Flat Plate Heat Ex. internal buildup

[zadwit]

A local boilermaker told me today that when a flat plate heat exchanger is used in elevated temps around 180-200(f) with well water, that it will coat up the inside within a year and I will have to flush the heat exchanger out with muriatic acid or the like. Has anyone had this problem and if so how do you prevent it?
He said that the water has magnesium and calcium and that at lower temps like a regular water heater uses, no problem, but when the temps are elevated to 180 to 200, then the trouble starts. He said to use a "shell and tube" heat exchanger.
Just wondering.

 

[jebatty]

I'm going to take a leap on this one and express an opinion. Yes and No and maybe not significant.

1) to the extent you have minerals in your water, they may cause scale somewhere, and the hx is one place where this will happen.

2) in a home system, closed system especially and probably also in an open system, the water is static, that is, you are not continually adding new water to the system in any or at least great quantity. Therefore, any scale that forms is limited to the mineral content of your starting water. Once that mineral has scaled or precipitated out, scale formation is over. Whether this is enough to cause an hx problem I don't know, but it should be limited. Will depend on the hardness of your water. If you use soft water, the issue should be greatly reduced.

3) in a commercial system, especially a large system, I believe water is added at a much higher level than a home system, so new minerals are being introduced all the time. Large systems condition the water to remove minerals before being added to the system. They also do boiler blow downs and other procedures to remove accumulating scale. This should not be necessary in a home system.

4) I would plumb an hx system with isolation valves on the hx, as you also do on the circ. This would allow easy removal and cleaning, if needed. Plate hx's are designed to turbulate the water passing through, causing a scrubbing action, both to maximize heat transfer and also to minimize any mineral buildup.

Shell and tube hx's would not be immune to scale deposits either; nor would a coil hx. If you have ever looked at an old hot water pipe in a domestic system with hard water, after many years there is lots of scale in the pipe. A shell and tube hx and a coil hx would be similar. But keep in mind that scale accumulation is related to continuing introduction of new hard water.

5) muriatic acid I believe is the cleaning agent of choice. My plate hx has now been on one full heating season, and before starting this season, I likely will remove it and clean it with muriatic acid. I don't know if there is a way to actually tell how much scale, build-up, if any, is inside the hx. If anyone knows how to determined this, let me know.

6) I conclude that scale is not likely to be a major issue, if an issue at all, in a home heating system that does not continually add quantities of new hard water to the system. Plumbing isolation valves to allow cleaning of the hx would be good practice, as would an occasional cleaning, at least after the first season, and probably much less frequently after that. Coil hx's also are not immune to scale build-up, if any.

 

[sdrobertson]

The only problem I've seen is if you have a plate exchanger hooked up to your make your domestic hot water, it will eventually plug up and need to be flushed out. The reason is the boiler water is always moving and not allot of water is added to that side of the system. The domestic hot water side is not moving except when your using hot water. This side just sits in the plate exchanger "cooking" and eventually it will plug. Like the post before states - just hook up a couple of valves so you can take the exchanger out and clean it out when it slows down on the hot water production and it will be a good as new.

 

[heaterman]

A local boilermaker told me today that when a flat plate heat exchanger is used in elevated temps around 180-200(f) with well water, that it will coat up the inside within a year and I will have to flush the heat exchanger out with muriatic acid or the like. Has anyone had this problem and if so how do you prevent it?
He said that the water has magnesium and calcium and that at lower temps like a regular water heater uses, no problem, but when the temps are elevated to 180 to 200, then the trouble starts. He said to use a "shell and tube" heat exchanger.
Just wondering.

A. He's right, at least partially. Water @ 180+ is a completely different animal than water @150*. It becomes much more corrosive and any minerals present will precipitate out of solution at a much higher/faster rate.

B. We ALWAYS set up our piping so that the HX can be isolated and flushed. Simply put a ball valve on each of the four in/out connections with a boiler drain in between the B/V and the HX. Now you can easily isolate it and connect a small pump to circulate CLR, muratic acid or whatever through the HX to clean it. There is no way to prevent it really, just ways to deal with it when it occurs. The frequency will depend on the amount of mineral and dissolved solids in your water and the operating temp. There is much to be said for designing your system to operate at lower temps.

C. I wouldn't advise a shell and tube HX. Not worth the cost as you can buy several plate type for usually the same $$ as a shell and tube. Those are used about strictly in large commercial apps.

D. Keeping track of the water quality in your system will go a Long way toward increasing HX life expectancy. I've seen gobs of the little plate types fail on domestic water situations in less than 2 years. Use a wye strainer on both sides of the HX and keep tabs on the boiler water side as much as possible.

 

[Bob Rohr]

Probably, if you have a well system with new fresh water flowing through, maintenance will be required. Test the hardness and TDS of the well water to get an idea.

Webstone an a few others make service valves for that application. It is a ball valve with a built in flush port. Install one on both sides of the HX to run a de-scaling product through them. They are also sold with instantanous water heaters which also suffer from hard water build up.

Performance will drop as HXers scale up. Check the incoming and outgoing delta t to know when this is happening.

Another customer of mine removes his plate HX every year an d takes it to a radiator repair shop. They boil it out for around 10 bucks. Install it with unions if you planon removal for service.

If you have hard water a water softener will help keep the HX cleaner. I soften just the hot water my home. So the softener is piped just upstrem of my solar pre-heat tank. Y strainers will not do much for hard water conditions, just the larger "chunks"


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[zadwit]

Thanks everyone for the replys. I priced shel and tube exchangers and they are expensive. Up here in Alaska, stuff is expensive enough due to freight to get it here.
I have "Y" strainers and can fill the system with soft water. I will start plumbing this all together in the next few days. It has been raining for the last week and the rivers are approaching flood stage. At least my propane tanks are dry in their new shed!
Thanks again.
Mark

 

[pybyr]

from reading manufacturers' literature a while ago on flat plate exchangers, I got the impresssion that the "turbulence" supposedly created by the patterns on the plates in the exchanger are supposed to largely deter or delay accumulation of deposits.

that's the theory, although I can't speak to how well it works in practice, since I haven't used one yet- but since I have pretty hard water, this topic is of concern to me

That (turbulence within the HX as a way to prevent or slow deposits) has me wondering, could one help keep a flat plate HX cleaner inside by designing the system around it to push water through both sides of the HX at higher velocities than the 2-4 feet per second that are considerered the "norm" for residential hydronics

(my understanding from what I've digested so far is that 2 fps is the lower end to keep air bubbles moving and then allow them to be purged, and 4 fps is supposedly the upper limit due to potential for noise for flow through or near habited spaces, and also because pressure drop in non-smooth pipe like iron tends to go up fast above 4 fps- but it seems like that'd be less of a factor with PEX)

can those of you who really know what you're doing with hydronics tell me whether this concept (faster flows through a plate HX to increase turbulence and minimize deposits) has possible practical merit, or has downsides that I am not thinking of?

Thanks

 

[in hot water]

from reading manufacturers' literature a while ago on flat plate exchangers, I got the impresssion that the "turbulence" supposedly created by the patterns on the plates in the exchanger are supposed to largely deter or delay accumulation of deposits.

that's the theory, although I can't speak to how well it works in practice, since I haven't used one yet- but since I have pretty hard water, this topic is of concern to me

That (turbulence within the HX as a way to prevent or slow deposits) has me wondering, could one help keep a flat plate HX cleaner inside by designing the system around it to push water through both sides of the HX at higher velocities than the 2-4 feet per second that are considerered the "norm" for residential hydronics

(my understanding from what I've digested so far is that 2 fps is the lower end to keep air bubbles moving and then allow them to be purged, and 4 fps is supposedly the upper limit due to potential for noise for flow through or near habited spaces, and also because pressure drop in non-smooth pipe like iron tends to go up fast above 4 fps- but it seems like that'd be less of a factor with PEX)

can those of you who really know what you're doing with hydronics tell me whether this concept (faster flows through a plate HX to increase turbulence and minimize deposits) has possible practical merit, or has downsides that I am not thinking of?

Thanks

It all has to do with "what's in your water" sediment, minerals, dirt, etc. And the operating temperature as mentioned above. Plate HX manufacturers offer special HXers for what they consider "brackish" water. Like sea water, etc.

I have cut some old plate heat exchangers in half on my band saw to see what makes them tick. For sure you will see some build up after years of service. it only takes a few thousands of a build up to really start limiting heat transfer ability.

Excessive velocities can lead to erosion of the plates. Often called erosion corrosion. The HX manufacturers will have guidelines as to acceptable velocity. Typically continuous circ hydronics are limited to 4 FPS. DHW systems, being an intermittent use, will sometimes design to 8 fps.

Faster flows do allow increased heat exchange rates, it's not about the "dwell time" but as always there are some limits.

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