Figure 1. In a vertically fed coil, the refrigerant flows from the bottom to the top, or the top to the bottom.

The art and science of troubleshooting finned coil evaporator performance varies as much as the applications in which this refrigeration component is placed. To understand the performance of an evaporator, you must first understand what you are dealing with. Evaporator design, refrigerant feed and coil engineering all must be factored into evaluating performance. And, don’t forget system design in evaluating evaporator performance. This includes refrigerant feed arrangement, refrigerant piping configuration and sizing, as well as valve selection and setting.

Evaporator Basics

The evaporator has several important performance elements, including:
  • A finned coil.
  • Fans.
  • Motors.
  • Drain pan and casing.
  • Refrigerant feed design and system.
  • System piping and valves.
In addition, for evaporators operating in environments below freezing, a method to remove frost accumulation on the coil surface is required.

Evaporator coils are constructed of tubes and fins, and multiple factors influence evaporator coil performance. These include:
  • Tube diameter.
  • Type of tube and fin material.
  • Fin spacing.
  • Coil circuiting.
  • Tube pitch/geometry.
  • Refrigerant feed.
  • Volume of air flowing over the coil.

Figure 2. A cross-fed coil balances the refrigerant flows either parallel or counter to the flow of air. Coil pressure drop is controlled by the number and lengths of circuits.

Tube.The tube is the medium by which the refrigerant is contained within the evaporator and is known as the prime surface of the coil. Its diameter and construction materials will influence its performance. The tube diameter ranges from 0.625 to 1", and each diameter will have various performance capabilities. Smaller diameter tubes (0.625 and 0.75") are better suited for lower capacities at medium-to-higher temperature applications. Small-diameter-tube evaporators perform better when fed with direct expansion and liquid overfeed refrigerant feed. This is due to limits in circuiting and control of refrigerant velocity and circuit pressure drop.

Larger diameter tubes (0.875 and 1") perform better in high capacity (ton) units. Liquid overfeed or flooded refrigerant feed as well as low temperature (-40°F [-40°C]) applications also are applicable with larger diameter tubing. The range of tube diameters can be successfully applied outside of the above characterized performance ranges.

Fin Type, Material and Spacing.The fin is the medium that extends the refrigerating capacity of the evaporator and is known as the secondary surface of the coil. Fin spacing, material and thickness all affect unit performance. Fin spacing ranges from 1.5 FPI (fins per inch) to 6 FPI; coils with the wider-spaced 1.5, 2 and 3 FPI are more likely applied in colder applications while narrower 4 and 6 FPI are applied in higher temperature applications.

Coil materials of construction currently available vary by manufacturer, but the user has a choice of steel tube and fin; aluminum tube and fin; and stainless steel tube and aluminum fin. Fins are bonded to the tube via two methods:
  • Mechanical bond, where the tube is expanded to the fin. Mechanical bond is used for aluminum tube and fin coils as well as stainless tube and aluminum fin coils.
  • Chemical bond, where steel tube and fin coils are bonded through a metallurgical process known as hot dip galvanized. During the bonding process, the coil is immersed in a bath of molten zinc.
Coil Circuiting.For effective evaporator design, each coil is circuited to maximize the performance within a given set of operating parameters. Coil circuiting is designed to control refrigerant pressure drop within the coil, manage internal gas velocity and provide proper wetting of the inside tube surfaces. All these factors affect the performance of the coil.

Coils circuited for low temperature applications have multiple circuits that are shorter in length; medium-to-high temperature coils have fewer circuits, but they are longer in length. This is due to the volume of vapor a coil operating in a low temperature application produces vs. a coil operating in a medium or high temperature application. Ammonia at -40°F (-40°C) produces 24.86 ft3/lbm of vapor vs. 5.91 ft3/lbm at 20°F (-6°C). This is greater than four times the volume of vapor produced for a similar sized evaporator. The additional vapor must be accounted for in the coil circuiting, header sizing, valve sizing and refrigerant pipe sizing.

Tube Geometry.Evaporators are circuited using two arrangements with airflow: vertical and cross-feed. Each type of circuiting arrangement has benefits and limitations. For the sake of this article, the information is being presented to provide an understanding that these arrangements exist and to recognize their existence when evaluating a coil.

In a vertically fed coil, the refrigerant flows from the bottom to the top, or the top to the bottom (figure 1). Cross-over piping is utilized to balance refrigerant pressure drop and circuit load. Vertically fed coils in liquid-recirculated and flooded applications do not utilize feed-metering devices (orifices) to meter refrigerant feed to each circuit, so the circuit is unrestricted.

A cross-fed coil balances the refrigerant flows either parallel or counter to the flow of air. Coil pressure drop is controlled by the number and lengths of circuits (figure 2). A cross-fed liquid-recirculated coil will utilize an orifice to meter the refrigerant feed to each circuit and will graduate the size of the orifice to compensate for the static head exerted by liquid within the coil header. The size of the orifices will vary based upon feed temperature and capacity.

The evaporator coil tube design as well as the tube pitch affects performance. Coils are supplied with inline or staggered design and vary by manufacturer. Tube pitch is measured by the distance between the tube centers in a vertical and horizontal plane. Coil tube design and pitch vary by manufacturer and tube diameter; it should be understood that in the process of troubleshooting an evaporator, there are different types of coil designs.

In the next issue, this article will continue with refrigerant feed arrangements, airflow over the coil, and evaporator troubleshooting.