I need more PSI for my RO/DI

[AZDesertRat]

Adding additional filters is counteproductive. Use fewer better filters and keep and eye on the pressure drop and you will be fine. Everthing you add in front of a RO membrane reduces the pressure available due to headloss. If sediment filter fouling is a problem then a pleated filter with 10x the surface area may be a better choice as it will last much longer and you can rinse them a few times if you are careful.

 

[Loki]

I've not had any problems with my filter array. Just the water pressure at this house. Our water at my old house was 92tds and I changed out my DI maybe every 6 months or so. My other filters never got changed because I sold the unit and upgraded before they were spent. I haven't checked this house for he TDS because I have misplaced my TDS meter. Once I find it ill test.


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[BeardedReefer]

As far as aiding at your RODI, if your water line runs very far you can try increasing the size of the tubing from the connection to the RODI system. If it isn't very far there is not much you can do about it. The reason increasing the size helps in long runs is because of line resistance. When a fluid runs through a line, velocity equals a pressure drop. So a larger line (tubing) has less velocity and therefore less pressure drop. Though you must realize this is not going to give you a huge boost when you are only talking about the volume per hour of the membrane and the bypass. If your line is really long it can make a difference though.

Your logic here is backwards, if you want to increase water pressure in a line you reduce the diameter of the line, thus increasing water pressure. Think of a water hose, slow flow, when you put your thumb over the opening "Velocity" increases equaling more pressure.

 

[AZDesertRat]

Actually Fragmatic is correct, increasing the line size increases the pressure available to the membrane. It has to do with resistance to flow and the C factor of the tubing or pipe. Velocity and pressure are two entirely different things. Velocity kills pressure, when we design a water system we keep the velocity down to like 3 to 5 feet per second for that reason.

Here is an exercise where you can calculate the pressure at the end of a line based on line size, pressure at the starting point and the velocity. Velocity and friction loss cause dramatic drops.

Lesson 4: Dynamic Pressure (Hazen-Williams Formula)

And here is the Darcy-Weisbach formula or equation for calculating it:

Darcy?Weisbach equation - Wikipedia, the free encyclopedia

You always want to upsize a line if pressure drop is a problem. Most if not all pump manufacturers recommend stepping pipe and fittings up a size or two if your line will be longer than normal.

 

[bct15]

Actually Fragmatic is correct, increasing the line size increases the pressure available to the membrane. It has to do with resistance to flow and the C factor of the tubing or pipe. Velocity and pressure are two entirely different things. Velocity kills pressure, when we design a water system we keep the velocity down to like 3 to 5 feet per second for that reason.

Here is an exercise where you can calculate the pressure at the end of a line based on line size, pressure at the starting point and the velocity. Velocity and friction loss cause dramatic drops.

Lesson 4: Dynamic Pressure (Hazen-Williams Formula)

And here is the Darcy-Weisbach formula or equation for calculating it:

Darcy?Weisbach equation - Wikipedia, the free encyclopedia

You always want to upsize a line if pressure drop is a problem. Most if not all pump manufacturers recommend stepping pipe and fittings up a size or two if your line will be longer than normal.

Ah, a good ole incompressible flow lesson.


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[BeardedReefer]

Actually Fragmatic is correct, increasing the line size increases the pressure available to the membrane. It has to do with resistance to flow and the C factor of the tubing or pipe. Velocity and pressure are two entirely different things. Velocity kills pressure, when we design a water system we keep the velocity down to like 3 to 5 feet per second for that reason.

Here is an exercise where you can calculate the pressure at the end of a line based on line size, pressure at the starting point and the velocity. Velocity and friction loss cause dramatic drops.

Lesson 4: Dynamic Pressure (Hazen-Williams Formula)

And here is the Darcy-Weisbach formula or equation for calculating it:

Darcy?Weisbach equation - Wikipedia, the free encyclopedia

You always want to upsize a line if pressure drop is a problem. Most if not all pump manufacturers recommend stepping pipe and fittings up a size or two if your line will be longer than normal.

From the Same lesson plan you are using to support your "logic" I give you:

Lesson 9: Static Pressure and Velocity Pressure

Lesson 9
This theory, which forms part of the study of hydraulic principles, states that the static pressure of a moving fluid varies inversely as its velocity, which means that as velocity increases, the static pressure decrease.
The principle of the relationship between velocity and pressure is illustrated below. Two vessels (a) and (b), of the same shape and cross-sectional area, are joined together by a small-bore pipe, (c). If fluid is added to vessel (b) and gradually find its own level, this level will still be maintained if two positions (d) and (e) of the same size and weight are placed on the liquid in the vessels (a) and (b), respectively.
If a force of 1N is exerted on piston (d), in a downward direction, fluid in (a) will flow through the pipe (c) into vessel (b). The velocity of the fluid will increase as it passes through (c), because the cross-sectional area of the pipe (c) is much less than that of (a), and the same volume of fluid must pass through (c) as leaves (a) in the same time.

Also going by your first post from lesson 4, if you use the formula when you "DECREASE" the Diameter of the pipe your PRESSURE DROP also DECREASES!!!! If your going to post an equation and lesson plan make sure you actually try the formula to see if it supports your theory.

 

[BeardedReefer]

Here is a video describing how "decreasing" your pipe/hose size "increases" velocity with formula demonstration and explanation.

 

[AZDesertRat]

You are confusing the issue.
The goal is maximum delivery pressure and maximum volume AT THE MEMBRANE. Yes, decreasing the line size will raise the pressure due to fricton loss at the entry to the pipe or tubing, BUT, it decreases both the pressure and the important GPM or flow at the point of delivery or usable end of the pipeline. The goal is to reduce velocity which in turn reduces the headloss due to friction and increases the delivery pressure at the point of use. You can't have both. For our purposes we don't really care what the pressure is at the point of entry, only the point of use.

 

[BeardedReefer]

No not confusing anything, Fragmatic's initial post was about increasing the "inlet" side line to the RODI unit itself which is not good practice, thus reducing usable PSI. Thats why most booster pumps are inline prior to the entire unit unless it is waste driven.

 

[BeardedReefer]

Also water Flow does not slow down by reducing the line size, it stay exactly the same thus increasing PSI.

 

[AZDesertRat]

Where are you coming up with this?
You always increase a line size if headlosses are suspected so you have sufficient pressure at the point of use. If that were not the case why do we increase a homes service line from 3/4" to 1" to keep the pressure losses down and the delivery rates up? Why do we increase the size of a water main from 6" up to 8" to maintain fire flows and required minimum pressures during dermands. Or whey do we increase an RO systems tap water line from 1/4" to 3/8", again to maintain the required flows and pressures?
I think you are way off base here.

 

[bct15]

I think someone is getting a little confused here, I am not a fluids expert but I do know a thing or two about fluids and math and see this as a good place for me to chime in as I love intellectual type discussions.

First off, what you guys are debating is the flowrate at the inlet...not necessarily the velocity. The volumetric flowrate V at the inlet is described by the following equaition.

V= Vel*A

Where Vel is the average velocity of the fluid and A is the area of the pipe inlet described by the following equation.

A = (Pi/4)*D^2

Now, solving for the velocity, and keeping the volumetric flowrat constant

Vel = (4*V)/(Pi*D^2)

And bundling all constants into one to make the equation look better

Vel = X/(D^2)

As can be seen by the above equation if the flowrate is kept constant the velocity of the fluid at the inlet must go up as the diameter of the pipe goes down. Because as the number you divide by gets smaller, the result gets bigger.

Now what is being discussed here is the pressure at the end of the system. As the water travels through a circuit it will encounter a pressure loss, therefor to achieve a required pressure at a certain point in the circuit the pressure loss must be known and added to the inlet pressure. This is often in industry described as head loss and is described by the following equation.

h = f*(L/D)*(Vel^2/2*g)

Where h is the head loss, f is a constant, L is the length of the pipe, D is the diameter, Vel is the velocity of the fluid, and g is the gravitational constant. Substituting the equation for velocity at a constant gpm from earlier you get.

h = f*(L/D)*(16*V^2/(Pi^2*D^4*2g))

To make this equation look a lot cleaner we will bundle all of the constants into one constant (V,f,Pi,g,L), and since diameter is the only thing in question we will leave it there.

h = X/(D^5)

It can be clearly seen that as diameter goes up the headloss quickly goes down (diameter is raised to fifth power) using the same principal as the velocity diameter relationship. The headloss can be seen as the amount of pressure that must be added at the inlet to achieve the desired pressure at a certain point in the circuit. So as the headloss goes up, the pressure required at the inlet will go up to and is explained by the following simple relationship.

Pressure - Pressure drop = Pressure at outlet

As can be seen by the above relation that since increasing the diameter decreases the pressure loss, increasing the diameter will increas the pressure at the outlet and every point in the circuit. Decreasing the diameter will increase the pressure drop, reducing the pressure at the outlet and everypoint in the circuit. The equation to convert headloss to pressure drop is

Pressure drop = fluid density * g * h

 

[Flatbrook]

Sorry, lost me on the first sentence, I think I'll take your word for it!


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