Handbook of air conditioning and refrigeration / Shan K

7.12 CHAPTER SEVEN
Pipe Joints
Copper tubes are also categorized as hard and soft copper. Soft pipes should be used in applications
for which the pipe will be bent in the field. Table 7.2 lists the dimensions of copper tubes.
Thermoplastic plastic pipes are the most widely used plastic pipes in air conditioning. They are
manufactured with dimensions that match steel pipe dimensions. The advantages of plastic pipes include
resistance to corrosion, scaling, and the growth of algae and fungi. Plastic pipes have smooth
surfaces and negligible age allowance. Age allowance is the allowance for corrosion and scaling for
plastic pipes during their service life. Most plastic pipes are low in cost, especially compared with
corrosion-resistant metal tubes.
The disadvantages of plastic pipes include the fact that their pressure ratings decrease rapidly
when the water temperature rises above 100°F (37.8°C). PVC pipes are weaker than metal pipes
and must usually be thicker than steel pipes if the same working pressure is to be maintained. Plastic
pipes may experience expansion and contraction during temperature changes that is 4 times
greater than that of steel. Precautions must be taken to protect plastic pipes from external damage
and to account for its behavior during fire. Some local codes do not permit the use of some or allplastic
pipes. It is necessary to check with local authorities.
Steel pipes of small diameter (2 in. or 50 mm less) threaded through cast-iron fittings are the most
widely used type of pipe joint. For steel pipes of diameter 2 in. (50 mm) and more, welded joints,
TABLE 7.3 Maximum Allowable Pressures at Corresponding Temperatures
System
Maximum
allowable
Fitting
Temperature,
pressure at
temperature,
Application
Recirculating water
Pipe material Weight Joint type Class Material °F psig
2 in. and smaller Steel (CW) Standard Thread 125 Cast iron 250 125
Copper, hard Type L 95-5 solder — Wrought copper 250 150
PVC Sch. 80 Solvent Sch. 80 PVC 75 350
CPVC Sch. 80 Solvent Sch. 80 CPVC 150 150
PB SDR-11 Heat fusion — PB 160 115
Insert crimp — Metal 160 115
2.5–12 in. A53 B ERW Standard Weld Standard Wrought steel 250 400
steel Flange 150 Wrought steel 250 250
Flange 125 Cast iron 250 175
Flange 250 Cast iron 250 400
Groove — MI or ductile iron 230 300
Refrigerant
PB SDR-11 Heat fusion PB 160 115
Copper, hard
A53 B SML
Type L or K Braze — Wrought copper — —
steel Standard Weld Wrought steel — —
Note: Maximum allowable working pressures have been derated in this table. Higher system pressures can be used for lower temperatures and
smaller pipe sizes. Pipe, fittings, joints, and valves must all be considered.
Note: A53 ASTM Standard A53
PVC Polyvinyl chloride
CPVC Chlorinated polyvinyl chloride
PB Polybutylene
Source: Abridged with permission from ASHRAE Handbook 1988, Equipment.