AIR DRYER CONTROL

Abstract

Control system for an air dryer with two desiccative towers (46, 48) which can switch the flow and counterflow of air between each of two drying circuits, each of which is assigned to a corresponding one of the two desiccative towers, comprising: a first temperature sensor (50) for arrangement in an inlet (28) of the air dryer, which is configured to output a first signal corresponding to the temperature of an inlet airflow; a second temperature sensor (72) for arrangement in an outlet (66) of the air dryer, which is configured to output a second signal corresponding to the temperature of an outlet airflow; a humidity sensor (68) for arrangement in the outlet (66) of the air dryer for outputting a third signal corresponding to the humidity of the outlet airflow; and a controller (40) connected to the first temperature sensor (50), the second temperature sensor (72) and the humidity sensor (68) and programmed to calculate, based at least partially on the humidity of the outlet airflow, whether one of the two desiccative towers (46, 48) is saturated, and to instruct the air dryer to switch the airflow to the other of the two desiccative towers when one of the two desiccative towers is saturated, characterized in that the control (40) is programmed to determine whether one of the two desiccative towers is saturated by calculating an instantaneous dew point suppression as the difference between the dew point at the inlet (28) and the dew point at the outlet (66) and comparing it with a predetermined threshold.

Description

REFERENCE TO RELATED REGISTRATIONS

[0001] The present application claims priority from the preliminary US application 62 / 236960 dated October 4, 2016. BACKGROUND OF THE INVENTION 1. AREA OF THE INVENTION

[0002] The present invention relates to desiccative air dryers and in particular to a system and a method for controlling the regeneration cycle of a twin-tower desiccative air dryer. 2. DESCRIPTION OF THE STATE OF THE ART

[0003] Air dryers for railways are typically pressure fluctuation adsorption dryers, also known as twin-tower desiccative air dryers. The basic control scheme for switching between the two desiccative columns features a fixed timer triggered by a "compressor ON" signal from the compressor control unit. When the compressor is running, the air dryer cycles between the two desiccative columns in a fixed time interval. It passes wet product air through one column to remove the water vapor, producing dry product air, while simultaneously taking a portion of the dry product air and passing it counter-currently through the other, previously saturated desiccative column to remove accumulated moisture. This control scheme is simple and robust, but it is also inefficient and wastes considerable energy.

[0004] An air dryer with a fixed time cycle and also a dew point monitor at the air outlet, which continues the drying phase as long as the dew point is better than a specified value, is disclosed in US 7279026 B1, from which the preamble of claim 1 is derived. Related prior art is disclosed in CN 104122834 A.

[0005] A typical AAR locomotive air supply system consists of a compressor and two main reservoirs in series, MR1 and MR2. The air dryer is usually installed between MR1 and MR2, so that dry air is supplied to MR2. MR2 is used as the sole air source for the train's braking system and is protected by a check valve between MR1 and MR2. The air in MR1 is used for other locomotive air consumers such as windshield wipers, horn, sandblaster, etc. When air is needed from MR1 or MR2, the compressor operates to replenish the system. If the air pressure in MR1 is lower than in MR2, the compressor operates so that air flows into MR1, refilling it; but until the pressure in MR1 is higher than that in MR2, no air flows into MR2. In this situation, the air dryer regeneration cycle is triggered by a compressor ON signal. However, because no air flows between MR1 and MR2, no air flows through the air dryer either.Therefore, the dry product rinse air consumed by the air dryer regeneration cycle is wasted.

[0006] A second inefficiency of the existing fixed-timer regeneration control scheme lies in the assumption that the water content of the incoming wet air is constant, and the fixed time cycle is based on the worst-case scenario with maximum flow and maximum wet air. The amount of water vapor in air is directly proportional to the saturation water vapor partial pressure, which has a strongly nonlinear, exponential-like relationship with temperature. For example, the saturation water vapor partial pressure at 0 °F is 0.01857 psia (pounds per square inch absolute); 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. By comparison, air at 125 °F can contain 5.35 times more water vapor than air at 70 °F, and air at 150 °F can contain 10.2 times more water vapor than air at 70 °F.Air at 125 °F can therefore contain 105 times more water vapor than air at 0 °F, and air at 150 °F can contain 200 times more water vapor than air at 0 °F.

[0007] It is therefore clear that a fixed-cycle regeneration cycle for an air dryer designed for a desiccative bed water absorption capacity and saturated air moisture content at the maximum air inlet temperature of, for example, 150 °F, will complete the cycle much more frequently than necessary at lower temperatures, thus wasting dry product rinse air. For example, a system designed for saturated air at 150 °F will complete the cycle 10.2 times too often at 70 °F and 200 times too often at 0 °F. At 70 °F, this translates to approximately (17% - 17% / 10.2) = 15% of product air and compressor energy being wasted.

[0008] Therefore, there is a need for an air dryer with a more efficient control system for the regeneration cycle. SUMMARY OF THE INVENTION

[0009] The present invention solves this problem with a control system for an air dryer and a method for controlling the air dryer according to the attached patent claims.

[0010] One embodiment relates to a control system for an air dryer that switches the flow and counterflow of air through each of two desiccative towers in response to the actual humidity of the air being dried by the air dryer. The air dryer incorporates a humidity sensor, a temperature sensor in the outlet air stream, and a temperature sensor in the inlet air stream to determine when to initiate desiccative regeneration. A fully regenerated desiccative bed provides maximum drying performance; as the desiccative bed becomes increasingly saturated, the drying performance decreases until it reaches zero when the desiccative bed is completely saturated with water. The air dryer system is generally specified to deliver a minimum drying performance, for example, a dew point suppression of 40°C.

[0011] The air dryer's control system is configured to measure the temperature of the inlet air, the temperature of the outlet air, and the relative humidity at the outlet. The control system then calculates the instantaneous dew point suppression (DPS). If the measured DPS is equal to or greater than the minimum target DPS, the air dryer continues to allow air to flow through the designated drying circuit (for example, circuit A). The measurement and calculation process continues and is repeated at regular intervals. If the calculated DPS temperature is lower than the minimum target DPS, the control system switches from drying circuit A to circuit B and begins a regeneration cycle in drying circuit A.

[0012] This closed-loop control system for the regeneration cycle, utilizing the humidity sensor, takes into account the temperature-dependent water volume in the air and a variable flow rate through the air dryer. The dryer initiates a regeneration cycle only when the desiccant is saturated with water to such an extent that the DPS at the outlet is less than or equal to the minimum setpoint. Because the drying capacity also depends on the dwell time of the air in the desiccant bed, air flowing through the desiccant bed at a low flow rate can achieve a higher DPS than air flowing through it at a high flow rate, given the same relative desiccant bed condition. By measuring and calculating the DPS at the outlet, regeneration is optimized for each flow rate.

[0013] A maximum time interval of, for example, 30 minutes between regeneration events and a minimum time of, for example, two minutes between regeneration intervals may be provided to ensure minimum performance in the event of a sensor or component failure. Brief description of the views in the drawings

[0014] The present invention will become more fully understood by reading the following detailed description in conjunction with the accompanying drawings. The drawings show: Fig. 1 schematically a locomotive air supply system with a double-tower desiccative air dryer in which a closed-loop control system according to the invention can be used; Fig. 2 schematically a double-tower desiccative air dryer in which a closed-loop control system according to the invention can be used; Fig. 3 a flowchart of a closed-loop control system for an air dryer according to the present invention; Fig. 4 a diagram of the water vapor partial pressure versus the ambient temperature for use in the programming of the control system of the present invention; Fig. 5 a diagram of an environmental distribution profile for the United States, which is used to model the improved efficiency of the air dryer of the present invention compared to conventional air dryers; Fig. 6 a diagram of the amount of purge air used by the air dryer of the present invention compared to conventional air dryers. DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to the drawings, in which the same reference numerals consistently denote the same parts, it is stated in Fig. Figure 1 shows a locomotive air system 10 comprising an air compressor 12, an aftercooler 14, a first and a second main reservoir MR1 and MR2, and a twin-tower desiccative air dryer 16 with a desiccative regeneration control according to the present invention, as described in more detail below. The second main reservoir MR2 is connected to a brake system 18 and a check valve 20 arranged between the first and the second main reservoirs MR1 ​​and MR2. The air dryer 16 is associated with a pre-filter stage 22, which can be separate or, as described here, integral and includes a drain valve 24 that is operated according to a drain valve flushing cycle time.

[0016] With reference to Fig. Figure 2 of the twin-tower desiccative air dryer 16 includes an inlet 28 for receiving air from the first main reservoir MR1. The inlet 28 is connected to an integral pre-filter stage 30, which is shown with a water separator 32, a coarse separator 34, and a fine separator 36 or coalescer. Accumulated liquids in the water separator 32, coarse separator 34, and fine separator 36 are discharged by the drain valve 24. Downstream of the pre-filter stage 30, a pair of inlet valves 42 and 44 is arranged to direct incoming air to one of two lines, each associated with one of two desiccative towers 46 and 48. Upstream of the inlet valves 42 and 44 and downstream of the pre-filter stage 30, a temperature sensor 50 is arranged. The first line downstream of the first inlet valve 42 leads to an exhaust valve 52 and a first desiccative tower 46.The second line downstream of the second inlet valve 44 leads to a second exhaust valve 54 and a second desiccative tower 48. The first line also includes a first check valve 58 and a first purge port 62 downstream of the first desiccative tower 46, and the second line also includes a second check valve 60 and a purge port 64 downstream of the second desiccative tower 48. A single outlet 66 is connected to the end of both the first and second lines, and a humidity sensor 68 and a second temperature sensor 72 are located upstream of the outlet 66. The inlet valves 42 and 44 and the outlet valves 52 and 54 are controlled by a controller 40.

[0017] The control unit 40 operates the inlet valves 42 and 44 and the outlet valves 52 and 54 such that compressed air present at the inlet 28 is directed through one of the desiccative towers 46 or 48 for drying. The other desiccative tower 46 or 48 can be regenerated by allowing dry air to flow back through the purge port 62 or 64 and out of the exhaust valve 52 or 54 as needed. The control unit 40 is also connected to the temperature sensor 50, the humidity sensor 68, and the temperature sensor 72. A heating element 70 can also be connected to the control unit 40 and located in the air dryer 60 to heat the outlet valve 24, the inlet valves 42 and 44, and the outlet valves 52 and 54 when the temperature is below freezing.

[0018] With reference to Fig. Controller 40 is programmed to provide a closed-loop humidity feedback process 80, which switches the flow and counterflow of air through each of the desiccative towers in response to the actual humidity of the air dried by the air dryer 16. Controller 40 collects data 82 from temperature sensor 50, humidity sensor 68, and temperature sensor 72 in the outlet air to determine when to initiate desiccative regeneration. A fully regenerated desiccative bed provides the highest drying performance; as the desiccative bed becomes progressively saturated, the drying performance decreases until it reaches zero when the desiccative bed is completely saturated with water.Since an air dryer system is generally specified to deliver a minimum drying performance, for example, a dew point suppression of 40 °C, the controller 40 can be programmed to ensure that the air dryer 16 meets this requirement and, if it does not, to switch the drying circuit from the saturated circuit to the unsaturated circuit and perform a regeneration cycle on the saturated desiccant in the saturated circuit so that it is ready for future use. According to the invention, the controller can calculate the instantaneous dew point suppression (DPS) of the air dryer 16 using the data collected by the temperature sensor 50, humidity sensor 68, and temperature sensor 72. The DPS can be calculated using a measured air inlet temperature and the measured outlet humidity, using the known relationship between temperature and water vapor partial pressure, as described in [reference]. Fig. The four shown can be calculated. For example, the August-Roche-Magnus approximation calculates the dew point temperature in °C (TD) as a function of the temperature °C (T) and the relative humidity (RH) as follows: TD=243.04*(((LN(RH / 100)+((17.625*T) / (243.04+T))) / (17.625−LN(RH / 100)−((17.625*T)*(243.04+T)))). Other equations and methods for calculating the dew point temperature using air temperature and relative humidity are well known in the field of psychometrics. The August-Roche-Magnus approximation is therefore shown as an example, and the invention is not limited to its use, as other approximations would also be sufficient.

[0019] The inlet air is assumed to be at 100% RH, i.e., with a dew point temperature equal to the inlet temperature due to the air system's 10:1 compression ratio. Dew point suppression is calculated as the difference between the inlet dew point temperature and the calculated outlet dew point temperature. If a test 86 determines that the measured DPS is equal to or greater than the minimum setpoint DPS, the air dryer 16 continues to allow air to flow through the currently used drying circuit, such as circuit A. The controller 40 then repeats the collection of relevant data at regular intervals until the measured DPS is no longer equal to or greater than the minimum setpoint DPS. If the calculated DPS temperature for test 86 is lower than the minimum setpoint DPS, the controller 40 switches from drying circuit A to circuit B at 88 and initiates a regeneration cycle on drying circuit A.The target DPS should be understood as a threshold, and System 10 could also be configured to initiate a regeneration cycle when the calculated DPS is equal to or less than the target DPS, and not to initiate it when the DPS is higher than the target DPS. The threshold DPS could also have a certain tight tolerance.

[0020] The closed-loop control of the regeneration cycle by the controller 40, using temperature sensor 50, humidity sensor 68, and temperature sensor 72, takes into account the temperature-dependent water volume in air and a variable air flow rate through the air dryer. The air dryer 16 thus initiates a regeneration cycle only when the desiccant in the circuit becomes saturated with water to such an extent that the outlet DPS is less than or equal to the minimum threshold. Because the drying capacity of the air dryer 16 also depends on the residence time of the air in the desiccant bed, air flowing through the desiccant bed at a low flow rate can achieve a higher DPS than air flowing through the desiccant bed at a higher flow rate, given the same relative desiccant bed condition. By measuring and calculating the outlet DPS, regeneration is optimized for each flow rate.

[0021] It is understood that the control unit 40 can be programmed to calculate the specific amount of water content in the outlet 66 using the humidity sensor 68 and to compare this water content with predetermined standards or a maximum permissible content for a specific brake system 18. According to Fig. 3. A maximum time interval 90 between regeneration events, independent of the calculated saturation, for example 30 minutes, and a minimum time 92 between regeneration intervals, independent of the calculated saturation, for example 2 minutes, can be provided in order to deliver a minimum performance level in the event of a sensor or component failure, so that the air dryer 16 continues to remove moisture from the compressed air, although then less efficiently than with a functioning closed control loop.

[0022] Because the air dryer 16 is typically installed between MR1 and MR2, the actual airflow through the air dryer 16 also depends on the relative charge levels of MR1 and MR2. For example, if both MR1 and MR2 are equally depleted when the compressor is running, half of the compressor flow will fill MR1 and half will pass through the air dryer to MR2. Depending on the motor speed during this charging event, the air dryer might see, for example, between 50 SCFM and 92 SCFM, standard cubic feet per minute. For example, if the pressure in MR1 is depleted and the pressure in MR2 is at full charge (note that there is a check valve between MR1 and MR2), 100% of the compressor flow will recharge MR1, while there will be no flow through the air dryer.Finally, with the compressor switched off, air can flow from the fully charged MR1 through the air dryer to MR2 (and on to the train brakes). It is thus evident that the flow through the air dryer can vary from 0 to 100% of the nominal compressor output. The desiccative bed in the air dryer 16 can hold a fixed amount of water before it becomes saturated, and the time until the bed becomes saturated depends on both the air temperature (warmer air can hold more water vapor) and the volume of air that has flowed through the desiccative. The variable regeneration control in a closed-loop system by the controller 40 takes both the temperature effect and the flow rate into account and regenerates only when the desiccative bed approaches saturation. EXAMPLE 1

[0023] With reference to Fig. 5. The purge air savings of the air dryer compared to a conventional control scheme can be estimated by using an environmental distribution profile of a proposed geographic area, such as the United States, representing the time a given locomotive is exposed to environmental temperatures in a typical year. The comparison assumes a locomotive year of 8141 hours (95% of a calendar year), a compressor 12 with a runtime of 1625 load hours in a locomotive year, and a flow rate through the air dryer 16 of 100 SCFM. A continuous train leak of 20 SCFM is assumed. A conventional air dryer is modeled with a 65-second drying cycle and a 65-second regeneration cycle consisting of 48 seconds of purging and 17 seconds of pressure rebuild. It is also assumed that the conventional air dryer only has cycles when the compressor is running.

[0024] The flushing loss is conventionally assumed to be 15% (15 SCFM), which is intended to take into account a memory effect and a flow-dependent flushing volume.

[0025] The air dryer 16 according to the present invention is modeled with a temperature-dependent variable drying cycle and includes a fixed regeneration time of 110 seconds, followed by a 10-second pressure rebuild. To calculate the drying cycle time, the water load rate is calculated for each temperature range for both 100 SCFM (recharging of MR2 when the compressor is switched on) and 20 SCFM (flow through the air dryer to MR2 and out as a BP leak, brake line leak). The drying cycle time is then calculated based on the known water capacity of the desiccator tower and the water loading rate. In this example, the maximum drying cycle time of the combined cycle A plus B is limited to 3600 seconds (1 hour), although this time could be extended further at lower temperatures. The air dryer 16 operates independently of the compressor 12 and regenerates as needed, regardless of the compressor's operating state.

[0026] With reference to Fig. Calculations show that, over a year of operation in an environmental profile typical for the continental United States, air dryer 16 requires 513,500 cubic feet less purge air than a conventional air dryer. At a cost of $0.32 per 1,000 ft... 3 For compressed air, this represents an annual saving of USD 164.

[0027] Another advantage of the air dryer 16 is that it overcomes a deficiency in the conventional air dryer control scheme. A conventional air dryer only regenerates when the compressor is running; however, the compressor is off 80% of the time. A train leak, however, is continuous, so when main reservoirs MR1 ​​and MR2 are full, the compressor shuts off, and the air in both main reservoirs MR1 ​​and MR2 supplies the brake lines BP until the pressure in MR1 drops to the switching point of the pressure regulator, at which point the compressor restarts. The air flowing from MR1 through the air dryer to MR2 while the compressor is off is thus not accounted for by the conventional air dryer control scheme and can overload a conventional air dryer desiccative bed at higher temperatures, rendering the air dryer ineffective.However, the air dryer 16 of the present invention will still provide sufficient drying because it performs a cycle based on the moisture content of the air being dried, and not simply according to a predetermined time period when the compressor is running.

[0028] The Air Dryer 16 also offers a significant advantage at low temperatures. At sub-zero temperatures, the Air Dryer 16 uses considerably less purge air than a conventional air dryer. This is particularly beneficial because overall draft leakage increases at low temperatures, and the reduced air consumption of the Air Dryer 16 effectively compensates for this. EXAMPLE 2

[0029] In field trials, the air dryer 16 was able to maintain a minimum dew point suppression of 40 °C (72 °F), with more than 80% of purge cycles occurring at a cycle maximum of one hour. In this case, one cycle is defined as the cycle time for tower A and tower B consecutively. For comparison, the same cycle on a conventional air dryer takes just under two minutes. Because the air dryer 16 uses a closed humidity feedback control loop to initiate the purge cycle, it takes into account both a much lower moisture load at low temperatures and an actual airflow through the dryer. The optimized purge provides both energy and air savings by preventing the waste of purge air and advantageously reduces wear and tear on the air dryer components, thus extending the service life of the air dryer 16.For example, the tested air dryer 16 operated for 3427 hours and completed approximately 3000 purge cycles during this time. Had the air dryer 16 instead used the traditional 2-minute purge (1 minute A, 1 minute B), it would have performed 38,000 purge cycles, assuming a memory mode operation in which it only performs cycles when the compressor is on and there is a load (20% of 3427 hours). Furthermore, the air dryer 16 has a capacity of nearly 450,000 cubic feet (ft.). 3 ) uses less purge air over the given period than a conventional air dryer. In addition to the clear savings resulting from less wasted purge air, the significantly reduced number of purge events means that the valves in the air dryer 16 are subject to much less wear and tear, resulting in a longer service life and greater reliability.

Claims

[1] Control system for an air dryer with two desiccative towers (46, 48) which can switch the flow and counterflow of air between each of two drying circuits, each of which is assigned to one of the two desiccative towers, comprising: a first temperature sensor (50) for arrangement in an inlet (28) of the air dryer, which is configured to output a first signal corresponding to the temperature of an inlet airflow; a second temperature sensor (72) for arrangement in an outlet (66) of the air dryer, which is configured to output a second signal corresponding to the temperature of an outlet airflow; a humidity sensor (68) for arrangement in the outlet (66) of the air dryer for outputting a third signal corresponding to the humidity of the outlet airflow; and a controller (40) connected to the first temperature sensor (50), the second temperature sensor (72) and the humidity sensor (68) and programmed to calculate, based at least partially on the humidity of the outlet airflow, whether one of the two desiccative towers (46, 48) is saturated, and to instruct the air dryer to switch the airflow to the other of the two desiccative towers when one of the two desiccative towers is saturated, characterized by , that the control (40) is programmed to determine whether one of the two desiccative towers is saturated by calculating an instantaneous dew point suppression as the difference between the dew point at the inlet (28) and the dew point at the outlet (66) and comparing it with a predetermined threshold. [2] System according to claim 1, wherein the control (40) is programmed to determine whether one of the two desiccative towers (46, 48) is saturated when the instantaneous dew point suppression is lower than the predetermined threshold. [3] System according to claim 2, wherein the control (40) is programmed not to determine that one of the two desiccative towers (46, 48) is saturated when the instantaneous dew point suppression is at least as large as the predetermined threshold. [4] System according to claim 3, wherein the control (40) is programmed not to instruct the air dryer to switch the airflow to the other of the two desiccative towers (46, 48) unless a predetermined minimum time period has been exceeded. [5] System according to claim 4, wherein the control (40) is programmed to instruct the air dryer to switch the airflow to the other of the two desiccative towers (46, 48) when a predetermined maximum time period is exceeded. [6] System according to claim 5, wherein the control (40) is programmed to calculate the instantaneous dew point suppression using the August-Roche-Magnus approximation. [7] Method for controlling an air dryer to switch the flow and counterflow of air between each of two drying circuits, each of which is assigned to a corresponding of two desiccative towers (46, 48), comprising the following steps: Measuring the temperature of an inlet airflow in the inlet (28) of the air dryer; Measuring the temperature of an outlet airflow in the outlet (66) of the air dryer; Detecting the humidity of the airflow at the outlet (66); based at least partially on the humidity of the airflow in the outlet (66), calculate whether one of the two desiccative towers (46, 48) is saturated; and Switching the airflow to the other of the two desiccative towers, (46, 48) when one desiccative tower is calculated to be saturated, wherein the step to calculate whether one of the two desiccative towers (46, 48) is saturated, based at least partially on the moisture content of the airflow at the outlet (66), includes calculating a present dew point suppression as the difference between the dew point at the inlet (28) and the dew point at the outlet (66) and comparing the present dew point suppression with a predetermined threshold. [8] Method according to claim 7, wherein the step of calculating whether one of the two desiccative towers (46, 48) is saturated further comprises determining that one of the two desiccative towers is saturated if the current dew point suppression is lower than the predetermined threshold. [9] Method according to claim 8, wherein the step of calculating whether one of the two desiccative towers (46, 48) is saturated further comprises not determining that one of the two desiccative towers is saturated if the current dew point suppression is greater than the predetermined threshold. [10] Method according to claim 9, wherein the step to switch the airflow through the other of the two desiccative towers (46, 48) is not carried out unless a predetermined minimum time period has been exceeded. [11] Method according to claim 10, comprising a step to switch the airflow through the other of the two desiccative towers (46, 48) when a predetermined maximum time period is exceeded. [12] Method according to claim 11, wherein the step to calculate whether one of the two desiccative towers (46, 48) is saturated comprises calculating the current dew point suppression using the August-Roche-Magnus approximation.

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