Understanding Evaporator Coils: Types, Function & Troubleshooting Tips
TL;DR – Quick Summary
Evaporators are one of the four main components in refrigeration and AC systems
Their primary function is to absorb heat energy from the refrigerated space or medium
Dry Expansion is the most common type of evaporator feed
Five major types: Plate-Surface, Finned Coil, Bare Tube, Chillers, and Plate Heat Exchangers
Each type has specific applications – from residential AC to industrial freezers
Low-temperature applications often require defrost methods to remove frost buildup
Fin spacing increases as operating temperature decreases (to prevent frost blockage)
Regular maintenance and cleaning are essential for optimal system efficiency
The “ABC principle” (Airflow Before Charge) is key for troubleshooting evaporator issues
The Heat Absorber
One of the Four main Components of a Refrigeration and Air Conditioning System is the Evaporator. The other three Components are the Condenser, Compressor, and Metering Device.
The Evaporator’s purpose is to absorb Heat Energy from the Refrigerated/Conditioned Space, or Medium.
We’ll first look at the operation of an Evaporator, with foundational principles of its function. Afterwards, we will dive into different Evaporator types, and gain further understanding of their operation and specific application.
How Do Evaporators Work?
All types of Evaporator coils are supplied from their system’s Metering Device. The Metering Device is fed with liquid, but the rapid pressure drop through this valve causes “Flash Gas”. This causes partial Evaporation of the liquid refrigerant, as it becomes a Saturated Mixture at the Evaporator’s inlet. With this said, the great majority of refrigerant flowing into any correctly operating Evaporator is liquid. When I teach Evaporators, I say this Refrigerant is “Effectively Liquid”.
Types of Refrigerant Feeds
The most common “Evaporator Feed” type is Dry Expansion. In this type of feed, the intent is to have the last droplet of liquid refrigerant evaporate and pick up Superheat before leaving the Evaporator coil. This ensures that no liquid refrigerant will return to the Compressor through the Suction Line.
Most Refrigeration applications like fruit require High Relative Humidity in their Refrigerated Space. This ensures the product stays in good condition, and prevents it from aging prematurely or drying out. Refrigeration Evaporators end up “leaving” Latent Heat in the air, so that not much of it is removed from the space compared to an Air Conditioning Application.
Note: Both Air Conditioning and Refrigeration applications also leverage Evaporator Fan Speed Control as a method to adjust the space’s Relative Humidity. However, Evaporator Fan Speed Control is more pertinent to Air Conditioning Applications.
Defrost Methods and Timing
Depending on the Temperature Range at which an Evaporator Operates, it may require a Defrost (the coil in the image below sure does!) With this said, this only applies to Refrigeration Evaporators which serve Low Temperature Coolers, or Freezers.
“Frost” can be removed from the Evaporator in a normal Defrost Cycle. Ice, however, cannot. If there is ice on a coil, something is not functioning properly. Common causes are:
Not running enough defrosts in a day
Defrosts being too short
Refrigerated Space’s door left open to a warm ambient, allowing moisture infiltration
Defrosts can occur once, or many times a day depending on the application. Common defrost methods include Electric Defrost, Hot Gas Defrost, “Kool Gas” Defrost, Off Cycle Defrost, and Off Time Defrost.
Types of Evaporator Coils
Plate-Surface Evaporator
This Evaporator Type is composed of two thin pieces of Sheet Metal. They are each “Stamped” in a mechanical press to create their refrigerant flow path from inlet to outlet. The two plates are then joined together to create the refrigerant’s passage. These also get called “Stamped Evaporators”.
These Evaporators are useful for their low profile, and versatility for certain applications. Their most common applications are acting as the freezer portion of a Mini-Fridge (see above image), a Reach-in Chest Freezer, or a sandwich/prep counter. If you’ve ever been to Cold Stone Creamery or another place that makes ice cream sandwiches, the prep counter is refrigerated by a Plate-Surface Evaporator!
Finned Coil Evaporator
This is by far the most common Evaporator type. It is everything from the coil in a Home Furnace/AC, to a large Commercial Refrigeration Cooler Coil as pictured above. They will commonly utilize many different refrigerant circuits with the coil to reduce pressure drop.
The refrigerant transfers its heat energy to the “Coil”, then the Coil transfers its heat energy to the “Fins”. The Fins then transfer their heat energy to the surrounding air. The Fins are a method to increase this Evaporator Coil’s Surface Area, which increases its ability to transfer heat.
The space between the Fins increases as the temperature application reduces. This is to allow adequate space for airflow between the Fins and ensures that frost does not form fully between the Fins in low-temperature applications. A car’s radiator is the most common example of a Finned Heat Exchanger.
Finned Evaporators are commonly equipped with fans to improve the rate at which heat is transferred. The speed of the air generated by the fan is dependent on the application. An Industrial Freezer with its Evaporator Coils 80 feet in the air will have a very high Air Velocity. A Supermarket’s Flower Cooler may have a very low Fan Speed, or no fan at all (Gravity/Convection Coil).
Bare Tube Evaporators
Similar to Finned Evaporators, but without the fins, Bare Tube Evaporators have their applications. They can be used to cool a Refrigerated Space where frost formation may be an issue, and/or very Low Air Velocities are required. They can also be submerged in a liquid such as Glycol to cool this Secondary Refrigerant for use in a Food Process Application.
Note: With similar Process Applications to Bare Tube, a “Shell and Coil” type Evaporator can alternately be used. It is instead a “Coil” of (usually) copper with the refrigerant flowing inside, submerged in a “Shell” filled with the Secondary Refrigerant.
Chillers
The Evaporator Coil of a Traditional “Chiller” (see above image) is of Shell and Tube construction. The “Shell” is “Flooded” with liquid refrigerant: filled with liquid from the bottom, and sucked off the top as vapour. The “Tubes” traveling inside the Shell house the Secondary Refrigerant, which circulates in/out of the End Bell of the Chiller’s Shell.
Most commonly, Water is the Secondary Refrigerant and is used for Air Conditioning Applications as this Water is circulated throughout a building where cooling is required. This is then piped to a “Fan Coil Unit”, to have air blown over it for cooling. The Secondary Refrigerant can also be used to cool Process Applications in the form of again Water, or Glycol (Ethylene or Propylene).
Note: a “Chiller” can also be used to describe the Shell and Tube Evaporator which cools Brine (salt water) or Glycol to form the surface of Ice Rinks.
Plate and Frame Heat Exchangers
The main benefit of Plate and Frame Heat Exchangers is having a large number of channels where there is heat exchange between the Primary and Secondary Refrigerant. This creates a great ability to transfer heat, which is why they are so efficient.
Serviceable Plate Heat Exchangers are formed of various Stamped plates with Gaskets between them, and then sandwiched together by mechanical means. In the image above, you can see nuts that are tightened onto Threaded Rods, against the “Frame”. You cannot see the plates here, as they are covered by a sheet of metal for protection from the weather.
Note: Plate and Frame Heat Exchangers may also be used to cool the Secondary Refrigerant for Ice Rinks (Brine or Glycol).
Note: a very similar Evaporator type is the Brazed Plate Heat Exchanger. Their stamped plates however are brazed together and are non-serviceable.
Maintaining Evaporator Efficiency
Proper maintenance of evaporator coils is essential for system efficiency. Dirty evaporator coils can lead to less heat transfer, which reduces the system’s overall performance. Regular cleaning and inspection should be part of any preventative maintenance program, especially in commercial refrigeration applications where food safety depends on consistent cooling.
When troubleshooting evaporator issues, always remember the ABC principle: Airflow Before Charge. Check that fans are operating correctly and coils are clean before suspecting refrigerant charge problems. For systems with persistent issues, consider whether non-condensable gases might be affecting system performance.
Summary
Being knowledgeable of the operating principles for Evaporator Coils is most useful when working on HVAC/R Systems. With many different Evaporator Types, continuing to gain knowledge on specific applications leads to more efficient Maintenance and Troubleshooting of Refrigeration/AC Systems.
By understanding the various evaporator designs and their applications, technicians can better diagnose system issues and recommend appropriate solutions to maintain optimal performance for customers’ specific cooling needs.
