what is the best method to increase holdover time in less no of ice packs?
TechNet-21 - Forum
I am not positive I am understanding your question. Are you asking how you might increase holdover time in cold boxes using fewer ice-packs? If so, I would first note that you should use the number of ice-packs prepared as specified on the equipment by the manufacturer if at all possible. However, in the case of freeze-preventive cold boxes, one helpful strategy will be to make sure your freezer is functioning very well and your ice-packs are fully frozen down to -25C. We have seen in many locations that power cuts, freezers in need of routine maintainence like defrosting, use of domestic (i.e. non-PQS pre-qualified freezers), and other factors can all lead to ice-packs being warmer than -25C and sometimes not even fully frozen in the freezers.
If you are NOT using freeze-preventive cold boxes, making sure your ice-packs are this cold will not be as helpful since you will condition them to 0C before use anyway to avoid freezing your vaccine load. But it could still be helpful to be sure that the ice-packs come out of the freezer fully-frozen, even in this case, before you condition them.
Perhaps post again if I have misunderstood your question.
Thanks Steven for your response.
That makes much more sense, sorry my response wasn't exactly relevant. We did work at PATH on some design and modelling of freeze-preventive cold boxes. We have not developed a finished design. As you likely know, Qingdao Leff Trading has a prequalified freeze-preventive cold box that has similar design to many of the prequalified vaccine carriers, only larger. As shown in the PQS product sheet, that cold box just barely stays within the weight requirements. In terms of the PCM used (we have generally tried to only use water inside the FP equipment functioning as a PCM), more optimization both volume-wise and location-wise is probably your best chance for lowering weight. The PCM/water in the FP liner isn't so much to help holdover as to prevent freezing. That said, if you can optimize and minimize the amount (and the thermal mass) of PCM/water you use in the FP barrier, that should actually extend holdover because that verification test starts with the cold box conditioned to +43C and the ice-packs you load then have to cool down that PCM/water. Also, using insulation to slow heat transfer instead of more PCM/water may allow some decrease in weight depending on design.
Another design space we explored was focused on locational thermal conductivity variations as opposed to thermal mass variations (latent and sensible heat). In thermal computer modeling we found that if we changed some internal wall materials to aluminum instead of plastic, we could more quickly transfer heat inside the cold box between the ice-packs and the PCM/water in the FP barrier. This has two noteable benefits (at least in modeling) - 1) it allows you to again decrease the amount of water/PCM needed to effectively condition your ice-packs inside the cold box because the heat transfer is more effective and quick and 2) we didn't expect it, but it also helped to distribute heat more evenly throughout the internal volume of the cold box, so there was less thermal gradient within that volume. In production, I'm not sure how feasible changing some plastic walls to metal ones of much higher thermal conductivity might be, but the idea of changing the rate of heat transfer preferentially in certain locations might allow you to use fewer ice-packs or less internal PCM/water.
The last possibility I'm thinking of that you could consider has to do with both conductivity and latent heat. From our interactions with Global Good (now name change to Global Health Labs), I understand they had developed a "solid PCM" that could be used in certain designs and might help make the equipment lighter and more thermally conductive at preferential locations. If you would like, let me know, and I can find a contact for you to see if they are able to give you more information.
Hopefully some of that can help a little bit, and good luck! It would be great to have more FP equipment and cold boxes available and weight and size are very difficult issues to address.
Thank's Steven for valuable suggestions. I am working on it and very close to the result also share the result with you very soon.
parallel, I also working on ILR.
There is a little bit of confusion on the calculation of the Holdover time.
Can I take the average temperature while calculating the Holdover time because nowhere is specifically mentioned in PQS for ILR .
Also, kindly suggest that is there any portal of WHO, where I can check when and why the amendment was done in PQS of CCE.
Good to hear development is going well.
I was understanding that your questions now were regarding ILRs. I'm not as well-versed on the fridge specifications and testing, but I blieve the current verification protocol is WHO-PQS E003 RF03 VP.4 for what are referred to as ILRs. Test 9 is for holdover. My paraphrasing of that test is:
1) stabilize the equipment after the previous test so the liner should be completely frozen.
2) cut power
3) measure time until the first "load" sensor gets warmer than 8C.
There should be no need to average as the test is concerned with the time until the first location where vaccines might be stored goes out of range, i.e. the thermally safe holdover time for vaccine storage with no compressor running/power. I believe this does not include the "surface" sensors as they are assumed to not necessarily represent vaccine storage.
To your other quesion - I don't know of anywhere that PQS documents the reasoning for all specification changes. There is a Revision History at the end of each specification or verification protocol that should document when changes were made. But this doesn't give a lot of detail in terms of reasons. You could try contacting PQS directly to see if they can give you any more information, but I don't know what they might be able to provide. If you are concerned about a specific change, you could ask them about that specifically and it's possible the could provide some details.
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