Air drying systems are well known and practiced in a wide variety of technical fields including the railroad industry.  An air drying system is designed to provide cleaned and dried compressed air to a pneumatic system.  One example of such a pneumatic system is an air brake system of a railroad train. 
An air drying system of a train typically includes a compressor, an air dryer unit and an aftercooler device typically situated between the output of the compressor and the intake port of the air dryer unit.  Usually driven by an engine or an electric motor either directly or through a belt and pulley mechanism, the compressor is essentially an air pump that supplies the compressed air to operate the air brake system and other air actuated equipment on the train such as contactors, switches, reversers, bell ringers, etc.  The compressed air discharged from the compressor, however, typically holds or entraps an unacceptably high quantity of moisture in the form of vapor.  This untreated air should ideally be stripped of all of its moisture before it is inducted into the air dryer unit.  An example of an air dryer unit can be found in U.S. application Ser.  No.  08/597,076 which is incorporated herein by reference. 
While air dryer units are quite efficient in removing condensed moisture and other airborne particulates from a stream of compressed air passing through them, they are not particularly effective in removing vaporized moisture suspended in the compressed air.  Consequently, an aftercooler device is typically inserted between the compressor and the air dryer unit to condense the vapor suspended in the air stream.  One example of an aftercooler device can be found in U.S. Pat.  No.  5,106,270 which is incorporated herein by reference. 
An aftercooler device typically includes a radiator unit and, perhaps, a bypass line along with a bypass valve.  The radiator unit includes an inlet connected to the output of the compressor and an outlet connected to the intake port of the air dryer unit of the air drying system.  The untreated air discharged from the compressor flows into the inlet and through the radiator unit.  The radiator unit has sufficient radiating surface between the inlet and outlet to cool the compressed air from the temperature at which it was discharged from the compressor to generally that within approximately 5 degrees Fahrenheit of ambient air temperature.  Air flow from a fan or other mechanism is directed over the radiating surface of the radiator unit thereby cooling the radiator unit and the untreated compressed air flowing through the radiator unit.  This causes the vaporized moisture necessarily entrapped in compression to condense.  From the outlet of the radiator unit passes the compressed air less the vapor that previously was suspended in it.  This stream of dryer compressed air, hereinafter referred to as aftercooled air, also forcibly carries with it the condensed moisture into the air dryer unit.  The air dryer unit then removes from the stream of aftercooled air passing through it the condensed moisture and the other airborne particulates. 
While operating in environments in which the temperature is sufficiently above the freezing point of water, the aftercooler device described above works quite well.  When operated at temperatures near or below the freezing point, however, the condensed moisture tends to freeze within the passages of the radiator unit thereby restricting the flow of air through the radiator unit.  One solution to this freezing problem is just to stop the flow of air through the aftercooler device until the frozen condensate within the radiator unit melts.  It is obviously quite unacceptable, however, to disrupt long the flow of compressed air to such a critically important system as the air brake system of a railroad train.  Consequently, practitioners in the air drying art have employed the bypass line along with the bypass valve mentioned previously to address this problem. 
The bypass line and bypass valve of prior art aftercooler devices are used to bypass the radiator unit under these circumstances.  The bypass line in these prior art systems connects at one end between the output of the compressor and the inlet of the radiator unit and at the other end between the outlet of the radiator unit and the intake port of the air dryer unit.  The bypass valve connects to the bypass line at the one end at which point it controls whether the untreated air from the compressor flows into the radiator unit or into the air dryer unit. 
Regarding the operation of the prior art aftercooler device in cold environments, as the condensed moisture starts to freeze within and thus restrict the passages of the radiator unit, the pressure drop across the radiator unit increases.  The bypass valve senses this difference in pressure between the inlet and the outlet of the radiator unit.  While the pressure difference is below a threshold value, the bypass valve remains closed.  This allows the untreated air to flow through the radiator unit.  The aftercooled air from the outlet of the radiator unit then passes to the air dryer unit thereby carrying therewith the condensed moisture for expulsion from the air drying system.  When the pressure difference rises above the threshold valve, the bypass valve opens.  The open bypass valve completely diverts flow of the untreated air through the bypass line and directly into the air dryer unit.  The bypass line via the bypass valve thus completely circumvents the radiator unit until the pressure difference reduces to an acceptable level (i.e., until the radiator unit thaws). 
There are several disadvantages inherent to the prior aftercooler device.  Perhaps the most apparent disadvantage is the amount of time that it takes to thaw the radiator unit.  The time it takes to defrost the radiator unit has proven to be quite unacceptable, especially, in colder environments.  Of greatest concern is the quantity of untreated air that is inducted into the air drying system while the radiator unit thaws.  Without the aid of the aftercooler device by which to condense the vapor in the incoming stream of untreated air, the air drying system is unable to remove all, or nearly all, of the moisture from the incoming air stream.  This excess moisture contaminates the air brake system and the other air actuated equipment on the train and can, especially over time, adversely affect their operation. 
Practitioners in the air drying art understand that it is necessary to defrost the radiator unit when air flow through its passages becomes overly restricted.  The pneumatic system to which the air drying system is connected, however, should not be deprived of a steady supply of cleaned and dried compressed air for too long a time.  Under cold environmental conditions, the prior art aftercooler device does deprive the pneumatic system of such cleaned and dried air for an unacceptably long time.  The performance of the prior art aftercooler device has thus proven to be quite unsatisfactory in the rail transportation industry under the aforementioned circumstances. 
It would therefore be quite desirable to devise an aftercooler apparatus that would reduce the amount of time that is needed to defrost the radiator unit.  An air drying system featuring such an aftercooler apparatus would not spend so much time defrosting its radiator unit.  Consequently, an air drying system so equipped would provide to the pneumatic system to which it is connected aftercooled air more often.  Equally important is that an air drying system so equipped would not induct as much moisture bearing air because it need not defrost itself so often.  An air drying system equipped with the present aftercooler invention would more efficiently exsiccate air than an air drying system having the prior art aftercooler device.  The pneumatic system to which the improved air drying system is attached would encounter less moisture and be less likely to suffer the adverse affects of repeated exposure to such moisture. 
The detailed description set forth below makes clear that the aftercooler apparatus of the present invention can be applied to a wide variety of pneumatic systems.  Typical of the types of pneumatic systems to which the aftercooler apparatus could be applied include the air brake systems of passenger and freight railroad trains, subway trains and various other types of rail related transportation systems.  Further examples include the air brake systems of various truck transport vehicles.  Other types of pneumatic systems to which the aftercooler apparatus could be applied may be found outside the transportation field.  Obvious modifications may be necessary, though, depending upon the specific application in which the present invention is employed. 
It should be noted that the foregoing background information is provided to assist the reader in understanding the instant invention.  Accordingly, any terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.