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air in glycol (18 Posts)
air in glycolHow do you get the air out of a glycol system? I have a Honeywell/Spirovent air eliminator on the hottest part of our solar install. I ran the system for 24 hours with water only to get the air out, then pumped in glycol to avoid air in the first place. I am running it at 65PSI and keeping the high temp around 150F. I pulled off a sample and it came out frothy! (think a good, rich, German beer frothy).
Will these air bubbles eventually get scrubbed out by the air eliminator? Or should I drain it out and pump it back in to let the air leave? (other brilliant ideas welcome...). I did not pipe in air elimination at the panels, as I was counting on the Spirovent. As a glycol system, taking it down is nearly impossible (and very bad politics with this customer).
You should be able to get most the air outwith using the charging pump. Using water first is always best. Do you have coin vents/or any type of vents at the highest point of the system?
As for coin vents - no, I was counting on the spirovent. What do you mean by getting the air out with the charging pump?
The spirovent is at the hottest point of the system, but not the highest (ie inside, when the hot returns from my two rooftop arrays come together).
How did you get the glycol into the solar loop?
Bubbles grow a lot when you take a sampleIf the system has 65 psi, every tiny bubble grows to be 5 times bigger when you release it from the system. I think that explains the frothiness.
But yes, the Spirovent should get the air out. I'm not sure how long it should take.
The charging pump is powerful, so it certainly pushes all the bubbles down to the reservoir that you are charging from. All the largest bubbles then stay in the reservoir and bubble up without getting sucked back into the charging pump. Just the little ones are left.
If you had a coin vent at the highest spots, another large percentage of the air will rise to it overnight. Of course, you must vent that air before the pump starts again. After the second night, you won't get much air.
I know that's a hassle, so I think experienced installers would develop their own "system" to save time. Some manufacturers and installers say to install an automatic air vent at the high spot, but to tighten the metal cap after a few months to eliminate any problems if it fails or leaks.
Now here's something I verified while testing pressurized drainback systems, and it may sound like heresy to boiler guys:
If you only have one collector array (I realize you have two so this doesn't apply) and it's piped correctly, and if the collector pump is rated for, say, 6ft of head, then it can easily push through a big bubble. The bubble could extend down from the top of the collectors up to about 4ft. It just pushes the bubble up and around and down, then around again, etc. It doesn't affect solar collection performance very much, because the bubble gets munched up quickly.
Think of it this way - how often have you had a single zone hydronic system airlock on you?Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experimentsThis post was edited by an admin on September 21, 2011 1:25 AM.
my fill processMichael, after I fill with water and run that for a few days, I use a utility pump in the bucket, an inline high head pump and the system pump to also pull the glycol in. This usually translate to very little air in the system.
The customer rushed us and we didn't have enough time with just water in the system. I believe that is our problem.
it is tough to eliminatethe dissolved or entrained air from a glycol mix. Big bubbles and micro bubbles can be eliminated with a good power purge. That "milky" entrained air can take days to eliminate even with the best micro bubble eliminator.
Those clear pipe air elimination demos that you see at the shows, that are glycol filled, often have an air entrainment chemical added to aid in the air removal for display purposes. A little "slight of hand" trick.
Be sure the eliminator is at the highest temperature point in the system. Use an air eliminator rated for solar temperatures with a 300F listing, to handle stagnation temperatures. Keep the fill pressure high, with the relief valve setting, and expansion tank capacity in mind, of course. Give the air eliminator a few days to do it's thing.
If your system has a flow setting device or flowmeter with a clear window you can monitor the milky fluid condition. You need to get the fluid clear of that disolved air to get the very best heat transfer in the collector and HX.
The float type air vent at the high point of the system, often at the collector, will help with any large bubbles that make their way to the high point, but they are really a high point vent, not really a good device for ongoing entrained air removal.
patience was never my strong suitThanks Hot Rod,
I think I just need to wait a few more days. I have an analog flow meter in there and the fluid looks great. As Kevin pointed out, they get bigger when you let some out to examine them.
I am not having any problems, other than slightly more noise in the pipe than we would like, and a desire for the system to be operating at optimum efficiency.
tomI charge a system first with water. When charging with water first is when you get most the air out.
I use 2 five gallon buckets when charging the water. one bucket is for the discharge off the pump (if there is a drain near by I will use that instead of a bucket) and the second is for the water and where I put in the suction hose (Obvious).
Using the water, I push out all the air through the discharge hose. Once the major air is out of the loop, I put the discharge hose in with the bucket water. This I will let circulate for 15-30 minutes, while open and closing valves to force pressure up and down in the system and to burp any remaining air out.
Once you've done this it is the same procedure for your glycol mixture.
The coin vents or auto air vents are absolute in a closed loop system. Every time I have checked the air vents, I have had just a spit of air (less then a second worth) come out.
Knock on wood, I have not had a system go to stagnation because of cavitation caused by improper charging. I was taught well.
noise?what type of piping? If you use that corrugated stainless solar tubing you need to be sure you have good velocity. Ideally 4 fps to push the air through all those ridges in the tube. It's a stubborn tube to purgecompletely.
Excessive velocity in that corrugated tube can present some unusual noises also. Try blowing through a short section some time. Then look outside to see if any elk answer the call :)
Drainback is in my bloodHot Rod,
I don't usually do glycol systems - too complicated, too many risks, too expensive. So I treat them like drainback (rigid copper in this case) but put in the foul elixer. Part of my problem now is I forgot the coin vents at the top (not necessary with drainback).
But no elk call from the piping. Funny image though.
Dish Soap... MAGIC Elixir...Inject a few ounces of Dawn liquid dish detergent into the loop, and your problems will magically disappear....
Think surface tension and surfactant...
Bye Bye Bubbles :-)
MEIt's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
suds for sudsMr. Eatherton,
The possibility arise that I will owe you a beer. I will try this next week and report back. More ideas are still welcome.
I do like the elegance of fighting bubbles with bubbles, thus I think froth in mug in exchange for removal of suds as a thank you is entirely appropriate.
air in glycol and efficiencyOK - so it has been 6 months. I went to check on that system today - exactly the same as when I left it. Froth in glycol (65PSI) - a slightly louder pump than we would like (but it is a Grundfos 150XL - so that just may be the motor noise).
It is checking out EXACTLY as I predicted for this customer (I was a little worried as my models are optimized for drainback evacuated tube).
However, the utility incentive was based on the utility's rather optimistic estimate of what solar panels can do (we have to find 12 million BTUs (per year) - or else accept a reduced incentive.
So, I thought both Michael and Mark Etherton had great suggestions above - and I am ready to put that into practice. The question I have though, is does entrained air reduce efficiency of heat transfer. It seems intuitive that it would - but at 65PSI I am not sure the amount of air is worth removing (frothy like a good German beer is the bubble level - after it decompresses from 65PSI).
Thoughts appreciated. Actual knowledge treated with excitement and respect.
Pretty high pressureI'd take a careful look at michael's instructions above, with a few options:
- 65 psi is pretty high, and will entrain air. Keep the pressure more in the 20s during purging. Not sure why you run that high normally, but another discussion.
- My charge station is a 25 gallon trash barrel. This allows me to mix on the fly for large systems, and has enough volume so that the transfer pump inlet at the bottom and the drain hose in the middle create a nice spin in the bucket.
- My pump is 1/2 hp. This means that I can purge at high flow rates and low pressure - higher pressure pushes the bubbles into solution. Purge low and for 15 mins or more. If your pump can take it, try 1/2 hr. Cycle the system pump and bleed it's rotor, purge the x-tank line, the PRV body, every nook and cranny. I usually do that during the water phase. Do not drain the system of the water - use the glycol to push it out.
- Yer not done purging air with the transfer pump until the glycol is totally clear at 20 psi. If you have manifolded copper coil HXs, or a big flat plate HX, or multiple flat plate collectors with skinny risers, or corrugated tubing, (...) you should try letting it rest for 5 min intervals with the drain/fills closed and maybe 10 psi at the pump station gauge.
air and expansion tankThe location of the expansion tank and the charge ports affects how the flow goes through the loop when charging the system. The best place for the expansion tank is right after the input charge port, That way when you build pressure in the system the expansion tank helps with the surge to move air bubbles out.
It can take up to an hour to purge small bubbles out of the glycol depending on the size of the array. Doing the charge when the system is cold makes easier for the bubbles to come out. I have sometimes had to let the bucket sit and wait for the air to come out the charge the system until the glycol in the bucket gets foamy again.
Good thoughtsThanks gentlemen. Both points look like good advice. For the record I did have the glycol push the water out. I think the 65# comes from Tom Lane, but I can lower it some when I try to get these bubbles out.
Any insight on the issue of whether these bubbles are having a notable effect on heat transfer?
RebootI typically use between 30-40 lbs. This covers most homes in terms of height of system piping and keeps the bubbles moving. I always include an air vent at the highest point and use a good microbubble scrubber.
I would recharge the system honestly. Run the charge pump for a long time in excess glycol and make sure the fluid in your bucket has NO bubbles. Shut off the return hose to jack up the pressure and quick open it. It creates a large surge and sometimes dislodges stuck air pockets. Also run the system pump to make sure no bubbles are hiding in the impeller.
Yes the air is having some effect on heat transfer. Glycol/water is far superior at both heat capacity and heat transfer. And it also affects your flow rates.This post was edited by an admin on March 9, 2012 10:51 PM.