tube coils, finned, heat exchanger coil

Understand the basics of finned tube coils and maximize coil capacity and efficiency.

Finned-tube coils are found in many process heating systems and many times are the first major component to show symptoms of improper upstream filtration.

In a dirty atmosphere, they can actually cause many systematic problems, including pump, compressor and fan failures.

Finned-tube coils (sometimes known as heat exchanger coil) have two important parts that convey heat transfer.

The first is the tubes, which also are called the “primary surface.” The second is the finned surface, which also is known as the “secondary surface.”
Both are ultra-important to creating the designed heat transfer with balanced air- and water-side resistance.

The fins on finned tube coils increase heat transfer by what is going through the tubes and what is on the outside. (As we all know, there is no such thing as “cold” – only the absence of heat – but for purposes of clarity, I’ll use the word cold to describe the lower temperature stream.) In heat transfer coils, the transfer goes from the hottest to coldest of the two streams. In a chilled water coil, the air temperature is higher than the water temperature; therefore, heat is removed from the air and given to the water. In a hot water heating coil or thermal liquid coil, the liquid is a higher temperature than the air; thus, the water or thermal liquid gives its BTU/hr capacity to the air.

The fins on a coil accomplish two major functions. First, they straighten the air out as it comes in contact with the entering-air side of the coil. Normal process fans tend to distribute air through a unit in a spiral pattern. The fins straighten this wavy air and allow the air a more uniform path across the primary surface. Second, they actually do some heat transfer of their own. The heat transfer is taken from the tube to the fin, thus raising the overall capacity of the coil. This is the reason the fins are called a secondary heat transfer surface.

Basics of Coil Selection.
Coil selection is now done primarily by computer, but many do not understand the dynamics of coil selection. Proper selection is predicated upon meeting three major requirements:

  • Heat transfer (BTU/hr load).
  • Fluid pressure drop.
  • Air pressure drop.

There is a unique balance that is required by the selector – you cannot meet one or two of these requirements and be way off on the other(s).

Take an example where a coil is selected and meets both the heat transfer load and water-side pressure drop requirements but is high on the airside pressure drop. In this example, the fan that has been selected to move the air is based on amount of air against a required total static resistance (pressure).

If the coil resistance is too high, the fan will produce less air and the capacity of the system is then reduced as well.
It is very important for all owners and designers of process equipment to understand that the selection of coils almost never carries a significant service factor in capacity. This means that system changes during the life of the coil can dramatically change the efficiency and capacity of any system.

SYSTEM CHANGES THAT CAUSE COIL INEFFICIENCY

A multitude of system changes can affect heating and cooling coils.

Among them are:

  • The fluid temperatures that are supplied to the unit are higher or lower than what was specified.
  • The air temperatures that are supplied to the unit are higher or lower than what was specified.
  • The fluid volume that is supplied to the unit is higher or lower than what was specified.
  • The air volume that is supplied to the unit is higher or lower than what was specified.

Notice that in each of these examples, the actual operating conditions vary from that which the coil was designed for. Because coils do not carry a significant service factor, if the process conditions are appreciably different from those for which the coil was designed, process inefficiencies will result.

Fluid Temperature Variations. If the actual fluid temperatures are higher or lower than those specified when the coil was designed, heat transfer is reduced. For example, suppose a process coil system is designed to heat air from 95°F (35°C) to 350°F (177°C), and it is selected based on 400°F (204°C) entering thermal liquid supplied to the coil’s tube side. Based on this process data, the coil selected is 6 rows, 12 fins per inch. However, suppose the actual fluid temperature is 380°F (193°C). This reduced fluid temperature may reduce BTU/hr capacity by as much as 15 percent and lower the leaving air temperature significantly.

This happens all of the time based on inferior insulation, faulty valves and other circulating characteristics.
Fluid Volume Variations. Fluid volume deficiency causes BTU/hr problems as well. Pumps are sized just like fans: They produce a volume of water or thermal liquid vs. a known maximum resistance. When that resistance is more than what was specified, then the pump produces less volume, and the coil is trying to produce BTU/hr with less flow (gal/min).

When looking at a coil, it is apparent that the space between fins and around tubes is limited.

Air Volume Variations.
Air volume that is not as specified is very much a symptom of too much resistance in the system. Certainly, there can be design problems from the outset, but many systems build up extra resistance because of air-side fouling. Coils are easily loaded with dirt, particles and bacteria because of their density. This increase in pressure reduces airflow and is a major contributor to decreases in overall system efficiency.

COIL FOULING DECREASES EFFICIENCY

Air-side fouling can cause a small decrease in efficiency early on, and if it is allowed to accumulate, efficiency can be cut in half (BTU/hr output). Dirt, debris and scale act like insulators in the system. (In addition, they negatively affect efforts to meet air volume requirements.) Insulating foreign material decreases efficiency because the two streams (water, steam or refrigerant inside the tubes; and air or gas on the fin side) cannot transfer heat as well. Just this dynamic can cause a 5 percent to 10 percent decrease in efficiency, and in extremely dirty situations, a 20 percent to 25 percent decrease in efficiency is possible. Foreign material also has a huge effect on airflow volume because the material does not allow a coil to pass the airflow from the entering side to leaving side. When looking at a coil, it is apparent that the space between fins and around tubes is limited. Actually, that is a good thing, because you want the air to be in contact with the primary and secondary surfaces as much as possible. However, if fouled, the very nature of the coil’s design can cause airflow restrictions and eventually airflow volume decrease.

Many mechanics and process systems operators deal with fouled coils by just adjusting the fan drives, speeding up everything so that the system can supply the design airflow (cfm). While effective in the short term, this approach is costly because the mechanical brake horsepower (BHP) goes up, and this hits the owner right in the pocketbook. Also, an operator can only speed up his drive for so long before he reaches the limit on motor horsepower and cannot adjust any further. At this point, the airflow amount will start to decrease, and so will the overall cooling or heating capacity of system.

SYSTEM PROBLEMS RELATED TO AIR-SIDE FOULING

There are many system problems that can be traced back to fouled coils, but here are a few:

  • The equipment must run longer to meet the design conditions. As a result, maintenance, emergency breakdowns and   replacements are more prevalent.
  • Operating and service personnel must spend more time with equipment to set and reset controls to allow the system to run   somewhat according to design.
  • With thermal fluid liquid systems, fouled coils can cause control, valve and pump issues.
  • With steam systems, valve cycling is possible, leading to condensate corrosion and inefficiency.

WHAT ABOUT CLEANING THE COIL?

Understanding some basic principles will help ensure that when you clean your coils, it is done correctly.

Understanding some basic principles will help ensure that when you clean your coils, it is done correctly. The most important part of cleaning a coil is knowing the type of product used and at what pressure. You must understand that some coils have very thin fin surfaces, and high pressure cleaning may clean the coil but close off the fin surface by bending the outer edges. Also, once the coil is cleaned, make sure that the cleaning agent is removed completely. I have seen hundreds of coils destroyed by corrosion caused by cleaning agents.

Clean your coils on a regular basis, and never let airborne particles find their way into the internal rows of any coil. You have a chance to clean a multi-row coil if the material is still on the outside area of a coil, but you won’t have much success if the material has moved more than two rows into the center of the coil.

And remember, prevention is worth its weight in gold. Why deal with fouled, inefficient coils when you can remove these foreign agents before they ever arrive at the surfaces? High quality filtration may seem like a huge expense, but it is pennies on the dollar in comparison to the costs of downstream contamination.

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

where

Q is amount of heat transferred over time (BTUs/hr).
U is heat transfer coefficient (BTU/ft2oF-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.

Figure 1. The temperature difference between the two mediums is the driving force for heat to be exchanged. The wider the temperature difference, the greater the rate of heat transfer between the mediums. By properly circuiting a coil for thermal counterflow, the log mean temperature difference between the two mediums is maximized.

Designing a Coil to Maximize Q

Maximizing 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 drain ability and vertical vs. horizontal orientation, and options such as protective coatings and many others that go into finned tube coil design.

tube coils, finned, heat exchanger coil

Finned Tube Coil Heat Exchangers

What do they do?

Finned tube coils transfer heat energy from a steam or liquid (inside the tubes) into air (across the fins) or vice versa.

We produce finned tube coils from one off small applications to much larger commercial and industrial projects. The list of applications are endless!

Typical Applications:

  • HVAC / Air Handling Units
  • Heat Pumps 
  • Boiler Economisers
  • Process Drying 
  • Agricultural Drying 
  • Space heating 
  • De-humidification 
  • Process cooling 
  • Heat recovery
  • Air cooling in food manufacture
  • Air heating using steam / thermal oil  

Materials:

We have various material options: 

  • Tubes in copper, steel or stainless steel 
  • Fins in aluminum, copper, steel or stainless steel
  • Frames in galvanized steel, stainless steel, Aluzinc or copper
  • Connections in brass, steel or stainless steel
  • We also supply various coatings to protect against corrosive environments

Coil Types:

  • Finned block – tubes expanded into rectangular fins
  • Spiral wound – crimped fin wound along tube length
  • Water to air 
  • Steam to air
  • Refrigerant to air
  • Oil to air 
  • Run-around coils (air to air heat recovery

Connections:

  • BSP threaded
  • PN16 flanged
  • ANSI flanged 
  • Plain tube

Replacement Coils:

  • On receipt of your damaged coil or drawing, we can produce a replica coil to fit into your existing duct work and match up to your existing pipe-work connections when possible.
    We also offer an on-site coil measuring service to measure up the existing coil. Many clients have saved both time and money by avoiding unnecessary modifications to their existing installations.

Your best solution in Heat Exchange solution!

[email protected]

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