I've started a new job at the hospital here in town. It has roughly 200 heat pumps in the facilities. Recently, I've been working on cleaning out a coaxial heat exchanger on a 2 ton system. The hose kit and heat exchanger have had some sort of scaling inside them that restricted flows enough to cause the loop's delta T to be 20-24 deg. F. in the cooling mode. The pressure drop across the HX was 16 psi. After flushing the HX and cleaning the scaling from the valves/hoses, it now has a pressure drop of 7 psi, and has a loop delta T of 11 deg. F. We know the scaling is metallic -- it can be picked up by a magnet. I have a few questions on it: 1. What would be the likely culprit of where this scaling/residue/deposit/corrosion came from? 2. If it's corrosion, what might we do to stop it? How might we know if it were corrosion vs. contaminants just left in the system? 3. Is there a procedure you would recommend on cleaning each heat exchanger of this scaling--more than just flushing water through it. In the past, an engineer recommended muric acid (or muriatic acid, I can't remember which) for cleaning a heat exchanger. However, that was for a Domestic Water heat exchanger that was using well water... Any help is appreciated. Thanks.
I would not do the acid routine. I have had some unpredictable bad results with it. If possible take a piece of the hose kit and expirement with a milder solution that is effective at cleaning it. As to where it came from, I once had a commercial circ pump with a cast iron housing and a bronze impellar get a bad bearing and it polished shaved bits of the housing out and introduced the material into several coax coils. Eric
Sometimes one can shock a coil clean by hitting the reversing valve in the middle of a cycle to cause rapid heat exchanger expansion. j
Thanks for the info. We haven't had any pumps do that, to our knowledge. What else might be causing it? We're now working on a second unit with the problem. The group of heat pumps these are in has about 30 different units on it. They just recently (within the last 4-5 weeks) brought on a new building/addition and tied the loop of it to the loop of these 30 heat pumps together. The new building has around 35 heat pumps right now, and more will be added in the next phase...maybe this is just more info than is needed. I'm told that there was a leak in a ground loop connection out in the ground. It was found and fixed. I'm also told that when that was done, the loop system got a lot of dirt in it. That definitely explains the pile of dirt found in the strainers so far. Could the abrasiveness of the dirt in the system be eating away at the metal pipe and cause this scale material? The loop water seems very clean. What is recommended to test the loop water's cleanliness/particulates, etc? Sorry if these questions seem like newbie questions. I've been involved with geothermal since 1995, but we were mostly doing residential. The commercial jobs we did only required us to install units and attach duct. Everything else was another subcontractor. And not seeing the owner's side on the commercial projects, I've not really seen this type of problem a lot. Again, any and all information is greatly appreciated.
What material is the building loop made of? If it is iron the marriage of new work to old would explain alot. Cutting and threading or welding or even extruding flanges would yield metalic trash that could have migrated to the unit(s) in question. Eric
The building loop is like any other newer building loop. Black iron pipe. Some is cut/thread, some is welded, some is victaulic... This scaling is not new stuff. It has been building up in these heat exchangers much longer than the few weeks since the other building has been tied in together. It seems to be in every one of the 2005 units. While we haven't checked all of them yet, there's nothing to suggest that it's not in all of them. With the break in the ground loop that they had a few years ago, they got a lot of dirt in the system. The strainers we have checked have sandy/silty mud in them. I'm wondering if it was the abrasiveness of that sand/silt has worn on the iron parts of the loop system. I'm most concerned about getting the scale off of the walls of the heat exchangers. We haven't had luck in overriding controls to keep the compressor running and hitting the reversing valve mid-cycle. Is there no cleaner that will not hurt the system, but will remove the scaling? I really don't want to have to replace the heat exchangers...what a nightmare that would be, cause you know the architect, engineer, and builder/installer left us enough access room to get to them.... Thanks guys.
Scaling Most anything acidic will remove the scaling. Milder acids like vinegar (acetic acid) are options. edit - I know milder is a horrible qualifier for an acid.
How much do you think this scaling will affect the efficiency of the units? If we still get the water flow through the HX that's required, is it going to affect the bottom line? Are the pumps working harder because of extra resistance from it?
Sorry.... .... been busy. If the water is just getting through the heat X due to scaling I am concerned about the longevity of the system. I would get the water tested. I would take the test results and act accordingly on de-scaling on a plained basis.
I "second" the vote for getting the water tested. I would also try to take some of the scale with just for kicks to see if they can determine the minerals. I have one customer that sees heavy calcification on his heat exchanger plates in a closed hot water loop. Testing his water yields very little dissolved calcium but due to Kosher requirements he needs to change his water frequently thus a new fresh source of minerals quite often. We have switched one unit to utilize R-O water which has dissolved a lot of the build up BUT that R-O water can be corrosive itself. In your case, I'm pretty sure it would eat your iron pipe for lunch and cause WAY MORE problems than some partially plugged up heat exchangers. I would say that first you have to determine the source of the minerals. (and if it is still adding) Then dissolve the existing scale buildup. The tricky question is do you isolate each unit when you clean it so the dissolved scale doesn't just go back into your loop to find a new home? That answer will kind of depend on what you use to clean them. Then you will want to monitor your water for a while to determine if the problem is gone. My gut thought would be the leak with all the mud contamination also brought with it a lot of minerals. Each strainer that has any crud remaining in it is really another source of dissolved minerals as the water flushes through the partially plugged strainer. I found a 1/4" hex nut in a high purity water system that caused so much grief you can't imagine. It must have fell into a tank lid or something during construction. The entire circuit was cleaned and passivated a number of times and it only took a couple days before the entire loop was filthy again. I can't even fathom how much that stupid little nut cost them (and the installer).
