Cavitation issues

[Marshy]

Watching a fire burn right now.

Reading right on the pipe going into the pump is 199°F. With the pump on speed 2, I get cavitation. With the pump on speed 1, there's nothing.

I'm reading a 25° delta T at my plate HX on the boiler side.

The pump is a bell and gossett NRF-36.

Should I still go through with the replumbing in the boiler shed or just run on speed 1?

I thought you said it was a single speed. My bad, I'd recommend running it on low and not messing with the plumbing at all. Any reason you would need the higher flow?

Let me see if I understand this. You have a 25F delta from boiler outlet (199F) compared to the cold side outlet of the htx? Or was thst measured at the return pex in the boiler cabinet?

What's the temp drop from boiler outlet to htx inlet (ie loss between boiler to htx)?
What's the temp drop from the cold side of the htx to the return in the boiler cabinet?

Pick your poison I guess, running on low speed or on medium and throttle the discharge slightly if you need a little more flow. Even throttled on medium it's probably more flow than slow speed unthrottled.

 

[maple1]

I am thinking the only down falls to running the boiler circ slower would be increased boiler idling, or not getting enough btus to the house to keep up with demand.

 

[warno]

I thought you said it was a single speed. My bad, I'd recommend running it on low and not messing with the plumbing at all. Any reason you would need the higher flow?

Let me see if I understand this. You have a 25F delta from boiler outlet (199F) compared to the cold side outlet of the htx? Or was thst measured at the return pex in the boiler cabinet?

What's the temp drop from boiler outlet to htx inlet (ie loss between boiler to htx)?
What's the temp drop from the cold side of the htx to the return in the boiler cabinet?

Pick your poison I guess, running on low speed or on medium and throttle the discharge slightly if you need a little more flow. Even throttled on medium it's probably more flow than slow speed unthrottled.

Yeah the bell and gossett NRF-36 is a 3 speed pump.

From boiler outlet to HX inlet there is less than 1° loss. So at 199° boiler the HX is getting ~199°. The 25° delta was on the HX only. So the inlet of the HX was 199 and outlet was 174. Then boiler return is just a few degrees less then that.

It took alot of throttling of the discharge valve to get the cavitation to stop. But switching to lower speed it was instant. The reasoning i want to run a higher speed is for more flow through the HX.

I am thinking the only down falls to running the boiler circ slower would be increased boiler idling, or not getting enough btus to the house to keep up with demand.

My boiler only flows to the plate HX in the garage which charges storage. It doesn't go to the house. All heat loads are pulled from storage. So the only load I need to worry about with the boiler is storage charging.

 

[Marshy]

Yeah the bell and gossett NRF-36 is a 3 speed pump.

From boiler outlet to HX inlet there is less than 1° loss. So at 199° boiler the HX is getting ~199°. The 25° delta was on the HX only. So the inlet of the HX was 199 and outlet was 174. Then boiler return is just a few degrees less then that.

It took alot of throttling of the discharge valve to get the cavitation to stop. But switching to lower speed it was instant. The reasoning i want to run a higher speed is for more flow through the HX.



My boiler only flows to the plate HX in the garage which charges storage. It doesn't go to the house. All heat loads are pulled from storage. So the only load I need to worry about with the boiler is storage charging.

Got ya. What about the other side of the htx, what is the temp delta?

You can still try previously discussed in an effort to get it to run on medium speed. Hope it works out for you. Keep us up to date.

 

[warno]

Got ya. What about the other side of the htx, what is the temp delta?

You can still try previously discussed in an effort to get it to run on medium speed. Hope it works out for you. Keep us up to date.

The storage picks up about 20 degrees to the boiler loss of 25.

I'll go for broke and increase supply piping size and lower boiler pump to the floor like discussed above. It'll have to wait a few months because I'll have to drain the boiler water down below that level to get the welding done.

 

[maple1]

I think then that with a 20dT to storage, and not hearing of any boiler idling increasing, you should be good to go running on the slower speed. Also sounds like your boiler dT is a bit more than 20 - actually closer to 30. I'm not sure you could improve things very much in all that, really. I think I would be happy to leave things just as they are.

 

[Marshy]

If he's looking to increase the delta T on the storage side he could slow that pump down and it should pick up more heat I believe.

@warno, what are the actual temps on the storage side of the htx in and out?

 

[warno]

Here's is a boiler log a did the other day. You can see the delta T temps for both sides of the HX.

 

[warno]

Well @maple1 Is correct. I tried a burn tonight with the boiler pump on speed 1 and it is idling way more then it ever has. The delta T looks good at the HX but the BTUs are not getting out of the boiler fast enough. The difference between the delta T on speed 1 vs 2 is only a few degrees so if I can remain burning and not idling then I need to run at least speed 2.

So with that knowledge, I'm back to relocating my pump to the floor and plumbing in larger supply to it.

 

[Marshy]

@warno, what is the htx rating?

 

[maple1]

Well @maple1 Is correct. I tried a burn tonight with the boiler pump on speed 1 and it is idling way more then it ever has. The delta T looks good at the HX but the BTUs are not getting out of the boiler fast enough. The difference between the delta T on speed 1 vs 2 is only a few degrees so if I can remain burning and not idling then I need to run at least speed 2.

So with that knowledge, I'm back to relocating my pump to the floor and plumbing in larger supply to it.

I wish I wasn't. If that helps any.

 

[warno]

@warno, what is the htx rating?

This is my HX

I wish I wasn't. If that helps any.

It's ok. I'm good with changing things up. Like i said the plumbing pieces aren't much money for me and the welding and fitting is my pass time anyway.

 

[maple1]

That's not a very big hx. Marshy might be on to something.

 

[warno]

That's not a very big hx. Marshy might be on to something.

I'm thinking I'm going to buy a 30 plate.

 

[maple1]

I would likely go even bigger.

 

[warno]

I would likely go even bigger.

I'm running at about 20ft of head at about 10 gpm with the current setup on the boiler side. My boiler can more then keep up with the delta T at this rate. And if I can get it to burn better with next years modifications it should keep up even better yet. How big of a HX would you recommend? Keep in mind i have 22 ft of underground piping to the HX that is only 1" pex.

 

[Marshy]

I'm not versed is sizing htx's for boiler systems but I believe the temp deltas you have shown us is indicating the htx is way too small. A 20-25 delta T is about half of what you should be reasonably shooting for. Maybe another member can help with that aspect. I saw some good comments from @Karl_northwind in another thread.

 

[maple1]

I'm also a bit over my head if I try to size your HX. I have one, same size as yours actually, doing just my DHW. But I just used seat of the pants to pick it, based simply on what I read others posting about on here. It works great for me, but I only use it for my DHW. Which is maybe 1/30 of my heating load. I can't remember what I see for dTs, but I have very slow flows going through it.

I don't think I would go less than a 40. Then from memory budget might kick in, I think they can get kind of expensive. But bigger would be better - the bigger it is, the more ability to get the heat from your boiler to your storage.

Now, to throw a couple more curve balls at you, I had a couple more thoughts come to me as I was trying to drag my butt out of bed a while ago.

How is your return temp protection mixing valve doing? Have you been watching the temps of return water entering the boiler? If it is letting more bypass flow through than it needs to, or still letting some in when it shouldn't be - that would also likely contribute to some increased idling.

And I notice in your chart that you kept both circs on the same speed. Have you tried boiler circ on speed 1 & storage circ on speed 2? If you already mentioned that, I either missed it or forgot.

Would really like to see more from others on the HX sizing aspect.

 

[hondaracer2oo4]

http://flatplateselect.com/site/pub/login.aspx?ReturnUrl=/site/default.aspx

You will need to create a account but it is free. Allows you to input different flows and deltas and see what you get.

 

[Marshy]

@maple1 is right. I would absolutely run the storage side pump on speed 2 and run the boiler side on 1 for the time being. That might help you maximize the heat transfer.

Thanks for the link @hondaracer2oo4, I'm playing with it now.

 

[Karl_northwind]

If you are using the flatplate select software, know that all 5x12x30 plate HX's (for example) are not created equal. make sure you compare the actual square footage of the HX with whatever you are using if you don't use the AIC flatplate HX. If I remember correctly I have a 30 plate 5x12 from one premium manufacturer that has 13 square feet of transfer area, and another cheapo chinese that has 8 square feet. YMMV

 

[hondaracer2oo4]

What size is your flat plate? Is it a 20? If so that’s tiny. Use the calculator I posted to play around but there is no draw back to oversizing your plates other then cost and you can get some cheap plates. I have a 50 plate which is feeding my primary loop on my hydro air system. No problems with the plate after 3 years. I would run a 70-100 plate hx if I was heating a water store. You can Always throttle your owb pump or tank pump to dial in th delta thst you aim for.

 

[Bob Rohr]

Any way to put some coils inside the boiler, and internal HX that could be pressurized would solve all the issues.

Keep in mind at high operating temperatures you may cavitate in the piping above the water level in the boiler. Cavitation is not limited to the volute of a pump. Anything or place where you drop below atmospheric pressure, especial;lc at high operating pressures can cause cavitation. You can hear cavitation in a garden hose when you kink it off! Any restriction in the piping that causes that pressure reduction can set up cavitation.

Is that fan coil above the water level? Here is the match and explanation for cavitation in open systems.

 

[Marshy]

If you are using the flatplate select software, know that all 5x12x30 plate HX's (for example) are not created equal. make sure you compare the actual square footage of the HX with whatever you are using if you don't use the AIC flatplate HX. If I remember correctly I have a 30 plate 5x12 from one premium manufacturer that has 13 square feet of transfer area, and another cheapo chinese that has 8 square feet. YMMV

The e bay ad he provides doesn't disclose any useful information to be able to determine it's performance. It only says it's a 20 plate htx with plate dimensions of 20"x4.24".

If I used the software correctly, a more appropriate size htx would be a 30 plate 10"x20" htx.

I assumed boiler supply temp to htx 185F, and outlet from htx 140F at 10gpm. On the storage side of htx, inlet 135F and outlet of 170F.

That being said, I'm making more than a few assumptions. First I'm assuming a larger delta T than he listed in his excel sheet 185-140 on the boiler side, and 135-170 on the storage side. I'm also assuming a flow rate of 10gpm from the boiler pump because the pumps maximum flow on speed 1 is ~26gpm unrestricted.

This probably doesn't add a whole lot of value other than big picture concept that I believe @warno needs a larger htx.

Here is the htx he has.

 

[Bob Rohr]

Water inside the evac tube solar collectors boils at around 100- 110 due to the vacuum in the tube.

o