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General Refrigerant Piping System Information
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Under normal operating conditions, the vapor pipe temperature of
a Duct Free System can vary as much as 280°F. With this large
variance in pipe temperature, the designer must consider pipe
expansion and contraction to avoid pipe and fitting fatigue failures.
Refrigerant pipe along with the insulation jacket form a cohesive
unit that expands and contracts together. During system operation,
thermal heat transfer occurs between the pipe and the surrounding
insulation.
If the pipe is mounted in free air space, no natural restriction to
movement is present if mounting clamps are properly spaced and
installed. When the refrigerant pipe is mounted underground in a
utility duct stacked among other pipes, natural restriction to linear
movement is present. In extreme cases, the restrictive force of
surface friction between insulating jackets could become so great
that natural expansion ceases and the pipe is “fixed” in place. In this
situation, opposing force caused by change in refrigerant fluid/vapor
temperature can lead to pipe/fitting stress failure.
The refrigerant pipe support system must be engineered to allow
free expansion to occur. When a segment of pipe is mounted
between two fixed points, provisions must be provided to allow pipe
expansion to naturally occur. The most common method is the
inclusion of expansion Loop or U-bends. Each segment of pipe has
a natural fixed point where no movement occurs. This fixed point is
located at the center point of the segment assuming the entire pipe
is insulated in a similar fashion. The natural fixed point of the pipe
segment is typically where the expansion Loop or U-bend must be.
Linear pipe expansion can be calculated using the following formula:
1. From the Table on the next page find the row corresponding with
the actual length of the straight pipe segment.
2. Estimate the minimum and maximum temperature of the pipe.
In the column showing the minimum pipe temperature, look up the
anticipated expansion distance. Do the same for the maximum
pipe temperature.
3. Calculate the difference in the two expansion distance values.
The result will be the anticipated change in pipe length.
General Example:
A system is installed and the design shows that there is a 100 foot
straight segment of tubing between an indoor unit and the outdoor
unit. In heating, this pipe transports hot gas vapor to the indoor units
at 120 °F. In cooling, the same tube is a suction line returning refrig-
erant vapor to the outdoor unit at 40 °F. Look up the copper tubing
expansion at each temperature and calculate the difference.
Vapor Line
Transporting Hot Vapor: 100 ft. pipe at 120 °F = 1.40 in.
Transporting Suction Vapor: 100 ft. pipe at 40 °F = 0.40 in.
Anticipated Change in Length: 1.40 in. – 0.40 in. = 1.00 in.
Liquid Line
The liquid temperature remains relatively the same temperature; only
the direction of flow will reverse. Therefore, no significant change in
length of the liquid line is anticipated.
When creating an expansion joint, the joint height must be a
minimum of two times the joint width. Although different types of
expansion arrangements are available, the data for correctly sizing
an Expansion Loop is provided in the Table on the next page. Use
soft copper with long radius bends on longer runs or long radius
elbows for shorter pipe segments. Using the anticipated linear
expansion (LE) distance calculated, look up the Expansion Loop or
U-bend minimum design dimensions. If other types of expansion
joints are chosen, design per ASTM B-88 Standards.
LE = C x L x (T
r
– T
a
) x 12
LE
=
Anticipated linear tubing expansion (in.)
C
=
Constant (For copper = 9.2 x 10
-6
in./in.°F)
L
=
Length of pipe (ft.)
T
R
=
Refrigerant pipe temperature (°F)
T
a
=
Ambient air temperature (°F)
12
=
Inches to feet conversion (12 in./ft.)
Copper Expansion and Contraction
See table on next page for precalculated anticipated expansion for various pipe sizes and lengths of refrigerant tubing.
To find the anticipated expansion value:
1. From the table below, find the row corresponding with the actual feet of the straight pipe segment.
2. Estimate the minimum and maximum temperature of the pipe.
3. In the column showing the minimum pipe temperature, look up the anticipated expansion distance corresponding to the segment length.
Do the same for the maximum pipe temperature.
4. Calculate the difference in the two expansion distance values. The result will be the change in pipe length.
COPPER EXPANSION AND CONTRACTION
