportional to, among other variables, the amount of desiccant material in the column of the tower, the geometry of the column, and the air velocity in the column. Eventually, the first tower becomes saturated with water and is no longer effective in removing water from the inlet air stream. 
     When the active tower becomes saturated, the control system for the air dryer switches the inlet air stream to the other tower, which is nominally the same construction as the first tower, so that drying may continue. Concurrently, some of the dry air discharged from air dryer outlet is redirected through the first tower to atmosphere. This counter-flow of dry air removes the accumulated moisture from the desiccant column of the first tower and transports it to atmosphere. When the second tower eventually becomes saturated, the inlet air is switched back to the first tower and the second tower is purged, and the cycling between the two towers is repeated as needed. This counter-flow operation to remove moisture from a saturated column is referred to as a purge cycle and typically consumes 15-20% of the dry air discharged from the air dryer. 
     In an ideal locomotive air system having a constant air flow through the air dryer, the air dryer purge cycle and switching between the two desiccant columns can be done with a simple timer. In practice, however, the flow of air through the air dryer is never constant. For example, the air flow to the brake system can vary from very low flow rates needed to maintain the system against brake pipe leakage to very high air flows to recharge the train brake system after brake release. As a result, switching according to a fixed time schedule wastes energy and compressed air as a fixed-time purge cycle results in purge cycling before the desiccant in a particular column is fully saturated. While an air dryer system could include a flow meter or a humidity meter that more accurately determines when the purge cycle should be implemented, these technologies are expensive and often unreliable in the severe environment of a locomotive. Accordingly, there is a need for an inexpensive and reliable approach for determining when to perform the purge air cycling in a twin tower air dyer. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention comprises a purge cycle control system for a twin tower air dryer in a locomotive braking system. The system includes an air dryer having first and second towers positioned between a first main reservoir and a second main reservoir of the locomotive braking system and being configured to switch the flow of air through one of the first and second towers to the other of the first and second towers in response to a purge control command. The system further includes a computer controlled braking system that is coupled to the locomotive braking system and to the air dryer by a purge control line. For example, the computer controlled braking system is coupled to a 13 pipe of the locomotive braking system, a 20 pipe of the braking system, locomotive brake cylinders and a brake pipe of the braking system. The system also includes a first pressure transducer positioned to determine the pressure on one side of a brake pipe charging orifice and a second pressure transducer positioned to determine the pressure on the opposing side of the brake pipe charging orifice. An air compressor link is also used to provide a signal to the computer controller braking system that indicates when the air compressor is operating. 
     The computer controlled braking system is programmed to send a purge control command to the air dryer when the total flow of air through the air dyer has exceeded a predetermined threshold. The predetermined threshold is the wet air capacity of one of the towers in the air dryer and is determined based on the particular design of the air dyer being controller by the system. The total flow of air is determined by calculating and summing the amount of air used for a bail, the amount of air used for an independent brake, the amount of air used for charging the brake pipe, the amount of air used for charging a second main reservoir. The total amount of air used may then be compared against the wet air capacity of the air dryer tower to determine whether it is appropriate to perform a purge cycle. 
    
    
     
       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 an air dryer purge control system according to the present invention; 
         FIG. 2  is a flow chart of a method of controlling an air dryer 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 purge cycle control system  10  for a twin tower desiccant air dryer  12 . Air dryer  12  is typically positioned downstream of the locomotive air compressor  14  and first main reservoir MR 1 . The inlet  16  of air dryer  12  is connected to the first main reservoir MR 1  via a back flow valve  18  and the outlet  20  of air dryer  12  is connected to the second main reservoir MR 2 . A computer controlled brake (CCB) system  22  is connected to the outlet of the second main reservoir and comprises a microcomputer-based system that provides full automatic and independent brake control for locomotives. CCB systems  22  are known in the art, such as the CCB II and CCB  26  available from New York Air Brake, LLC of Watertown, N.Y. 
     CCB system  22  includes a 20 pipe circuit  24  interconnected to the 20 pipe, also referred to as the independent application and release pipe, and a 13 pipe circuit  26  interconnected to the 13 pipe, also referred to as the actuating pipe. In a locomotive consist (i.e., two or more locomotives connected together) having CCB system  22 , pressure developed in 20 pipe circuit  24  provides brake cylinder pressure to the locomotive brake cylinders and thus applies the independent locomotive brakes. Pressure developed in 13 pipe circuit  26  actuates a control valve to provide bail-off command pressure. Control system  10  further includes a 19/64″ charging orifice  28  located between a second main reservoir MR 2  of the locomotive braking system and a brake pipe relay  30  that connects the locomotive air system to the train brake pipe. Pressure transducers  32  and  34  are positioned on either side of charging orifice  28  for sampling and reporting the pressures at those locations. 
     As seen in  FIG. 1 , control system  10  further includes a purge control link  40  between CCB system  22  and air dryer  12  so that CCB system  22  can signal air dryer  12  to execute a purge cycle to switch the primary air flow and counter-flow between the two towers of air dryer  12 . In addition, CCB system  22  is connected to air compressor  14  by a link  42  so that CCB system  22  can receive a signal indicating that air compressor  14  is on, thus providing CCB system  22  with an indication that the air supply system is in a charging state. This information is particularly useful during a dry charge of the locomotive when first main reservoir MR 1  and second main reservoir MR 2  are being charged from an exhausted state because the flow rates are very high during a dry charge so the purge cycle needs to occur more frequently to keep up with the saturation of the desiccant. As explained below, when the compressor is on and the second main reservoir pressure is low (such as below the pressure governor setting, which is typically 120 psi) and increasing, the purge cycle can be adapted to occur more frequently to handle the large volume of air moving through air dryer  12 . 
     Referring to  FIG. 2 , CCB  22  is programmed to implement a purge cycle process  50  to determine the optimal time for initiating a purge cycle in air dryer  12  after a continuing recalculation of flow volume determines that the wet air capacity of a particular air dryer  12  has been reached. The first step in process  50  is to perform a check  52  whether there is a bail command. If so, the flow volume is calculated  54 . Flow volume is calculated in step  54  based on the amount of air consumed for a bail, which can be approximated based on the volume of the 13 pipe, the average number of locomotives in a consist (for example, five), and the charge pressure in the 13 pipe, typically MR 2  pressure. For example, the following formula may be used:
 
Air Vol 13  ft 3 =(No. Locos)*(Locomotive Length ft)*(Vol 13 Pipe ft 3 /ft)*(Pressure 13  psi/Pressure atm  psi).
 
Thus, five locomotives, where each locomotive is 75 ft long and has a ¾″ ID pipe and a 13 pipe charge pressure of 145 psi, would require approximately 11.3 ft 3  of air as the formula above would result in Air Vol 13  ft 3 =(5)(75)((0.75 2 *π/4)/144)(145/14.7)=11.3 ft 3 .
 
     After flow calculation  54 , of if there was no bail command at check  52 , a check is performed  56  to determine whether CCB system  22  has received an independent brake command. If so, flow volume is calculated  58 . The calculated flow volume in step  58  is based on the amount of air consumed in an independent brake application, which can be approximated based on the volume of the 20 pipe, the average number of locomotives in a consist (for example, five), and the charge pressure in the 20 pipe, which is known by CCB system  22  on the lead locomotive and can be either measured by CCB system  22  at the trailing locomotive or assumed to be the typical maximum independent pressure, 45 psi. For example, the following formula may be used:
 
Air Vol 20  ft 3 =(No. Locos)*(Locomotive Length ft)*(Vol 20 Pipe ft 3 /ft)*(Pressure 20  psi/Pressure atm  psi).
 
Thus, five locomotives, where each locomotive is 75 ft long and has a ¾″ ID pipe with a 20 pipe charge pressure of 40 psi, would require approximately 3.1 ft 3  of air.
 
     After flow calculation  58 , or if there was no independent brake command at check  56 , a check is performed  60  whether there is brake cylinder (BC) flow. Air is supplied to the brake cylinders on the locomotive in response to either an independent brake application or an automatic brake application. If both an independent and automatic brake application are made, the brake cylinder pressure is determined to be the greater of the two inputs. The logic of brake cylinder pressure development is well known to those skilled in the art. For each brake application, regardless of independent or automatic, the calculated flow volume in step  62  is a function of the brake cylinder pressure and the total brake cylinder volume on that locomotive as follows:
 
Air Vol BC  ft 3 =(Brake Cylinder Volume locomotive  ft 3 )*(Pressure BC  psi/Pressure atm  psi).
 
Thus, 5.1 ft 3  of air is needed to pressurize the locomotive brake cylinders to 40 psi if the locomotive has a total BC volume of 1.86 ft 3 .
 
     After flow calculation  62 , or if there was no brake cylinder flow at check  60 , a check is performed  64  whether there is brake pipe flow. Brake pipe flow occurs when the brake pipe relay maintains the brake pipe pressure against leakage, and during a brake release and recharge. Check  64  may thus be performed by comparing the outputs of pressure transducers  32  and  34  to determine whether there is flow through the 19/64 orifice. If check  64  determines that there is brake pipe flow, the flow volume is calculated  66 . Flow volume in step  66  is calculated based on the brake command state and by the pressure differential across 19/64″ charging orifice  28 , which is positioned between second main reservoir MR 2  and brake pipe relay  30 . Air flow is a function of the pressure drop across an orifice, the size of the orifice, and the upstream pressure. Thus, the outputs of pressure transducers  32  and  34  as well as the known size of 9/64″ charging orifice  28  allows for an estimation of the brake pipe flow. The formula for air flow through an orifice is well known and generally of the form:
 
 Q=C   f   A   o *SqRt(2Δ P /ρ)
 
where Q is flow rate, A o  is the orifice area, C f  is the flow coefficient, ΔP is the pressure across the orifice, and ρ is the air density. The total flow volume through the 19/64 orifice over a period of time can thus be calculated as:
 
Air Vol 19/64  ft 3 =( Q  ft 3 /min)*(Time min).
 
For example, if the measured flow rate is 15 SCFM over a period of 5 minutes, then the total air volume can be calculated to be 75 ft 3 .
 
     After flow calculation  66 , of if there was no brake pipe flow at check  64 , a check is performed  68  whether second main reservoir MR 2  pressure is increasing. Check  68  may be performed by checking air compressor link  42  to determine if compressor  14  is operating, i.e., in the “on” state, and then using the output of pressure transducer  32 , which is in communication with and downstream of second main reservoir MR 2 , to determine any pressure increase in second main reservoir MR 2  over time. If second main reservoir MRS pressure is increasing, flow volume is calculated  70 . For example, the following formula may be used:
 
Air Vol MR2 Charge  ft 3   =V   MR2 *( P   MR2 increase   /P   atm ).
 
For example, it would take 19.7 ft 3  of air to increase the pressure in MR 2  from 125 psi to 145 psi, where MR 2  is 14.5 ft 3 .
 
     After flow calculation  70 , of if there was no second main reservoir MR 2  pressure increasing at check  68 , a final check is performed  72  whether the calculated flow sum from all sources, i.e., the calculated total flow volume, is greater than or equal to the wet air capacity of air dryer  12 . The wet air capacity is defined as the average volume of wet air which will saturate a desiccant tower in air dryer  12 , which is a function of the amount of desiccant in the desiccant column, the chemistry of the desiccant, and related physical characteristics of the particular design of air dryer  12 . Based on these factors, a total wet air capacity can be calculated for the particular air dryer  12  and used for check  72 . For example, railway air dryers are typically rated at inlet conditions of 100% RH, 100° F., and 100 SCFM. At these inlet conditions, the desiccant beds are typically designed to be saturated in approximately 2 minutes. For an air dryer with these design characteristics, the desiccant bed will be saturated after approximately 200 ft 3  has flowed through it. The purge control logic of the present invention may be programmed to switch desiccant beds when the total air, as notionally calculated by purge cycle process  50 , exceeds 200 ft 3 . The wet air capacity of air dryer  12  is preferably a user configurable setting that can be changed to accommodate the particular design of air dryer  12  that is in use. Alternatively, system  10  can be pre-programmed with a list of available air dyers or standard components, and their corresponding wet air capacities, and a user can select the appropriate air dryer or components being used in a particular system  10 . 
     If check  72  determines that the wet air capacity has been reached, the air dryer purge cycle is initiated  74 , such as by sending a purge cycle control signal from CCB system  22  to air dryer  12  via link  40 . The calculated flow volume is then reset to zero  76  and processing returns to the beginning to measure the wet air capacity of the newly active tower in air dryer  12 . If check  72  does not determine that the calculated flow is greater than or equal to the wet air capacity of air dryer  12 , processing returns to the beginning and is repeated with any new flow calculations added to the results of prior flow calculations, thus accumulating the amount of wet air being processed by air dryer  12 , until check  72  determines that enough wet air has passed through air dryer  12  such that it is time to initiate a purge cycle. 
     Using process  50 , control system  10  can thus determine when there is flow through air dryer  12  and the approximate total volume of air that has flowed through air dryer  12  during a given measurement interval. When the volume of air calculated to have flowed through air dryer  12  approximates the wet air capacity of a tower in air dryer  12 , CCB system  22  can command the initiation of a purge cycle based on actual conditions rather than an arbitrary time period. As a result, air dryer system  10  provides very high purge air efficiency because the purging occurs on the basis of the air volume that has actually been processed by air dryer  12  rather than a fixed time interval that may bear no relationship to the actual usage of air dryer  12 .