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.
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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.
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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
ft
3/lbm of vapor vs. 5.91 ft
3/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.
