50% propylene glycol concentration for roof make-up air units 2

[oooaaa]

The winter design temperature is -12deg. F in my case, some people suggested that we should us 50% propylene glycol for roofrop make-up air units, but I am so hesitate with this figure. Except increasing pressure loss and risk of corrision of the water loop system, decreasing equipment efficiency, what are other disadvantages of using such high concentation solutation?

 

[ChrisConley]

If you use inhibited glycol you're reduce the risk of corrossion on the water loop.

Unless you plan on circulating the fluid at -12F you don't need to have that much glycol.

The glycol is protection from burst pipes (in this case) and 25% glycol will provide burst protection beyond your outdoor design condition.

The glycol in solution will 'slush' during a shutdown, but won't burst the pipes. Once you start your boiler up again the warm fluid will melt the slush and allow your system to start-up.

 

[stanlsimon]

We generally use 35% ethylene glycol maybe 40% in Minnesota where the design winter temp may be -40.

Also it helps to provide a dedicated pump to circulate water continuosly through the outside air coil. This helps prevent freeze ups but also solves a control problem of tripping freeze stats when a hot water(glycol) coil receives a modulated amount of heat. In this situation hot air comes out of the top of the coil and cold air comes out of the bottom of the coil. The freeze stat sees a ribbon of cold air which causes the unit to trip out.

So the moral of the story is provide a run around loop with a three way valve piped in such a way that the coil always gets flow flow.

 

[magicking]

Just a reminder: higher the percentage of glycol, lower the efficiency of the heat transfer from the coil to the air. But in your case the incoming temperature is only -12°F, 50% glycol will not be significant for the coil size. But in Calgary with -40°F ambient we still use 40% glycol only.

 

[sterl]

If this air handler is associated with a process space or trying to maintain a "tight" WB/DB/Dewpoint relationship, permitting the glycol to go slush may cause controls to drive your system pretty crazy.

If your design ambient is Minus 12 and that's a 1% number: check your weather history for the 50-year low and make sure that is not much lower...and providing that's as low that's as low as your cooling system can ever go, even on pump down to standby: There's nothing that says you need to be 50% Prop. 10 to 12 deg of freeze protection is pretty standard for stationary systems.

Keep in mind though: If there is a small volume of charge involved, the guys mixing it find it a lot easier to do either 50-50 or 33-67 because of the 1: 1 or 2: 1 proportions...Asking them to field mix 37% is going to cause a phone call on the procedure unless your field guys can do math better than mine...