Open Storage Concept with External Flat Plate HX

[brant2000]

I just came up with the idea and was wondering if anyone may have done something similar or has thoughts they'd like to share. What I was thinking about would be to place a flat plate HX as the first user in a simple pressurized loop (near boiler outlet) and use a circulator on the storage side of the HX to control charging/discharging of storage. This would be using an open storage tank. Not sure if you could do it with a single pump or not, but was thinking the flow could be reversed to optimize this (draw off the bottom to store / draw off the top to charge).

The benefit I see to doing this is really that flat plate HX's are cheap and this could be done with one vs. two that are normally required. Also, may provide simpler control of charging/discharging.

 

[jebatty]

A plate hx can operate in both directions, but heat transfer efficiency (approach temperature) is better in one direction vs the other. I have not seen data that shows this difference.

 

[tom in maine]

There is data online. It drops significantly in the wrong direction.

 

[mustash29]

Pumping through a HX in the right direction, typically you are talking counter flow. - Hot supply flowing left to right, cooler fluid flowing right to left.

If you go the "wrong" way you will be in parallel flow. - Hot supply flowing left to right, cooler fluid also flowing left to right. Initially there is a greater delta T, but as the fluids flow through the HX, the delta T gets lower as the two fluids approach the same temperature. The best way to make parallel flow work good is to seriously overpump the heat source side while slowly pumping the heated fluid through the other side.

 

[jebatty]

Tom:

There is data online. It drops significantly in the wrong direction.

Please provide a link. I'm normally pretty good at finding info, but this one escaped me. Thanks.

 

[tom in maine]

I did not book mark it, but here is a link that shows the same data for a tube in tube hx. The curves were very similar.
Check page 46.

http://books.google.com/books?id=Zs...v=onepage&q=plate heat exchanger flow&f=false

 

[maple1]

The benefit I see to doing this is really that flat plate HX's are cheap

They are?

I must have been looking in the wrong places when I got my little one.

 

[maple1]

A plate hx can operate in both directions, but heat transfer efficiency (approach temperature) is better in one direction vs the other. I have not seen data that shows this difference.

I checked out Toms link, and now I'm confused. Although pg. 46 shows a shell & tube, it seems to show parrallel flow is more efficient? Or am I not reading things right?

I'm getting ready to plumb in a plate HX for my DHW, and was setting up for counterflow - but is that the 'right' way to plumb it? Planning for quite low flows on both sides - but both sides do have some adjustability in their flows with variable speed pumps on each.

Maybe I'll go read more of that book - but I was getting bad flashbacks from my university days seeing all those formulas.

 

[mike van]

They are?

I must have been looking in the wrong places when I got my little one.

What he said -

 

[DaveBP]

They are?
I must have been looking in the wrong places when I got my little one.

Perhaps compared to multiple rolls of copper tubing of equal heat exchange capacity. Cheap is relative.

 

[brant2000]

What he said -

I've always thought so. Just like DaveBP mentioned, compared to the cost of multiple rolls of copper tube I would think they're a bargain. Haven't done the calcs yet, but on ebay I see plenty of HX's that would likely be large enough for under $100.

In most industries they have become pretty much the standard for two reasons, lower cost than alternative (shell and tube) and a much smaller footprint.

Regarding the efficiency of reversed flow - that is a good point that I hadn't really considered. I suppose it could be overcome by oversizing the HX (obviously negating some of the previously discussed benefit).

I'm always concerned when dealing with coils of copper tubing, as it has a tendency to not want to coil and bend well for me. And then there will be the challenge of suspending the uper HX in the top of the tank. That is one of the benefits I see in avoiding them entirely. Does anyone have tips on how to handle this in a lined/wooden tank?

 

[Karl_northwind]

check this out. it has a bit on FPHE use for this:
https://www.hearth.com/talk/threads...g-for-hydronic-wood-fired-heat-sources.83606/

 

[Karl_northwind]

I have a system installed (effecta 35KW boiler, 2 330 gallon vertical tanks) with a 5x12x30 plate Hx for interaction with the heating system (glycol) that uses 2 taco 00R circulators facing each other to remove heat from the tanks (pulls from top, returns to bottom) and to deliver solar heat to the tanks (removes from bottom of tanks, returns to middle of tanks. accomplished with a fancy control system (solar logic) and a couple of Flow checks.
has worked well this winter.

karl

 

[heaterman]

Ahh yes...."cheap" is a relative thing. Being that Karl is in the HVAC biz full time and works with projects well into 5 figures routinely, a $350 HX is cheap when considering what it does and what it adds to the overall project.
When you work with this stuff everyday, you tend to focus on what works, what goes in with the least amount of labor and how much maintenance is needed down the road.

 

[brant2000]

Ahh yes...."cheap" is a relative thing. Being that Karl is in the HVAC biz full time and works with projects well into 5 figures routinely, a $350 HX is cheap when considering what it does and what it adds to the overall project.
When you work with this stuff everyday, you tend to focus on what works, what goes in with the least amount of labor and how much maintenance is needed down the road.

Agreed, but as a professional DIY'er and long time tinkerer, I generally welcome the challenge. I try to respect my limitations when it comes to things that I just can't do myself or that it wouldn't make sense to do without professional tools. Otherwise, trying to design/construct something is what I get excited about.