Glycol vs. Water 1

[mena1]

Newbie question:

When designing a system, how do you know when to select glycol over water or vise versa?

What comes to my mind is cost, freeze protection.
How does selecting glycol over water affect your system (your piping, pumps, etc)?

Thanks for the feedback.

 

[cdxx139]

Shoot for a water system everytime. You get better heat transfer, easier pumping etc.

I use glycol, if I cant protect the coils for whatever reason. But is its an AHU, use a freeze protection pump. When OA is greater than 40 degrees, pump provides full flow through coil, and the modulating valve bleeds in hot water as needed to maintain air temperature. The important thing here is to size the coil for a minimum of 3 fps. Running water doesnt freeze (or so someone told me).

If you have an air cooled chiller outside, you almost have to put glycol. Maybe use heat trace instead, but that can deteriate ocver time.

If I do gio glycol, I will try to have a heat exchanger, and just use the glycol in the coils required for freeze protection, so you dont lose efficicney thorughout the system.

knowledge is power

 

[317069]

Thank you for your reply
if we use a water system, can we add the glycol later to this system if we found that we need it?

 

[IRstuff]

Yes, but if you even think that you're going to use glycol, you should design for glycol. You'll need to account for:

> viscosity differences, being that glycol will get extremely sluggish the closer you are to freezing point, and it'll be substantially more sluggish than water. This will drive pump and plumbing performance.

> corrosion protection, being that EG is slightly corrosive, and standard antifreeze is supplemented with corrosion inhibitors, so your plumbing will need to be corrosion resistant.

TTFN

FAQ731-376

 

[GMcD]

For building hydronic systems, inhibited propylene glycol is the product to use (Dowfrost is one brand). A 20% to 25% solution will give you decent freeze protection down to about 15F without compromising the pump head, and heat transfer capability of the system. If you are in a cold climate like the prairies in Canada, a 40% maximum solution is the normal good practice, and at that solution concentration, yes, there will have to be some accounting for the de-rating of the pump head (increased friction) and heat transfer capacity of your heating terminals. ASHRAE has good charts and recommendations in the Handbooks.

 

[rmw]

If you use Glycol, you have to account for density differences as well as viscosity differences. Depending on your concentration of Glycol, that can change your pressure drops through piping and Hx's. I'd say as a rule of thumb, if you use very low concentrations of EG or PG, you can just about treat it like water.

But, if your design calls for you to use high concentrations, say 40-50-60% EG or PG, you will require different pumps and your system pressure drops will be quite different at each point.

As IRstuff states, the colder the fluid, the more viscous it gets and you will get to a point where it may not freeze, but you can't pump it either. I had to cajole Dow to give me properties for 60% PG by assuring them that I didn't intend to pump it - just keep it from freezing. It is nasty stuff at that concentration and -40 F (or C).

Get a Crane Technical Paper 410 and find the Darcy equations and do some pressure drop calculations to see the difference.

rmw

 

[desnov74]

Good practical info on Chillers.com

http://www.chillers.com/AntifreezeMain.htm

http://www.chillers.com/AntifreezeProductInfo.htm

http://www.chillers.com/AntifreezeFreezepoints.htm

http://www.chillers.com/WatertreatmentInfo.htm