Getting the best out of my plate HX

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...

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.

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 said:

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?

Click to expand...

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.

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

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

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.

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

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 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.

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

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.