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.
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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.
