Tube Shell Exchanger Output Temps

[nt30410]

Hello again all,

Biomass 40 is running top notch now, thanks to tips and tricks of everyone here. It's cycling nice, clean, and I coulnd't be happier.

My next problem is a whole other can of worms (and has lead me down the rabbit hole already trying to diagnose).

My Heating/Cooling guy and I are trying to figure out why I've got low output temps on my tube shell exchanger. Pic follows with current readings after about 48 hours of continuous burn. Bottom line is, I can't crack around 150F. I have 3 water heaters used as storage/buffer, so only about 150 gallons. I have a 16x20 air/water HX in the furnace and a solitary 5 foot slat-fin rad in the upstairs bedroom. 3 speed pumps for each (storage, furnace, rad). Have experimented with all speeds, found that has little impact.

Now all this to say, at around 10F outside, I can keep the 1800 sqft farm house at around 68F, but based on the output numbers, I know there's something amiss. There's got to be more heat there.

Last piece of the puzzle is the tube shell - it's 85,000 but/h. Is it possibly undersized?

Needless to say, I'm happy that the 80,000 btu propane furnace has been silent for over 30 days.View attachment 156187

A few things don't seem right. Like already mentioned, the boiler connections should be reversed. Secondly, the domestic water connections should also be reversed. I think this will make the difference. Enclosed is a suggestion for the changes. Good luck.

At 3 GPM you should be able to make it work with a little wiggle room if you find a much larger HX. The load isn't that high but you need a close approach HX to meet your heat emitter requirements. Pay attention to the head loss in the new HX if you go that route.
They generally have high head at rated output especially the flat plate type. If you go much larger the head loss will be much less at the lower flow rate and the delta T will be much tighter.
I'm thinking something along these lines and your existing pump may work:
View attachment 156364
I would definitely calculate your HX before you buy one as they are not inexpensive!

Hello again all,

Biomass 40 is running top notch now, thanks to tips and tricks of everyone here. It's cycling nice, clean, and I coulnd't be happier.

My next problem is a whole other can of worms (and has lead me down the rabbit hole already trying to diagnose).

My Heating/Cooling guy and I are trying to figure out why I've got low output temps on my tube shell exchanger. Pic follows with current readings after about 48 hours of continuous burn. Bottom line is, I can't crack around 150F. I have 3 water heaters used as storage/buffer, so only about 150 gallons. I have a 16x20 air/water HX in the furnace and a solitary 5 foot slat-fin rad in the upstairs bedroom. 3 speed pumps for each (storage, furnace, rad). Have experimented with all speeds, found that has little impact.

Now all this to say, at around 10F outside, I can keep the 1800 sqft farm house at around 68F, but based on the output numbers, I know there's something amiss. There's got to be more heat there.

Last piece of the puzzle is the tube shell - it's 85,000 but/h. Is it possibly undersized?

Needless to say, I'm happy that the 80,000 btu propane furnace has been silent for over 30 days.View attachment 156187

 

[nt30410]

A few things don't seem right. Like already mentioned, the boiler connections should be reversed. Secondly, the domestic water connections should also be reversed. I think this will make the difference. Enclosed is a suggestion for the changes. Good luck.

Sorry, the attachment didn't get with the reply.

 

[maple1]

Secondly, the domestic water connections should also be reversed.

What DHW connections? I don't think DHW was even mentioned before, all that was mentioned/described that I can see is storage, furnace HX & one rad. So not sure what you mean? I also don't think I would use a 15-58 as you show in your diagram - I don't think they're meant to pump fresh potable water.

Plus, if you reverse both circuit connections thru a HX you will be back where you started - but again not exactly sure what you meant.

OP is doing pretty good since he first posted - we've added a pool and DHW to his circuit already. lol....

 

[AaronM]

Secondly, the domestic water connections should also be reversed.

What DHW connections? I don't think DHW was even mentioned before, all that was mentioned/described that I can see is storage, furnace HX & one rad. So not sure what you mean? I also don't think I would use a 15-58 as you show in your diagram - I don't think they're meant to pump fresh potable water.

Plus, if you reverse both circuit connections thru a HX you will be back where you started - but again not exactly sure what you meant.

OP is doing pretty good since he first posted - we've added a pool and DHW to his circuit already. lol....

lol thank you for that Maple1. That did make me chuckle

A thank you though to all taking a stab at this. I can assure you no DHW and no Pool. I was hoping for DHW at some point, but not until everything was running smoothly. As mentioned, this will probably be a sidearm of sorts, possibly even my existing tube shell exchanger - it is infact made of 316 stainless.

 

[AaronM]

That changes things!

I re-ran the calcs with glycol on one side and the bigger pump allows you to drop a few sizes in HX. Again I don't know your exact head loss in your plumbing but I am estimating it based on a 150' of 1" PEX and my assumptions are captured in the picture below. The head values are for a boiler loop flow of 6 GPM yielding 8' head for 150' of 1" PEX, 5' head in HX, and allows a loss budget of 14' head in boiler head, near boiler head, and near HX head (26-99 speed 3). This HX is only slightly over-sized in this example even though it's rated at 175K BTU/hr. The output side of the HX is engineered for 4 GPM allowing 1 GPM to charge your tanks with everything else running. You may be able to go slightly smaller in the HX but the calculator won't go smaller then the FG5X12-16 so i'm not sure. This GEA (AKA Flat Plate) HX is available locally or sold at SupplyHouse.com and are well built SS units.

The old HX could be used as a sidearm for domestic hot water if it's construction can handle oxygenated water (SS,Cu, etc) otherwise it will eventually leak. If it leaks it could possibly flow into the secondary HX loop being lower pressure and hopefully not the other way poisoning the household! A Pressure Relief valve somewhere in the HX secondary loop should be installed to make sure nothing will get over pressurized.

Hats off to you for re-doing these calculations! Not sure why I neglected to mention the boiler loop was different. Apologies.

I will hunt for the FG5x12-16. I am leaning toward keeping the glycol and the boiler loop isolated as a closed system. Will tackle a 'vacation' and 'extended winter power failure' setup over the summer. Before anyone asks, I do have a dump zone in the shed to prevent overheating in a power failure event.

At this point, is it of your (and everyone else's) opinion that the tube shell HX I have is just simply grossly undersized and/or not as efficient as the aforementioned FG5x12-16?

 

[cpeltier]

Seems like a lot of electricity just to go from about 6.5 gpm to 9.5 gpm. If you can make do with 6.5 gpm then you could do it with an 85 watt 15-58 conventional pump or a 45 watt Alpha ECM pump, instead of the 197 watt 26-99. (You can run three freezers on less than 200 watt.)

He does have a loss budget of 14' head in my calcs so dropping to speed 2 or 1 can be experimented with. It takes a lot of electricity to make up for the cost of a new pump IMHO but it may be worthwhile.

 

[cpeltier]

Hats off to you for re-doing these calculations! Not sure why I neglected to mention the boiler loop was different. Apologies.

I will hunt for the FG5x12-16.

Much less expensive options with similar specs are the B&G BP400-30 or it's knock-off the Brazetek BT3x8-30.

 

[nt30410]

Secondly, the domestic water connections should also be reversed.

What DHW connections? I don't think DHW was even mentioned before, all that was mentioned/described that I can see is storage, furnace HX & one rad. So not sure what you mean? I also don't think I would use a 15-58 as you show in your diagram - I don't think they're meant to pump fresh potable water.

Plus, if you reverse both circuit connections thru a HX you will be back where you started - but again not exactly sure what you meant.

OP is doing pretty good since he first posted - we've added a pool and DHW to his circuit already. lol....

As one maritimer would say to another ..you got to use you imagination bye. He is using DHW tanks as storage / buffer. He is heating the tanks via his boiler through a heat exchanger. The differance I see is what is labeled as dhw is actually his heating supply to his house. I think he could see through that. If you look at the pdf in my other post, the boiler connections are on the end of the heat exchanger rather that on the side.The other concern was the heat exchanger should be piped "counter flow" ie the tank side connection out (to the tank) should see the hottest water from the boiler. His 15-58 should do fine in his application..

 

[maple1]

Agreed on the counterflow.

I have a sidearm that helps with DHW - which is a tube shell exchanger. I have since added a FPHX because the sidearm wasn't cutting it when heating DHW just from storage. The boiler water on it flows through the larger side connections, and I think that's the same way I've seen it on any other tube/shell HX I've seen info on. Could be wrong though...

But on the FPHX - I got a Bell & Gossett for my DHW. A 20 plate. Was reasonably priced. If the tube/shell will be replaced by a FPHX, I would tend to the oversized end of the spectrum (bigger is better?) - mine works awesome.