ASHRAE Journal - July 2019 - 46

COLUMN ENGINEER'S NOTEBOOK

damage from subsequent excavation can cause the insulation jacket to be penetrated, leading to external corrosion and eventual pipe failure. An underground distribution piping that will not experience corrosion and will
provide a long service life (>50 years) is one of the most
important design and installation objectives.

PEX Piping

TABLE 1 Carbon Steel and PEX-a physical and mechanical properties.
MATERIAL

CARBON STEEL

PEX-A

Modulus of Elasticity (E)

27.9 × 106 psi

180°F: 87,000 to 130,000 psi

Pipe Roughness Value (e)

0.00015 ft

0.000002 ft

Coefficient of Thermal Expansion 6.5 × 10-6 in/in·°F

212°F: 2.72 × 10-4 in/in·°F

Thermal Conductivity (K)

25 Btu/h·ft·°F

2.86 Btu/h·ft·°F

Engineering Properties
Crosslinked polyethylene (PEX) is a flexible plastic
pressure pipe with over 40 years of successful use in the
European market in hot water applications, including
extensive testing for durability and material performance. It was first introduced in North America in 1984
and is widely used for plumbing, water service, fire
protection, hydronic heating and cooling (particularly
radiant floors), snow and ice melting and ground source
geothermal piping systems. PEX began being used in the
1990s for underground applications.
PEX is a polyethylene material, which has undergone
a change in molecular structure using a chemical or a
physical process whereby the polymer chains are chemically linked. The primary reason for cross-linking PE is
to increase the material's elevated temperature internal
pressure performance. Crosslinking also improves the
pipe's environmental-stress crack resistance, resistance
to slow crack growth, chemical and corrosion resistance,
toughness, and abrasion resistance. Finally, crosslinking
makes PEX a "semi-thermoset" polymer, providing good
long-term stability.
In general, 140°F (60°C) is the maximum service temperature for thermoplastic high-density polyethylene
(HDPE) pressure pipe applications. With PEX, however,
the in-service temperature can be raised to at least 180°F
(82°C) and sometimes as high as 200°F (93°C), depending on the starting density, degree of crosslinking, type
of crosslinking and operating pressure. PEX pipes are
also tested to ensure short-term exposure to a temperature of 210°F (99°C) is also tolerated.
PEX-a tubing with an oxygen diffusion barrier is most
commonly used for underground heating hot water
applications. The oxygen diffusion barrier limits oxygen
permeation through the tubing wall in hydronic heating
applications, which prevents corrosion of ferrous metal
parts in the heating system. PEX-a tubing is produced
using the Peroxide (or, "Engel") method, named after
the inventor Thomas Engel. During the manufacturing
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TABLE 2 PEX elastic modulus versus temperature.
TEMPERATURE (°F)

ELASTIC
MODULUS (PSI)

LONG-TERM
MODULUS (PSI)

COEFFICIENT OF THERMAL
EXPANSION, IN/IN·°F

220

6,815

889

2.94E-04

200

8,700

1,056

2.39E-04

180

11,165

1,260

1.89E-04

160

14,065

1,513

1.61E-04

140

19,720

1,827

1.28E-04

120

27,115

2,221

1.11E-04

100

36,540

3,612

1.00E-04

61,915

3,612

7.94E-06

process, free radicals are created when HDPE polymer is
melted and cross-links between molecules occur at temperatures that exceed the decomposition temperature
of the polymer. Table 1 shows physical and mechanical
properties comparison of PEX-a and carbon steel. The
PEX-a coefficient of thermal expansion is substantially
higher than steel pipe.
PEX is not subject to galvanic action and does not rust
or corrode. PE has no nutritional value and will not support microbiological induced corrosion and fouling as
seen in carbon steel. PEX also has an extremely smooth
surface compared to carbon steel, which provides minimal opportunity for the precipitation of minerals such
as calcium carbonate onto the interior surface. This
smoothness also reduces pressure drop and associated
pump energy.
The modulus of elasticity is related to the Poisson ratio
for plastic materials. It is the ratio of normal stress to
corresponding strain. A material with a low modulus of
elasticity is more flexible and expands with less force
than a material with a higher modulus of elasticity. For
PEX pipes, the Poisson ratio is 0.4. Table 2 shows the
effect of temperature on short-term and long-term
modulus of elasticity, and the coefficient of thermal

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ASHRAE Journal - July 2019

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