Pumping water vs. glycol/water solution

[Microwatt]

This may be more of a mechanical question but you guys are smart so I figured you could answer it. Let's say you have a fairly large heating/cooling system that uses only water. Now let's say you make that water a 30% glycol/water solution. Would you expect the pumps to draw more current or less current with a mixed solution vs just water? The way I look at it the solution is heavier so it should create more head pressure on the pumps which would mean less volume and less volume would mean less work to be done, hence less current draw. Just like a fan. The less air you move the less work the fan has to do. Or, would increase in viscosity cause the pump to have to do more work with less volume?

 

[iwire]

It sounds like a closed loop so any differance in weight should cancel out. I think it will be all about viscosity but I have no guess how that will change things.

 

[G._S._Ohm]

This may be more of a mechanical question but you guys are smart so I figured you could answer it. Let's say you have a fairly large heating/cooling system that uses only water. Now let's say you make that water a 30% glycol/water solution. Would you expect the pumps to draw more current or less current with a mixed solution vs just water? The way I look at it the solution is heavier so it should create more head pressure on the pumps which would mean less volume and less volume would mean less work to be done, hence less current draw. Just like a fan. The less air you move the less work the fan has to do. Or, would increase in viscosity cause the pump to have to do more work with less volume?

What is the viscosity and density of the solution vs. that of water?

 

[Microwatt]

What is the viscosity and density of the solution vs. that of water?

Let's say both are more than that of the water, but I can't say for sure. I'm not really looking for any type of number I'm really just wondering, theroetically, if a pump would have to work harder to move a glycol solution rather than just water.

The story is, years ago the system was coverted from water only to a glycol system. All pumps were left as is. We have thermal imaging done and this year the OL block for this pump made the report so we had it replaced and set to the nameplate of the motor and now we have a problem with it tripping. A little history, the plant operators won't hesitate to turn overloads up themselves if they are having problems so the OL block that burnt up was set too high so this pump could have been over-amping for years.

We are just trying to figure out if the coversion to glycol years ago could be the reason this pump is working harder. It's a 40hp pump with a nameplate of 52 amps. Right now it's drawing 57 amps but that is with the valve only 2/3 open. The more the valve is closed the amps decrease so I know volume does have something to do with it.

We are out of our cooling season so we can handle less volume now, but in a few short months we will need to open this valve all of the way.

 

[fmtjfw]

This may be more of a mechanical question but you guys are smart so I figured you could answer it. Let's say you have a fairly large heating/cooling system that uses only water. Now let's say you make that water a 30% glycol/water solution. Would you expect the pumps to draw more current or less current with a mixed solution vs just water? The way I look at it the solution is heavier so it should create more head pressure on the pumps which would mean less volume and less volume would mean less work to be done, hence less current draw. Just like a fan. The less air you move the less work the fan has to do. Or, would increase in viscosity cause the pump to have to do more work with less volume?

Once in the dim dark past I emptied a 55 gallon barrel of glycol in to a cooling system through a 3/4 or 1 inch hose. It took forever. I asked if I could "pressurize" the barrel with a little compressed air. I was told no, just find something to read for the afternoon. This leads me to believe that either it is fairly viscose or somehow cause the interior of the hose to narrow. It was also the heaviest 55 gallon barrel I have ever moved.

 

[G._S._Ohm]

It's a 40hp pump with a nameplate of 52 amps. Right now it's drawing 57 amps but that is with the valve only 2/3 open. The more the valve is closed the amps decrease so I know volume does have something to do with it.

Yes, the amount of fluid moved increases with hp, probably hp is some function of the speed^3 of the fluid.

If both the viscosity and density of the solution are more than that of water (this info must be available on the Web) I'd say this reason is the best candidate for the cause of the overload.

How difficult is it to go back to water temporarily and make add'tl measurements to confirm that this change is not due to the passage of time?

 

[gar]

121127-1741 EST

See http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html for a table of viscosity of water and glycol of various ratios relative to temperature.

The addition of glycol increases the viscosity. Increasing viscosity produces a need for greater power to move the liquid in a pipe.

When the liquid is used for cooling it is much more viscous than when used for heating.

.

 

[Jraef]

The concept of "In a centrifugal pump system, less flow = less work" is true, but only assuming that the specific gravity of the fluid is the same. Glycol has a higher specific gravity than water, ergo it takes more work to move the same amount of fluid through the same space / head etc., or with the same amount of work force applied, less will flow, which ever way you want to look at it. In your case with the system designed for glycol, changing it to water would allow MORE fluid to flow and thus the motor will work harder, draw more current, possibly over load.

 

[gadfly56]

The concept of "In a centrifugal pump system, less flow = less work" is true, but only assuming that the specific gravity of the fluid is the same. Glycol has a higher specific gravity than water, ergo it takes more work to move the same amount of fluid through the same space / head etc., or with the same amount of work force applied, less will flow, which ever way you want to look at it. In your case with the system designed for glycol, changing it to water would allow MORE fluid to flow and thus the motor will work harder, draw more current, possibly over load.

Ethylene glycol is 1/2 of what goes into dynamite grade nitroglycerin, and I can tell you that the viscosity is much greater (10X) than that of water. Once you mix it with water, it's a lot closer to water than glycol. gar mentioned The Engineer's Toolbox, and if you poke around you'll find a little table that shows the extra power you need to push the same volume of glycol solution as water. For 30% glycol it looks like about 25% more power, so you should definitely see higher amps on the motor. This also assumes there's some "head room" in the pump. If you're operating close to the max flow on the pump curve, you'll pull more amps until you max the pump and then you won't get any increase in flow. This is for centrifigal pumps.

 

[ActionDave]

...OL block that burnt up was set too high so this pump could have been over-amping for years.

We are just trying to figure out if the coversion to glycol years ago could be the reason this pump is working harder. It's a 40hp pump with a nameplate of 52 amps. Right now it's drawing 57 amps but that is with the valve only 2/3 open. The more the valve is closed the amps decrease so I know volume does have something to do with it.

I don't think the glycol has any bearing on the problem from what you describe here. It may be that you have too much pump and not enough motor.

 

[weressl]

The concept of "In a centrifugal pump system, less flow = less work" is true, but only assuming that the specific gravity of the fluid is the same. Glycol has a higher specific gravity than water, ergo it takes more work to move the same amount of fluid through the same space / head etc., or with the same amount of work force applied, less will flow, which ever way you want to look at it. In your case with the system designed for glycol, changing it to water would allow MORE fluid to flow and thus the motor will work harder, draw more current, possibly over load.

The 30% glycol solution increases the specific gravity by 1-4% depends on operating temperature. In short average runs in our aplications, friction is negligible. So the only issue will be the head-pressure increase and along with it the power requirements. See pumping affinity laws for detailed answers.

 

[G._S._Ohm]

With this site
http://www.mcnallyinstitute.com/14-html/14-04.htm
you can put some numbers on the difference.

 

[Microwatt]

121127-1741 EST

See http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html for a table of viscosity of water and glycol of various ratios relative to temperature.

The addition of glycol increases the viscosity. Increasing viscosity produces a need for greater power to move the liquid in a pipe.

When the liquid is used for cooling it is much more viscous than when used for heating.

.

Great link. I've learned this pump is a chilled water pump for a chiller and 40 deg F is what they try to maintain.

 

[Microwatt]

With this site
http://www.mcnallyinstitute.com/14-html/14-04.htm
you can put some numbers on the difference.

This is good info too. I will do some studying and try to wrap my head around this.

 

[weressl]

With this site
http://www.mcnallyinstitute.com/14-html/14-04.htm
you can put some numbers on the difference.

As I said viscosity difference will not make any significant difference. Even the specific gravity difference is minor.

 

[GeorgeB]

The 30% glycol solution increases the specific gravity by 1-4% depends on operating temperature. In short average runs in our applications, friction is negligible. So the only issue will be the head-pressure increase and along with it the power requirements. See pumping affinity laws for detailed answers.

It appears he is using momentum transfer (centrifugal) pumps, so it may be of no concern, but there is "considerable" lubricity to the glycol, enough that we use it in hydraulic systems (positive displacement) where it is the only lubricant. At "normal" operating temperatures, ethylene glycol at 60% concentration (hydraulic "standard") will be in the 10-100 cSt range ... 130F to 60F range, roughly. The 30% concentration ... I don't know. Head loss due to viscosity would probably be more significant that the 1-4% sg penalty as weressl says.

I'd strongly suggest the glycol manufacturer be in the loop. Water from systems using chlorine to eliminate bad things often gives problems.

George

 

[weressl]

It appears he is using momentum transfer (centrifugal) pumps, so it may be of no concern, but there is "considerable" lubricity to the glycol, enough that we use it in hydraulic systems (positive displacement) where it is the only lubricant. At "normal" operating temperatures, ethylene glycol at 60% concentration (hydraulic "standard") will be in the 10-100 cSt range ... 130F to 60F range, roughly. The 30% concentration ... I don't know. Head loss due to viscosity would probably be more significant that the 1-4% sg penalty as weressl says.

I'd strongly suggest the glycol manufacturer be in the loop. Water from systems using chlorine to eliminate bad things often gives problems.

George

George,

I was saying the opposite, that the friction loss is insignificant and the head pressure increase due to the small change in specific gravity is of minor impact. Look at the tables that someone linked to see the impact of viscosity.