Why is RODI so much faster when it’s hot?

[Slocke]

So today my RODI did 5 gallons in under 16 minutes. When I got this RODI in January (after leaving my old one outside on a very cold night causing it to shatter) it took over an hour. I’ve heard say that it’s to do with pressure and yes I’m aware of PV=nRT but it is not 4xhotter (certainly not 4x hotter Kelvin). (I did check and I my ppm was still 0)

Why is this? And share how fast yours is.

 

[JoJosReef]

Last couple of times I set my time to the usual and it was already overflowing when I got back. It sits in a large lab sink, so it just overflows into the sink. Not good, but not a disaster. Will have to do periodic checks.

 

[Tamberav]

It’s the increased viscosity of the water at lower temps that slow it down and its a dramatic change.

 

[Solo McReefer]

PV=nRT

Does that apply to liquid water?

I don't know

 

[Extremeengineer]

Wow, 16 minutes for 5 gallons is some serious output. That’s 450 GPD actual, not rated. I get 1 gallon every 20 minutes, 204 ppm to 0 ppm, 60psi, unit rated 100 GPD, so I’m getting about 75% of rated. Applying that to yours, would be rated for 600 GPD!!! Doubt that unit would fit on wall between wash basin and shelf in my setup lol

 

[BeanAnimal]

The efficiency of the RO membrane is highly temperature dependent.

 

[Solo McReefer]

The efficiency of the RO membrane is highly temperature dependent.

Insane in the membrane

Someone needs to invent a membrane warmer

 

[Extremeengineer]

Here’s a chart with correction factors, my ground water is approx. 60° F, so their stated factor is about spot on with my real world numbers.

So what unit are you using to get that output?

 

[BeanAnimal]

Flow rates are quoted at 77F. Use the multiplier below to determine approx flow at other temps

 

[BeanAnimal]

Ahh I see somebody beat me to it,

FWIW 100 is too hot and is going to be hard on the membrane.

 

[Sassafras]

An other interesting set of charts of the effect of various factors:

Figure 2 - Effect of Feedwater Pressure on Flux and Salt Rejection​	Figure 3 - Effect of Feedwater Temperature on Flux and Salt Rejection​

Figure 4 - Comparison of Operating and Cleaning Parameters for Thin-Film Composite Membrane and a CA Membrane​

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Figure 5 - Effect of Increasing Salt Concentration on Flux and Salt Rejection​	Figure 6 - Effect of Increased Recovery on Flux and Salt Rejection​

Figure 7 - Effect of Feedwater pH on Water Flux and Salt Rejection​

 

[Cichlid Dad]

I guess I'm weird, but I set the temperature before I make water.

 

[TangerineSpeedo]

In the summer I have to run the tap for 5 mins because some plumber ran the cold water pipe through the attic. It is so hot I can't even take a shower with just the cold on.
But after the pipes are clear:
5g
22 mins winter
18 mins summer
8 stage double membrane w/pump and it fits under the bathroom sink.

 

[Slocke]

Thanks all!
It is just a simple, cheap, and common 4 stage RODI filter. I know 16 minutes is crazy and that’s why I asked the question. I’ll look at the pressure gauge and temperature of the water tomorrow.

 

[Slocke]

Does that apply to liquid water?

I don't know

Not perfectly because water is far from an ideal fluid but while above freezing there would be a relationship between pressure, volume, and temperature that is at least close to the equation.

 

[Malcontent]

Use desalination RO hardware which runs at 800+ psi...problem solved.

 

[BeanAnimal]

Cold water is denser (more viscous) and therefore harder to push through the membrane. Likewise, the membrane is made of a polymer, as it gets colder it becomes less flexible and the pores will tend to contract, also slowing down transfer.

The word "push" denotes that higher pressures moves more water through than lower pressures...


So warm and high pressures makes more water than cold and low pressure. Pretty simple... rejection rate (efficiency) is a different subject.

 

[Dburr1014]

I know in the winter my production is much better because the input line passes over(near) my wood stove in the basement. The water is warmed.
Summer is cooler in the basement.

 

[BeanAnimal]

I (long ago) stopped paying attention. The RO unit fills a reservoir using float switches and logic. It just does its thing without interaction.

 

[Dburr1014]

I (long ago) stopped paying attention. The RO unit fills a reservoir using float switches and logic. It just does its thing without interaction.

Same, love it.