text: fig1 through 6 depict exemplary embodiments , for purposes of illustration , of a refueling apparatus according to the present invention . one skilled in the art will readily recognize that the principles of the present invention are equally applicable to embodiments of gaseous fuel refueling systems other than the particular embodiments shown in the drawings . referring to fig1 a refueling module or apparatus 10 is preferably enclosed by a housing 12 , which includes louvered portions 14 to allow for the circulation of air therethrough and a control panel 16 thereon . it should be noted at the outset that although the control panel 16 is located on the housing 12 in the exemplary refueling apparatus , the invention also contemplates a remote control panel mounted separate from the refueling module , such as inside the user &# 39 ; s home , for example . a flexible outlet conduit 20 , with a suitable connector 22 at its free end , is adapted to be releasably connected to a vehicle or other gaseous fuel consuming device in order to discharge the gaseous fuel thereto . as will be discussed below in more detail , the exemplary refueling module 10 is preferably constructed to be housed in a small , unobtrusive module - type package and is designed to operate on ordinary residential electrical supply systems ( e . g . 110 - 230 volt systems ) in order to provide a convenient and easy - to - operate system for home refueling of a gaseous fuel powered vehicle or other device . one skilled in the art will readily recognize , however , that the principles of the present invention are equally applicable to larger versions of a refueling module , which are adapted for commercial use and which are capable of simultaneous multi - vehicle refueling , for example . referring to fig2 and 3 , the exemplary refueling module 10 is generally illustrated both in perspective and in schematic views . the refueling module 10 includes an inlet 28 adapted to be connected to a gaseous fuel supply system 30 by means of a conventional connector device of the type known to those skilled in the art . preferably , the gaseous fuel supply system 30 comprises a natural gas supply system such as that commonly found in many residential and commercial facilities . the exemplary refueling module 10 also preferably includes a pair of manual shut - off valves 32 for shutting down the module during extended periods of non - use or for isolating the module from the supply system 30 for purposes of servicing or repairing the module . the gaseous fuel from the supply system 30 is typically at 1 / 4 psig ( 1 . 72 kpa ) for example , and flows through a preferably electrically - operated solenoid valve 34 , a desiccant filter apparatus 36 , and a check valve 38 , into the suction 40 of a first stage gas compressor 42 . although various types of desiccant filters may be employed for removing water vapor or other moisture from the incoming gaseous fuel , the desiccant filter 36 preferably employs a sorbent material , such as an activated carbon , a zeolite material , a silica gel - type material , or various clays , for example . the first stage gas compressor 42 compresses the gaseous fuel , and thereby increases its pressure to a predetermined desired pressure level . in one actually - constructed prototype embodiment of the present invention , such predetermined gas pressure at the discharge 44 of the first stage gas compressor 42 was generally in the range of 5 to 60 psig ( 34 to 414 kpa ). the exact pressure in any particular application of the present invention will , of course , depend upon the pressure in the discharge line 44 and may vary in accordance with various operating conditions and design factors such as whether or not an additional compressor is included in the refueling module to futher compress the gaseous fuel , as well as other factors readily recognizable by those skilled in the art . from the discharge 44 of the first stage gas compressor 42 , the gaseous fuel flows through a gas cooler 50 , which is preferably a cooling coil over which ambient air is forced by a cooling fan 52 . in one actually - constructed prototype embodiment of the refueling module 10 , the temperature of the gaseous fuel entering the gas cooler 50 was a maximum of approximately 240 f . ( 116 c . ), and the gaseous fuel temperature on the outlet of the gas cooler 50 was substantially at ambient temperature . although the gas cooler 50 has been depicted in the drawings as comprising the cooling coil apparatus described above , one skilled in the art will readily recognize that other types of heat exchanger apparatus or cooling means may alternately be employed to reduce the temperature of the compressed gaseous fuel from the discharge 44 of the first stage gas compressor 42 . from the gas cooler 50 , the compressed gaseous fuel preferably flows through a lubricant filter and separator 56 , an interstage accumulator or pulsation chamber 58 , and a check valve 60 , into the suction 62 of a second stage gas compressor 64 , where it is further compressed to further increase its pressure to another predetermined pressure level . the lubricant filter and separator 56 may comprise any of a number of known filter - type devices adapted to remove lubricating oil or liquids from a gas stream passing therethrough . the lubricant filter and separator 56 functions to return compressor lubricants to the suction 40 of the first stage gas compressor 42 in a manner which will be described in detail below . the pulsation chamber 58 is an accumulator - type vessel , which serves to damp out any gas pressure surges or pulsations from the first stage compressor 42 . unexpectedly , it was found that hermetically - sealed gas compressors of the type commonly employed in refrigeration apparatus were ideally suited for use as the first and second stage compressors described above . such compressors are inexpensive , durable and readily available as off - the - shelf items . one skilled in the art will readily recognize , of course , that other compressors may alternatively be used . the second stage gas compressor 64 further compresses the previously - compressed gaseous fuel to a pressure in a range of approximately 100 psig ( 689 kpa ) to approximately 400 psig , such discharge pressure preferably being approximately 300 to 350 psig ( 2070 to 2415 kpa ). the exact discharge pressure from the second stage gas compressor 64 will , of course , depend upon the pressure in the discharge line 66 . one skilled in the art will , of course , readily recognize that either a single two - stage compressor , or even a single one - stage compressor , may be substituted for the two compressors 42 and 64 in order to compress the gaseous fuel to the desired pressures . preferably in such case , the single two - stage compressor would include appropriate inlets and outlets for interstage connection of the gas cooler 50 . alternatively , if only a single , one - stage compressor is employed , the gas cooler 50 would be connected to the discharge side of such a compressor . from the discharge 66 of the second stage gas compressor 64 , the compressed gaseous fuel preferably flows back through the gas cooler 50 , where it is again cooled from a maximum temperature of approximately 240 f . ( 116 c .) to substantially ambient temperature at the outlet of the gas cooler . the cooled and compressed gaseous fuel then flows through a second lubricant filter and separator 68 , which is substantially similar to the previously - mentioned lubricant filter and separator 56 , and which serves the same function of returning compressor lubricants to the suction 62 of the second stage gas compressor 64 as will be described in detail below . since at ambient temperatures most gaseous fuels are capable of containing vaporized or entrained lubricants or moisture , an optional moisture - removing means may be included downstream of the lubricant filter and separator 68 . the inclusion of such moisture - removing means is , of course , optional if desired or deemed necessary . two of several alternate embodiments of the optional moisture - removing means are shown in fig5 a and 5b , and will be described in detail below . the previously cooled compressed gaseous fuel next preferably flows through a check valve 70 and into one or more adsorbent filters 72 . the sorbent filter 72 preferably comprises a chamber containing a sorbent material , such as a zeolite , an activated carbon , a silica gel , or various clays , for example . the sorbent filter 72 preferably functions to remove any remaining compressor lubricants or other materials , and also to remove the so - called &# 34 ; heavy end &# 34 ; constituents of the gaseous fuel . generally speaking , such heavy end constituents include propane and other constituents that are heavier than methane . the purpose of removing such heavy end constituents is to maximize the capability of the storage tanks on the vehicle ( or other gaseous fuel consuming device ) to sorptively store the constituents of the gaseous fuel for which the vehicle &# 39 ; s engine or other fuel consuming device is designed . from the sorbent filter 72 , the compressed gaseous fuel flows through a check valve 76 , and then flows into an inlet 78 for one or more optional , but preferred , sorbent storage vessels 82 , or flows into the refueling module discharge system 86 . the path of the fuel flow depends upon the function of a control system ( described below ) which automatically opens or closes various solenoid valves in response to various gas pressure conditions throughout the system . it should be noted that the sorbent storage vessels 82 are optional , but preferred . an even less expensive refueling apparatus according to the present invention may be provided that eliminates the sorbent storage tanks ( and their associated controls and apparatus ) altogether , or that includes storage tanks that do not contain a sorbent material therein . if the outlet conduit 20 is disconnected from the vehicle or other gaseous fuel consuming device , and the gas pressure in the sorbent storage vessels 82 is below a predetermined pressure level , the compressed gaseous fuel will flow through the inlet 78 and a preferably electrically - operated solenoid valve 80 to recharge the sorbent storage vessels 82 . such recharging of the storage vessels 82 will also occur even if the outlet conduit 20 is still connected to the vehicle storage system 100 and the pressure in the vehicle storage vessels or tanks is generally at or above a desired pressure level , generally approximately 300 to 350 psig ( 2070 to 2415 kpa ). alternatively , if the pressure in the vehicle &# 39 ; s storage tanks is below such desired pressure level , the control system ( described below ) will cause the compressed gaseous fuel to flow through a preferably electrically - operated solenoid valve 90 and into a manual discharge valve 96 , which may be either in the outlet conduit 20 or incorporated into the connector device 22 . preferably , a check valve 101 is included in the vehicle storage system 100 to prevent the gaseous fuel from returning to the refueling apparatus . downstream of the solenoid valve 90 , a vent line 88 is preferably provided for venting compressed gaseous fuel from the outlet conduit 20 when the connector 22 is disconnected from the vehicle or other gaseous fuel consuming device after the refueling operation is completed . as will be described in detail below , the preferred control system automatically causes a preferably electrically - operated solenoid valve 94 to open in order to allow such vented compressed gaseous fuel to be released into a holding chamber or vessel 98 . once the pressure in the outlet conduit 20 has been reduced to a sufficiently low level , the solenoid valve 94 closes , to isolate the holding chamber 98 between the solenoid valve and a check valve 48 . upon later actuation of the gas compressors , the gaseous fuel in the holding chamber 98 is caused to flow through the check valve 48 into the suction 40 of the first stage gas compressor 42 . the above - described automatic venting system for the outlet conduit 20 is optional , but is preferably included in the refueling module system in order to relieve the pressure in the outlet conduit 20 , thereby allowing for ease of disconnection of the connector 22 . it should be noted , however , that the manual discharge valve 96 is preferably a three - way manually - operated valve which also allows the operator to manually vent the outlet conduit 20 directly to the atmosphere or to other gas collection or disposal means in order to allow for ease of disconnecting the connector 22 . the refueling module 10 also preferably includes a pressure relief valve 37 , which is upstream of the check valve 38 and which opens in response to an unacceptably high gas pressure level and relieves such pressure by venting the gas to the atmosphere through an atmospheric vent 25 . similarly , a pressure relief valve 53 is provided on the discharge 44 of the first stage gas compressor 42 , and a pressure relief valve 73 is provided at the outlet of the adsorbent filter 72 . the pressure relief valves 53 and 73 are adapted to open in response to undesirably high gas pressures and to relieve such pressures by venting gaseous fuel through their respective vent lines 55 and 75 and the atmospheric vent 25 . the lubricant filter and separator 56 on the discharge side of the first stage gas compressor 42 is adapted to collect compressor lubricants from the compressed gaseous fuel and return them to the suction 40 of the first stage gas compressor 42 through a return line 57 and a preferably electrically - operated solenoid valve 61 . similarly , the lubricant filter and separator 68 is adapted to collect compressor lubricants and return them to the suction 62 of the second stage gas compressor 64 through a return line 69 and a preferably electrically - operated solenoid valve 71 . because of the well - known difficulty in starting or activating a gas compressor without first balancing the pressure between the suction and discharge sides of the compressor , the control system ( described below ) is also adapted to cause the solenoid valves 61 and 71 to remain closed whenever the first and second stage gas compressors 42 and 64 are activated , and further adapted to cause the solenoid valves 61 and 71 to open whenever the first and second stage gas compressors 42 and 64 are deactivated . such opening of the solenoid valve 61 and 71 provides fluid communication between the suctions and discharges of their respective gas compressors in order to balance the gas pressures across the compressors , thereby allowing the compressors to restart upon reactivation thereof . furthermore , the flow of gas between the suctions and discharges of the respective compressors , through their respective return lines 57 and 69 , also provides sufficient motivating force and pressure to forcibly urge the collected lubricants from the separators 56 and 68 to the respective suctions 40 and 62 of the respective gas compressors 42 and 64 . referring to fig4 an exemplary sorbent storage vessel 82 includes a sorbent material 83 that is contacted by the gaseous fuel flowing through the inlet 85 . as referred to herein , the terms &# 34 ; sorbent &# 34 ; and &# 34 ; sorptive &# 34 ; are intended to refer to both adsorbing or absorbing , or both . preferably , the inlet 85 of the sorbent storage vessel 82 includes an inlet filter 87 , which may comprise a screen mesh - type filter , a fibrous - type filter , or other filter means known in the art that is suitable for substantially preventing the introduction of particles or other impurities into the sorbent material 83 . the sorbent material 83 may comprise any of a number of sorbents , such as activated carbon , zeolite compounds , silica gels , or various clays , for example . such sorbent materials may be in the form of pellets , spheres , granulated particles , or other suitable forms whereby the surface area of the sorbent material is optimized in order to maximize the amount of gaseous fuel adsorbed or absorbed ( or both ) thereby . the present invention also contemplates the use of liquid absorbents , such as a liquid coating on an adsorbent material , for example . the sorbent filter 72 includes a similar sorbent material and has a somewhat similar tank - like construction and configuration to that shown in fig4 for the sorbent storage vessel 82 , except that the filter 72 has separate inlets and outlets . although columbia grade 9lxc activated carbon pellets were used as a sorbent material in one actually - constructed prototype embodiment of the exemplary refueling module 10 , and is generally regarded to be the preferred sorbent material for the sorbent filter 72 and the sorbent storage vessels 82 , other sorbent materials may be alternatively employed . specific examples of such sorbent materials are listed below : __________________________________________________________________________adsorbent material manufacturer product designation__________________________________________________________________________activated carbon calgon corp . bpl 4 × 10 mesh coal baseactivated carbon calgon corp . pcb 4 × 10 mesh coconut baseactivated carbon american norit sorbonorit b4 pellets company , inc . activated carbon westvaco chemical nuchar s - a companyactivated carbon westvaco chemical nuchar wv - a 14 × 35 wood company base granularactivated carbon witco chemical columbia grade 9lxc division powder low ash coal baseactivated carbon witco chemical columbia grade 9lxc division pellet low ash coal baseactivated carbon american norit norit rb - 3 companyzeolite ( natural ) anaconda minerals 2020a / d1 companyzeolite ( synthetic ) union carbide corp . metal alumino silicate linde division 13x 8 × 12 beadszeolite ( synthetic ) union carbide corp . metal alumino silicate linde division 4a 8 × 12 beadszeolite ( synthetic ) union carbide corp . metal alumino silicate linde division 5a 1 / 8 &# 34 ; pelletszeolite ( synthetic ) union carbide corp . metal alumino silicate linde division 13x powderzeolite ( natural ) anaconda minerals 5050 l companyzeolite ( natural ) double eagle clinoptilolite petroleum and mining companysilica gel -- -- clay -- -- __________________________________________________________________________ it should be noted that it has been found advantageous to activate the preferred carbon sorbent material before putting the refueling module 10 into use . specifically , the sorbent material is first packed into the sorbent filter 72 and the sorbent storage vessels 82 to the maximum extent possible , and each filter or storage vessel is evacuated . then each of the filter or storage vessels is placed in an oven otherwise heated and then again evacuated . as mentioned above , most gaseous fuels are capable of containing vaporized or other entrained lubricants or liquid materials even at ambient temperatures . therefore , if found to be necessary or desired , an optional moisture - removing means may be included downstream of the lubricant filter and separator 68 . one of several alternate embodiments of such optional moisture - removing means is illustrated schematically in fig5 a . an exemplary moisture removal system 110 generally includes a heat exchanger 112 operatively connected to a conventional refrigeration system 114 and adapted for cooling the gaseous fuel in order to cause vaporized or entrained lubricants or other liquids to be precipitated out from the gaseous fuel stream . the heat exchanger 112 includes a gas inlet 116 and a gas outlet 118 , with refrigerant from the refrigeration system 114 being conveyed therethrough . the refrigerant from the refrigeration system 114 and the gaseous fuel are isolated from one another in the shell - and - tube heat exchanger 112 ( or other suitable heat transfer device ). as the gaseous fuel is cooled , the lubricants or other liquids are precipitated out and conveyed from the heat exchanger 112 through a drain line 120 and a check valve 122 , and are then either returned to the suction side of one of the gas compressors or otherwise conveyed to a suitable disposal means . another alternate embodiment of the optional moisture - removing means is shown schematically in fig5 b . an examplary alternate moisture removal system 130 generally includes a vortex apparatus 132 having a gas inlet 134 and a gas outlet 136 . devices such as the vortex apparatus 132 are well - known in the art and are adapted to impart a vortex or otherwise generally spinning flow path to the compressed gaseous fuel flowing therethrough , thereby cooling the fuel so that lubricants or other entrained liquids may be separated out . an example of such a vortex separator is the so - called &# 34 ; vortex tube &# 34 ;, manufactured and sold by vortec corporation of cincinnati , ohio . the inclusion of one of the moisture removal systems 110 or 130 , or other suitable moisture removal apparatus is optional in the refueling module 10 , but may be deemed desirable or necessary in order to reduce contamination of the sorbent filter 72 , thereby prolonging the useful life of the sorbent material therein . the operation of the exemplary refueling module 10 may best be described with reference to the schematic flow diagram of fig3 . in order to discharge gaseous fuel from the refueling module 10 into the vehicle storage system 100 , the outlet conduit 20 is connected to the vehicle storage system by way of the connector device 22 , the manual discharge valve 96 is opened , and the refueling module is energized ( as described below ). when the refueling module 10 is energized , the solenoid valves 80 and 90 are caused to open and the solenoid valve 94 is caused to close . the pressurized gaseous fuel in the sorbent storage vessels 82 is thereby discharged through the outlet conduit 10 to the vehicle storage system 100 . simultaneously , a timer device ( described below ) starts and runs for a predetermined period of time . at the end of such time , the pressure in the storage vessels 82 and the vehicle storage system 100 are approximately equalized , and the solenoid valve 80 is caused to close , the compressors 42 and 64 are started , the cooling fan 52 is started , the solenoid valve 34 is opened , and the solenoid valves 61 , 71 and 94 are closed . the compressors 42 and 64 , and the cooling fan 52 , continue to run until the pressure in the vehicle storage system 100 is pressurized to a predetermined pressure level . when the predetermined pressure level in the vehicle storage system is reached , the pressure switch 92 causes the solenoid valve 90 to close and the solenoid valve 80 to open , thereby allowing the storage vessels 82 to be recharged and pressurized to their predetermined pressure level . it should be noted that the control system preferably prioritizes the functions of the refueling module such that if the storage vessels 82 are charged to their predetermined levels , the vehicle storage system 100 receives gaseous fuel from the storage vessels first and then from the gas compressors 42 and 64 . if the storage vessels 82 are not so pressurized when the refueling module is connected to the vehicle storage system , the gaseous fuel bypasses the storage vessels to first supply the vehicle storage system before recharging the storage vessels . when the desired pressure in the storage vessels 82 is reached , the pressure switch 84 causes the solenoid valves 34 and 80 to close and causes the compressors 42 and 64 and the cooling fan 52 to be deactivated . simultaneously , the pressure switch 84 causes the solenoid valves 61 and 71 to open in order to balance the gaseous fuel pressure on the suction and discharge sides of their respective gas compressors . as discussed above , compressed gaseous fuel flows from the discharge side to the suction side of the first stage gas compressor 42 and simultaneously forcibly urges collected compressor lubricants in the lubricant filter and separator 56 through the return line 57 and into the suction 40 of the first stage gas compressor . similarly , when the solenoid valve 71 is opened , collected compressor lubricants are forced from the lubricant filter and separator 68 through the return line 69 and into the suction 62 of the second stage gas compressor 64 . when the refueling apparatus shuts off , the pressure switch 92 causes the solenoid valve 94 to open in order to vent the gaseous fuel from the outlet conduit 20 to the holding chamber 98 , thereby allowing the outlet conduit to be easily disconnected . the pressure switch 74 may optionally be included and set at a pressure level slightly higher than that of the pressure switches 92 and 84 . thus the pressure switch 74 may function as part of a safety shut - off system that automatically shuts down the entire refueling module system in the event of an unacceptably high gas pressure level therein . as a further safety feature , the pressure relief valves 37 , 53 and 73 may be set at a pressure slightly higher than that of the optional pressure switch 73 ( if included ) and serve to automatically relieve the pressure in the system if the pressure continues to build even after the system is shut down . fig6 illustrates an exemplary electrical control system adapted to cause the refueling module to function as described above . one skilled in the art will readily recognize , of course , that other control systems , either electrical or of other types , may alternatively be employed . in order to initiate operation of the exemplary refueling module 10 , an on - off breaker switch 154 is closed . the closing of the on - off switch 154 causes the preferably red indicator lamp r to illuminate , indicating that the gas compressors are not activated , and further causes the preferably green indicator lamp g to illuminate , indicating that the sorbent storage vessels 82 are charged and pressurized to their predetermined level . in order to discharge gaseous fuel from the refueling module 10 into the vehicle storage system 100 , the outlet conduit 20 is connected to the vehicle storage system by way of connector 22 , the manual discharge valve 96 is opened , and the vehicle fill switch 156 is closed and held closed by the operator until the indicator lamp g shuts off . the closing of the vehicle fill switch 156 completes a circuit to cause the relay r1 to be energized , which closes the contacts r1a and r1b and opens the contacts r1c , thereby causing the solenoid valves 90 and 80 to open , the solenoid valve 94 to close , and the timer t1 to start . the compressed gaseous fuel in the sorbent storage vessels 82 is thus allowed to discharge through the outlet conduit 20 into the vehicle storage system 100 . the decrease in the pressure of the sorbent storage vessels 82 initially causes the pressure switch 84 to close , thereby shutting off the indicator lamp g and energizing the relay r2 in order to close its contacts r2a , r2b and r2c . at this time , the vehicle switch 156 may be released to its open position because both of the contacts r1a and r2a are closed . after a predetermined time established by the timer t1 , the pressures in the storage vessels 82 and in the vehicle storage system 100 are approximately equalized , and the timer t1 causes its contacts t1a to close and t1b to open , thereby energizing the compressor relay c and closing the solenoid valve 80 to isolate the storage vessels 82 . the relay c closes its contacts ca and cb , causing the compressors 42 and 64 , as well as the cooling fan 52 to start . at the same time , the preferably amber indicator lamp a is illuminated ( to indicate that the compressors and fan are running ), and the relay r5 is energized . the energized relay r5 opens the contacts r5a and thus shuts off the indicator lamp r . the compressors 42 and 64 continue to run until the pressure in the vehicle storage system is pressurized to a predetermined pressure level , causing the pressure switch 92 to open and thereby closing the solenoid valve 90 . the opening of the pressure switch 92 also energizes the relay r4 , which causes its contacts r4a and r4b to close and r4c to open . once the storage tank 82 has been recharged to its predetermined pressure , the pressure switch 84 again opens to de - energize the relay r2 and illuminate the indicator lamp g . the de - energization of the relay r2 opens the contacts r2a and r2b , thereby de - energizing the relay r1 , and opens the contacts r2b . the de - energization of the relay r1 in turn opens the contacts r1a and r1b to reset the system and to de - energize the timer t1 and close the solenoid valve 80 . as a result , the contacts t1a simultaneously open to de - energize the relay c ( and thus deactivate the compressors and cooling fan ) and to open the normally open solenoid valves 61 and 71 in order to balance the pressures across the compressors and force the compressor lubricants back to the suction thereof . also , the solenoid valve 94 is caused to open to vent the gaseous fuel from the outlet conduit 20 into the holding chamber 98 . it should be noted that if for any reason the power is interrupted during the refueling operation , the vehicle fill switch 156 must be pushed again in order to resume refueling . also , if for any reason , the system becomes overpressurized , the optional pressure switch 73 automatically shuts down the system as discussed above . on some occasions , the refueling operation may be interrupted or terminated before the pressure in the vehicle storage system 100 reaches the predetermined pressure that opens the pressure switch 92 , thereby preventing the storage tanks 82 from being recharged . one example of such an occasion is when the vehicle is needed by the user before the refueling operation is completed . in such a case , the control system provides for recharging or refilling the storage tanks 82 other than as described above at the completion of a full refueling operation . to recharge the storage vessels , operation of the refueling module is initiated by closing the on - off breaker switch 154 as described above , thereby illuminating the indicator lamp r and energizing the relay r2 through the closed pressure switch 84 . as a result the contacts r2a , r2b , r2c are closed . the optional storage tank fill switch 160 is depressed to energize the relay r3 , thus closing the contacts r3a , r3c and r3d , and opening the contacts r3b . this allows the optional switch 160 to be released to its open position , causes the solenoid valve 80 to open , and starts the compressors and cooling fan by way of the relay c and its contacts ca . simultaneously , because contacts cb are closed and the relay r5 is energized to open the contacts r5a , the lamp a is illuminated and the lamp r is shut off . the compressors and cooling fan then continue to run until the pressure in the storage tanks 82 reaches their predetermined level and the system is automatically shut down by the opening of the pressure switch 84 and the remainder of the sequence described above . it should be noted that if power is interrupted during the recharging of the storage tanks 82 , the compressors and fan will be deactivated and thus the solenoid valve 80 will close . to resume the operation , the optional switch 160 must again be depressed . it should be noted that the switch 160 is optional , and if included , it functions as described above . finally , it should also be noted that a stop switch 170 is provided to allow the operator to shut down the system manually if desired or necessary to do so . the foregoing discussion discloses and describes exemplary embodiments of the present invention . one skilled in the art will readily recognize from such discussion that various changes , modifications and variations may be made therein without departing from the spirit and scope of the invention as defined in the following claims .