The gas furnace heat exchanger
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When we burn natural gas or propane, the products of combustion are heat, carbon dioxide and water. If we simply burned it inside the structure, the efficiency of the burn would be 100%. However, carbon dioxide exhaust would consume and displace the oxygen in the room. So we put the fire inside a box that has a vent pipe that exhausts those gasses outdoors.
That box started out as just that, a box. Soon we understood that most of the heat was was going out the vent. The stack temperature was over 1000F (538C).
We made the walls of the heat exchanger thinner. There was some experimenting with different materials. Aluminum and copper move heat faster than steel or cast iron. However, because expand and contract more that cast iron and steel there were longevity problems. They also were not as able to withstand the high temperatures. Cast iron was the first material used. It was more stable than pretty much all other materials. However, cast iron could not be made in thin sections. Steel became the material of choice.
The thinner the steel, the faster the heat moves. Then we had to make a convoluted path for the hot gasses so they would "scrub" the heat from the gas. In the higher efficiency furnace the length that the gasses had to pass thru was lengthened and the thickness was reduced.
If you watch the slideshow above, you can see length to which we have gone to extract as much heat as possible from the gas.
The video below gives an idea of how the temperature drops as the gasses rise thru the heat exchanger.
That box started out as just that, a box. Soon we understood that most of the heat was was going out the vent. The stack temperature was over 1000F (538C).
We made the walls of the heat exchanger thinner. There was some experimenting with different materials. Aluminum and copper move heat faster than steel or cast iron. However, because expand and contract more that cast iron and steel there were longevity problems. They also were not as able to withstand the high temperatures. Cast iron was the first material used. It was more stable than pretty much all other materials. However, cast iron could not be made in thin sections. Steel became the material of choice.
The thinner the steel, the faster the heat moves. Then we had to make a convoluted path for the hot gasses so they would "scrub" the heat from the gas. In the higher efficiency furnace the length that the gasses had to pass thru was lengthened and the thickness was reduced.
If you watch the slideshow above, you can see length to which we have gone to extract as much heat as possible from the gas.
The video below gives an idea of how the temperature drops as the gasses rise thru the heat exchanger.
The 2 stage heat exchanger
At this point, we have reached an impasse. Any further reduction in stack temperature and we have a problem. Remember the products of combustion? CO2, heat and water. In the heat exchangers above, the stack temperature is above the boiling point of water. Stack temperatures of these furnaces are around 250F (121C). This limits the thermal efficiency to around 80%. This means 20% of the heat produced by the combustion of the gas goes out the stack. So, what happens if the heat exchanger pulls more heat from the gasses?
The water that is produced when the fuel burns is steam. When we drop the temperature below the boiling point of water, two things happen. One, the latent heat of vaporization releases its heat when the steam condenses. This heat can be utilized to heat the structure. Two, you get water. Not only water, but carbolic acid. Carbolic acid is carbon dioxide mixed with water. This acid is corrosive. When this water contacts the steel heat exchanger, (most of the newer primary heat exchangers have an aluminum coating, but that does not protect the steel) it will rust out the heat exchanger.
So, what do we do? Make the heat exchanger out of stainless steel? Stainless steel is less affected by acids. Well, yes and no. One of the problems with stainless is that it is a poor conductor of heat.
The solution became two heat exchangers. The primary heat exchanger is made from steel to transfer heat from the flame quickly, which may have a temperature as high as 3500F (1926C).
Then, as the temperature drops, but before condensation, the gasses enter the secondary heat exchanger. This heat exchanger is made from stainless steel. Most of these heat exchangers are designed somewhat similar to an air conditioning coil. A series of small diameter tubes with a swirl plate inside that makes the gasses scrub the surface of the tubes.
Also, when water is condensed from the gas, there has to be a provision for draining the water. Thus we have drainage systems on these high efficiency furnaces.
The water that is produced when the fuel burns is steam. When we drop the temperature below the boiling point of water, two things happen. One, the latent heat of vaporization releases its heat when the steam condenses. This heat can be utilized to heat the structure. Two, you get water. Not only water, but carbolic acid. Carbolic acid is carbon dioxide mixed with water. This acid is corrosive. When this water contacts the steel heat exchanger, (most of the newer primary heat exchangers have an aluminum coating, but that does not protect the steel) it will rust out the heat exchanger.
So, what do we do? Make the heat exchanger out of stainless steel? Stainless steel is less affected by acids. Well, yes and no. One of the problems with stainless is that it is a poor conductor of heat.
The solution became two heat exchangers. The primary heat exchanger is made from steel to transfer heat from the flame quickly, which may have a temperature as high as 3500F (1926C).
Then, as the temperature drops, but before condensation, the gasses enter the secondary heat exchanger. This heat exchanger is made from stainless steel. Most of these heat exchangers are designed somewhat similar to an air conditioning coil. A series of small diameter tubes with a swirl plate inside that makes the gasses scrub the surface of the tubes.
Also, when water is condensed from the gas, there has to be a provision for draining the water. Thus we have drainage systems on these high efficiency furnaces.
So, what can we expect to gain in efficiency by using a secondary heat exchanger? 98% efficiency is possible and the lowest allowed by the Federal government in the northern states of the United States is 95%. Below is a video of the 2 stage heat exchanger.
Problems with heat exchangers
The heat exchanger is subjected to widely varying temperatures, so it must be designed to handle the expansion and contraction of the metal without cracking. In earlier furnaces, the steel used in furnaces was thick enough to resist cracking. However, thermal efficiency was reduced because it took so long for the heat to move thru the metal. As efficiency increased, the heat exchanger thickness was reduced so the heat would move faster. This changed the requirements for installation of furnaces to keep the firing rate closer to the manufacturer's specs. What I mean by this is when a modern furnace is installed, the firing rate must be set as close to factory specs as possible. An overfired furnace will burn out the heat exchanger. The pic below shows a tube type heat exchanger that was overfired.
Below you can see cracked heat heat exchangers. Some of these are easy to see. Others may be much harder to find unless the furnace is disassembled.
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An underfired furnace is as bad as on that is overfired. If the furnace is underfired, the the burned gasses lose heat faster than they should and may condense in the primary heat exchanger. Because the primary is not stainless, the heat exchanger may rust out.
So, what happens when the heat exchanger cracks or rusts thru? Because the burning gasses inside the tube are now mixing with the air circulating across the outside, combustion is upset and the burn may not be complete. The video below shows the flame change as the furnace blower starts. In the first part, the static pressure from the fan forces the flame back out of the burner. In the second, the flame on the right is swirling when the fan starts.
The problem here is the incomplete combustion of the gasses produces carbon monoxide gas. This is a poisonous gas. If this gas, even in small amounts, are present in the air we breathe, death occurs quite soon.