Technology Brief - May 2008

    As a manufacturer of steam autoclaves, we frequently hear feedback from current and prospective customers alike, regarding condensate issues (wet loads) in their chamber post-sterilization impeding satisfactory processing results. In fact, it could very well be the single leading technical issue we have run into over the past 30+ years of manufacturing such systems. To understand why that is, you really have to understand the myriad of potential causes, to appreciate the complexity of solving this simple problem.

    First and foremost, in helping our clients identify the source of the issue, we begin by evaluation of the most common suspects first.  Foremost among those suspects is the steam supply.  Frequently we find that steam quality is the culprit in leading to excessive condensate formation within the autoclave.  Our first recommendation to clients is to examine their steam circuit/supply and look for potential area of condensate formations such as dead legs or improperly trapped or insulated supplies.  The duty of a steam trap is to discharge condensate while disallowing the escape of live steam to escape the circuit/supply.  The further away the steam line is from the heating source/medium, the more likely condensate is to form, and require removal from the system.  If you or your staff have no experience with proper steam supply design/piping, we would recommend that you contact your local supplier (Armstrong, Spirax/Sarco,…) for an evaluation.  One quick test you could employ, to identify if your steam supply is the culprit, is to measure the condensate discharge just ahead of the sterilizer in two locations.  Calculate steam quality or dryness fraction if overall steam flow rate is known and compare the result.

    Calculating the “dryness fraction”is accomplished as follows.

    
    
    where

     ζ = dryness fraction

    ww = mass of water flow (kg, or lb / unit time)

    ws = mass of steam (kg or lb / unit time)

    If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95.

    Once you’ve established that the steam supply is not the source, it’s time to look more closely at what is being loaded into the vessel, the design of the sterilizer itself and perhaps the operation.

    Another common cause of wet loads is the load itself.  However tempting it may be to pack the chamber full of product, in order to keep up with demand, this is frequently a cause of excessive condensing of steam which can not then be flashed-off by subsequent steam injections.  Large quantities of hard-goods or even complex packaging can make proper steam circulation a challenge.  Steam enters the chamber and contacts the product, it is essential that the steam collapse (condense) on the product in-order for the heat to be released to the intended load.  But ultimately the water formation must be discharged through condensate management or re- vaporize in order to prevent contamination of the product.  Removal of the raw water is crucial to prevention of insulating the load against the steam.  That said, too much steam, at too fast a rate can result in excessive water formation which might overwhelm or “swamp” steam traps.. 

    To “dry” the load after the fact, many customers are tempted to employ excessively long deep vacuums at the end of the cycle. Because the chamber is heated (via the jacketing) and the fact that water will “flash” off into steam at a lower temperature under vacuum, this is relatively successful.  But what ultimately results is an unnecessarily long cycle time, and potentially a non-sterile load as water can collect in areas, and insulate the intended product from achieving temperature.  Deep post-sterilization vacuum “drying” phases are like “closing the barn door after the horses escaped.”

    When we are asked to assist under these conditions, we typically look to prevent (or greatly reduce) the formation of condensate by making sure that the load itself is brought up to a certain temperature prior to the introduction of steam.  This can be accomplished in a couple of different ways. 

    First, accomplishing this can be as simple as defining a “load heat up” phase at the beginning of the cycle.  The phase would simply function at the dictate of jacket setpoint, chamber drain (or load thermocouple) setpoint and a phase timer.  The phase would not advance until the setpoint(s) were achieved and the phase timer elapses, where it would then pass to the Purge (or Pre-Vac in the instance of a vacuum equipped sterilizer). 

    A second approach might be to add an actual heated air in-bleed phase at the beginning of the cycle.  A heat exchanger can be installed to the air in-bleed assembly to enable a combination of vacuum, and air in-bleed pulses that would bring the product up to temperature more quickly, reducing the condensate formation at the beginning of the cycle.  While a heat exchanger is not absolutely necessary, it may prove useful for particularly large chambers or dense loads.  As an alternative to the heat exchanger, you might wish to consider simply increasing the jacket temperature setpoint, as long as it does not interfere with maintaining the temperature uniformity within the chamber during the sterilize dwell phase.  A word of caution, merely increasing the jacket temperature could result in a “super-heating” of the steam in the case that your condensate issue is not particularly bad, so use this strategy cautiously.

    Having worked on thousands of autoclaves (some designed and manufactured by ETC, other’s by practically every other manufacturer), ETC has seen virtually every imaginable approach to system design over our 38+ years in business.  It truly is fascinating that the design approach of various suppliers can differ so greatly, despite the fact that we are all dealing with the same basic physics.  Over this time, we have identified some unique approaches that can be causes, or solutions, to the condensate formation issue.  These include:

    Charge Rate Control- Many autoclaves are not equipped with a proportional control valve, or the necessary control system capability, to control the “ramp up” or “charge” rate of steam injection during the cycle.  If a chamber does not have some means (valving configuration, type of steam control valve, or software limitations…) to control the rate that steam is added to the chamber, the result could be a flooding of the chamber with steam that results in instant condensate formation.  If the steam injection were to be ramped up more slowly, the amount of steam (and subsequent condensate) can be more easily controlled.

    Filter Sterilization- Many pharmaceutical-grade autoclaves are fitted with an air in-bleed filter casing that can be sterilized “in situ” (in place).  Some more popular manufacturers of autoclaves will sterilize this filter during each cycle, other’s (like ETC) make this a unique cycle that is only run when requested.  In many cases, the former design does not come with an isolation valve for the filter, while the latter (ETC’s approach) includes an isolation valve as well as a condensate bleed valve.  The latter approach is superior in ETC’s opinion because a) it reduces the wear on the filter element, resulting in a much longer life-time/lower running cost and b) prevents repeatedly saturating the filter with steam, which would result in excessive moisture being brought into the chamber at the completion of the cycle.  To that end, you may wish to look how your current system is configured.  

    There are many causes for condensate formation, in fact, condensing of steam is essential in heat transfer (and hence sterilization) but too much condensate is detrimental.  If you are experiencing such problems, try some of the above in identifying the cause, or contact ETC for assistance.