After a steam sterilization holding phase for porous material, there are two options to dry the load I see in most softwares:
A long vacuum hold to dry on the load (e.g. hold at 70 mbar for 20 minutes)
Multiple vacuum pulses to dry the load (alternate between 1 minute 70 mbar, 1 minute 800 mbar (pressure raise using filtered inlet air), again 1 minute at 70 mbar etc…)
To me only the first makes sense because the load’s internal heat will help the condensate to evaporate. Using pulses will pull in “cold” air and cool down the loads internal temperature with less efficient evaporation as a consequence during vacuums. The drying time will also be longer(due to ramping up/down pulses).
What is your opinion? Does the choice depend on the type of load or is one of both always better than the other?
As per my practical experience on this the “multiple vacuum pulse” approach will help you. In past I got a trouble while validation, on garment dryness value, as per EN 285 the limit is NMT 1% the weight shall be increased (after sterilization) from initial weight (of before cycle). We had taken trial by long vacuum hold and not get the satisfied result. When we have taken pulsed approach three pulse-10 minutes of each, the value comes below 1%. The reason could be that, by applying single long pulse hold , the vacuum in chamber get created soon after the vacuum pump start, within few seconds/minutes depending upon the load. Since then continuous hold of the vacuum will not result in the further removal of moister. While with multiple pulse, you get chance the load to get settle and soon we vacuum to withdraw the moister. I think the best way is to link the concept with primary drying and secondary drying phase of lyo (however not exactly the same process but can be conceptualize )
When the item to be dried has limited heat capacity then pulse drying will be more efficient. Thus stoppers, linens and similar items benefit from their use. For stainless steel, where there’s a substantial amount of heat retained by the item a deep vacuum approach will suffice. The residual heat in those types of items keeps the condensate hot enough that it can be removed with a single deep vacuum.