The short answer
A standard economizer takes sensible heat out of flue gas and stops at about 250°F so nothing condenses. A condensing economizer keeps going below the water dew point, about 135°F for natural gas, and recovers the latent heat in the water vapour too. It needs corrosion-resistant construction and, above all, a cold stream of water to dump the heat into.
The standard economizer
This is the finned-tube heat exchanger most industrial boilers already have, or should have. Hot flue gas passes over tubes carrying boiler feedwater, the water warms up, and the boiler burns less fuel to make steam.
It is designed to stay dry. Exit gas is held at roughly 250°F or higher so the water vapour never condenses on the tubes or in the stack. That protects carbon-steel equipment from acid corrosion. It also puts a hard floor on how much heat you can recover.
The condensing economizer
A condensing economizer, also called a flue gas condenser, is built to get wet. It cools the gas below the dew point on purpose, so the water vapour turns back into liquid and gives up its latent heat. For natural gas, that latent heat is a big prize. In one DOE example it was about 9 percent of the boiler's firing rate. See how much heat your boiler sends up the stack for how that adds up.
There are two main designs:
- Indirect contact. Flue gas and water are separated by tube walls, like a standard economizer but in corrosion-resistant material. The DOE notes these can heat water to around 200°F.
- Direct contact. Water is sprayed straight into the gas stream. Heat transfer is very effective, but the water leaves at a lower temperature, around 140°F per the DOE, and it has touched the flue gas.
Side by side
| Standard economizer | Condensing economizer | |
|---|---|---|
| Recovers | Sensible heat only | Sensible and latent heat |
| Typical gas exit temperature | About 250°F or higher | Below about 135°F on natural gas |
| Materials | Carbon steel is usually fine | Stainless or other corrosion-resistant |
| Condensate | None, by design | Collected and drained, often reusable after treatment |
| Heat sink needed | Boiler feedwater | A cold water stream, the colder the better |
The one question that decides it: where does the heat go?
A condensing economizer can only condense water if something cold is on the other side. That is the question to answer before anything else. Good heat sinks include:
- Cold boiler makeup water, especially on plants with poor condensate return
- Process water and wash-down water, which is a big one in dairy and food processing
- Low-temperature space heating or domestic hot water loops
- Greenhouse heating, which is part of the Sidel CREN clean-stack vision
The catch is timing. The boiler makes heat when it runs, and the plant needs hot water when it needs it. If those two schedules don't line up, the recovered heat has nowhere to go. That is the job of thermal storage: Novacab TESS banks recovered heat in a phase-change material and releases it later, so the condenser keeps working even when demand is low.
What about the water?
Burning 1,000 cubic feet of natural gas makes roughly 10 to 12 gallons of water. A condensing unit turns part of that back into liquid. It is mildly acidic, so it is usually neutralized before it is reused or discharged. On plants that are short of water, or paying a lot for it, that recovered water is a second benefit on top of the fuel savings.
Other fuels
Natural gas is the easiest case. Its condensate is relatively clean and its acid dew point sits below the water dew point. Coal, oil and biomass bring sulfur and particulate into the gas, which makes condensing harder on materials and fouls heat transfer surfaces. That is why Sidel CREN pairs heat recovery with particulate control upstream. The Rotary Particulate Collector cleans the gas before it reaches the SRU condenser.
The Sidel SRU
The SRU is a flue gas condensing heat recovery unit, in industrial use since 1978. It recovers sensible heat, latent heat and condensed water from boiler and furnace exhaust. See how the SRU works.
Quick answers
What is the difference between a condensing and a non-condensing economizer?
A standard, non-condensing economizer heats boiler feedwater and keeps flue gas above roughly 250 degrees F so no water condenses. A condensing economizer deliberately cools the gas below its water dew point, about 135 degrees F for natural gas, to recover the latent heat in the water vapour as well. That requires corrosion-resistant construction and a cold water stream to absorb the heat.
What do you need for a condensing economizer to work?
You need a steady supply of water cold enough to cool flue gas below its dew point, such as boiler makeup water, process water, wash water or a low-temperature heating loop. Without a cold heat sink, a condensing economizer cannot condense anything.
Can you recover water from boiler flue gas?
Yes. Burning natural gas produces roughly 10 to 12 gallons of water per 1,000 cubic feet of gas, and a condensing heat exchanger recovers part of it as liquid condensate. The condensate is mildly acidic, so it is typically neutralized before reuse or discharge.