Abstract:
An air dryer having a heater element associated with its valves to prevent freezing at cold temperatures. The air dryer includes a temperature sensor and an electronic controller that reads the temperature sensor and inhibits actuation of the valves whenever the temperature of valves is below freezing or a predetermined temperature that indicates a risk for freezing until the valves have been sufficiently warmed by the heater to avoid freezing during operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to railway air system air dryers and, more particularly, to an air dryer having a heating control system for preventing freezing of valves. 
     2. Description of the Related Art 
     A typical “twin-tower” desiccant-type air dryer includes two drying circuits that are controlled by valves. Wet inlet air flows through one circuit to remove water vapor, while dry product air counter flows through the other circuit to remove the accumulated water and regenerate the desiccant. Inlet and outlet valves for each pneumatic circuit are responsive to controlling electronics to switch the air flow between the two circuits so that one circuit is always drying while the other is regenerating. The air dryer may include a pre-filtration stage with a water separator and/or coalescer positioned upstream of the drying circuits. The pre-filtration stage removes liquid phase and aerosol water and oil that can accumulate in air supply system as a result of the compression of ambient air by the locomotive air compressors. A pre-filtration stage includes a drain valve that is used to periodically purge any accumulated liquid. For example, a typical pre-filtration drain valve actuation cycle might command a purge (open) for two seconds every two minutes. 
     The air dryer valves, including any pre-filtration drain valve, are constantly subjected to wet air and thus prone to freezing at low temperatures. In order to counteract this problem, a heater element may be provided to warm the valves sufficiently to prevent freezing. Unfortunately, it takes time to sufficiently warm the valves when the air supply system is powered up from a cold temperature. If any of the valves are commanded open before they are sufficiently warmed, the valves can freeze in the open position. If a valve remains in an open position when it should otherwise be closed, there is a risk of an uncontrollable venting of the compressed air from the locomotive air supply system. Further, due to the high volume of air flowing through the frozen valve, the heater may not have sufficient power to thaw the frozen valve, if it is frozen open. Thus, there is a need for a heating control system that ensures that the valves are sufficiently warmed before they are operated so that they do not freeze. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention comprises an air dryer having an inlet for receiving compressed air, a series of valves positioned in a valve block for controlling the movement of the compressed air through a desiccant, a heater configured to warm the valve block, a temperature sensor for outputting a signal indicating the temperature of at least a portion of the air dryer, and a controller piloting the series of valves. To prevent a risk of the valves freezing when operated, the controller is programmed to inhibit operation of the series of valves until the signal received from the temperature sensor indicates that the series of valves are warm enough that they will not freeze when operated. The series of valves may include a pair of inlet valves and a pair of exhaust valves associated with a twin-tower desiccant air dryer. The series of valves may also include a drain valve associated with a pre-filtration stage. The temperature sensor is preferably positioned to determine the temperature of air flowing through the air dryer, but may be installed in the valve block or positioned to detect the outside temperature. 
     The present invention also comprises a method of preventing frozen air dryer valves that involves the use of an air dryer comprising an inlet for receiving compressed air, a series of valves positioned in a valve block for controlling the movement of the compressed air through a desiccant, a heater configured to warm the valve block, a temperature sensor for outputting a signal indicating the temperature of at least a portion of the air dryer, a controller piloting the series of valves. The signal indicating the temperature in the air dryer is received by the controller from the temperature sensor, and then the controller inhibits operation of the series of valves if the signal received from the temperature sensor indicates that any of the series of valves could freeze when operated. 
    
    
     
       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 having an air dryer having a heated valve block according to the present invention; 
         FIG. 2  is a schematic of an air dryer with integral pre-filtration stage and a heated valve block according to the present invention; 
         FIG. 3  is a schematic of a heated valve block of an air dryer with pre-filtration state according to the present invention; and 
         FIG. 4  is a flowchart of a heater control process for an air dryer having a heated valve block. 
     
    
    
     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 a two-tower desiccant air dryer  16  having heater control according to the present invention, as more fully described below. Second main reservoir MR 2  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 drain valve purge cycle time. 
     Referring to  FIG. 2 , two-tower desiccant air dryer  16  comprises an inlet  28  for receiving air from first main reservoir MR 1 . Inlet  28  is in communication with pre-filtration stage  30 , shown as comprising a water separator  32 , a coarse coalescer  34 , and a fine coalescer  36 . Any accumulated liquids in water separator  32 , coarse coalescer  34 , and fine coalescer  36  are expelled through drain valve  24 . A pair of inlet valves  42  and  44  are positioned downstream of pre-filtration stage  30  for diverting incoming air between one of two pathways, each of which is associated with one of two dessicant towers  46  and  48 . A temperature sensor  50  is positioned upstream of inlet valves  42  and  44  and downstream of pre-filtration stage  30 . Optionally, the temperature, or a second temperature sensor may be located in the valve block housing the series of valves. The first pathway downstream of first inlet valve  42  leads to an exhaust valve  52  and first desiccant tower  46 . The second pathway downstream of second inlet valve  44  leads to a second exhaust valve  54  and second desiccant tower  48 . The first pathway further includes a first check valve  58  and first bypass orifice  62  downstream of first desiccant tower  46 , and the second pathway further includes a second check valve  60  and bypass orifice  64  downstream of second desiccant tower  48 . A single outlet  66  is coupled to the end of the first and second pathways, and a humidity sensor  68  is positioned upstream of outlet  66 . Inlet valves  42  and  44  and outlet valves  52  and  54  are piloted by controller  40 . Controller  40  operates inlet valves  42  and  44  and outlet valves  52  and  54  so that compressed air provided at inlet  28  is directed through one of desiccant towers  46  or  48  for drying. The other of desiccant towers  46  or  28  may be regenerated by allowing dried air to reflow through bypass orifice  62  or  64  and out of exhaust valve  52  or  54  as needed. Controller  40  is also in communication with temperature sensor  50  and humidity sensor  68 . A heating element  70  may also be coupled to controller  40  and positioned in air dryer  16  to warm drain valve  24 , inlet valves  42  and  44  and outlet valves  52  and  54  if the temperature is below freezing. 
     As seen in  FIG. 3 , the air dryer pathways seen in  FIG. 1  are arranged so that drain valve  24 , inlet valves  42  and  44 , and outlet valves  52  and  54  are commonly located along with heater element  70  in a valve block  72 . As explained above, air dryer  16  includes temperature sensor  50  for determining the approximate temperature of valve block  72  and thus drain valve  24 , inlet valves  42  and  44 , and outlet valves  52  and  54 . Temperature sensor  76  is shown as being positioned to detect the temperature of air passing through air dryer  16 , but may be positioned to detect the temperature of valve block  72 , the temperature of the inlet air, the temperature of ambient air, or some combination of the above. 
     Referring to  FIG. 4 , air dryer controller  40  is programmed to implement a heater control process  80  to ensure that valve block  72  is sufficiently heated to a temperature that avoids the likelihood that drain valve  24 , inlet valves  42  and  44 , or outlet valves  52  and  54  can become frozen. First, controller  40  reads the temperature  82  such as by using temperature  76  positioned in valve block  72 . Next, a check  84  is performed to determine whether the temperature is below freezing (or any other predetermined temperature selected to be indicative of a risk that drain valve  24 , inlet valves  42  and  44 , or outlet valves  52  and  54  will become frozen). If the temperature is below the threshold at check  82 , controller  40  inhibits valve operation  86 , such as by inhibiting the operation of drain valve  24 , inlet valves  42  and  44 , and/or outlet valves  52  and  54  until such time as the temperature has risen above the threshold. Thus, if air dryer  16  is turned on after an extended cold soak at low temperature, controller  40  will affirmatively inhibit actuation of drain valve  24 , inlet valves  42  and  44 , and/or outlet valves  52  and  54  until heater element  70  has warmed valve block  72  sufficiently to prevent any of drain valve  24 , inlet valves  42  and  44 , and outlet valves  52  and  54  from freezing in an open position and causing an undesired venting of compressed air from locomotive air supply system  10 . Preferably, inlet valves  42  and  44  are normally open and exhaust valves  52  and  54  are normally closed in the unpowered state, so that compressed air may flow through air dryer  16  to MR 2  when all valves are in an unpowered state. By using closed loop temperature feedback to control inhibit the operation of the series of valves, the start-up time for a cold air dryer is proportional to the starting temperature. Alternatively, a simple system which uses a fixed time delay calculated to allow the valve block to warm to above freezing for the worst case condition may be provided. 
     The same sensor and controller may be used to turn off the heater, when the temperature of the valve block is at or above the target temperature, thus regulating the temperature of the valve block to a temperature above freezing when the ambient temperature is below freezing; and turning the heater off completely when the ambient temperature, as indicated by the temperature of the valve block, is above freezing.