Coltri MCH 11, Electric or Gas driven?

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Hi Pao,
I envisioned going that route in the future if all goes well. But for now, the goal is for me and my 2 sons(both certified) be able to dive, gain experience and complete the dive log requirements of at least 100 dives for all 3 of us. During the process, we may offer discover local diving as our beachfront is just 2 km away from Agsalin Fish Sanctuaryhttps://www.youtube.com/watch?v=F_FDMVio3nI in Gloria. If things don't go very well and I become jobless, at least I have my dive equipment with me to use to catch my meals.
 
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Hi Lain,
I'm not good at wires. But I can get my way around engines, fuel tanks/pumps, frames and the like. For my families intended use, I might just be running the compressor for the 6 Al-80 per day. Do you have a figure how much fuel does the the GX 240 consume in an hour? I can buy used automotive EFI fuel tank w/ pump assembly cheaply ( if the need arise) and be able to utilize it to store and refill the compressor engine safely.

As for the running cost, I will try to make some money out of my dive equipments to be able to at least cover the running costs. Watching videos of Coltri teardown and assembly looks not much more complicated than a 4 banger honda civic engine. I will try to do most of the maintenance for added savings.

Regarding electric motors, it will be difficult to find a 4kw single phase motor or its price will be unpractical. If I get my hands on lets say a 2kw motor and size the pulley to drive the pumping unit at half the rated speed of 1350 RPM, will there be any other negative effects besides the longer fill time? Does the low pressure oil pump require a minimum speed to deliver minimum required pressure? Cooling fan speed will be reduced but the dwell time will be longer and adding additional fans is not so difficult.
 
To answer your first part.
A good rule of thumb with petrol engines is to choose around 2HP bigger than the electric motor equivalent so the 8 BHP Honda GX240 would be the equivalent of a 6HP electric motor. So well in your requirement.
Second is that the rated Power (torque) is based on full throttle 3600 RPM yet in practice a good engineer will choose a bigger motor than necessary for the sake of the $30 extra in cost and this allows a lower throttle RPM setting say 3000 rpm (with consideration for users overclocking the thing. But the bigger motor gives sufficient power required for the flow and pressure.

Full consumption is a graph depending on RPM and load but for practical purposes assume around 2.2 litres per hour (at full 3600 RPM) giving you around 2 hours running without sucking fumes to stall.
 
Running a lower RPM than the rated RPM is to my mind a good idea but again with consideration. Generally speaking if you half the speed you half the flow and to all intent and purpose you also half the required cooling so pretty much all in all equals out.

The one exception is the oil lubrication and for that you need the compressor engineer opinions from the manufacturer as I cant speak for Coltri.

From a general application a typical industrial designed breathing air compressor runs at 700 to 800 RPM with a minimum around 350RPM and a maximum of 950RPM with the the engineers only getting excited when it gets down to around 300 RPM Now that is for both a gearoter oil pump design on taper roller bearings or a splash lubricated crankcase design using ball bearings.

But these recreational scuba units are running around 1200 to 1500 RPM you really need the company engineers advice on the particular Coltri model.
 
Hi Lain,
I'm not good at wires. But I can get my way around engines, fuel tanks/pumps, frames and the like. For my families intended use, I might just be running the compressor for the 6 Al-80 per day. Do you have a figure how much fuel does the the GX 240 consume in an hour? I can buy used automotive EFI fuel tank w/ pump assembly cheaply ( if the need arise) and be able to utilize it to store and refill the compressor engine safely.

As for the running cost, I will try to make some money out of my dive equipments to be able to at least cover the running costs. Watching videos of Coltri teardown and assembly looks not much more complicated than a 4 banger honda civic engine. I will try to do most of the maintenance for added savings.

Regarding electric motors, it will be difficult to find a 4kw single phase motor or its price will be unpractical. If I get my hands on lets say a 2kw motor and size the pulley to drive the pumping unit at half the rated speed of 1350 RPM, will there be any other negative effects besides the longer fill time? Does the low pressure oil pump require a minimum speed to deliver minimum required pressure? Cooling fan speed will be reduced but the dwell time will be longer and adding additional fans is not so difficult.

One note...don't use the EFI fuel pump to feed the engine. It's intended for, well, EFI pressures and not gravity fed carbs. As a transfer pump it's probably OK but slow. (If you know your way around engines you know this already, but other readers might not.)
 
Hi John.
Good points you mentioned. I will be using the tank mounted pumps as a transfer pump to minimize the spillage and/or fumes. Its a bit slow using the common 79 L/hr pumps might take around 4 mins for a 4 Liter transfer and 1 minute using the 255L/hr Type-R pumps. Anyway for now, 2 hours of run time per day of the compressor is more than enough for our needs. So next day fuel refill will be on a cold engine.

I don't recommend others to modify any system unless they are confident on what they are doing and they take the risk alone.
 
Mindoro? Gas would definitely be your first choice. Whitetip has a shop in PG. You might consider getting your system from them so you don't have to go off island for support. Oriental or Occidental?

I inquired yesterday from my LDS for the gasoline version MCH-13. They want 1100 US$ extra for the gas version. Although Ray mentioned same price from him for the gas or electric version, shipping and taxes will make the prices higher compared to buying it from my LDS.

The ending, I ordered the the 230v single phase MCH-11. For 1100 US$, I can choose to upgrade my power supply or buy a honda engine and modify the frame. As mentioned by others, its good to have both electric and engine power capability.

Hopefully by April 2019, we can start to provide very affordable diving package for everyone. You are all welcome to come.
 
https://www.shearwater.com/products/swift/

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