What Does "1 Meg Ohm-cm Specific Resistivity" Mean?

The electrical relationship that you are looking for is

ρ = RA/l

where you measure the resistance of a sample (let's say a column of water 1 cm X 1 cm (A = 1 cm²) and 10 cm long. If you measure 10,000,000 Ω, you have

ρ (the resistivity) = (10⁷Ω)(1 cm²)/10 cm = 10⁶ Ω cm

!!!!!!!!!!

not wat i was looking for and it made no sense!!! i had to think hard... THANKS ALOT!!!

NOT USE FULL!!!!!!!!!!!!!!!!

PWSlack wrote:

snip

> That's not the whole story. In order for the boiler to operate in the
> long term with minimal mechanical replacment, the water must be
> maintained slightly alkaline, usually by dosing something like
> caustic soda in small quantities.

Thanks. I presume that the boiler manufacturer would be able to address
that, such as the exact quantity/concentration.

snip

> So the course of action w.r.t. this boiler is to match the
> manufacturer's feedwater requirements with the local water sources
> via a feedwater treatment plant of some sort. There are many
> reputable specialists in this field and a Google search locally will
> doubtless provide a number of telephone contacts that could be
> approached for further advice and an outline proposal for a package
> plant that would do the job. Go talk to them!
>
> Will that do for now?

Yes. I rated this as a 'GOOD ANSWER'; not only have you confirmed my
initial impression, but you've provided some good additional info, too.
Thank you VERY much. I do have a brief follow-up, if you don't mind.

When you mentioned "dosing something like caustic soda in small
quantities", I presume that ... if this boiler were to be used
exclusively for steam ... that there would merely be an initial dosing
and that the caustic should remain in the boiler since it wouldn't be
contained in the steam (perhaps the manufacturer even does that prior to
shipping). However, most of the water from the boiler we will be using
will actually exit as hot water rather than as steam, which would carry
the caustic with it, although much of it will also exit as steam for
various uses during the brewing process. Is it a fair guess to expect
that, despite any steam usage, we should still be able to maintain the
proper level of caustic in solution by monitoring the concentration in
the water leaving the boiler, and that the levels should not build up to
a level that would eventually require a blow-down?

Thanks in advance.

Bill Velek

<...caustic soda in small quantities......the boiler manufacturer would be able to address that, such as the exact quantity/concentration...>

Nope. Contact a specialised boiler feedwater treatment chemicals specialist. Nalco (usual disclaimer) springs to mind as a chemicals supplier, and Prominent (usual disclaimer) as a dosing equipment supplier. These and others all have a web presence.

<...there would merely be an initial dosing and that the caustic should remain in the boiler since it wouldn't be contained in the steam...>

Nope. The caustic will react with certain species in the boiler, and any residual will be lost from the system during blowdown, and will need to be replenished. A boiler water treatment chemicals specialist will be able to advise and supply the necessary.

<...the levels should not build up to a level that would eventually require a blow-down...>

Blowdown is always required (both in boilers and in cooling towers), as the remaining dissolved substances in the feedwater, as well as the dosed caustic, will concentrate-up as steam/vapour is drawn off. The interval between blowdowns will be determined largely by the concentration ratio as suggested in the earlier post. It is common to blow a boiler down at, say, 3000-6000μS/cm, and to omit this operation will result in the boiler priming, eventuallk; priming is when water droplets get carried over into the steam system - the boiler starts to 'froth'.

The cleaner the feedwater in terms of the absence of dissolved substances, which is indicated by a high resistivity/low conductivity, the less frequent the blowdown operations will be, and the better the economics.

PWSlack wrote:

snip

> Blowdown is always required (both in boilers and in cooling towers), as
> the remaining dissolved substances in the feedwater, as well as the
> dosed caustic, will concentrate-up as steam/vapour is drawn off. The
> interval between blowdowns will be determined largely by the
> concentration ratio as suggested in the earlier post.

snip

Thank you for taking the time to respond and explain this science to me; I am most grateful for your help and so I don't mean to offend you in any way whatsoever, but please let me get some clarification.

I'm still having difficulty understanding why a blowdown would _EVER_ be required in _MY_ situation, considering that regular "water heaters" are _never_ blown down ... or at least I've never heard of it ever being necessary before ... so I think I've probably just failed to make myself clear.

First, we will be using R/O water, so the mineral content will be _substantially_ below precipitation levels for any and all minerals.

Second, the relatively small amount of steam that we will draw from the boiler (probably not ever more than 15% of the water we add) ... should not leave behind enough minerals to elevate levels in the remaining water to anywhere even remotely close to precipitation levels; e.g., if R/O cut calcium to 5% of what was dissolved in municipal water, and I boil away HALF of the water as steam, I would still only have a concentration of 10% of what was dissolved in the municipal water, so wouldn't it all remain in solution in the hot water?

Third, the rest of the water will be drawn off and used as hot water, which should carry the dissolved minerals with it, and therefore, I would suspect, have the exact same effect as a blowdown -- the minerals are removed from the boiler.

To give you something specific, for a typical 1.5 barrel batch of beer, we will use 56.5 gallons of _water_ and then use steam generated from about 6 gallons of water. In addition, we will use a fairly nominal amount of steam to purge and clean our lines, and some to steam-clean our fermenters, tanks, and any kegs (since we plan to 'can' most of our beer, kegs should comprise a fairly low percentage of our production). I don't know for certain how much steam will be used for the cleaning, but I can't imagine that it would use more than a couple of gallons of water. So I think a good estimate is that the ratio will probably be about 56.5 gallons as water : 8 gallons as steam -- so about 12.4% of the water will be converted to steam, and the rest drained as hot water. If, in such a situation, a blown down and/or addition of a bit of caustic soda is still required, please let me know.

Thanks.

Bill Velek

<...difficulty understanding why a blowdown would _EVER_ be required...>

Because it's a separation process. The dissolved substances in the feedwater remain in the boiler drum and do not leave with the steam stream; they concentrate-up over time. Species such as calcium sulphate ("gypsum"), calcium carbonate ("limestone") and magnesium carbonate ("marble") are things that are not wanted in the boiler drum as their solubility decreases with rising temperature. "Kettle scale". The tendency is for these to precipitate-out onto heat exchange surfaces, adding insulation to them in the water space, reducing the thermal efficiency of the unit, and thereby making the combustion-side of the wall run hotter, leading to creep, fatigue and eventually material failure. Further, these things increase the tendency of the boiler to "prime", i.e. carry-over water into the steam stream, with rising concentrations of dissolved substances. This is why regular blowdown is necessary and periodic washout is a valuable planned preventative maintenance [PPM] procedure.

Reducing the concentrations of dissolved substances in a boiler feedwater, often with reverse osmosis equipment among other things, is therefore a Good Thing.

<..."water heaters" are _never_ blown down...>

This would be due to there being no separation process taking place within them. Cold water in. Hot water out. The End.

Periodic washout of <...water heaters...> as a PPM will still be found valuable.

Frame of reference:

18 megohm water is very highly purified, devoid of any substance but H20, ultrapure water used in the electronics industry for chip manufacture, for example.

10 megohm water is highly purified with typically an ion exchange demineralization process (could be reverse osmosis/demin, electrodeionazation, or similar) that is a typical goal for high pressure boiler operation such as in utility central stations.

The answers so far have answered the question. However, please do not add caustic soda to the feed water in a high pressure boiler. Usually a boiler that calls for meg ohm water is a high pressure boiler, 1000 to 1750 psig operating pressure, and adding caustic soda to the feed water will cause a boiler failure, and it may be fail before leak. Contact your local feed water treatment specialist such as Nalco or Calgon and get a good coordinated treatment program.

harralrw wrote:

> snip ... please do not add caustic soda to the feed water in a high
> pressure boiler. Usually a boiler that calls for meg ohm water is a
> high pressure boiler, 1000 to 1750 psig operating pressure, and
> adding caustic soda to the feed water will cause a boiler failure,
> and it may be fail before leak.

Thanks. I just happened to come across that statement about resistivity while I was searching for a potential boiler, and didn't know what it meant or when it applied; it no doubt was not with the same information.

The largest unit that we are considering (or something similar) is the "Reimer's Electra Steam" model "HL-265" which has the following specs:

BTUH Out Put: 895,000
KW: 265
Water Capacity: 71 gallons
Maximum temperature is 240F, which I believe creates about 10 p.s.i. ... which I'm sure is considered a low pressure system. But THANKS for the safety warning.

Cheers.

Bill Velek

"...go into the beer we'll be making..."

Right! All very helpful, I'm sure, all very critical information to have. Now, about that beer...

If you are lucky enough to have good feed water you may be able to use polishing filters instead of Reverse osmosis. They are a lot cheaper.

Really pure water is a great solvent for almost everything. You can get your water too pure for some types of pipe.

Thanks for the advice. I'll get the specs for our municipal water and do some research. Saving money is always good for business.

Cheers.

Bill Velek

Sorry, but "polishing filters" and "reverse osmosis" is comparing apples and oranges. In fact, it is likely that the reverse osmosis plant that you are considering will itself require "polishing filters" as one form of pretreatment prior to pumping through the RO membranes. You may require additional treatment steps.

Filters remove particulate matter that could foul the RO membranes. They do not reduce the saline content of the feedwater.

The RO system will take out well over 90 % of the dissolved solids content of your feedwater. This will likely not enable reaching your 1 Megohm target unless you have unusually low dissolved mineral content in you water supply (not likely).

1 megaΩ/cm resistivity is the boiler specification of the minimum quality of water purity. 18 megaΩ/cm resistivity is a much more purify water, it is often use in electronic industry. The water at 1 megaΩ/cm resistivity is highly corrosive, stainless steel piping or PVC.... should be used instead of regular copper pipe.

If you are commercial brewery, you should contact your local water treatment company to install a water treatment system, it is best to leave it to the specialist so you can concentrate on your field.

If you are an hobbyist like I am, you may try my set up (I made my own beer and wine) as following:

incoming water - catalytic conditioner - Sediment tank with up ward charcoal filter - RO - boiler (in my case a converted pressure cooker)