ACHRNEWS

Go deep: Introducing the energy-efficient 'Deep' coil

April 26, 2000
The design of finned tube heat exchangers has stayed relatively the same over the past several decades. These heat exchangers are commonly used in air conditioning and refrigeration units as condenser-evaporator coils. They are also routinely used in air handlers as hot- water, steam, or chilled-water coils.

To be efficient, the heat exchanger should transfer maximum heat (absorb-reject), offer minimum resistance to airflow, have minimum resistance to liquid flow inside the tubes, and be cost effective.

There are many variables, which are constraints, in the design of these heat exchanger coils. The more prominent of these are air resistance, liquid resistance inside the tubes, fin density, tube density, air velocity, and face area. All these variables are interdependent.

For example, higher fin density will yield higher heat transfer, but will also increase the air resistance, requiring a bigger fan motor for the same air movement. The higher the velocity, the higher the heat transfer, but it will also increase air resistance.

Smaller tube spacing will reduce the face area, but will also increase the air resistance. A riffled cross section will increase the heat transfer, but it will also increase the resistance to liquid movement, requiring a more powerful motor.

Weaving through all these constraints, a typical configuration has evolved and remained on the scene for several decades. Using this configuration, the heat exchanger coil has typically high fin density (12 to 16 fins per in. on the condenser side, 8 to 12 on the evaporator side), a low number of tube rows (four or less), and smaller tube spacing (typically 1- by 1-in. staggered).

The coil is typically flat-slab shape. For condensers, often this slab is then bent into a “U” shape to wrap around the fan. For evaporators, the coil remains in the flat shape.

Enhancements

Now, there is a new way to design the coils to enhance its objectives. According to this new method, the fin density is drastically reduced, and the tube spacing as well as the tube rows are drastically increased.

Consider the specifications in Table 1. Coil A represents the conventional design coil and Coil B represents the redesigned equivalent, called a “Deep” coil. All other details not specifically mentioned remain the same between A and B.

Tests would show these two coils would provide the same or similar performance as shown in Table 2.

In these tests, an inlet water temperature of 135°F was chosen as an approximate simulation of an R-22-based refrigerant cycle condensing unit, based on enthalpy considerations.

As can be seen from Table 2, the air pressure drop across the coil declines by nearly 75% in the redesigned Coil B without any sacrifice in the heat transfer and without adding any new copper tubing or fin material. This means that the fan motor hp size can be reduced by half. In conventional air conditioning units with one outdoor and one indoor motor, this saving multiplies.

Reducing the fan motor hp by half means higher energy efficiency for the overall machine, reduced oem cost, and a drop in consumer operating cost. In some instances, it may mean reduced sound levels.

One caution: In the absence of proper tooling, making a sample of Coil B, which is an 8-row, 1- by 2-in. tube pattern, 6 fins per in. (fpi), will be a challenge. These difficulties can give false or misleading results.

There are a number of other benefits inherent in the Deep coil design.

Reducing the footprint

In many instances, you can reduce the footprint of the overall machine, which means reduced storage/shipping charges, reduced installation floor space, and easier handling.

The reduced fin density also means reduced incidence of moisture being carried over into the airstream — a serious problem in some installations. Moisture outside the confines of the unit means stained ceiling tiles, wet electrical insulation, as well as annoyance if the moisture falls on people.

There is another common problem with large-face-area coils. As the coil face area gets larger and larger relative to the fan, the air distribution gets uneven. Most of the air passes through the coil face area that is close to the fan, relatively starving the far corners of the coil. Though the fan is moving the designed quantity of air, the heat transfer is short due to uneven distribution of air.

It is possible the Deep coil will enhance coil efficiency beyond the prior art levels. This can be accomplished by drastically increasing the tube rows to a high number of 8, 10, or 12, by selecting the proper fin density and suitably high tube spacing.

In short, the Deep coil converts the current short air path, high fin density, low tube rows, small tube spacing, “slab” shape configuration into a long air path, low fin density, high tube rows, high tube spacing, “cube” or “duct” shape configuration. It is contrary to conventional wisdom, but you will get used to it.

The new coil has been covered by patent application.