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.The stages of operation are common to both and can be summarized as air removal andsteam admission, heating-up and exposure, and drying or cooling.Many modificationsof design exist and in this section only general features will be considered.Fullertreatments of sterilizer design and operation can be found in Health TechnicalMemorandum 2010 (1994).General design features.Steam sterilizers are constructed with either cylindrical oroblong chambers, with preferred capacities ranging from 400 to 800 litres.They can besealed by either a single door or by doors at both ends (to allow through-passage ofprocessed materials; see Chapter 22, section 3.2.3).During sterilization the doors areheld closed by a locking mechanism which prevents opening when the chamber isunder pressure and until the chamber has cooled to a pre-set temperature, typically80C.In the larger sterilizers the chamber may be surrounded by a steam-jacket whichcan be used to heat the autoclave chamber and promote a more uniform temperaturethroughout the load.The same jacket can also be filled with water at the end of thecycle to facilitate cooling and thus reduce the overall cycle time.The chamber floorslopes towards a discharge channel through which air and condensate can be removed.Temperature is monitored within the opening of the discharge channel and bythermocouples in dummy packages; jacket and chamber pressures are followed usingpressure gauges.In hospitals and industry, it is common practice to operate sterilizerson an automatic cycle, each stage of operation being controlled by a timer respondingto temperature- or pressure-sensing devices.Operation1 Air removal and steam admission.Air can be removed from steam sterilizers eitherby downward displacement with steam, evacuation or a combination of the two.In thedownward displacement sterilizer, the heavier cool air is forced out of the dischargechannel by incoming hot steam.This has the benefit of warming the load during airremoval which aids the heating-up process.It finds widest application in the sterilizationof bottled fluids where bottle breakage may occur under the combined stresses ofevacuation and high temperature.For more air-retentive loads (i.e.dressings), however,this technique of air removal is unsatisfactory and mechanical evacuation of the air isessential before admission of the steam.This can either be to an extremely high level(e.g.2.5 kPa) or can involve a period of pulsed evacuation and steam admission, thelatter approach improving air extraction from dressings packs.After evacuation, steampenetration into the load is very rapid and heating-up is almost instantaneous.It isPrinciples and practice of sterilization 395 axiomatic that packaging and loading of articles within a sterilizer be so organized asto facilitate air removal.During the sterilization process, small pockets of entrained air may still be released,especially from packages, and this air must be removed.This is achieved with a near-to-steam thermostatic valve incorporated in the discharge channel.The value operateson the principle of an expandable bellows containing a volatile liquid which vaporizesat the temperature of saturated steam thereby closing the valve, and condenses on thepassage of a cooler air-steam mixture, thus reopening the valve and discharging theair.Condensate generated during the sterilization process can also be removed by thisdevice.Small quantities of air will not, however, lower the temperature sufficiently tooperate the valve and so a continual slight flow of steam is maintained through a bypassaround the device in order to flush away residual air.It is common practice to package sterile fluids, especially intravenous fluids, inflexible plastic containers.During sterilization these can develop a considerable internalpressure in the airspace above the fluid and it is therefore necessary to maintain aproportion of air within the sterilizing chamber to produce sufficient overpressure toprevent these containers from bursting (air ballasting).In sterilizers modified or designedto process this type of product, air removal is therefore unnecessary but special attentionmust be paid to the prevention of air 'layering' within the chamber.This is overcomeby the inclusion of a fan or through a continuous spray of hot water within the chamber to mix the air and steam.Air ballasting can also be employed to prevent bottle breakage.2 Heating-up and exposure.When the sterilizer reaches its operating temperatureand pressure the sterilization stage begins.The duration of exposure may include aheating-up time in addition to the holding time and this will normally be establishedusing thermocouples in dummy articles.3 Drying or cooling.Dressings packs and other porous loads may become dampenedduring the sterilization process and must be dried before removal from the chamber.This is achieved by steam exhaust and application of a vacuum, often assisted by heatfrom the steam-filled jacket if fitted.After drying, atmospheric pressure within thechamber is restored by admission of sterile filtered air.For bottled fluids the final stage of the sterilization process is cooling, and this needs tobe achieved as rapidly as possible to minimize thermal degradation of the product andto reduce processing time.In modern sterilizers, this is achieved by circulating waterin the jacket which surrounds the chamber or by spray-cooling with retained condensatedelivered to the surface of the load by nozzles fitted into the roof of the sterilizerchamber.This is often accompanied by the introduction of filtered, compressed air tominimize container breakage due to high internal pressures (air ballasting).Containersmust not be removed from the sterilizer until the internal pressure has dropped to a safelevel, usually indicated by a temperature of less than 80C.Occasionally, spray-coolingwater may be a source of bacterial contamination and its microbiological quality mustbe carefully monitored.4.3 Dry heat sterilizationThe lethal effects of dry heat on microorganisms are due largely to oxidative processeswhich are less effective than the hydrolytic damage which results from exposure tosteam.Thus, dry heat sterilization usually employs higher temperatures in the range160-180C and requires exposure times of up to 2 hours depending upon the temperatureemployed (section 10).Again, bacterial spores are much more resistant than vegetative cells, and theirrecorded resistance varies markedly depending upon their degree of dryness.In manyearly studies on dry heat resistance of spores their water content was not adequatelycontrolled, so conflicting data arose regarding the exposure conditions necessary toachieve effective sterilization.This was partly responsible for variations in recommendedexposure temperatures and times in different pharmacopoeias.Its application is generally restricted to glassware and metal surgical instruments(where its good penetrability and non-corrosive nature are of benefit), non-aqueousthermostable liquids and thermostable powders (see Chapter 21).In practice, the rangeof materials which are actually subjected to dry heat sterilization is quite limited, andconsists largely of items used in hospitals [ Pobierz całość w formacie PDF ]
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.The stages of operation are common to both and can be summarized as air removal andsteam admission, heating-up and exposure, and drying or cooling.Many modificationsof design exist and in this section only general features will be considered.Fullertreatments of sterilizer design and operation can be found in Health TechnicalMemorandum 2010 (1994).General design features.Steam sterilizers are constructed with either cylindrical oroblong chambers, with preferred capacities ranging from 400 to 800 litres.They can besealed by either a single door or by doors at both ends (to allow through-passage ofprocessed materials; see Chapter 22, section 3.2.3).During sterilization the doors areheld closed by a locking mechanism which prevents opening when the chamber isunder pressure and until the chamber has cooled to a pre-set temperature, typically80C.In the larger sterilizers the chamber may be surrounded by a steam-jacket whichcan be used to heat the autoclave chamber and promote a more uniform temperaturethroughout the load.The same jacket can also be filled with water at the end of thecycle to facilitate cooling and thus reduce the overall cycle time.The chamber floorslopes towards a discharge channel through which air and condensate can be removed.Temperature is monitored within the opening of the discharge channel and bythermocouples in dummy packages; jacket and chamber pressures are followed usingpressure gauges.In hospitals and industry, it is common practice to operate sterilizerson an automatic cycle, each stage of operation being controlled by a timer respondingto temperature- or pressure-sensing devices.Operation1 Air removal and steam admission.Air can be removed from steam sterilizers eitherby downward displacement with steam, evacuation or a combination of the two.In thedownward displacement sterilizer, the heavier cool air is forced out of the dischargechannel by incoming hot steam.This has the benefit of warming the load during airremoval which aids the heating-up process.It finds widest application in the sterilizationof bottled fluids where bottle breakage may occur under the combined stresses ofevacuation and high temperature.For more air-retentive loads (i.e.dressings), however,this technique of air removal is unsatisfactory and mechanical evacuation of the air isessential before admission of the steam.This can either be to an extremely high level(e.g.2.5 kPa) or can involve a period of pulsed evacuation and steam admission, thelatter approach improving air extraction from dressings packs.After evacuation, steampenetration into the load is very rapid and heating-up is almost instantaneous.It isPrinciples and practice of sterilization 395 axiomatic that packaging and loading of articles within a sterilizer be so organized asto facilitate air removal.During the sterilization process, small pockets of entrained air may still be released,especially from packages, and this air must be removed.This is achieved with a near-to-steam thermostatic valve incorporated in the discharge channel.The value operateson the principle of an expandable bellows containing a volatile liquid which vaporizesat the temperature of saturated steam thereby closing the valve, and condenses on thepassage of a cooler air-steam mixture, thus reopening the valve and discharging theair.Condensate generated during the sterilization process can also be removed by thisdevice.Small quantities of air will not, however, lower the temperature sufficiently tooperate the valve and so a continual slight flow of steam is maintained through a bypassaround the device in order to flush away residual air.It is common practice to package sterile fluids, especially intravenous fluids, inflexible plastic containers.During sterilization these can develop a considerable internalpressure in the airspace above the fluid and it is therefore necessary to maintain aproportion of air within the sterilizing chamber to produce sufficient overpressure toprevent these containers from bursting (air ballasting).In sterilizers modified or designedto process this type of product, air removal is therefore unnecessary but special attentionmust be paid to the prevention of air 'layering' within the chamber.This is overcomeby the inclusion of a fan or through a continuous spray of hot water within the chamber to mix the air and steam.Air ballasting can also be employed to prevent bottle breakage.2 Heating-up and exposure.When the sterilizer reaches its operating temperatureand pressure the sterilization stage begins.The duration of exposure may include aheating-up time in addition to the holding time and this will normally be establishedusing thermocouples in dummy articles.3 Drying or cooling.Dressings packs and other porous loads may become dampenedduring the sterilization process and must be dried before removal from the chamber.This is achieved by steam exhaust and application of a vacuum, often assisted by heatfrom the steam-filled jacket if fitted.After drying, atmospheric pressure within thechamber is restored by admission of sterile filtered air.For bottled fluids the final stage of the sterilization process is cooling, and this needs tobe achieved as rapidly as possible to minimize thermal degradation of the product andto reduce processing time.In modern sterilizers, this is achieved by circulating waterin the jacket which surrounds the chamber or by spray-cooling with retained condensatedelivered to the surface of the load by nozzles fitted into the roof of the sterilizerchamber.This is often accompanied by the introduction of filtered, compressed air tominimize container breakage due to high internal pressures (air ballasting).Containersmust not be removed from the sterilizer until the internal pressure has dropped to a safelevel, usually indicated by a temperature of less than 80C.Occasionally, spray-coolingwater may be a source of bacterial contamination and its microbiological quality mustbe carefully monitored.4.3 Dry heat sterilizationThe lethal effects of dry heat on microorganisms are due largely to oxidative processeswhich are less effective than the hydrolytic damage which results from exposure tosteam.Thus, dry heat sterilization usually employs higher temperatures in the range160-180C and requires exposure times of up to 2 hours depending upon the temperatureemployed (section 10).Again, bacterial spores are much more resistant than vegetative cells, and theirrecorded resistance varies markedly depending upon their degree of dryness.In manyearly studies on dry heat resistance of spores their water content was not adequatelycontrolled, so conflicting data arose regarding the exposure conditions necessary toachieve effective sterilization.This was partly responsible for variations in recommendedexposure temperatures and times in different pharmacopoeias.Its application is generally restricted to glassware and metal surgical instruments(where its good penetrability and non-corrosive nature are of benefit), non-aqueousthermostable liquids and thermostable powders (see Chapter 21).In practice, the rangeof materials which are actually subjected to dry heat sterilization is quite limited, andconsists largely of items used in hospitals [ Pobierz całość w formacie PDF ]