how a core /rad /coil works

[thorilan]

For those of you that want to learn the workings and considerations of heat exchange in a core/rad and how some basic principles may improve your performance. The info for this comes from many different sources and hopefully this will be less confusing than some of the other explanations that are out there
The purpose of a finned tube heat exchanger (the technical name also known as a rad core or coil) is to isolate two different mediums so that they do not touch or mix together, and to transfer the heat from one medium to another.
To achieve heat transfer, there must be a difference in temperature between the two mediums (gas and/or liquid which refurs to the air outside of your rad and the water inside your loop), a pathway made of materials that allows conduction of heat so it can convey from one location to another( the surface material of the core which is usualy copper or aluminum with some brass), and a means of exposing the heat to the fluid medium (the water block ). If any of these items are missing, heat transfer will not occur.

This is reflected in the basic relationship from which all heat transfer equations are derived:

Q = U A DeltaT

where
Q = Amount of Heat Transferred Over Time (BTUs/hr)
U = Heat Transfer Coefficient (BTUs / ft2-?‹F-hr)
A = Area Available for Heat Transfer (ft2)
DeltaT = Temperature Difference (?‹F)


Changing any one of these values affects Q (the amount of heat that is transferred).


U =Overall Heat Transfer Coefficient is affected by the thermal conductivity of the materials that the tubes and fins are made of, the viscosity and thermal conductivity of the two mediums and the velocities at which these mediums move through the rad/core . NOTE in larger systems Medium velocity (Medium as in the liquid and gas , aka air and water) is important because a thin layer adheres to the rough surface of the metal thereby slowing the movement of the medium. This creates a laminar layer that insulates the bulk of the medium from touching the tube and/or fin surfaces. As a general rule, the faster the medium moves, the more turbulence is created, which breaks down this insulating laminar layer.

A =Heat Transfer Area of the fin and tube material exposed to the mediums. Air and gases are poor thermal conductors so more surface area is needed for heat transfer. Both sides of a single fin are exposed to the air or gas so 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. Tubes are the Primary Heat Transfer Surface and 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.

DeltaT =The temperature difference between the two mediums. "U" has to do with conveying (conduction and convection) the heat and "A" relates to exposure to the heat. But the DeltaT between the mediums as they move through the rad/core is the driving force that makes the heat want to move from one medium to the other. The greater the difference , the more heat that will be transfered .


Some of the coil's physical attributes that affect the heat transfer coefficient, like the tube pattern, 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 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 . Even with the best heat transfer coefficient and largest heat transfer surface area configuration it is STILL the temperature difference that drives the heat transfer. Its importance can be seen by looking back at the basic heat transfer equation Q=UA DeltaT . In larger systems IF only the DeltaT in the equation is changed from 1?‹ to 2?‹ the heat transfer is increased by 100%. Changing the DeltaT to 10?‹ increases heat transfer by 1000%.

Obviously it is very important to maintain a wide temperature difference between the air and water (gas and liquid). Making sure this occurs in a rad requires that the liquid in the tubes moves through the coil in the opposite direction (counter flow) to how the gas, on the fin side, moves through the rad. This even applies to chambered cores but to a much smaller extent as cores are a much smaller system and the difference is very small .
The circuit path of the medium should also be designed for the liquid to enter the coil at the lowest point and exit at the highest point. This allows any air in coil tubes and/or in the incoming liquid to float upward with the liquid flow and out of the coil providing automatic air venting or " Bleeding". If a core/rad is not bled, air may become trapped in several tubes, insulating the liquid from touching the tube wall, which lowers core performance .