Heat exchanger in Detail

Heat exchanger:

A heat exchanger is a device designed to efficiently transfer or "exchange" heat from one matter to another. When a fluid is used to transfer heat, the fluid could be a liquid, such as water or oil, or could be moving air. The most well known type of heat exchanger is a car radiator.

How Does a Heat Exchanger Work:

Heat exchangers, metal shells and tubes, work by transferring heat from one place to another. When a furnace burns natural gas or propane fuel, its exhaust/combustion by-products (also known as flue gas) enter and travel through the heat exchanger. The hot flue gas heats the metal as the gas makes its way to the exhaust outlet of the furnace. As this is happening, the hot metal heats the air circulating over the exterior of the heat exchanger.

Primary Heat Exchanger:

This part contains the hottest flue gas, found closest to the burners in a furnace. As a result, the flame and heat subject it to the most stress, which can cause cracking and heat stress over time. Furnaces that are 70-80% efficient have one heat exchanger.

Secondary Heat Exchanger:

If you have a furnace that is considered high-efficiency (90%+ efficient), it contains both a primary and secondary heat exchanger. As the combustion exhaust leaves the primary heat exchanger, it travels into a secondary heat exchanger where more heat is released from the flue gas and water vapor begins to form. This change of state from water as a vapor to a liquid releases latent heat in the secondary heat exchanger, bringing the furnace to an even higher level of efficiency. This is why high-efficiency furnaces are sometimes referred to as condensing furnaces. Secondary heat exchangers are generally constructed from stainless steel or a coated steel material capable of withstanding heat, moisture and acid.

Health Risks:

Because the heat exchanger contains the flue gas inside of itself, it is important there are no holes, cracks or other deterioration. This type of deterioration that permits leakage and mixing of flue gas with the air being heated can result in incomplete combustion and the formation of carbon monoxide and other harmful by-products. Although your furnace may not immediately leak carbon monoxide into the living space, high CO levels make it unsafe to operate. Something as simple as the chimney getting plugged or damaged exhaust pipe can result in a very dangerous situation.

Detect Heat Exchanger Problems:

Unless your furnace is malfunctioning or your carbon monoxide detector is going off, it’s nearly impossible to know if your heat exchanger has developed problems without direct inspection or testing for CO. This is why regular maintenance and inspections are very important. An inspection and a combustion analysis/CO test  is the best way to determine whether the furnace is operating safely.

Visual Inspection:

Some heat exchangers can be inspected visually. Others require specialty tools to examine the unit more closely. Our technicians are equipped with a camera with a flexible shaft that can see into inaccessible areas of your furnace to ensure a thorough inspection. We sometimes find internal issues in heat exchangers that would otherwise be hidden and appear in good condition from the outside.

Operation of the heat exchanger, for air-to-air heat transfer:

The operation of heat exchangers occurs by transfer of energy in the form of heat from one medium (air, another gas, or a liquid) to another. The operation of heat exchangers in which there is complete separation between the two media and where no intermediate storage of heat occurs is known as "recuperator heat recovery". Recair develops and manufactures recuperator units for air-to-air heat transfer.

Operation of the heat exchanger with the maximum possible efficiency:

In order to maximize the heat transfer in the operation of a heat exchanger, the shape of the intermediate wall in the heat exchanger is designed such that the distance which the heat flow must pass through is minimized. In this connection it is necessary to take into account the permissible pressure drop in the heat exchanger.

The two air streams move countercurrently but parallel to each other along the intermediate wall, which wall is in principle infinitely large (in practice maximally large). The cold air stream can be heated to the temperature of the exiting hot air stream, and vice versa (the hot air stream can be cooled to the temperature of the exiting cold air stream).

In order to achieve uniform distribution of the air streams over the width of the heat exchanger, triangular channels are formed in the intermediate wall; these channels are of low diameter, and of like resistance. In this way, the air flows in all of the channels are identical. Each triangular channel is surrounded by three similar channels, in which the counter-flow takes place. In this way, one can reach the extremely high efficiency of 93% in the heat exchanger. Because of the fact that the heat exchanger results in only a very small temperature difference between the in-flowing external air and the out-flowing internal air, the result is a very comfortable living and working climate.