Air Cooled Heat Exchangers Put the Heat On

In Greek mythology Prometheus was credited with stealing fire from the gods and bringing it to man. Similarly in various Native North American tribes the mythology talks of fire being stolen by different life forms — Coyote, Dog, and the Hare as well as others and bringing it to man. Fire was such an important to tool to early man that stories were told to explain how it came to be in our possession.

There is no doubt that the harnessing of fire has transformed our world. Because without fire there could not have been an industrial revolution. Industrial processes consume a great deal of energy and specifically energy from fire. Heat is often required to convert materials from a primary form into secondary, secondary into tertiary and higher forms. Common processes are ore into iron or primary metals, iron into steel, and steel into hot rolled sheets for automobiles.

Once the transformation process is complete in most cases the heat must be dissipated. Either so that the process can continue, or so that the finished product can be shipped safely. Sometimes being left in a warehouse to cool naturally is all that is required, but more often than heat must be dissipated rapidly.

When cooling condensate from a power plant, equipment such as shell-and-tube heat exchangers may be used to remove heat from the saturated steam/condensate to cold water – often to a body of water like a lake or river. As regulations tighten over thermal pollution more plants are moving towards rejecting their heat to the atmosphere especially if there is a risk of the hot fluids, say from a chemical process, leaking into and contaminating the water body. In these cases air-cooled heat exchangers, or “fin fans”, can be used to transfer waste heat from a hotter liquid to the cooler ambient air.

Whether a shell and tube exchanger or a fin-fan, the physics of the heat transfer are fairly similar.

At the simplest level when heat is transferred whatever leaves a system it must go into its surroundings. For a fin-fan, the heat removed from a process liquid goes into the atmosphere.

The energy transactional relationship expressed in words is:

The Mass Flow of Liquid multiplied by the Specific Heat of the Liquid multiplied by the Change in temperature in the Liquid (incoming compared with outgoing) MUST EQUAL The Mass Flow of the Cooling Air multiplied by the Specific Heat of the Air multiplied by the Change in temperature of the air.

This is represented in equation form as…

Energy transaction represented in equation form

Energy transaction represented in equation form

Where:

Energy transactional relationship

Incidentally, this also applies to cooling of solid materials, such as red-hot steel, coming off a hot rolled line where air is blown on hot surfaces. These very hot solid forms will have a blast of ambient air or chilled air to take away the temperature of the metal.

(In the case of red-hot steel, heat will also be carried away by conduction through the rollers, as well as radiation to the surroundings.)

Looking at the above equation the mass of cooling air drives the heat removal transfer. Often engineers and operators will speak in ‘air volume’ but ultimately it is the mass of the air that does the “work” of heat removal.

For a fin-fan designer moving the largest mass of air over the smallest fin-tube surface area is the main goal in order to minimize capital costs. Other considerations however are low noise and low pressure drop through the cooling coils. This means increasing the coil area and slowing the air velocity. So competing parameters and considerations exist.

To increase the heat transfer through the metal of the fin tubes the thinner the walls of the tubes the better the heat conductivity and therefore the higher the heat transfer. With corrosion (and to a degree erosion) taking place inside the tubes, the thicker the tube wall the longer the life of the fin coils. So again other competing parameters and considerations exist and a balance of heat transfer to longevity must be struck.

Sometimes both shell-and-tube heat exchangers work in conjunction with air-cooled heat exchangers. For example say hot smelter acid needs to be cooled to go back into the smelting process. Because the acid is very corrosive, a shell-and-tube exchanger made from corrosion resistant materials suited for extreme environments (C276, Hastelloy, Inconel, etc.) will be used to transfer its heat to a secondary cooling loop. Often a glycol solution is used to avoid freezing within the secondary loop and because of the liquid they are also called “glycol coolers”.

The secondary loop goes through the air-cooled heat exchanger to return the secondary cooling fluid back to its lower temperature state, so it can do work of heat removal in the shell-and-tube exchanger.

The movement and control of heat in processes is critical to optimizing industrial processes and a great deal of thought goes into the technologies.

If you have an air-cooled heat exchanger application you would like to talk about please contact www.aircooledheatexchangers.net to discuss how to size one that will fit area limitations and take into account life-cycle costs, reliability, maintainability and noise considerations.


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