Getting the Most From Your Coil
When searching for a new heat exchanger, consider a custom-designed coil. The following tips will help you determine if a custom-designed unit will benefit your process.
Heating and cooling coils are offered as off-the-shelf standard units or can be designed to fit a specific application. At first glance, a coil does not look complicated, and attempting to enhance its performance may seem unnecessary and impractical. But, in some applications, custom-designed coils allow you to meet stringent process heating demands and minimize performance problems. Before specifying a new coil, consider the following tips to determine if a custom-designed unit can provide benefits to your process.
Tip 1: Keep Performance in Mind
A coil has no moving parts, and no performance-improving adjustments are required -- and few can be made -- once it is installed. It must be built with the required performance in mind. Therefore, performance optimization at the design stage is critical.
Tip 2: Consider a Variety Of Materials
Assess the coil's operating environment, including both the external and internal environments so the proper materials and coatings can be selected. Other factors to consider include the temperature and pressure to which the tubes and fins will be subjected. Coils can be designed and built with the standard copper tube/aluminum fin construction or with other materials such as aluminum, copper, carbon steel, stainless steel, cupro/nickel, Hastelloy or titanium. In addition, various coatings can be applied to enhance corrosion resistance.
Tip 3: Recognize Material Affect On Performance
A material's thermal conductivity largely determines a coil's total heat transfer capability. Of the materials listed in the previous tip, copper tubes provide better heat transfer than steel tubes, and aluminum and copper fins provide better heat transfer than steel fins. Therefore, if the application requires steel components to withstand corrosive, high pressure or high temperature environments, the coil must be made larger than a coil using more heat-transfer-efficient components.
Tip 4: Consider Coil Shape
The basic shape (length, width and thickness relationship) is determined by the envelope into which the coil must fit. Usually, there are height, width or depth restrictions inherent to the installation that define the space available for the coil. Therefore, this also should be an initial consideration in the coil's design.
Tip 5: Optimize Heat Transfer
The purpose of a coil, or finned tube heat exchanger, as it sometimes is called, is to transfer heat between two mediums while isolating those mediums so they do not mix. To achieve heat transfer, there must be:
A temperature difference between the two mediums.
A pathway of material that allows conduction of heat so it can be conveyed from one location to another.
A means of exposing the heat to the fluid medium.
If any of these items are lacking, heat transfer will not occur. This is reflected in the basic relationship from which all heat transfer equations are derived. Changing any one of these values affects the amount of heat that is transferred.
Q = U x A x DT
Q is the amount of heat transferred over time (BTU/hr)
U is the heat transfer coefficient (BTU/hr-ft2-oF)
A is the heat transfer area of the fin and tube material exposed to the mediums
DT is the temperature difference between the mediums (oF)
Tip 6: Supply the Engineer With The Required Parameters
To ensure good design for your application, supply the engineer with the required parameters to design the optimum air cooling/heating coil for a specific application. Required design parameters include:
The temperature (both dry and wet bulb) and pressure of the air entering the coil.
The airflow rate available at the coil inlet side.
Description of the tube-side medium, whether it is water, a defined water/glycol
The entering tube-side fluid temperature and flow rate (gal/min).
In addition, the designer must be made aware of any contaminants in the airstream that may collect on the fin surface. This allows the designer to enter a fouling factor to account for the reduction in heat transfer resulting from the contaminated surface.
Tip 7: Realize Air and Gas Temperature And Velocity Affect Coil Design
The greater the temperature difference between the two mediums, the greater the heat transfer rate. This is the ?T in the preceding heat transfer relationship. However, the actual air velocity also must be considered in the coil's design. A low velocity may result in inefficient heat transfer while too great a velocity (usually greater than 500 ft/min) will carry the condensed moisture with the airstream leaving the coil. High velocities create noise and high air friction, translating to higher fan energy usage. Also, forces developed by extremely high velocities could damage a coil if it is not properly designed to withstand them.
Therefore, the designer must consider the air velocity through the coil and strike the proper balance of air velocity, air friction and heat transfer efficiency. This is accomplished by coordinating the coil surface area with the fin configuration, fin density and rows-deep configuration. Optimum design will provide the best heat transfer performance with an acceptable velocity and air pressure drop.
Tip 8: Recognize That Tube-Side Fluid Temperature and Flow Velocity Affect Coil Performance
The fluids must contain the necessary potential to achieve the amount of heat transfer required. This potential comes from the temperature difference and flow rate. If the required potential is not available, either the temperature difference or the flow rate -- or both -- must be increased. In addition, the fluid's flow velocity through the tubes is critical to the coil's performance. Again, the design engineer is looking for the optimum combination of fluid velocity and fluid pressure drop. These factors are affected by the internal diameter of the tube, the number of tube circuits (flow passages) and the total lengths of the circuits.
Tip 9: Provide Structural And Mounting Information
Good coil casing design requires knowledge of the type of installation and the shock, vibration and thermal loads to which the coil will be subjected as well as the type of environment to which it will be exposed. In addition to the overall size and weight of the coil, casing material, casing thickness and coil configuration will be selected according to these factors.
Tip 10: Purchase a Coil Suitable for Your Process
You can expect a custom-designed coil to fit the first time you attempt to install it. This is because you can specify the type, size and location of the piping connections, the casing configuration and the mounting bolt hole locations.