Plate Heat Exchanger BTU Ratings

[jebatty]

I picked up the following on a plate hx with these published specs:

3.62 Square feet of surface area
392F High temp / - 320F Low temp
Weight; 4.21 lbs
Size; 3" x 7.5" x 4" - 30 plate
Ports; 3/4" Male NPT

5 gpm each side
90,000 BTUs per Hour..........................215,000 BTUs per Hour
Side 1 Temps.......Side 2 Temps............Side 1 Temps..........Side 2 Temps
In /130.....................In / 70...........................In / 160.................In / 50
Out / 93...................Out / 115.......................Out / 67................Out / 140

My comments:
1) I would opt for a nominal 5 x 12 x 30 plate or more with 1" ports as minimum for a gasification boiler system.
2) BTU calculation: gpm x 8.35 x 60 x delta T
3) Above calcs (side 1 - side 2) seem to be based on a cooling application. I assume heating would be similar. Using the 90,000 calc, based on delta T, btu's side 1 = 92685; side 2 = 112725; 82% efficiency. Using the 215000 calc, based on delta T, btu's side 1 = 225450; side 2 = 232965; 96% efficiency.
3) I don't think the typical boiler application (5 gpm each side) would approach these btu's/hr. Note in the specs above that the closer delta T (90000 btu) is less efficient that the wider delta T (215000 btu). A typical boiler application might be, and I think these are optimistic with this smaller hx (these specs would come pretty close to my experience with a 5 x 12 x 30 plate hx; assumed 5 gpm both sides):

Boiler......................System
138,000 btu/hr.......100,000 btu/hr..........72% efficiency
Side 1 Temps..........Side 2 Temps
In / 185..................In / 130
Out / 130...............Out / 170

or

62625 btu/hr...........37575 btu/hr.............60% efficiency
Side 1 Temps...........Side 2 Temps
In / 185...................In / 160
Out / 160................Out / 175

Other posts suggest a 70 plate likely would be better, and I agree, for a gasification boiler system, especially at the high temp, closer delta T, applications, such as baseboard and charging storage. For radiant without storage the 30 plate might be considered.

 

[WoodNotOil]

Jim

My experience has been the same with my 30 plate. Mine only has 3/4" ports though and I get a lot of idling. I will eventually replace mine with a 70 plate and probably have larger ports on the boiler side to be able to transfer more btus faster. I would not suggest 30 plates for anyone doing storage applications.

 

[DaveBP]

Back before I found some propane tanks and was considering HXs I remember seeing specs like you're quoting, Jim, that gave me the impression that there is more to BTU exchange than just plate size and numbers.

Lot of talk on the forum lately that seems to just compare plate number.

I remember that the port sizes made a lot of difference. And that diifference was still there between units with the same 5 X 12 size and plate number. I don't think I ever saw through the numbers to understand what else was different in addition to plate size and number. Or I may have been just totally baffled. Again.

The price certainly went up with the port size as I remember.

 

[mwk1000]

I agree with Dave. I would recommend you look at the GEA http://www.flatplate.com/resources_SOFT.htm Selection software site. It is online and can help you understand the impact of changing the input and output temperatures, flow rates. Especially at the higher temps if you are trying to heat a tank up to 175 the performance drops off quickly.

I have a 5x12 / 70 that would not get above 155 until I re-ran the numbers and saw that the best way to fix the issue was to raise the GPM up to 10-12 on the boiler side. Never would have figured it out without the selection software. It was recommending more a 10x20 - 40 plate unit until I alter the flow rates.

Good stuff, use it and keep changing the side B input temps to simulate a heating tank and watch what happens to the heat exchanger.

 

[justforme]

i'm looking into running a 3” x 7.5” x 4” - 30 plate with 3/4” ports.

i want to heat an area that can and will freeze. i want to run a couple of cast iron radiators filled with a antifreeze solution.

the other side will be a zone on the boiler

i'll be using 12v el sid pumps (it's a off grid application) as i recall there around 3-4 g/m

what do you think? will one of these heat up one or two good size cast iron radiators?

i run my boiler ( a tarm mb solo 55) at around 200º


let me know
thanks
jim

btw: thanks for a great site!
i set the boiler up and the 500 gallon propane storage tank with info i found here.

 

[Nofossil]

Not to be pedantic, but 'efficiency' is a different critter when it comes to heat exchangers. They're all 100% efficient in that the energy that goes in on one side all comes out the other. If it doesn't look like that, then the calculations are off, perhaps from rounding errors or other factors. In any event, the energy doesn't disappear.

The important rating is what percentage of the possible heat transfer is actually accomplished. If you have two identical sides, with the boiler side providing hot water at 180 and the storage providing water at 100. A perfect exchanger would provide return to the tank at 180 and return to the boiler at 100 at an infinite flow rate. In the real world, exchangers miss the perfect return temperatures by increasing amounts as flow rates increase.

 

[jebatty]

nofo: Not to be pedantic, but ‘efficiency’ is a different critter when it comes to heat exchangers. They’re all 100% efficient in that the energy that goes in on one side all comes out the other....

Nothing wrong with being pedantic; in fact, I often prefer it!

Need to talk about this a bit. Skipping the really fine points, isn't this correct: BTU's into Side A + BTU's into Side B = BTU's out of Side A + BTU's out of Side B. With other variables remaining the same, as delta T of Side A and Side B becomes less, the BTU transfer between Side A and Side B also becomes less. The BTU transfer rate may be increased by changing flow rates and/or heat transfer area (# and size of plates).

mwk: Especially at the higher temps if you are trying to heat a tank up to 175 the performance drops off quickly.

I have a 5x12 / 70 that would not get above 155 until I re-ran the numbers and saw that the best way to fix the issue was to raise the GPM up to 10-12 on the boiler side.

I think this is what you were observing on the tank/system side but is not exactly a correct read on hx performance. I believe that based on your original operating parameters, Side B output from the hx was above 155 but that tank/system heat loss on Side B was equal to BTU's being transferred to Side B. Thus, tank/system did not rise higher than 155. Changing the parameters achieved an increase in BTU transfer.

Now I think I am being pedantic also.

Happy Holidays to all!

 

[mwk1000]

Not exactly, You have the case that as delta T decreases ( Your tank gets hotter ) The side B input temps increase and the overall efficiency of transfer side a-> side B decreases unless you are dynamically altering the flow rates. ( To much control work for a home system for me anyway ). So no -- the load did not change the efficiency of transfer is decreasing as the tank gets hotter. That is one of the downsides of my home made tank in that the water does seem to mix and the bottom to top temps are not very different. A well stratified tank would help this in delaying the temperature rise in side B until many more BTU's had been stored

Ahh, After looking at your reply again I see what you meant. I did not mean to imply that the HX was not able to go above 155 but rather my tank was unable to go above 155. It has been a wonderful science project in figuring the run time of the boiler, the HX transfer rates, the number of gallons to transfer to etc. If my EKO ran for 8-12 hours like I originally expected all would be good at the lower flow rates. I have a block of water 8'x6'x4' = 1436 gal. If my pumps work at 4 Gal/min that is 6 hours for one exchange of water -- not good enough to work in the real world. My EKO is only running 6-7 Hours a night so I needed to maximize the HX efficiency and the simplest way to get there was to raise the flow rates. In addition, when the Delta T is low my 5x12 heat exchanger will not deliver 175 degree water out of side B at the original flow rates of 4-6 GPM.

I have certainly learned a bit in this process, my original estimates were not bad but they were without a good understanding of the heat transfer process. I have been able to get 165-170 and I think that will be it for my tank with the HX I have now. I have pushed the flow rates as High as I care to with 1" pipe. The EKO is running about 3 hours less than expected, the house is drawing just a bit more than I figured so I have lost some BTU storage time and have a Higher load. On the up side, my tank will continue to heat the house all the way down to 95 before it just can't keep up. That works well for my HX since I am able to operate in the 165-100 degree range. Better for my tank liner & insulation, better for my HX delta T.

I did a spreadsheet when I was planning all this that I have adjusted to the real boiler times and my real house load.

Tank l w h Cu Ft Gal Lb BTU@70Deg BTU@65Deg
8 6 4 192 1436.16 11920 834408.96 774808.32


BTU@70Deg / BTU@65Deg / Outside Temp / BTU/Hr Load / Reload Hrs / Tank drop per hr
9.13 8.48 Hours of heat @ -15 91393 14.80 7.67
9.69 9.00 Hours of heat @ -10 86107 15.34 7.22
10.35 9.61 Hours of heat @ -5 80614 15.98 6.76
11.09 10.29 Hours of heat @ 0 75265 16.69 6.31
11.94 11.09 Hours of heat @ 5 69889 17.51 5.86
12.93 12.01 Hours of heat @ 10 64513 18.47 5.41
14.11 13.10 Hours of heat @ 15 59137 19.61 4.96
15.52 14.41 Hours of heat @ 20 53761 20.97 4.51
17.25 16.01 Hours of heat @ 25 48385 22.63 4.06
19.40 18.02 Hours of heat @ 30 43009 24.71 3.61
22.17 20.59 Hours of heat @ 35 37632 27.38 3.16
25.87 24.02 Hours of heat @ 40 32256 30.94 2.71
31.04 28.82 Hours of heat @ 45 26880 35.93 2.26
38.80 36.03 Hours of heat @ 50 21504 43.42 1.80



This last two weeks in the teens and 20's has shown me that my house is right on track loosing 4-5 degrees / hour and lasting about 14 hours + 6 Hours Boiler burn time. So close to my once a day goal !!