WO2000078365A1

WO2000078365A1 - Medical sterilizer

Info

Publication number: WO2000078365A1
Application number: PCT/US2000/017096
Authority: WO
Inventors: Jonathan A. Wilder
Original assignee: H & W Technology LLC
Current assignee: H & W Technology LLC
Priority date: 1999-06-22
Filing date: 2000-06-21
Publication date: 2000-12-28
Anticipated expiration: 2001-12-22
Also published as: EP1202751A4; AU5882600A; EP1202751A1

Abstract

A steam sterilizer (10) operable under various pressures and temperatures above ambient. The sterilizer (10) has a chamber (25) in valved communication with a surrounding jacket (30). Steam is input into the jacket (30) and removed from the chamber (25) through parallel valved circuits. The jacket (30) is desirably of larger volume than the chamber (25) to provide a step change in chamber temperature at cycle startup. Steam flow between the jacket (30) and chamber (25) is controllable at variable flow rates by manipulation of inlet (36, 40) and exhaust valves (34, 38) in combination to maintain the temperature or pressure setpoint with a fast response and without significant setpoint overshoot.

Description

MEDICAL STERILIZER

PRIORITY CLAIM

Thfs application claims the benefit of the filing date of United States Provisional Patent Application Serial Number 60/140,514, filed June 22, 1999 for " Medical Sterilizer."

Technical Field: This invention relates to steam sterilization. It is particularly directed to sterilizers for use in the medical field, and which incorporate steam under a range of pressures and temperatures as the sterilizing medium. Background Art: Sterilizers using steam as the sterilizing medium are taught by several patents, including US 4,108,601 and US 4,238,447 to Wolf, US 4,309,381 and US 4,372,916 to Chamberlain and Cook, US 4,759,909 to Joslyn, US 4,844,933 to Hsieh, Johnson and Dudek, and US 5,164,161 to Feathers and Ellis. These patents are felt to be representative of the current state-of-the-art in vessel design and process control. Wolff teaches an apparatus and method for a sterilizer having process control using a temperature feedback signal and supplies steam at two pressures. Wolffs pressure regulators inherently limit the minimum process temperature to that determined by the lower pressure regulator and feed steam source. Use of a temperature feedback signal as process control inherently slows system response and causes setpoint overshoot because temperature transducers inherently introduce a hysteresis and promote setpoint overshoot. The hysteresis is due to the innately slow response of temperature transducers relative to pressure transducers.

Chamberlain, et al. teach a pressure controlled sterilization cycle with a goal of verifying complete air removal from the chamber by comparison of measured temperature and pressure to steam table data. A device according to Chamberlain et al.'s teachings is limited in process control steam flow rate settings. Joslyn is similarly limited in steam flow rate settings. Hsieh et al. teach a sterilizer apparatus adapted to processing foodstuffs. Again, the steam flow rates for temperature control available in Hsieh et al.'s apparatus are limited. Feathers et al. teach a sterilizing apparatus and method having proportional control of temperature, but utilizing heating elements with duty cycles. The temperature differential from setpoint determines the duty cycle.

A need remains for an improved sterilizer that can be manufactured at a relatively low cost, and has a very fast response to step changes in setpoint temperature. It is desirable that the sterilizer chamber be able to attain and maintain any setpoint pressure or temperature over ambient, up to maximum values of the feed steam. Furthermore, the sterilizer should exhibit minimal overshoot or deviation from the setpoint. DISCLOSURE OF THE INVENTION

The present invention provides an apparatus and method for sterilizing objects using a steam heat source. A sterilizer constructed according to this invention can control a process at any temperature above ambient room temperature, limited only by the temperature of the feed steam. A representative device of the invention constitutes a test sterilizer, which finds exemplary use in the evaluation of biological and chemical indicator devices. Such indicator devices are typically positioned to monitor the performance of sterilizers used either in hospitals or in industrial settings, such as the production of finished medical devices. The sterilizing means used by the present invention is steam (water vapor) under pressure and elevated temperature. Vessels may be constructed in accordance with this invention as pressure vessels to meet or exceed the specifications of ISO 11138-2 and 11138-3, Subclause A.1.1, Annex A. A dual-loop control system is presently preferred, using feedback signals from both temperature and pressure sensors. The process temperature is controlled using feedback from a comparatively fast pressure transducer, and the process temperature is verified by a relatively slow temperature transducer. Temperature is changed and maintained within a desired band using one or more of a multiple available flow rates of process steam.

An apparatus for sterilizing objects, according to the instant invention, typically has a chamber defining a first volume and in which to receive a load for sterilization. The chamber may be covered along its length by a jacket defining a second volume. It is currently preferred that the second volume is at least twice the size of the first volume. A feed fluid circuit with a feed valve system is provided to admit steam, at a variable flow rate, from a steam source into the jacket. A valve may be located in an interconnecting fluid circuit to provide occludable fluid communication between the chamber and the jacket. An exhaust valve system may be located in an exhaust fluid circuit to control fluid flow between the chamber and an exhaust. Certain exemplary sterilizers may include a vacuum pump in fluid communication with the chamber to reduce pressure in the chamber below atmospheric pressure. The feed valve system may be formed from a plurality of inlet valves in a parallel flow circuit. The individual inlet valves may be opened and closed in various combinations to produce a variable flow rate to maintain a pressure and temperature condition in said chamber within a desired range. One exemplary feed valve system includes two inlet valves in a parallel flow circuit. In such a feed system, a first inlet valve may be characterized as having a slow steam flow rate, and a second inlet valve may be characterized as having a fast steam flow rate. An exemplary exhaust valve system also may include two exhaust valves in a parallel flow circuit, similarly characterized as having fast and slow flow rates. The paired exhaust valves may individually be opened in multiple combinations with the paired inlet valves, thereby to permit fifteen different rates of fluid flow through the interconnecting circuit to the chamber. Any number of valves may be arranged in the feed or exhaust circuits to increase the range of flow rates. Inlet and exhaust valves may also be proportional valves. The various valves are typically controlled by a computerized control system. The instant invention is typically used by: placing the objects to be sterilized into a sterilization chamber; evacuating the chamber to a first pressure reduced from atmospheric pressure; determining a second pressure for saturated steam from a sterilizing steam source; pressurizing the jacket to the second pressure by admission of steam; opening a valve communicating between the jacket and the chamber to permit equilibration of pressure in the jacket and the chamber to a third pressure; comparing a temperature feedback signal, after a first time interval at the third pressure, with a programmed temperature value; opening and closing one or more supply and exhaust valves in a combination to create a desired steam flow rate if the temperature feedback signal deviates from a programmed value by more than a tolerance band; and maintaining pressure in the chamber within a tolerance range about the third pressure.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which illustrate what is currently regarded as the best modes for carrying out the invention:

Figure 1 illustrates a representative piping schematic for a first embodiment of the invention;

Figure 2 illustrates an alternative piping and control arrangement.

BEST MODES FOR CARRYING OUT THE INVENTION Test sterilizers of this type are expected to produce a so-called square- wave process condition. Under such conditions, pressure and temperature excursions occur over essentially zero time, allowing evaluation of the performance of the indicators being tested without confounding factors, such as the effect of exposure during a relatively slow achievement of a setpoint pressure or temperature. Due to the extremely rapid pressure rise required by the standard and its intolerance to excursions beyond the defined setpoints, an innovative chamber design was conceived.

A first exemplary vessel, illustrated in Figure 1, includes a jacketed load- containing chamber 25. It incorporates a large jacket 30 of approximately four times the volume of the chamber 25. Jackets of other sizes are workable. However, the volume ratio is a compromise between pressure vessel code requirements that dictate the economics of the vessel production, the need to avoid superheated steam (Jacket pressure less than twice the desired chamber pressure-therefore the minimum ratio is greater than 1 :1) and the reliability of the slow steam valves. The maximum ratio that produces a vessel that is not too physically large for practical applications is the upper limit (i.e., there is no real upper limit-this is also a benefit in that the jacket pressure may not be much greater than the chamber target pressure, which makes life easier in pressure targeting algorithms and the ability to treat steam as an ideal gas).

The jacket 30 is pressurized with steam to a pressure greater than the eventual setpoint pressure. When the pressurization phase of the cycle is completed, one or more valves 34 or orifices are opened between the pressurization jacket 30 and the evacuated chamber 25. The chamber 25 will, in general, have been previously evacuated to a pressure of 4 kPa (equivalently 0.58 psia, 30 torr or 39.4 mBar). The pressure in the jacket 30 will have been selected so that if the chamber 25 were empty, (did not contain a load) the pressure in the combined two volumes when in communication would be the actual setpoint. This last point is not critical to the success of a cycle. The pressure in the jacket 30 prior to opening the valve(s) 34 can be any pressure greater than the pressure in the evacuated chamber 25 if the steam generator has sufficient capacity to provide sufficient steam to fill the communicating chambers to the setpoint within the time allowed by the ISO standard (currently 10 sec).

Pressure control near the setpoint may be done using a small-orifice inlet valve 36 and a small-orifice outlet valve 38. When the pressure in the chamber 25 has nearly reached the setpoint pressure, the larger orifice steam input valve 40 may be closed and the final approach to the pressure setpoint can be done using the small-orifice valve 36. If necessary, an application of a multivalve control scheme may suitably control the pressure. That is, various combinations of open and closed valves in a parallel flow arrangement may be used to change either or both input and exhaust flows. The combination of valves may have an assortment of flow conductances, or may be identical valves. Simple solenoid valves, typically used in this apparatus, and which either open or close are advantageously less costly than complex proportional valves.

With continued reference to Figure 1 , a sterilizer, indicated generally at 10, is instrumented for process verification and control feedback. Typical instrumentation includes chamber pressure transducer 44, jacket pressure transducer 46, chamber temperature sensor 48, and jacket temperature sensor 50. More sensors than illustrated may be used in a sterilizer 10, for example to verify temperature profile throughout the volume of the chamber 25. The jacket 30 is desirably provided a safety relief valve 55 to avoid jacket rupture due to overpressurization. Similarly, a pressure relief valve 57 is desirably placed in communication between the chamber 25 and a suitable venting arrangement. In certain sterilizers, an auxiliary override chamber vent valve 60 may also be provided in parallel with the safety relief valve 57.

The jacket 30, and also the chamber 25, may quickly be cooled by introduction of water from a cold water source W through water control valve 64. A drain valve 66 may be provided to drain water from the jacket. A steam trap 68 may be positioned in parallel with the exhaust circuits from the chamber 25. Besides the slow acting steam exhaust valve 38, a fast steam exhaust valve 70 may be incorporated in a sterilizer. Such a fast exhaust valve may also include a vacuum pump 74 in-circuit. It is preferred that the jacket 30 substantially covers the chamber 25 to have maximum effect on chamber temperature. Ends of the chamber 25 and jacket 30 may be insulated. The jacket 30 may also substantially entirely surround a chamber 25. In Figure 1, a hinged access door 77 is provided on one end of the sterilizer 10. A backhead 79 closes off the opposite end of the chamber 25 and jacket 30. The door 77 and backhead 79 may be heated with steam, or in a preferred apparatus, with silicone heating elements. The chamber 25 and jacket 30 function as pressure vessels, and are typically structured as cylindrical elements. However in some applications, other geometric configurations are appropriate. Sterilizers are typically manufactured from steam resistant metals, including stainless steels. Fittings and piping may be brass, steel, or inert materials such as PTFE. Alternatively, stainless steel may be used throughout to form a sanitary sterilizer.

Control valves used in the presently preferred embodiment are typically solenoid type valves to provide automated control. Such valves may be controlled and operated under computerized control, such as by programmable logic controllers (PLCs). Other type valves may be used also, including manual valves. Slow acting valves, such as steam exhaust valve 38, may operate with the flow therethrough "choked" by an orifice 72. Orifice 72 may be located either upstream or downstream of the valve 38. The individual valves also may alternatively be sized directly to provide the desired flow conditions. It is further within contemplation to use valves having controllable and variable flow characteristics. Such variable flow rate valves may replace the combination-in-parallel fast and slow acting valve arrangement, of the presently preferred embodiments, with a single valve. For purposes of this disclosure, a fast valve produces a fast flow rate functionally defined as one that is used to fill large volumes or to counter large excursions from setpoint. A slow valve has a slow flow rate typically used for control within or near the desired control band around setpoint. Additionally, fast and slow valves may be opened in combination to produce various rates of flow between the maximum and minimum fast and slow rates attainable by the system.

The currently preferred vessel design uses a jacket/chamber volume ratio to provide rapid initial temperature adjustment in the chamber 25. It is currently preferred that the volume of the jacket 30 is larger than the volume of the chamber 25. The volume ratio provides a temperature adjustment approaching a step change from room temperature to the desired setpoint temperature. In a second exemplary sterilizer, the volume of the jacket 30 is 8820 cu. in. (144.5 liters, 5.1 cu. ft.) and the volume of the chamber 25 is 1251 cu. in. (20.5 liters, 0.72 cu. ft.). These values provide a ratio of approximately 7:1 for jacket to chamber volumes. This second sterilizer has a jacket 30 with a 23.5" ID and a chamber 25 with an 8" ID to achieve these volumes. The lengths of the cylindrical chamber 25 and jacket 30 elements of this second sterilizer are 23.5". A sterilizer according to the instant invention can be made to any desired or convenient length or other size dimension. The volume ratio arrangement of preferred sterilizers advantageously provides the opportunity to: 1) preheat the chamber to near or above temperature setpoint (the latter precluding or at least limiting condensation), while not exceeding temperature setpoint by any great amount; 2) be able to do this for any temperature/pressure combination within the ISO spec while not exceeding a preliminary jacket pressure of 4.7 bar (69 psia); and 3) model the pressure-temperature relationships for control by using a linear fit to steam table data.

A second configuration to control temperature of a sterilizer 20 is illustrated in Figure 2. Sterilization chamber 25 is substantially surrounded along its length by a jacket 30. A chamber access door 77 is provided at one end of chamber 25. This door may be insulated only, or in fluid communication with the jacket 30. Alternatively, door and/or backhead 79 preferably are provided with separately controllable heating elements. A heater for backhead 79 is indicated generally at 81.

As illustrated in Figure 2, a cold water source, indicated by W, is in fluid flow communication with a feedwater pump 83. Fluid flow directions are indicated generally by arrows. Pump 83 supplies steam generator 85, which produces steam indicated by S. A steam generator may be used as illustrated, or alternatively, plant steam may be used in a sterilizer. A steam shut-off valve 87 may be provided to control steam delivery to jacket 30 through fast and slow flow rate valves 90 and 92, respectively. Fluid flow from the jacket to the chamber may be controlled by one or more interchamber valves 95. Fluid flow from the chamber may be controlled by fast and slow flow rate valves 97 and 99, respectively. The chamber may be evacuated or exhausted through an ejector 101, as illustrated. Ejector 101 is typically provided with a vent 103. The pressure at the ejector exit, and also in the chamber, may be modified by a vacuum pump 105. Discharged fluid then generally exits to a drain funnel 107.

It is preferred to have pressure relief valves, such as jacket relief valve 109 and chamber relief valve 112, in fluid communication to a safe exhaust location. A controlled ventilation of the chamber may also be performed through chamber vent valve 115 and vent 117. A cold water source W may be used to rapidly cool the jacket 30 and chamber 25. A typical arrangement for such cooling incorporates a strainer 120, a water shut-off valve 125, and a check valve 127. Water may be removed from the jacket 30 through a jacket drain valve 129. Water may be drained from the steam generator 85 through a water drain valve 130. The temperature in the jacket 30 may be monitored using one or more temperature sensors 132. Temperature in the chamber 25 may be monitored using one or more temperature sensors 134. Similarly, pressures in the jacket 30 and chamber 25 may be monitored with jacket and chamber pressure sensors 137 and 139, respectively. Typical safety valves used in preferred sterilizers are rated at about 50 psig.

Both high and low pressure setpoint pressure control may be done with sets of two valves, each valve in the respective set having different flow rates. One set of valves would be provided for inlet and one for exhaust/evacuation. The two valves in each set are typically sized empirically (based upon preliminary calculations) to allow rapid pressure excursions to both pressure and vacuum setpoints whole maintaining fine control necessary to avoid overshoot beyond the allowable tolerance on those setpoints. A form of proportional control is typically used to determine when the valves should be opened or closed. A control refinement to allow other intermediate or lower rates of pressure change is to open one or both of the inlet and exhaust valves simultaneously, assuming the process phase required or allowed admission of gas or vapor to the chamber. Table 1 presents design data representing control trends for fifteen useful combinations using four valves; two each for inlet and exhaust. Table 1 assumes that the steam generator has less capacity than the vacuum system and that equivalent orifices are of the same size. X signifies an open valve. Rate is indicated on a 1 -5 scale, where 1 is slowest. Therefore, as indicated by the data of Table 1, a proportional control with a rapid response time may be achieved using a multivalve configuration having orifices (or valves) of two sizes in the inlet and exhaust.

Either a steam table based algorithm may be used, or a lookup table may be used to determine operational parameters of a steam sterilizer according to principals of this invention. The control system may be operated by a computer, and have steam data stored in a memory. The algorithm or table is used to determine the pressure corresponding to a given desired setpoint temperature. This pressure is monitored at intervals of perhaps no more than one second. Depending on the degree of excursion off setpoint, the various combinations of open and closed valves described above and indicated in Table 1 are used to bring the pressure back to some defined dead band around the setpoint. Bands of excursion off setpoint are defined and assigned to different valve configurations empirically. That is, if the pressure drops below setpoint, steam is admitted to the chamber/jacket system using one or more of the positive flow (steam to chamber) valve configurations. If the pressure exceeds setpoint, steam is removed by use of one or more of the negative flow configurations (steam from chamber, or evacuation). As the pressure moves from a band further away from setpoint to one closer to setpoint, the configuration appropriate to the current band is activated, and the one previously used for the greater excursion away from setpoint is deactivated. This approach limits the possibility of overshoot. In reality, once setpoint is achieved, for a well-insulated vessel and a well behaved load (no outgassing or sudden volume change like implosions from the pressure) only the lowest flow bands and configurations (those closest to zero flow) are used. Once the desired setpoint pressure and temperature are reached, flow may even be terminated until an excursion away from the setpoint is experienced.

Chamber temperature and pressure are monitored and may be recorded at the intervals stated above. If the temperature is outside a predetermined limit from its correct saturation value, (normally +- 3%), an alarm condition may be signaled, with the response of the system to the alarm depending upon the specific programming of the control software. At a minimum, an alarm event may be recorded. At a maximum, the cycle may be aborted.

Table 2 presents steam temperatures, pressures and densities typically found in operation of the second mentioned exemplary sterilizer. The weight of steam to fill the jacket 30 and both chamber 25 and jacket 30 in combination were determined using the volumes of the jacket and chamber of the second preferred embodiment.

Note that these relationships may be developed for any chamber and jacket sizes. To use Table 2, one picks the desired setpoint temperature of the chamber 25 and notes the weight of steam required to fill the chamber 25 and jacket 30 at that temperature. Then, one observes the pressure necessary to fill only the jacket 30 with that amount of steam. The jacket 30 is filled to that pressure, shut off from the steam source and is allowed to communicate with the previously evacuated chamber 25, filling it to what should be the setpoint temperature and pressure. It is because the chamber 25 will contain medical devices or other test objects which act as a heat sink, as well as the inevitable inaccuracies of using a fitted curve, that one must also include a valve control system. Also, the effect of gradual cooling of the chamber due to thermal losses to the atmosphere must be overcome by the control system. A computerized control system 133 (see Figure 1) may operate any or all of the various valves in combination to produce a steam flow rate to obtain desired pressure and temperature conditions in the chamber 25. Such a control system responds to feedback signals from sensors such as temperature sensors 48 and 50, and pressure sensors 44 and 46. It is currently preferred, for faster response times, to control saturated steam conditions in a sterilizer apparatus by using a pressure feedback signal, and to verify process parameters are met by confirmation from the temperature sensors. The preferred control scheme may be characterized as a double loop. The feedback signals are then compared to a previously programmed range, and appropriate control signals are output to the various valves in response to deviation from the programmed range. A proportional response to control valve opening, and steam flow rate, may be generated based upon the magnitude of deviation of the feedback signals from the programmed range. Example 1 Desired Setpoint: 120.7 °C, 29.4 psia lb. Steam to fill chamber and jacket: 0.416

Jacket-only temperature and pressure: 125.1 °C, 33.81 psia

A more efficient approach to this process is to use the near-linear relation of pressure, temperature and weight of steam in a known volume. For the system described, the relation is as follows: P = 0.089017T - 8.384099

Where P is the pressure of steam required in the jacket alone and T is the temperature in

°C.

Example 2

Using the above mathematical relationship and the desired setpoint of Example 1, one obtains a jacket pressure of 34.7 psia, within 5% agreement with the table. The values for other process temperatures of interest in psia are shown in Table 3 for both methods. The calculated results of the mathematical relationship, as well as all tested results are within 5% of the table value and thus can be easily accommodated by the pressure control valving. The performance requirements of the ISO standards for test sterilizers include relatively rapid response times for pressure changes, as well as tight tolerances once setpoints are achieved. These two factors necessitate the development of an extremely rapidly responding control system. The approach developed for the instant invention involves using multiple valves of different orifice sizing connected in parallel to provide for a multiplicity of flow rates into and out of the system. In addition, opening both inlet and outlet valves simultaneously, coupled with the selection of a vacuum or ambient pressure on the outlet side further increases the number of options for flow rates and pressure control responses.

The results presented herein concern viscous flow regimes, in which the molecules of the fluid flowing through a piping system or orifice are in relatively close contact, and terms like viscosity have meaning. However, the conclusions developed do not depend upon the flow regime and apply to all regimes including molecular and Knudsen flow. For any flow system, there are two contributions to flow restriction. These are the conductance of the piping and the conductance of any valve or orifice in the system. Generally, one chooses the orifices (be they valves or proper orifices) to be the limiting element for flow restriction in the overall system.

The Tables show the flow rates for both air and steam through the system described previously. Note that conductance losses from piping are listed separately, with aggregate conductance provided for each branch.

The examples given below are based on a third embodiment having respective volumes of 220 1 (jacket) and 280 1 (chamber). The volume ratio is thus 0.79 : 1, but provides an illustration of the concepts. It is currently thought that a volume ratio of at least 2:1 or more is to be preferred to provide a rapid, step-like, change in startup temperature. One preferred embodiment has a volume ratio of 4:1.

Two solenoid valves of differing conductance will be used, in the present example, for both steam supply and evacuation. These are modeled as orifices of 0.125 and 0.5 inches (3.175 and 12.7 mm) diameter. Both the inlet and outlet sides of the system are fitted with a pair of such valves, each of which can be opened and closed independently. Conductance for air across an orifice is modeled by the following equation:

where C is conductance in liter/sec, d is the orifice diameter in meters, and the pressures are in any convenient absolute units. For steam, this equation is:

P setpoint

P source '

In the sonic limit, where the fluid is moving at a rate equal to or greater than the speed of sound, these expressions simplify, but the quantitative differences are small, so the complete expression given above was used. The sonic limit is determined by the expression: "

where (gamma) is the ratio of the specific heats at constant pressure and constant volume. Increasing the pressure differential across an orifice beyond the sonic limit does not increase flow over the sonic limit flow rate, since the downstream portion of the stream cannot communicate the existence of a lower pressure, as it is effectively disconnected from the upstream portion. This is because information travels only at the speed of sound and no faster. The simplified expressions for pressure ratios greater than the sonic limit are given below for air and steam respectively.

Δ ir"

Table 4 shows the conditions under which a system using the valve control system of Figure 1 were modeled. The pressures in Table 4 are given in absolute millibars. The alarm pressure in Table 4 is the pressure at which the system would be expected to respond to a deviation from the setpoint pressure. The source pressure for valve 40 and valve 36 is that of steam or air at 75 psia (5168 mbar). The source pressure for valve 38 and valve 70 is ambient pressure, or a standard atmosphere. For the vacuum range of the system, the conditions presented in Table 5 were used. In Table 5, the high-pressure source is the same, but a vacuum pump has been connected to the system and the low-pressure source is 20 mbar or -14.4 psig/0.3 psia. The setpoint and alarm ranges in each case are different, due to the greater imprecision of measurement at the low end of any given pressure transducer's range.

Tables 6-9 show the predicted flow rates for air and steam under the conditions presented in the preceding Tables 4 and 5. Note that a positive flow signifies net flow into the system and a rising pressure, while a negative flow signifies net flow out of the system (exhaust) and a falling pressure. Note also, that the entire system is extremely nonlinear, with equivalent Cv (measure of flow restriction by the valves/orifices) providing very different results depending upon the pressure and medium being transported. For steam, a very different set of results is found compared to air. An additional option to increase evacuation response is to valve off the jacket 30 from the chamber 25 by closing valve 34 (see Figure 1). The increased response is because the vacuum system then has to pump on a smaller volume than if the valve 34 was left open, as is the case in all of the previous examples. The response times for evacuation only for both air and steam are given in Tables 10-13. It may be seen that the response is increased linearly with the volume pumped. The nonlinear response time is calculated as the product of the volume, the natural logarithm of the ratio of the pressures (setpoint and startpoint), divided by the flow rate.

Table 14 compares the performance requirements of the relevant AAMI standard, ST45 and the ISO 1 1 138 part 2 (BI resistometer) and 1 1 140 part 2 (CI resistometer) standards with the measured response of a preferred sterilizer constructed according to the instant invention. Note that the 1 1138 and 1 1 140 requirements are identical, so only one column labeled, "ISO" is shown.

The sterilizer for which values are illustrated in Table 14 is nominally called a ILS20 series of steam BIER/test and validation sterilizers. Such sterilizers Typically have a stainless steel, hinged, single door 77. Typical chambers 25 have a 0.76 cu. ft. ( 0.021 m³) volume, and are covered along their length by a jacket 30. The door is typically preheated to at least 90°C. A vacuum system is generally included having a liquid ring pump with an atmospheric air eductor. The liquid ring pump may be a Siemens model 2BV2 071, with a nominal pumping speed of 71 cfm/. The vacuum pump, coupled with an air eductor provides a minimum pumping speed of approximately 30 cfm at 40 mbar (4 kPa, 0.58 psia). This described system meets or exceeds the pumpdown time requirement of ISO 11138/11140 part 2.

A steam source may include an internal generator using tap water feed, to provide 18 KW, 54 lb./hr. steam @ 100°C, including a booster pump for feed water. A clean steam generator may alternatively be supplied as an option. Controls typically include a SteriMagic control and data acquisition system and a PC and printers. An air compressor may be further included for valve operation and an air oveφressure source. Alternatively, plant air may be used if the source is available at 60 psig/5 bar. The door 77 and backhead 79 temperature is generally controlled with silicone heaters to facilitate reliable achievement of isothermal process temperature conditions. Pressure transducers used to monitor the chamber are typically of NIST-traceable calibration. A capacitance manometer may be used and controlled to eliminate thermally-induced variance with process temperature. The sterilizer requires utilities including tap water at 4 gpm/15 1pm maximum, and a drain suitable for at least 2x incoming flow rate. An electrical connection should provide 208VAC three-phase power, with lOOAmps per phase. An alternative sterilizer uses the principle of the multiple, different flow-rate valves to allow for two rates of inflow to the chamber prior to the sterilization exposure phase, also enabling the execution of a so-called gravity displacement cycle, which does not use a prevacuum phase, just pushes the air out of the chamber with steam. A sterilizer designated as "The emulator" is designed for additional flexibility in programming of sterilizer cycles. The emulator replaces all of the multiple, parallel valves with infinitely-variable proportional valves, allowing the sterilizer to emulate any sterilizer's evacuation and inflow rates, and thus their process characteristics. Such infinitely variable proportional valves permit a variable flow rate dependent upon the amount the valve is opened. Methods and apparatus to provide rapid, precise transitions in pressure and temperature for use in the field of sterilization of medical devices are disclosed herein. These methods and apparatus offer the advantage of cost and simplicity over the standard alternatives. Although the invention has been described with regard to certain preferred embodiments, the scope of the invention is to be defined by the appended claims. INDUSTRIAL APPLICABILITY

The way the invention can be made and used is obvious in the context of the preceding description.

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 8

Table 9

Table 10

Table 11

Table 12

Table 13

Table 14

Claims

CLAIMSWhat is claimed is:

1. An apparatus for sterilizing objects, comprising: a chamber defining a first volume in which to receive a load for sterilization; a jacket substantially covering a length of said chamber, said jacket defining a second volume exterior to said chamber and at least equal in size to said first volume; a first valve system for the puφose of admission of steam, having an elevated temperature and pressure, at a variable flow rate, from a steam source into said second volume; a valve providing occludable fluid communication between said chamber and said jacket; a second valve system to control fluid flow between said chamber and an exhaust.

2. The apparatus of claim 1, wherein said first valve system comprises a plurality of inlet valves, said inlet valves being opened and closed in combinations to produce said variable flow rate in response to signals received from a computerized control system, said control system being programmable and responsive to pressure and temperature inputs to maintain a pressure and temperature condition in said chamber within a desired range.

3. The apparatus of claim 1, wherein said first valve system comprises a first inlet valve characterized as having a slow steam flow rate, and a second inlet valve characterized as having a fast steam flow rate.

4. The apparatus of claim 1 , wherein said inlet valves have fluid flow conductances in combination operable to provide a fast and a slow inlet fluid flow.

5. The apparatus of claim 1, wherein said second valve system comprises a plurality of exhaust valves.

6. The apparatus of claim 5, wherein each of said exhaust valves may be opened in combination with each of said inlet valves, thereby to permit fifteen different rates of inlet fluid flow to said chamber.

7. The apparatus of claim 1 , wherein said first valve system comprises a proportional valve.

8. The apparatus of claim 7, wherein said second valve system comprises a proportional valve.

9. The apparatus of claim 1, said second volume being at least three times the size of said first volume.

10. The apparatus of claim 1 , said second volume being at least six times the size of said first volume.

11. The apparatus of claim 1 , further including a vacuum pump in fluid communication with said chamber to reduce pressure in said chamber below atmospheric pressure.

12. An apparatus for sterilizing objects, comprising: a chamber defining a first volume in which to receive a load for sterilization; a jacket substantially covering a length of said chamber, said jacket defining a second volume at least twice the size of said first volume; a first valve system for the puφose of admission of steam, having an elevated temperature and pressure, at a variable flow rate, from a steam source into said second volume; a valve providing occludable fluid communication between said chamber and said jacket; a second valve system to control fluid flow between said chamber and an exhaust.

13. An apparatus according to claim 12, further including a computerized control system oprerable to individually actuate valves of said valve systems; and wherein said first valve system comprises a plurality of inlet valves connected in a parallel flow circuit, said inlet valves being opened and closed in combination to produce said variable flow rate in response to signals received from said computerized control system, said control system being programmable to respond to pressure and temperature inputs whereby to output control signals to said inlet valves to establish a steam flow rate, through said valves in combination, to maintain a pressure and temperature condition in said chamber within a programmed range.

14. An apparatus according to claim 13, wherein said second valve system comprises a plurality of exhaust valves connected in a parallel flow circuit.

15. An apparatus according to claim 14, said exhaust valves comprising fast and slow conductances.

16 A method for sterilizing objects, comprising: placing the objects to be sterilized into a sterilization chamber; evacuating said chamber to a first pressure reduced from atmospheric pressure; determining a second pressure for steam from a sterilizing steam source; pressurizing said jacket to said second pressure by admission of steam from said steam source; opening a valve communicating between said jacket and said chamber to permit equilibration of pressure in said jacket and said chamber to a third pressure; comparing a temperature feedback signal, after a first time interval at said third pressure, with a programmed temperature value; opening and closing one or more supply and exhaust valves in a combination to create a desired steam flow rate if said temperature feedback signal deviates from a programmed value by more than a tolerance band; and maintaining pressure in said chamber within a tolerance range about said third pressure.