Hi Emile, The contact time is what appears to be the biggest issue with scrubbers and what is recommended is a long path through the “chalk”. In the Comsub I have two 200mm diameter x 250mm long packs with one 75mm centrifugal fan (forced draft) feeding them in parallel. However I note that Phil’s deepworker scrubbers break all the rules and only have a path through the lime of approximately 80mm but he has a very large area approximately 300 x 400 mm rectangular area and is using induced draft (fan sucking) 80mm computer fan. This is very interesting as the DW’s have a long service history and are obviously successful. So he uses two per person and has just used these times 5 = 10 units for the 5 people. This is a good safety issue because if one fails there are nine others all with their own fans. The other aspect is that they can probably use 2 at a time so short dives are not a penalty. My new ones I have designed to hold 10 kgs each and will keep another 10 kgs in a shaped cloth bag that is vacuum packed for emergency. The tests I have done I will need a 90mm fan which had better performance but noisier running through a resistor on 24 volts. If I were to start again I think I would try and copy Phil’s. Cant beat tried and true. Pardon Me Phil. Hugh From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Cliff Redus Emile When I build my boat, I to did some investigation into commercially available scrubber systems that would be applicable for a one man psub. I did not find any that were economically viable. While I agree with Leander that fundamentally the scrubber is not complicated, to make an efficient and practical scrubber takes some work. I went through several iterations on mine. One of the key items is, as you mention, the fan. ABS requires 72 hours of emergency life support after your primary mission time and this has to be able to run off the emergency battery bank. If you under size the fan to minimize the power requirement to the fan, then the fan can not generate sufficient differential pressure across the fan to circulate the contaminated air. This is particularly true if you use an axial flow fan rather than a radial centrifugal fan. Axial fans are commonly installed on most PC computers. They are designed for high flow rate but low head. I made the mistake of using an axial flow fan on my first iteration of my scrubber. While it would last 80 hours ( the current load is very small), the scrubber effectiveness was not great probably because the blades very stalling. My initial CO2 level would start at about 350 ppm but would build to 5000 ppm (0.5 mole %) over 30 minutes. My second iteration replaced the axial fan with a small radial centrifugal fan (squirrel cage) which developed four times the head at about 1/4 of the flow rate. The current demand was only slightly higher. This worked great while still meeting the low current requirment. The scrubber absorbent choice to me is a no brainier. Sodasorb HP is what is used most often these days for psubs and re-breathers for divers. It is easy to get and not very expensive. From a design perspective, after sorting out the fan, it comes down to how do want to handle the storage of the emergency Sodasorb. There are three options, 1) cartridges for the primary and emergency absorbent, 2) scrubber sized to hold the primary and emergency absorbent 3) scrubber sized for the primary absorbent and spare Sodasorb stored in bulk requiring refill when primary absorbent becomes saturated. There are advantages and disadvantages to each technique. While cartridges are easy to replace in an emergency situation, the spare cartridges don't pack as efficiently as bulk absorbent. For a small psub like mine, this rules them out; I just did not have the room. I note that the Deepworker uses the option 2) . For small one man psubs, I think option 2 is probably the best way to go. The disadvantage to this approach is that for non emergency conditions, you end of tossing a lot of partially used Sodasorb after each dive. For a one person sub, this is not an issue but for more than one, this makes this option not as attractive. Also, because it is inexpensive, this is not a big deal for a one person sub. This option provides the least stress in a stressful emergency as the operator does not have to worry about changing out the absorbent when it becomes saturated. On my boat, I use option 3 primarily because of the storage issue. On my next one person boat, I would go with option 2. Replacing 30 lbs of Sodasorb (80 hours of life support for one man) is not a big deal given that it simplifies life. While the manufactures of these absorbents add a chemical that turns the absorbent a different color than white when it becomes saturated, this is not very practical to use as lighting in subs make it hard to detect the color change. A better detection of a saturated absorbent is a good CO2 sensor with an audible alarm. So to me, it comes down to correctly specifying the fan and how to package the spare absorbent. Cliff
__________ Information from ESET NOD32 Antivirus, version of virus signature database 5950 (20110313) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com |