Low outlet temp as a goal for max efficiency

[Nofossil]

This is a repost from a PM discussion. Someone was wondering about my assertion that a lower outlet temperature is desirable in order to get higher efficiency. I figure that either this might be helpful to others, or perhaps someone will straighten me out if I have it wrong. Worth it either way.

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Any wood boiler is in effect a heat exchanger. The water jacket and/or fire tubes are an air-to-water heat exchanger. In virtually all boilers, the flow of water is bottom-to-top, with cold water coming in the bottom and exiting from the top. Virtually all of them also have the flue gas flowing bottom-to-top, with the hottest flue gas flowing upwards and losing heat to the water on its way to the flue pipe, which is at or near the top.

For any heat exchanger, you get more heat transfer if you have more ‘delta t’ - difference between the water on one side and air on the other side at any point. That’s one of the reasons why counterflow heat exchangers are preferred wherever possible. In a counterflow heat exchanger, the flow directions are opposite, so the hottest fluid on one side meets the coldest fluid on the other side, and vice versa. A well designed counterflow heat exchanger can achieve very good performance. For example, some homes have air-to-air heat exchangers that bring in fresh outside air and exhaust indoor air. Some of them can bring in outside air at 0 degrees and introduce it into the house at 50 degrees, while taking 70 degree indoor air and exhausting it to the outdoors at 20 degrees. In this application, we can talk about ‘efficiency’, as the heat being exhausted is lost. counterflow air-to-air heat exchangers used for house ventilation can achieve 70% efficiency or better.

In boilers, however, the water and flue gas both flow bottom to top - they are NOT counterflow heat exchangers. In that situation, the best that you can do is get your flue gas down to the temperature of your hottest water. That’s an impossible goal, but the flue temperature of any boiler is a measure of lost heat.

Here’s the nub of it - all other things being equal, the colder the water jacket, the more heat you will extract from the flue gas.

Since it’s not a counterflow exchanger, this is even more critical. With any given fire burning in the boiler, there are only two things you can do to reduce water jacket temperature:

1) Reduce the inlet temperature - bring in colder water

2) Reduce the outlet temperature by increasing the flow rate.

Condensation imposes a limit on how far you can go with the first approach.

You may not want to go too far with the second approach, since at some point the water isn’t hot enough to be useful. Still, if you’re using radiant or just heating a cold storage tank, I’d use maximum flow and keep the inlet as cold as I dared.

When Orlan had the EKO 80 tested in Europe, they had it supplied with 35gpm of water at 130 degrees. The outlet temp was about 140. They got better than 90% efficiency out of it. That’s how the game is played.

 

[kabbott]

For any heat exchanger, you get more heat transfer if you have more ‘delta t’ - difference between the water on one side and air on the other side at any point. That’s one of the reasons why counterflow heat exchangers are preferred wherever possible. In a counterflow heat exchanger, the flow directions are opposite, so the hottest fluid on one side meets the coldest fluid on the other side, and vice versa. A well designed counterflow heat exchanger can achieve very good performance.

Methinks you got the counterflow part backwords? I know what you saying though.

I agree with the concept based only on heat exchanger efficiency. My question is how does it effect combustion efficiency?
Maybe it doesn't in a gassifier but with my homemade conventional boiler i seem to take a hit at lower temps.

Lets not forget standby loss should be lower at lower operating temps no?
So do gassifiers burn as efficiently at lower temps and if yes how low?

 

[Nofossil]

Methinks you got the counterflow part backwords? I know what you saying though.

Lets not forget standby loss should be lower at lower operating temps no?
So do gassifiers burn as efficiently at lower temps and if yes how low?

Oops - you're right. Opposite flow direction is the key point. Need a picture to help make it clear.

Gasifiers need very high combustion temperatures to achieve their peak efficiency. I suspect that there's a combustion temperature that represents an inflection point - you need to be above it, but no diminishing benefits beyond that point.

However, combustion temp has little to to with water jacket temp. The combustion is shielded from the water jacket.

It would be interesting to see if running at lower output yields higher efficiency due to lower flue losses, or lower efficiency due to reduced combustion temps. My money is on the former as long as you have solid secondary combustion.

 

[Gooserider]

Interesting, but a thought I just had concerning the flow direction question - obviously there are gravity flow advantages to having the boiler run with cold water in the bottom and hot out the top, but how much does that really help a pumped flow? Has anyone experimented with running a boiler "backwards" in order to get whatever advantage there might be from creating a counterflow situation?

Might be an interesting experiment.

Gooserider

 

[Nofossil]

Interesting, but a thought I just had concerning the flow direction question - obviously there are gravity flow advantages to having the boiler run with cold water in the bottom and hot out the top, but how much does that really help a pumped flow? Has anyone experimented with running a boiler "backwards" in order to get whatever advantage there might be from creating a counterflow situation?

Might be an interesting experiment.

Gooserider

In a normal boiler, even a gasifier, it's probably not a huge factor. The flue gas is dropping from over 1000 degrees to around 300, and the water temp is only changing by 20 degrees. That's an average delta T of somewhere near 450 degrees. Counterflow would change that by 20 degrees.

Still, it would be an interesting experiment.

 

[jebatty]

Nofo, I think you were right the first time on counterflow. Wood boilers are not counterflow as you initially asserted. If you look at the diagram, this becomes clear. The point with counterflow is that as the hot input medium cools from being in contact with the water flow in the opposite direction, the coolest input medium (still very hot) remains in contact with the coldest water, thus continuing to transfer btu's. In counteflow the delta-T remains widest over the entire heat transfer flow zone.

 

[DaveBP]

The KOB wood boilers (is that the one that Viessmann recently bought?) use counter flow exchange in their fire tubes. Exhaust gases go from top down and out the draft inducer while the water comes in at bottom and out the top conventionally. I think that can be a significant difference.

I agree with nofossil's premise that increasing the flow through the boiler to extract more heat from the exhaust will improve efficiency. That raises the customary next question; How much?

The average exhaust temperature from bottom to top through the fire tubes must be at least 700F or more degrees. So the delta T between the gasses and water jacket is something like 500F. As a percentage of that difference would 20 degrees lower output water temp result in a real gain of efficiency? At what cost of circulator electricity?

You guys who have had your systems up and running and are familiar with your burn cycles could try this if you have a variable speed circulator through your boiler. At the most stable part of your burn just crank up the circulator speed. Watch the supply temp for a while. Does it go down? Lower the speed back down. Does the supply temp go back up?

Inquiring minds want to know!

 

[Hydronics]

In boilers, however, the water and flue gas both flow bottom to top - they are NOT counterflow heat exchangers. In that situation, the best that you can do is get your flue gas down to the temperature of your hottest water. That’s an impossible goal, but the flue temperature of any boiler is a measure of lost heat.

Guys, this is true- not counterflow but keep in mind: gasifiers are counter Delta T if you will. The hottest combustion gas (bottom) "sees" the coldest water jacket temps (bottom-return). As the comb. gas rises through heat exchangers and loses temp. it continually meets lower water temps. So in effect you do optimize heat transfer, flow direction is irrelevant.
Correct me if I haven't had enough coffee yet this morning...

 

[arrowe44]

I have a friend with a1978 installed tarm 140000 btu that is hooked up that way. It works fine, with small fires( heating less than 2000 sq ft, well insulated now) until really cold nights. He had overheat at 3am last winter, blew soldered pipe joint at boiler apart and filled cellar with steam and hot water. Top of boiler inlet pipe sloped down 6" in 3 ft and overheat aquastat on boiler failed to open dump zone valve.

 

[julien]

Has anyone experimented with running a boiler "backwards" in order to get whatever advantage there might be from creating a counterflow situation?

Might be an interesting experiment.

Gooserider

Biomax have return water direct on smoke exit.