Glycol Corrections

[jdmz]

When sizing a pump for a glycol system, I understand glycol has a pressure drop correction and well as a capacity correction as a function of the type, % volume glycol, and fluid temperature.

The question is when I give a rep the head(ft) to size a pump, do I give him the corrected pressure drop or the pressure drop as if it were water? The reason why I ask is because their selection software will be able to generate a selection based on the fluid and temperature. In other words, is the head I am giving them Head in Ft h20 or head in ft @ glycol conditions?

 

[HVAC-Novice]

For your system you need to calculate with the expected viscosity (temperature, and % of what type of glycol). Even if you don't use glycol, a chilled system has higher pressure drop than a hot system due to higher viscosity of cold water. "Standard" water charts in the US are based on 60°F water.

Provide them psi, or ftH2O (this as a unit is standard water head, not your actual fluid). Their selection software needs % and type of glycol and the temperature to account for what the pump does with more viscous fluids. Actually all manufacturers have online selectors to do the same thing. Don't need a rep.

For temperature, assume the lowest expected operating temperature (=highest expected viscosity). So for a hot water system, assume 90°F maybe. For chilled water 40°F. Chilled glycol (geothermal?) maybe 20°F. Be conservative.

 

[jdmz]

Thanks so that means I would want to apply any correction factors to the head for viscosity prior to arriving at the ft h20 scheduled on the pump schedule. I have been doing it correctly then.

 

[HVAC-Novice]

How do you do the calculation in the first place? If you use Revit or other software, you are able to enter the fluid properties and it will account for it.

If you use tables or charts that are based on standard water, then yes, you need to apply the correction factors.

I know this wasn't your question, but your devices also need to be corrected for glycol. There will be less heat transfer, heat capacity etc. Manufacturer have selection software that accounts for that. So, if you have a unit heater that requires 10 gpm and 2 psi with standard water, may require 11 gpm and 4 psi for a given heat output. Make sure the flowrate you use also accounts for glycol. Also make sure to decide on EG or PG early on or you re-do a lot.

I saw projects where a designer just assumed standard water for geo-exchange glycol. Not only they originally designed for 20% PG, they also had to increases that to 25% due to freezing issues. That lead to insufficient flow for the heat pumps and they cycled off. I can tell you, replacing the too small pumps and pipes was not cheap.

 

[jdmz]

I use standard K factors for water so I apply a correction factor for the viscosity of glycol.

I do account for the additional GPM required because of capacity derate, usually with equipment selection with the glycol conditions.

So if I am getting a selection for coil with 35% glycol and they tell me my head loss is 7 psi through the coil, is that 7 psi in water or in glycol? Meaning when I convert to ft h20, do I multiply the 7 psi by 2.31 ft to obtain head in ft?

 

[HVAC-Novice]

Typically ftH2O means standard water. It is basically just a unit o pressure. Same way in-H2O for air also is just a unit of pressure (and not related to the fluid in question, air). But to be sure, check with the manufacturer.

I use Modine equipment and their software doesn't show a unit for pressure, you just have to "know" it is psi.

To avoid confusion, use psi. Your pump selection software should accept either unit.

I grew up with metric units. So besides dealing with IP units, i also have to deal with all these inconsistencies. Like firs ti was confused because people said "pounds of pressure" and not Pounds per square inch or foot". or they referred to inches of pressure instead of in-H2O. I knew they talked about pressure, but only heard a unit of mass and a unit of length and couldn't understand why they seem to talk about pressure. Better use the full name of a unit

 

[nuuvox000]

Calculate pressure drops through piping/coils/etc using the glycol viscosity. If it makes it less confusing for you, you can just add it up in psi, then convert psi to ft w.c. (which is a pressure as well). Then you absolutely need to give the rep (or software) the information on the liquid you will be pumping.

 

[pierreick]

Hi,
I don't understand "glycol corrections", I've attached data for ethylene glycol aqueous solution.
Your favorite search engine should be the ultimate resource.
Note: Your comments about head are confusing.
Head loss should be expressed in Pressure units (Bar, Pa, Atm, PSI) then converted, if necessary, in height of liquid, P=Ro*g*H.
Good luck
Pierre

 

[ChasBean1]

Do you think your friction loss calcs are so accurate that you would need to correct selected pump head based on a slight change in density due to 25% glycol? No. Heat transfer is de-rated a bit with a glycol solution so go slightly larger on heat exchangers and a smidge larger on pumps, but otherwise these things are very small re: sizing/selection of systems.