Tube Shell Exchanger Output Temps

[AaronM]

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.

 

[ewdudley]

I guess the first question would be is why the tube shell in the first place? Aren't both sides pressurized?

 

[AaronM]

Sorry, by pressurized, you mean closed essentially yes? Boiler loop is closed, contains gylcol at 15%. The other side can be open or closed. The idea being a cold supply line and hot output line could offset domestic hot water to the second story. These are currently closed.

Would a plate style exchanger have been a better solution?

 

[Bob Rohr]

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

How much surface area in that HX? I've never had much luck with that type of HX for close temperature approach. Maybe not enough surface area or the design it too wide open?

Small plate style give much better performance, from my experience.

 

[AaronM]

How much surface area in that HX? I've never had much luck with that type of HX for close temperature approach. Maybe not enough surface area or the design it too wide open?

Small plate style give much better performance, from my experience.

Got nothing on the spec sheet. However, a 155K has 3.6 sqft and a 300K is 6.8 sqft. So I would estimate around the 1.9 sqft mark.

Right now, system is closed on both sides. Do not want to drag the temps down further by trying to supplement domestic hot water.

 

[woodsmaster]

Are both sides glycol ? Do you really need glycol ? im also wondering why the heat exchanger ? You should be able to heat DHW first and still have plenty of heat for the house. I see no need for the heat exchanger other than it don't look like you used oxygen barrier pex.

 

[jebatty]

A HX bare rating (85,000 btuh) is quite meaningless unless for that rating you know sides A and B gpm flow rates, sides A and B pressure drop, sides A and B entering and leaving temps, and approach temperature. And the you need circulators on both sides that can provide the required gpm's at the spec'd pressure drop, giving due consideration also to system pump head. Properly sized (spec'd), both tube and shell and flat plate can work well, but as always the devil is in the details.

 

[cpeltier]

As jebatty pointed out you need to know the specs on the HX to design for the flow rates and delta Ts. One thing I noticed in the picture is the boiler supply and return appear to be reversed. You may get slightly better performance and hotter water if you reverse them. If reversed and you slow the tube side flow the exiting temperature should increase. It doesn't mean you'll get any more BTU/hr as it's defined by: BTU/hr = GPM X delta T X 500. Looking at your system with a 22 delta T on the load side of the HX @ 85,000 BTU/hr = ~8 GPM flow rate which seems reasonable looking at the plumbing in the picture.

 

[tom in maine]

Yes,
your heat exchanger should have the flows going in opposite directions. They are in parallel. That drops performance.

 

[jebatty]

Looking at your system with a 22 delta T on the load side of the HX @ 85,000 BTU/hr = ~8 GPM flow rate which seems reasonable looking at the plumbing in the picture.

I would like to agree with you but I'm not sure I can. On the boiler side with a delta-T=9F, flow rate would need to be about 19 gpm for 85,000 btuh. The circ looks like a 15-58, and that circ won't move that volume of water. Without knowing the pressure drop on Side A + system head we don't know the Side A gpm, and it likely is materially less than 19 gpm, maybe 1/2 that? And this then is 1/2 the problem, because the same questions are present on Side B.

My semi-educated guess is that the HX is substantailly undersized to do the job desired. And I too would go with a properly sized plate HX if a HX is desired or needed.

 

[cpeltier]

I can't see the pump for the boiler side plumbing. He described 2 zones and the storage tank utilizing 15-58s. If the boiler side was then it would be marginal at 18.8 GPM assuming the delta T of 9F is accurate. This is also pushing the flow capacity of 1" pipe. At this flow rate significant head loss comes into play requiring an even larger pump.

Another minor consideration is the system derating for Glycol. The pump efficiency will go down and your HX performance will decrease. At 15% glycol this may be another 5% loss but it does add up.

A flate plate HX can be sized to move more heat and work in a "close approach" mode but the head loss is typically higher then these shell and tube HXs and proper pump selection and pipe sizing are very important.

 

[AaronM]

Sifting through all your replies, amazing insight. While searching for HX specification details on the limited instruction sheet, I too noticed that the flow should have been reversed. Thankfully it's easy to reverse a pump even with my plumbing knowledge. Mission accomplished in about 5 mins. I'll grab the infrared and take additional readings upon my return home today. However, I had 149F water at the intake of the Furnace based HX, but I believe that is simply because it is the first stop now, instead of the storage tanks. This made a drastic difference in house temperature though. 12F outside last night, 74F inside. Furnace fan running at 1500CFM.

Indeed I have 15-58s all around. Have them all running on the "hi" setting. This includes the boiler side, it's sitting approx. 70 feet away in the wood shed and it's 1" line. Temp readings were taken 6" down/up stream from the inputs and outputs of the tube/shell HX using an infrared thermometer. I could try the food/meat thermometer if this is believed to be more accurate.

To answer the previous question the boiler side is 15% glycol because I do a decent amount of traveling and my wife is on shifts - affords me a some peace of mind. In retrospect a constantly on circ going would have meant skipping the tube/shell HX all together as suggested. It's not too late I suppose, could delete this once I shut down for the season.

I too am leaning toward the HX being undersized, though installer leaning toward a blockage in the tanks (dip tubes), causing lack of flow.

Added a storage tank bypass before work this morning. The intent this evening is to cycle a load heating the lines to the rad and furnace exclusively to measure any changes to output temperatures.

I have requested HX specifications from the manufacturer.

Thank you all for your replies thus far, never ceases to amaze me.

 

[jebatty]

Indeed I have 15-58s all around. Have them all running on the "hi" setting. This includes the boiler side, it's sitting approx. 70 feet away in the wood shed and it's 1" line.

This identifies your fundamental underlying problem. 1" pex at 140 feet round trip from/to the boiler is undersized and there is no practical circulator or appropriate design that will move the volume of water needed to handle the 140,000 btuh output of your boiler or even the 85,000 btuh you desire at your hx for your pool.

No taking into account additional pump head from curves, fittings, the hx, etc., which could be quite significant, with your 15-58 the maximum volume of water you can move will be about 7 gpm, and at a delta-T of 20 that will move 70,000 btuh, far less than your boiler output. Add in pump head not accounted for and the actual volume of water that will move will drop rapidly. Use of glycol will make the flow somewhat less. Not what you want to hear, but your design with 1" pex is woefully inappropriate. IMO any attempt to correct this result with a high head circulator also is likely to produce unsatisfactory results.

Under these circumstances it makes no sense to me to discuss a different heat exchanger because that will not solve your underlying problem. If your system can provide needed heat with high delta-T and related low flow rates, your system may be able to work, but I don't see any way to solve your pool heating problem with your current design.

 

[AaronM]

This identifies your fundamental underlying problem. 1" pex at 140 feet round trip from/to the boiler is undersized and there is no practical circulator or appropriate design that will move the volume of water needed to handle the 140,000 btuh output of your boiler or even the 85,000 btuh you desire at your hx for your pool.

Under these circumstances it makes no sense to me to discuss a different heat exchanger because that will not solve your underlying problem. If your system can provide needed heat with high delta-T and related low flow rates, your system may be able to work, but I don't see any way to solve your pool heating problem with your current design.

Thanks for the insight. 2x1" insulated lines were recommended by the local wood boiler retailer/supply given my 70 foot run. It was this or 1 1/4". Would you suggest the deletion of the HX all together as Woodmaster suggested? As at this point, I'm stuck with the 2x1" pex for financial reasons.

 

[AaronM]

Also, minor point of clarification, no pool at any point in the system. It's a series of three 50 gallon water heater tanks as storage/buffer which now be isolated/excluded individually (manually), a water to air HX in the furnace and a solitary 5' slatfin rad.

When tanks are excluded, less than 3 gallons needs to be heated for the furnace HX and the Rad. Damn 9 volt died last night in the infrared, however upon bypassing the tanks, my trusty meat thermometer showed about 160F leaving the tube/shell HX after a 1hr burn. Will grab a fresh 9 tonight and get some better readings.

 

[jebatty]

I understand the Biomass 40 to be a wood gasification, pressurized boiler. Seems to me that a hx is needed to separate the pressurized boiler system from the unpressurized pool system. Also would be needed to separate a glycol boiler system from a non-glycol pool system. Further, IMO would be needed to keep chemically treated and oxygenated pool water, both corrosive to the boiler system, separate from essentially inert boiler water.

If you eliminate the pool heating and have a closed space heating system, then no hx should be needed. On the other hand, may be economical rather than having a glycol system with large storage with glycol. Rule of thumb with a properly designed hx system is that the Side B hx space heating temps will be 5-10F less than the Side A boiler supply side temps.

 

[cpeltier]

The 70' run of 1" PEX could be a killer at higher flow rates. I ran a quick calculation based on 150' of 1" PEX the 15-58 on speed 3 and see a 7.5 GPM flow with ~11.5' head. In addition you have the HX head loss, boiler head loss, other boiler and HX near side plumbing loss, and any check valves, etc. If you doubled up on the pumps (2 15-58s in series on speed 3) it would only improve the flow to ~10 GPM or so.

Fan coils generally don't need much GPM as you indicated but they do need to have very hot water to get the rated output. Often running the fan coil longer at a lower temperature will suffice and allow you to use the storage as it cools down. I'm not sure how your controls are setup but you should be able to take advantage of the lower tank temperatures. I've been successful in using water temps as low as 125F for usable heat.The radiator unless it is greatly over sized is a different story. It needs the hottest water it can get to produce meaningful heat (or switch to a different style - IE over sized cast iron).

So, I have another question. Is your furnace fan running continuously to keep the house warm at 74F at 12F with 149F inlet at 2-3 GPM?
What is the fan coil return temperature?

If your required flow is truly only 3 GPM and the fan is running continuously then removing the HX or replacing it with one over-sized to reduce delta Ts (close approach mode) should fix it immediately. I would however, be concerned that this system will only provide adequate heat with the boiler running. The 150 gallons of storage (if up to temperature) would only provide 50 minutes of run time in this scenario.

If the fan is not running continuously then some control work will make lower temperatures usable for the fan coil but not necessarily the radiator. You'd have to experiment with it to see if it's adequate.

I'm sure the HX will produce it's rated output but probably not at the delta Ts that you need.

 

[AaronM]

Thank you Jebatty, no pool in the system now or likely ever. That's what the pond is for I think the confusion lies in our choice of a tube/shell exchanger. Our heating/cooling guy happened to have one, yadda yadda yadda, it was a freebie.

To answer your questions cpeltier -

In terms of controls, the circ pump for the furnace HX is a thermostat on the mainfloor. Furnace fan controls are manual. For about 6 hours in the evening, I typically run the fan at 1500 CFM, and back it down to the default re-circulation speed of 500 CFM overnight and during the day while at work. At high speed, it achieves 74F, at recirc speed it maintains a constant 70F. So I think your 'lower, longer' theory is absolutely at work here.

Now for the rad, it's correctly sized for 180F, but certainly not for lower temperatures. I should have around 15'. Having said that... the bedroom it is located in seems to be maintaining 71F so long as the door stays closed. The second story has no duct work, and is heated by electric baseboards - 1500W per room. Going for proof of concept by replacing one with a slatfin hydronic, seems to be a good use of $100 so far. Same setup here, thermostat controls a circ pump.

The 50 minute scenario you describe is about what was expected from the get-go. My original intent was to use these as buffer/tempering tanks (hook one water heater to a 220v and maintain 120F at all times) and afford me an hour so once the boiler had shut down - little wiggle room should my spouse be late from her shift.

I am keeping my eyes open for larger storage, but it has to be of a specific size/shape to get down the staircase of this old farm house. If my repurposed water heater tank theory worked, I simply envisioned a small army of them down there - they are very easy to obtain, all under $50, easy to move, and are already somewhat well insulated. However, I know that each connection adds 2 elbows, restriction, etc. And needless to say, I have to get the above mentioned issues in check before moving on to bigger storage.

All that to say, I'm open to ditching the tube/shell or upgrading it. In either case, perhaps it could be used as a sidearm for DHW?

In the interim, I'm limping along just fine, I just want to make the best of what's there. And I'll join the club perhaps of folks who wish they'd gone 1 1/4"

 

[woodsmaster]

Only downfall to not having the hx is that you would have to have glycol in all your storage and house plumbing or eliminate the glycol altogether. If your not gone to much I'd insulate the boiler room, install an electric heater in shed for backup and get rid of the glycol. If your gone a lot the I'd get an over sized flat plat, larger pump and hope for the best,

 

[cpeltier]

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:

I would definitely calculate your HX before you buy one as they are not inexpensive!

 

[AaronM]

Only downfall to not having the hx is that you would have to have glycol in all your storage and house plumbing or eliminate the glycol altogether. If your not gone to much I'd insulate the boiler room, install an electric heater in shed for backup and get rid of the glycol. If your gone a lot the I'd get an over sized flat plat, larger pump and hope for the best,

Way I see it, I could well be out the $200 in glycol or the money for an oversized plate. Shed insulation is an option, it's getting re-sided anyway this summer (good bye 'nail on brick'!), but a back up battery would be an option too. I guess it's a cost-benefit-headache analysis from here on in. Existing HX was a freebie, so I can't complain!

 

[AaronM]

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!

Excuse me as I nerd out on this sweet calculator . Thank you for this, picture is worth a thousand and could save a few hundred The above would allow me to keep my existing setup and simply swap out the tube/shell if I understand correctly?

On the plate exchanger front, I understand not all are created equally. If I go this route (which is likely, as it retains all the logic/appeal of original config), I will start digging on here for recommended brands.

 

[AaronM]

Probably inconsequential now, but I said. 15-58s all around, but the boiler loop has a 26-99.

 

[cpeltier]

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.

 

[ewdudley]

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 ...

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.)