Maximize Your Finned Tube Heat Exchanger Know-How
What is the principle of heat transfer and how does it relate to coil design? A finned tube heat exchanger encourages energy to pass from one medium to another while not allowing the mediums to mix.
To achieve heat transfer in a finned tube coil, there must be a difference in energy concentration between the two fluid mediums. There also must be fluid pathways made of materials that allow passage of heat.
This is reflected in the basic relationship from which all heat transfer equations are derived:
Q = U x A x ΔT
Q is amount of heat transferred over time (BTUs/hr).
U is heat transfer coefficient (BTU/ft2-oF-hr).
A is area available for heat transfer (ft2).
ΔT is temperature difference (oF).
Q is directly proportional to U, A and ΔT. Changing any one of these values affects Q, the amount of heat that is transferred.
Overall Heat Transfer Coefficient. The overall heat transfer coefficient (Q) is affected by the thermal conductivity of the materials comprising the tubes and fins; the viscosity and thermal conductivity of the two fluids; and the velocities at which these mediums move through the coil.
In any heat exchanger, a thin layer of heat transfer medium adheres to the rough surface of the metal heat transfer surface, thereby slowing the movement of the medium. This creates a laminar layer that insulates the bulk of the medium from touching the tube or fin surfaces. As a general rule, the faster the medium moves, the more turbulence is created, which breaks down this insulating laminar layer. The face area size of the coil and number of coil tubes connected to the supply header (circuits) affect the velocity and turbulence of the gas or liquid moving through the coil.
Heat Transfer Area of the Fin and Tube Material Exposed to the Mediums. Air and gases are poor thermal conductors, so more surface area (A) is needed for heat transfer. In a fin-tube design, both sides of a single fin are exposed to the air or gas; when fins are stacked close together, they create a very large amount of heat transfer surface area. Liquids usually flow inside the tubes and are good thermal conductors, requiring less surface area. In a fin-tube coil, the tubes are the primary heat transfer surface and the fins are the secondary heat transfer surface. It is important to have good metal contact between the fins and the tubes because without it, there is not a thermal pathway for heat to move.
Temperature Difference Between the Two Mediums. Both U and A are important parts of the equation: U has to do with conveying (conduction and convection) the heat and A relates to exposure to the heat. But, the temperature differential (ΔT) between the mediums as they move through the coil is the driving force that makes the heat want to move from one medium to the other.
Designing a Coil to Maximize QMaximizing U. As mentioned previously, the thermal conductivity of the tube and fin materials affects a coil's thermal performance. Copper tubes with aluminum fins provide the most effective heat-transfer-to-cost combination. However, these materials are not suitable for many high temperature applications. Other coil materials that are stronger, more corrosion resistant, and temperature resilient -- for example, 90/10 copper/nickel, carbon steel and stainless steel -- also are less efficient in transferring heat, thereby requiring more heat transfer surface area within the coil.
For example, if replacing a copper tube and aluminum fin coil with a stainless steel tube and fin coil, the all stainless steel coil could require approximately twice as much heat transfer area to achieve the identical heat transfer capacity. This could be accomplished by using more coil face area (fin height times fin length), increased fins per inch, a greater number of rows deep or any combination thereof.
Increasing the medium's velocity causes more turbulence, which improves conductive heat transfer. However, as they move faster, the increase in heat transfer starts to level off and medium friction losses continue to rise. Eventually, the amount of energy needed to overcome friction loss is not worth the small thermal gain. If the velocities get too high, service life of the coil is decreased by erosion of the tube and fin material, or cracks occur from the dynamic stresses incurred. By varying the coil face area, tube size and number of circuits, the most efficient medium velocities are reached within the coil even though fixed air and water flow rates are supplied.
At lower medium velocities, enhancing the heat transfer surfaces can create the turbulence required to break down the insulating laminar fluid layer. This can be accomplished by use of a corrugated or enhanced fin surface in place of a flat, smooth fin surface or by adding devices to the inside of tubes to "turbulate" the liquid. The tube pattern arrangement through the fin surface and the distance between each tube also will vary the air turbulence and the amount of heat transfer area, which in turn affect the coil's thermal performance. These enhancements will increase the thermal capacity of a coil but they also affect the medium pressure losses through the coil and the coil cost.
Maximizing A. Some of the coil's physical attributes that affect the heat transfer coefficient -- the tube spacing, tube size and distance between the tubes -- also affect the amount of heat transfer area. Increasing the fin height, fin length and the number of tube rows deep increases the heat transfer surface of both the fin and the tube. Adding more fins per inch increases the surface area of the fin exposed to the air, and changing the tube pattern in the fin can add more tubes. Again, more is better to a point. Too much surface area could lower velocities through the coil and add cost. Too little surface area could raise velocities, thus friction losses, and shorten the coil's service life. Depending on in which dimensional plane (i.e., fin height, fin length, rows deep) the coil surface area is changed, there is increased or decreased cost involved.
Maximizing ΔT. Even with the best heat transfer coefficient and largest heat transfer surface area configuration, it is still the temperature differential that drives heat transfer. Its importance can be seen by looking back at the basic heat transfer equation. If only the ΔT in the equation is changed from 1 to 2oF, the heat transfer is increased by 100 percent. Changing the ΔT to 10oF increases heat transfer by 1,000 percent.
In many cases, it is beneficial to maximize the temperature difference between the fluid mediums. In order to maximize ΔT throughout the coil, the tube-side fluid (oil, water, glycol, etc.) and fin-side fluids (air, etc.) should form a thermal counterflow arrangement. By having the cold liquid enter the side of the coil where the cooled air exits, a thermal counterflow is created and a wide temperature difference is maintained between the two mediums as they move through the coil. Also, the temperature of the leaving air approaches that of the cold entering liquid temperature, further extending the cooled range of the air, which results in more heat transfer (figure 1).
In thermal parallel flow, the temperature difference starts off wide then quickly narrows. The leaving-air temperature can only approach the now warmer leaving liquid temperature, not the colder entering liquid temperature.
There also are factors such as drainability and vertical vs. horizontal orientation, and options such as protective coatings and many others that go into finned tube coil design.