Heat exchangers - tube in shell versus plates

[Ecky]

Looking for opinions on tube-in-shell heat exchangers versus plate. A friend of mine has a plate exchanger and is having problems with it plugging. (He removes the plate, cleans it and is good to go for another year or so.) I am assuming that plate exchangers are actually more efficient but obviously this changes if it becomes too plugged.

I am looking to transfer heat from a solar loop to a GARN. I am trying to determine whether to use an external tube-in-shell, a plate exchanger, or build one internal to the GARN made of pex, black steel, or corrugated stainless steel. An internal one would mean that I would only need the solar loop pump but may loss efficiency. There is also the question of whether the increased head would be too much for a single pump. (Currently looking a Laing DC5's - 6-7 gpm at low pressure head).

Comments / suggestions?

 

[slowzuki]

Your Garn and solar panels should be clean and work well with any type. Shell and tube would be good idea in untreated atmosperic system so it can run longer between clogging.

I'm looking into an in tank for a solar drainback system. Friction loss isn't any problem in my setup, I don't need many gpm. The head losses are pretty high though.

 

[Ecky]

Your Garn and solar panels should be clean and work well with any type. Shell and tube would be good idea in untreated atmosperic system so it can run longer between clogging.

I'm looking into an in tank for a solar drainback system. Friction loss isn't any problem in my setup, I don't need many gpm. The head losses are pretty high though.

Nice to hear from a fellow Canadian. Thanks for taking the time to advise. Any opinions on internal versus external?

 

[slowzuki]

I'm looking into using an internal, not on a Garn but in a stainless milk tank. Less pumps, simpler controls, more efficient if sized right. Copper is fairly cheap right now too. I'm also considering no HX and directly pumping the tank water into my collector to eliminate the drain back tank.

Haven't decided yet though.

 

[Ecky]

I'm looking into using an internal, not on a Garn but in a stainless milk tank. Less pumps, simpler controls, more efficient if sized right. Copper is fairly cheap right now too. I'm also considering no HX and directly pumping the tank water into my collector to eliminate the drain back tank.

Haven't decided yet though.

I hear ya'. Drain back is not an option for me unfortunately. I read in Ramlow (Solar Water Heating) that pumping treated water (not sure if yours is treated) can eventually lead to problems because there is an accumulation of deposits left every time the fluid drains out at night. Eventually the pipes become restricted. Something I hadn't thought about. The idea of less pumps and simpler controls is always more appealing of course. Good luck. I would be interested in hearing what you decide upon and how it works once implemented.

 

[slowzuki]

We have high mineral content that leaves deposits, a build up will likely happen here too eventually. A little acid flush will fix it right up. At such low flows with 1/2" I don't think it will clog for a long time either.

 

[Ecky]

We have high mineral content that leaves deposits, a build up will likely happen here too eventually. A little acid flush will fix it right up. At such low flows with 1/2" I don't think it will clog for a long time either.

Fair enough. Wondering (out loud) how much efficiency would be lost and how quickly. 10% in a season? I suspect no one knows for sure but it would be interesting to know.

 

[Nofossil]

This must be efficiency week....

I may be speaking out of turn here, but I think it's important to make a distinction between 'transfer efficiency' and heat loss.

When you're rating a heat exchanger, you care about how effective it is at exchanging heat. I'd like to use the word 'effectiveness' rather than 'efficiency' to describe how well a heat exchanger works. 'Efficiency' implies that something is lost, and that's not the case in this situation.

The way I think of it, a perfect heat exchanger with equal flow on both sides would have the primary side return temp equal to the secondary side supply temp and vice versa. In that case it has transferred ALL of the energy that could possibly have been moved, and I would suggest that it's 100% effective at transferring energy.

That doesn't happen in the real world. However, the energy that isn't transferred is not lost - it just stays on the same side of the HX. You'll see a higher return temp on the primary side and a lower return temp on the secondary side, but the energy hasn't been lost. A heat exchanger with a 50% rating does not lose half the energy - it just doesn't transfer it.

A higher primary return means that the heat source (solar panel, boiler, whatever) will have a higher inlet temp and therefore a higher outlet temp than it would have had with a more effective HX. That higher temp will lead to more transfer through the HX on the next loop.

HX effectiveness can affect efficiency, though. If higher temps in the heat source result in more losses to the environment, then a less effective HX will result in lower system efficiency. If the HX is capturing heat that would otherwise be lost, then HX effectivness does equal efficiency - a flue gas heat extractor would be an example. Finally, a less effective HX could result in other system problems. In my case, the immersed coil HX in my storage tank is not very effective at high storage tank temps. This results in my boiler idling when attempting to top off storage.

 

[Ecky]

An excellent explanation! Thank you.

 

[slowzuki]

And transfer inefficiency leads to thermal inefficiency. If your solar collect operates at higher temps as is can't transfer heat to the storage tank due to transfer inefficiency, it loses more heat to the outdoors.

 

[Ecky]

Makes sense. Thanks.