Question about the rationale behind RO feed water temperature

[KoleTang]

As winter is coming around I am realizing that I have been running my RO unit incorrectly.

RO membranes are rated for their performance at 77f.

RO system instructions say to hook up the system to the cold pipe (if using a saddle valve). (So the membrane is not destroyed)

Now it seems that some people also think that this should be translated to using the cold tap ONLY when you are running the system off a sink. This sounds wrong to me, since the sink gives you the ability to mix in some hot water to get closer to 77f.

So as long as you aren't going past 77f it is OK to mix the cold tap with the hot tap, correct?

Reference: http://forum.simplydiscus.com/showt...ture-and-Pressure-Correction-to-Determine-GPD

 

[billwill]

Now that's an interesting question! Seems like it might make sense. At the rate of RO production though, I wonder if you'd ever actually have hot water at that point?

 

[KoleTang]

Now that's an interesting question! Seems like it might make sense. At the rate of RO production though, I wonder if you'd ever actually have hot water at that point?

That's a valid point. I'll have to calculate the size of my water heater, and the power it uses to heat new water. Depending on that it may just be more cost effective to run a booster pump. But I'm still curious as to whether or not running some hot water in with the cold water is OK.

 

[FishGuyTy]

From what I understand, the temp affects the rate of production. Cooler water temps would produce less water and ideal or peak operating temp is 77f. The tdi reading will still be 0.0 with cold water, you will just get less water. I just bought a 100gpd RO/DI unit. The Aqua FX Barracuda and I read the instructions front to back. Here is the scale it gives for temperature versus production in GPD for a 50 gpd membrane.

77f = 50gpd with a factor of 1
68f = 42.85gpd with a factor of .875
59f = 36.5gpd with a factor of .730
50f = 31gpd with a factor of .620

I have no idea what the "factor of" is, but it was on the table provided in the instruction packet I received with the unit. I hope this helps Happy Reefing!

 

[Randy Holmes-Farley]

I'm confused about why you would want to.

FWIW, the output is lessened at lower temps, but the purity is a bit higher (although the purity effect is small).

So unless you are really pressed for time, I can't see why you'd want warmer water.

This article details the effects of temperature on output and purity:

https://web.archive.org/web/2001060...ures/BNP__Features__Item/0,2732,24877,00.html

 

[KoleTang]

I'm confused about why you would want to.

FWIW, the output is lessened at lower temps, but the purity is a bit higher (although the purity effect is small).

So unless you are really pressed for time, I can't see why you'd want warmer water.

This article details the effects of temperature on output and purity:

https://web.archive.org/web/2001060...ures/BNP__Features__Item/0,2732,24877,00.html

I'm more concerned about the higher amount of waste water at the lower temperature. But the cost of the extra water may very well be less than the cost it would take to use heated water or run a booster pump.

I didn't know there was a purity differential relative to temperature. Is there a point of diminishing returns where at a certain temperature the water production is slower, yet the quality difference is negligible?

 

[Randy Holmes-Farley]

OK, I can see that if waste water is a significant concern for you, that might be a benefit of higher temperature.

 

[AZDesertRat]

If the waste is an issue get two capillary tube flow restrictors and trim one for the winter months and one for summer months.
77 degrees is just a number the industry picked to compare membranes against each other, it is not the most efficient temperature since colder water actually treats better and gives you lower RO only TDS. Lower RO TDS means your DI will work better and last longer saving you money in the process.


You can use this form and calculate your membranes performance at different temperatures, pressures and incoming TDS levels.
http://spectrapure.com/manuals/CALC-FORMULA.pdf

 

[KoleTang]

If the waste is an issue get two capillary tube flow restrictors and trim one for the winter months and one for summer months.
77 degrees is just a number the industry picked to compare membranes against each other, it is not the most efficient temperature since colder water actually treats better and gives you lower RO only TDS. Lower RO TDS means your DI will work better and last longer saving you money in the process.


You can use this form and calculate your membranes performance at different temperatures, pressures and incoming TDS levels.
http://spectrapure.com/manuals/CALC-FORMULA.pdf

What do you mean by "trim"?

Where on does the valve go? On the waste hose?

 

[AZDesertRat]

A capillary tube flow restrictor looks like an uncooked spaghetti noodle with a small flange or keeper on one end and fits inside the waste line,out of sight. It is porous or has tiny microscopic holes in it so water passes through it as in capillary action. You trim the length with a sharp razor blade to change the waste ratio to suit your exact water temperature and water pressure. If your water temperature varies widely between summer and winter it is best to have two restrictors and change from one to the other as temperatures go up or down so the waste stays more constant.