Getting the best out of my plate HX

[warno]

I'm wondering if i can get more out of my plate HX i use to charge my storage. My system consists of a 20 plate HX with a grundfos 15-58 on the storage side running on low speed and another 15-58 on the boiler side running on medium speed. I run batch burns in my boiler and while firing my boiler stays about 10-20 degrees ahead of my storage until the fire starts dying down then they balance out at alittle lower then I'd like (about 165-170).

Is there a better way to run my system to keep the storage alittle closer to the boiler while it's in the peak of the burn? Would delta T pumps help with this?

 

[maple1]

You might be able to tune it in a bit by monitoring the ins & outs of the HX to see what the dTs are on each side, and maybe adjusting circ speeds. But a given sized HX can only transfer so much heat. Example, if you could slow the flow on the storage side you would be a bigger spread & higher exit temps but it would take longer for all the storage to circulate through. So net gain would be not much - total BTU transfer would be the same over a given time. I might have gone bigger on the HX - I have a 20 plate on my DHW and it does good at that but I think it would be slow heating up my storage. I forget how close the DHW out temp gets to boiler supply temp - I will have to watch it closer next time it is running. I have digital temp guages permanently tied to the ins & outs.

That likely didn't help much...

 

[warno]

I thought about going with a bigger HX but i was concerned about drawing too much from the boiler. I'll try to do alittle bit of watching the ins and outs on my temps periodically through a burn maybe this weekend.

 

[Bob Rohr]

Generally speaking the higher the flow rate the more energy you transfer. Staying below excessive velocity in the connected piping of course.
3/4 pipe can handle 7 gpm comfortably, 1" around 12 gpm.

If it is a 5X12 20 plate I suspect even speed 3 will not over pump it, depending on pipe size that you used.

The free online simulator at flat plate website allows you to enter all the various numbers and get output, flow rate, pressure drop etc. It's sometimes easier to click buttons and get various options and answers.

 

[warno]

I used the flat plate website to source my HX but without knowing the actual output of my boiler being homemade. So i was kind of guessing on some of the numbers.

My HX is a 5 x 12 and it's fed on both sides with 1" piping. Are you suggesting i turn up my speed on both pumps or just one or the other?

 

[Bob Rohr]

Generally speaking the higher the flow rate the more energy you transfer. Staying below excessive velocity in the connected piping of course.
3/4 pipe can handle 7 gpm comfortably, 1" around 12 gpm.

If it is a 5X12 20 plate I suspect even speed 3 will not over pump it, depending on pipe size that you used.

The free online simulator at flat plate website allows you to enter all the various numbers and get output, flow rate, pressure drop etc. It's sometimes easier to click buttons and get various options and answers.

I used the flat plate website to source my HX but without knowing the actual output of my boiler being homemade. So i was kind of guessing on some of the numbers.

My HX is a 5 x 12 and it's fed on both sides with 1" piping. Are you suggesting i turn up my speed on both pumps or just one or the other?

Turn them both up. If you hear a lot of rushing water you probably have flow at or over 5 feet per second. That is about the speed that you start to hear flow in piping and really don't want to exceed that. Ideally 4 FPS is a target max. flow.

Certainly no harm in oversizing a HX other than cost. The more surface area the better the exchange.

Close approach is a heat exchanger term used to describe a condition where A and B side are within a few degrees, when you design a close approach you will notice the size of the HX increases.

But without some hard data it's tough to get an exact size or predict performance. Invest in a BTU meter and see exactly what kind of work your boiler is doing

On the district energy systems in Europe you purchase and pay for your energy via BTU meters, very accurate devices.

 

[PassionForFire&Water]

Bob, any possible recommendations for BTU meters here in the US?

 

[Bob Rohr]

I saw this company at a trade show, they had some simple and affordable meters.

Basically you measure flow in GPM and the temperature difference between supply and return, that gives you the actual energy amount being transfered.

http://www.onicon.com

 

[vtwoodheater]

What is your plate exchanger rated for BTU wise? When I was running my outdoor wood boiler, I had a 40 plate exchanger. Don't recall the physical dimensions, but it was rated for 400,000 BTU's +/-. I sized it according to the maximum output of my boiler. Never had a problem with that aspect of the system.

A quick search found a 350,000 BTU exchanger for $185.

Cheap, throw it in, never worry about your heat transfer rate again. My 2 cents.

 

[warno]

What is your plate exchanger rated for BTU wise?

I can't remember the actual btu rating on my HX but based on last night's burn it's not nearly enough.

So correct me if I'm wrong here but is there anyway to have too much HX as far as in between the boiler and storage? Would a really large HX just mean the boiler and storage would be closer to the same temps at all times?

My problem is last night i over loaded the firebox and my boiler got up to over 200°F. With my storage at the high 170s it never seems to want to take anymore heat. So if i put in a much larger HX would the 2 be closer in temps together or no?

 

[TCaldwell]

I think there are some online flat plate calculators that you can plug in temps and flow rates

 

[warno]

Here's the numbers i put in on flatplateselect.com

And here's the options given

The highlighted option is almost identical to my plate HX. So I'm not sure where to go from here, but I think I'm greatly over pumping the boiler side. my boiler side is a bell and gossett NRF-36. I'm running around 12ft, if my calculations are correct, on my head loss. Right now I'm running on speed 2. Here's the curve for my pump on the boiler side. Top graph...

 

[jebatty]

Based on your flat plate specs, the maximum your hx can move is only 50,000 btuh (10 x 10 x 500). Notice that also is about the Side B load on the specs. Or are all of your specs guesses? 50,000 is really low for any wood boiler, and I see yours is homemade. It also is really low for the demands of almost all heating systems. Needed is a good estimate of your boiler output, flow rates and temperature differential needed to meet boiler output (Side A) and system requirements (Side B), and then size the hx accordingly. Without good info it is very difficult to give good advice. My gut reaction is that either or all of the boiler, the system and the hx are under-performing to meet your needs.

 

[warno]

Based on your flat plate specs, the maximum your hx can move is only 50,000 btuh (10 x 10 x 500). Notice that also is about the Side B load on the specs. Or are all of your specs guesses? 50,000 is really low for any wood boiler, and I see yours is homemade. It also is really low for the demands of almost all heating systems. Needed is a good estimate of your boiler output, flow rates and temperature differential needed to meet boiler output (Side A) and system requirements (Side B), and then size the hx accordingly. Without good info it is very difficult to give good advice. My gut reaction is that either or all of the boiler, the system and the hx are under-performing to meet your needs.

I built mine based on a design of one that is supposed to produce about 250,000 btu. And I'll bet is pretty close to that. My boiler has no problem keeping up with charging the storage. I think it's all in my HX that is limiting me to getting higher temps in my storage. Best i can get is high 170s through out my 750 gallon storage.

 

[jebatty]

I sense your frustration in not getting the performance that you expected. Here is some math that might help you start to understand and evaluate your system. If as you say your boiler output is maximum 250,000 btus, and if the Side A differential is 10 as you show in your specs, then you need to be moving 50 gpm (250,000 / 10 / 500) through Side A; and to move that same volume of btus through Side B to get a 10F differential (180 / 190), you also need to move 50 gpm through Side B. Obviously, if your boiler output is less than 250,000, the gpm needed to maintain a 10F differential goes down.

If the math is applied to your 750 gal of storage, and your goal is to raise storage from 140 to 180, for example, the btus needed to do this, by coincidence, is only 250,000 (8.34 x 40 x 750). In an efficient wood gasification boiler, the amount of seasoned wood to deliver these btus is just 49 lbs with an efficiency of 85% (250,000 / 6,040 / 0.85). 49 lbs is about 6 nice oak splits at 8 lbs/split.

As Bob stated above, 1" piping, if that's what you have, pretty much limits you to maximum 12 gpm flow rates, Sides A and B. What I don't see in your posts is the calculated pump head at any flow rates on both sides of the hx. With a good pump head estimate at various flow rates on each side of the hx, it's possible to make some real progress in sizing an hx.

Let's assume all of your piping on both sides of the hx is 1" pex, straight only, no L's, curves, valves, etc. How may feet of pex do you have on each side of the hx? Look at the charts for 1" pex and determine the pump head ( = psi drop x 2.3). Calculate pump head for 12 gpm and various other flow rates you think are realistic. Report back the feet of pex and the calculated pump head and, at least for me, I will have some info that can start the path to sizing an hx for your system. And also provide a realistic btuh boiler output you want to move from your boiler to storage. If you're thinking 250,000 through 1" pex, that's not possible in my opinion.

 

[Bob Rohr]

Hard to argue with Jim's math, and your actual results seem to prove it out.

The sizer math shows under 50,000BTU/hr with the numbers you put in.

The 1" piping will limit you to about 120- 140,000 BTU/ hr, realistically

 

[warno]

tomorrow I'm going to sit down and go through all my fitting on both sides to try to get a pretty accurate loss number.

But tonight i went out and switched both pumps to speed 1 and also throttled the boiler side down abit more with a isolation valve. My temp gun at the HX ports read a 32 degree drop on boiler side and a 25 degree gain on the storage side. Boiler water was coming in at 190.

It seems i can easily get the storage to 179°F with no trouble but that's as high as I've seen it ever go. So if my boiler is still firing pretty hard it seems the storage just won't take it and the boiler shuts down on high limit. Idle mode, boooo. I'd really like to get it into the high 180s if possible but honestly I've never seen it break 179. But like I said I'll try to get this numbers posted tomorrow.

I greatly appreciate all the help and advice everyone. Thank you

 

[warno]

I wrote down all the fittings i have on each side of the HX in the order of flow. I'm still working out my loss calculations but here's what i have.

All fittings are 1", Unless otherwise noted

"BV" is ball valve

The themomix valve has a 10CV

The diverter valve has a 7.5CV

Boiler side. Boiler side circ is a bell and gossett NRF-36

Storage side. Storage circ is a grundfos 15-58

 

[Bob Rohr]

Some basic heat transfer, we all know hot goes to cold, the basic laws thermodynamics. The rate at which you transfer is directly related to the temperature difference ∆T (delta T) When you have a cold tank and a hot boiler the exchange will be faster then when the temperatures get closer. So you will see a fast increase when firing from a cold start.

As the storage gets closer to the boiler output, that exchange rate slows.

The size and flow rate through the HX will also play a part in the exchange. It is possible to size and flow a heat exchanger to what they refer to as a "close approach", where the b side can get to within a few degrees of the A side. It takes a large sized HX and some good flow rate to do that, I think you observed that in the software simulations. Play with that approach number and deltas on your inputs and see how the size of the HX changes.

You will never get 100% there is always SOME efficiency loss thru a heat transfer. Consider heat loss from the tank also, regardless of how much insulation, the tank and boiler lose heat.

The faster the flow the more energy exchange, all things being equal. Reducing flow rate does not, cannot increase heat transfer, if you believe the numbers and laws of thermodynamics.

IF your goal it to get the tank within a few degrees of the boiler operating temperature, the constipation is either the flow rate or size of the HX, probably both. Assuming no flow blockage or mechanical issues. The pumping power and HX size required to do that? The juice may not be worth the squeeze!

Flow meters temperature, temperature differential readings, pressure drop across the circulators, stack temperature are all ways to measure and confirm heat transfer, actual flow rates, etc.

You can crunch numbers before you start the project to get a good idea how it will perform, or gather actual operating data to confirm real life performance.