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Post by rxc on Jan 25, 2015 14:47:46 GMT
Hoppy,
I have been looking at this Coltri unit for about 3 years, because the page you point to says that it can be started with a 4Kva genset, and the users manual says something similar (Maybe 5 kva - it has been a while since I read it). I asked Northern Lights, who make my 6KV genset if it would start this compressor, and they say "NO". No questions about it, they just say NO. I tried to contact Coltri to talk to someone, but have never succeeded in finding a knowledgeable person.
It is very frustrating to try to get this sort of info and run into a brick wall of silence. It is not like you can just buy it and try it out - we need to find someone who actually has one and runs it to find out.
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Post by sailbleu on Jan 26, 2015 8:07:35 GMT
The hose goes in between the first stage and the second stage . That last being in most cases a mouth piece and for me my full face mask. The first stage delivers around 10 bar overpressure to the surrounding pressure. That means it would be impossible for me to go deeper than 90 meter because of the stable 10 bar pressure due to the tank remaining on deck. As if it was possible to decent that deep with a simple ( only 10 meter long ) hose like this , but this just to illustrate the principle. Regards
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Post by rene460 on Jan 27, 2015 8:58:04 GMT
May I add some comments from my experience with heavy industry compressor drives.
There are two issues to be considered. The start up of an electric motor involves a surge current as mentioned above, typical up to 7 times the current for full load but out of phase with the voltage hence not 7 times power, but is the reason behind the 8 kVA for a 3200 watt compressor. As the motor reaches speed current drops but before back to steady state draw passes thorough a point about two times full load power. The generator requires VA capacity, power reserve and a control system to handle this surge. As mentioned above for motor starting a soft start or variable frequency drive (two different things!), can substantially reduce the peak, but the second issue then comes in. It is only really effective for centrifugal pump and compressor loads where the required torque reduces at low speed.
The compressor drive torque required unfortunately is not dependant on speed and requires full load torque at all speeds. An electric motor only provides typically half full load torque until it is well over fifty percent speed. Venting off during start up overcomes this and will not hurt the compressor valves for a short start up phase. The trouble occurs again however if the motor stops and starts to control pressure.
This may help you understand what you are up against, but others will have to describe how their system works in practice. You need to understand the surge specification of your compressor set, and also the surge handling capability of your genset.
rene460
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Post by rene460 on Jan 27, 2015 9:08:23 GMT
On the 12v system from ch. smith that hoppy mentioned above, my brother turned up with one yesterday. He is delighted with it for boat inspection, cleaning etc, even used it to fit a temporary replacement propellor, but he feels that it would not replace the full scuba gear for deep diving. He feels the limit might be in the range of 6 - 10 metres, not sure of exact limitations. But if this meets your purpose it was simple to provide the power supply from the onboard batteries. Always starts against zero back pressure then runs continually.
rene460
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Post by rxc on Jan 27, 2015 14:11:42 GMT
Rene,
THanks very much for your explanation. It is the first one I have ever heard, and I will try again to contact the Coltri people to see if their 4Kva spec is really correct. Maybe they have some sort of motor-controller soft-start capability or compressor startup sequence that allows a smaller genset to start their unit. It would be very helpful.
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Post by hoppy on Jan 27, 2015 15:01:33 GMT
Out of interest, what dive qualifications does everyone here have? I have the PADI deep diver (40m) and that's all. When I bought my regulator I got one that works in cold water and half expected to do the drysuit course but then I decided I'm happy with sticking to the tropics and didn't bother
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Post by hoppy on Jan 27, 2015 15:15:43 GMT
I just got out my dive bag. The regulator and gear still have the tags on them from when I got them serviced just over 5 years ago. I suppose I need to send it in again for a service.
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Post by Zanshin on Jan 27, 2015 15:28:41 GMT
I have the PADI open water and Nitrox certifications, but I am planning on getting my rebreather this season with my friends from Octopus Diving in St. Martin
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Post by sailbleu on Jan 27, 2015 16:30:36 GMT
May I add some comments from my experience with heavy industry compressor drives. There are two issues to be considered. The start up of an electric motor involves a surge current as mentioned above, typical up to 7 times the current for full load but out of phase with the voltage hence not 7 times power, but is the reason behind the 8 kVA for a 3200 watt compressor. As the motor reaches speed current drops but before back to steady state draw passes thorough a point about two times full load power. The generator requires VA capacity, power reserve and a control system to handle this surge. As mentioned above for motor starting a soft start or variable frequency drive (two different things!), can substantially reduce the peak, but the second issue then comes in. It is only really effective for centrifugal pump and compressor loads where the required torque reduces at low speed. The compressor drive torque required unfortunately is not dependant on speed and requires full load torque at all speeds. An electric motor only provides typically half full load torque until it is well over fifty percent speed. Venting off during start up overcomes this and will not hurt the compressor valves for a short start up phase. The trouble occurs again however if the motor stops and starts to control pressure. This may help you understand what you are up against, but others will have to describe how their system works in practice. You need to understand the surge specification of your compressor set, and also the surge handling capability of your genset. rene460 Hello Rene, Maybe I should of added this to my previous postings , but at the time I had my Coltri working (briefly) with my big diesel gen. , that passed away with only around 20 hours of running time ( water backflow) , I did not have a frequency controller fitted . This gen had a very hard time to start up the compressor , sometimes it worked , sometimes it didn't . Back then , and Even now i always open the water drain so no pressure can build up while starting. At the time I even made an alteration on my comp by installing a possibily for venting the discharge of the first stage . That seemed to be the solution to my startup (amps) problem , untill my diesel gen seized to perform that is. My petrol Kipor 2700 watt generator is no match for an electric motor on the comp. though . So my 5 hp honda petrol helps me out, be it with some noise. Regards
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Post by rene460 on Jan 28, 2015 12:00:05 GMT
Hi sailbleu,
I assume your Honda 5hp (3.73 kW) is rated around 4.6 to 5 kVA (based on a power factor of about 0.8), so that gives a good idea of what the Coltri requires. Presumably it is working quite hard by the time the bottles are fully charged, hence the noise. I assume that by "help out" you mean it does the job, and you do not try and run both generators at the same time. This would require special synchronising equipment!
Your idea to vent the first stage as well as the discharge is sound, and will further reduce the starting power and current demand. Large industrial compressors always require unloading for all stages for startup.
rxc, I wonder if Coltri provide recommended specifications for suitable generators as I am sure that you are not the only one wanting to operate a dive compressor away from mains power supplies.
rene460
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Post by sailbleu on Jan 28, 2015 15:23:06 GMT
Rene,
Yes of course , the honda does the job , it is the only source to get my comp. running I do not run two gens at the same time you know , my diesel is - after getting some water in the cilinder - since then , non active and stored in my garage untill , maybe , I'll have some time to (try) repair it. My petrol kipor gen only supplies the extra energy for domestical purposes , on the boat that is. No match for the comp. as you well know.
Regards
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Post by hoppy on Mar 16, 2015 8:43:06 GMT
Any thoughts on the type of bag to keep the dive gear in? I was thinking that a mesh bag could be good for when you don't have the chance to hang the gear up long enough to fully dry everything. On the other hand, a dry bag means the gear can be kept in a cockpit locker away from splashes of salt water when the locker is being used After writing this I'm starting to think both. Get the cheapest mesh bag for when I'm using the gear and the dry bag for storing.
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Post by hoppy on Mar 5, 2016 17:05:22 GMT
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