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Old 16-07-2009, 05:22   #1
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AC vs DC for Air Conditioning, Watermaker

Guys, another electrical question.

Spectra makes what looks to be an incredibly efficient DC watermaker, like the Newport 400 which produces 62 liters per hour on only 13 amps of 24v power. That's barely 300 watts, compared to five or six times more for an ordinary AC watermaker of similar capacity.

Does the additional complexity mean that these units are even less reliable than regular watermakers? Although an AC watermaker is a good load for a genset, it seems to me that a unit like this will give you a whole lot more flexibility about when and under what circumstances to make water. For example, you can make water whenever the main engine is running, without having to start up the genset.

A related question:

I hate the idea of running a genset all night to have air conditioning (neighbors in the anchorage hate it even more, I'm sure). What about small DC aircon units serving just the sleeping areas, or small AC aircon units powered by inverter? Mastervolt, among others, makes an inverter which will automatically start up your genset if your batteries fall below 50% charge. It seems much more peaceful to cool with DC power, and the units will draw a lot less than their theoretical load anyway since they will cycle on less and less during the night as the outside temp falls. If you can't make it through the night on one charge, it's still better having the genset on only for an hour or two, than have it running all night long.

I guess it will make the batteries work quite hard, but surely even from a cost point of view the saved genset hours will be more valuable than the additional battery charge/discharge cycles.
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Old 16-07-2009, 05:45   #2
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Spectra uses a clark pump ( a hydraulic intensifier) that uses lost high pressure energy ( ie that most watermakers loose ) to dramatically improve overall energy efficiency. Its not a question of AC versus DC, its just that the clark pump allows a small capacity watermaker to be run effectively from DC.
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Old 16-07-2009, 05:55   #3
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Thanks, I understood that more or less. My question is about the strategy -- does efficient DC operation help a whole lot in managing your power? Or might you as well use a simpler (more reliable?) conventional watermaker to give a good load to your genset?

Are the Spectra units less reliable, with all that added complexity?
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Old 16-07-2009, 06:10   #4
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From my experience, if you have a regular run/maintenance routine, the units are highly reliable. If you don't, there can be many issues.

Regularly running the units (twice a week or so)
Back flushing with fresh water whenever you have run them
Not running when there is oil in the water
Not using clorinated water to back-flush
Changing the pre-filters on a regular basis
Making sure you HAVE pre-filters on the intake side (We use two a 25 and 8 micron filter? if I remember correctly)

We ran ours regularly, we bought it used and one of the pumps we bought with it was shot. We replaced the pump and have never had an issue. The older units, used a special variation of an agricultural pump from Shurflo. These pumps are right on the edge of their performance envelope. That caused some failures in the pumps. The newer units use a much higher capacity pump and I don't hear of failures in them. They are much more expensive, so since I have not had any problems with mine, I have yet to replace them. The Clark pump is completely mechanical, (Okay, there are some sensors that can be on them) so they tend to be VERY reliable. Unless you attempt to run sand or other fine abrasives through them.

We got ours for the flexibility that having a DC driven, highly efficient would provide us. For us, it was well worth it.
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Old 16-07-2009, 06:15   #5
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If you want to provide a load for your genset, I'd guess you are also charging batteries? If you do have a charger, you are supplying a load by pulling the DC. You get the additional flexibility in that you can power them from excess solar energy, or wind energy, or water generator, or engine mounted generator, or batteries, or DC charger while using small generator or... you probably get the idea.
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Old 16-07-2009, 06:24   #6
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The Spectra watermakers are staggeringly expensive -- more than $10k (!) for the Newport 400. So that's another consideration. I guess that's about double or more the cost of a conventional watermaker of similar capacity.
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Old 16-07-2009, 06:38   #7
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That's the major reason I went with the older units! The only thing you really need from Spectra is the Clark pump! If you look at the specs for the 380c units, you'll see that all the components were originally off-the-shelf. In the later version there are LOTs of proprietary, stuff that Spectra certainly wants to recover the RD cost on. Membranes and housings are readily sourced, pumps can be acquired from Shurflo, Prefilters can be sourced from Home Depot or Lowes. The high pressure lines can be sourced from Spectra (You can get them from various sources, but they want you to buy a roll of 250 feet or so!) The rest are clamps, high pressure fittings and such. McMaster-Carr is a great source as well as Parker-Hannifin dealers. More work, but for a $4k-$7k savings, well worth it.

Ah, another point, but this is mostly hear-say from other Spectra owners. The MPC's sensors seem to fail on a not infrequent basis. They seem to cause the majority of the distress some of the Spectra users experience. But, this is just hear-say.
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Old 16-07-2009, 06:55   #8
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There are few things that are important to know before deciding on any watermaker. Spectras are expensive but they are by far the most energy efficient. Depending on your ability to recover the energy spent to run a watermaker, saving two thousand dollars on a watermaker then spending more than that on energy recovery can make the cheaper watermaker more expensive. Newport 400s are great units with plenty of convenient bells and whistles that many can't do without, but are merely conveniences. Add all the abilities that a Newport comes with to another equivalent watermaker and the price narrows. Spectras Cape Horn Extreme may be a better choice depending on your budget needs and desires. It's dual feed pump set up adds extra insurance in case of one feed pump failure you can still make water with just one pump. As Strygaldwir stated these Shurflo pumps are proprietary to Spectra and can be pricey but not as expensive as replacing a high pressure CAT style plunger pump other watermakers use. If you're a boater that calculates amp usage on every boat system to make sure your batteries and charging system can handle it then a Spectra should be on your list. If you're the type that runs a generator all the time or runs their engines many hours each week, other much higher energy using watermakers will fit the bill. Also I can not emphasis enough, after sales service should be a high priority, ask around. There is a learning curve that if you've never owned a watermaker before that needs to be learned. Another advantage is the Clark pump that Spectra uses is warranted for life to the original owner. A consideration when deciding if less expensive is cost efficient or not.
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Old 16-07-2009, 07:54   #9
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If I remember correctly you are looking at a larger boats. Probably you will have a gen set. We have a gen set that we run once a day to pull down the cold plates and charge batteries. No water maker yet but when added it will be ac for better load and probably the largest we can fit.

Seperate issue, very likely you will not need or want ac on the hook. Canvas and hatches keep the boat cool enough.
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Old 16-07-2009, 08:27   #10
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If I remember correctly you are looking at a larger boats. Probably you will have a gen set. We have a gen set that we run once a day to pull down the cold plates and charge batteries. No water maker yet but when added it will be ac for better load and probably the largest we can fit.

Seperate issue, very likely you will not need or want ac on the hook. Canvas and hatches keep the boat cool enough.
Thanks for that. Yes, it's around 50 feet and all the boats were looking at have gensets. I think clearly we can use a regular AC watermaker and won't die without the high efficiency of the Spectra, because when you've got the genset running anyway you want it loaded up and don't usually care so much about power consumption (plus or minus even a kilowatt).

Air conditioning maybe a different thing. Our current boat has aircon but no generator, so it is impossible to use the aircon anywhere but in a marina. But even that setup is very useful at times -- especially where it's really noisy at night (think Bodrum Marina with all the nightclubs) and you'd like to shut the hatches and shut out the noise. You might be able to sleep temperaturewise but you just don't want the hatches open.

I kind of don't like the idea of air conditioning on the hook because of situational awareness -- you can't hear anything with the hatches closed and I sleep with one eye open anyway on the hook. That might change with a new generation anchor, at least in good weather.
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Old 16-07-2009, 08:44   #11
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Same here, we have full instruments in our bunk. Nice to see what's going on without having to get up.

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I sleep with one eye open anyway on the hook.
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Old 16-07-2009, 08:47   #12
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Dockhead:

I could be wrong here, but...

Let us not forget Ohm's Law:

300w at 24v = 12.5 amps
1500w at 110v = 13.6 amps
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Old 16-07-2009, 08:59   #13
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Dockhead:

I could be wrong here, but...

Let us not forget Ohm's Law:

300w at 24v = 12.5 amps
1500w at 110v = 13.6 amps

You are certainly right, but what's your point?

A watt is a measure of energy; amps are just current. So 300w is one-fifth of the energy of 1500w and the voltage at which this energy is delivered is irrelevant. So a watermaker operating at 12.5 amps and 24 volts is indeed five times as efficient as one operating at 13.6 amps and 110 volts.
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Old 16-07-2009, 09:59   #14
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We have a Spectra Newport 400. In two years of cruising, running it quite often (except when on ICW) and putting several hundred hours on it in that time (we have a washing machine on board and boy, does that add the hours to the WM'er). In terms of reliability, we had to replace a salinity probe ($60, not required for operation, but good to have) and a feed water pump head ($350), as well as a few fittings, but those are cheap and I figure just part of what you expect on a boat. Always puts out high quality water at very close to spec (16 gall/hr).

In terms of power required, we would often run the genset when making water, not that it was needed, but just that we tended to do our energy pig operations at the same time. However, there were many times we ran it off the battery, especially if we were sailing. This is even more feasible if you have a decent amount of solar/wind power, or if you are going to be running an engine for a couple of hours. The bottom line, though, is that with the Spectra, you have the option of running on battery and with an AC watermaker, you don't. If your charging options are limited, you can always be conservative with power use and still have water to drink.

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Old 16-07-2009, 11:29   #15
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Cool. Do you mind telling me how much you paid for it, and how much you thought you were paying above a similar non-Spectra WM?
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