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PIPING DESIGN GUIDE
Copper Expansion and Contraction
Under normal operating conditions, the vapor pipe temperature of
a Multi V Mini system can vary as much as 280°F. With this large
variance in pipe temperature combined with a potential straight run
pipe of up to 492 feet and a segment length between fittings of up to
131 feet, the designer must consider pipe expansion and contraction
to avoid potential 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 the 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 expan-
sion to naturally occur. The most common method is the inclusion
of expansion Loop or U-bends. See Figure 43 on page 72. Each
segment of pipe has a natural fixed point where no movement oc-
curs. This fixed point is located at the center point of the segment
assuming the entire pipe is insulated in a similar fashion. The natu-
ral fixed point of the pipe segment is typically where the expansion
Loop or U-bend should be placed. Linear pipe expansion can be
calculated using the following formula:
LE = C x L x (T
Where
LE = Anticipated linear tubing expansion (in.)
C
= Constant (For copper = 9.2 x 10
L
= Length of pipe (ft.)
T
= Refrigerant pipe temperature (°F)
r
T
= Ambient air temperature (°F)
a
12 = Inches to feet conversion (12 in/ft.)
70
SYSTEM ENGINEERING
|
– T
) x 12
r
a
in./in.°F)
-6
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Refer to Table 28 and 29 for anticipated expansion distances for
copper pipe.
1. From Table 28, find the row corresponding with the actual length
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. Do the same for the maxi-
mum pipe temperature.
4. Calculate the difference in the two expansion distance values.
The resultant will be the anticipated change in pipe length.
For example,
A Multi V Mini heat pump system is installed and the design shows
that there is a 260 feet straight segment of tubing between a Y-
Branch and an indoor unit. In Heating mode, this pipe transports hot
gas vapor to the indoor units at 120°F. In Cooling mode, the same
tube is a suction line returning refrigerant 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: 260 ft. pipe at 120°F = 3.64 in.
Transporting Suction Vapor: 260 ft. pipe at 40°F = 1.04 in.
Anticipated change in length: 3.64 in. – 1.04 in. = 2.60 in.
Liquid Line
The liquid pipe temperature will not vary significantly. Only the direc-
tion of flow will change.
Creating an Expansion Joint
When creating an expansion joint, the joint height should be a
minimum of two times the joint width. Although different types of ex-
pansion arrangements are available, the data for correctly sizing an
expansion loop is provided. Use soft copper with long radius bends
on longer runs or long radius elbows for shorter pipe segments. Use
the anticipated linear expansion (LE) distance calculated, and look
up the Loop or U-bend minimum design dimensions. If you choose
other types of expansion joints, design per ASTM B-88 standards.
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