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Table 7—R--22 System Suction Pressure Drop
Nominal
Size
(Btuh)
Line
OD
(in.)
Pressure
Drop
(psi/100 ft)
Suction
Velocity
Fpm
R---22 Suction Line Pressure Drop (psig)
Total Equivalent Line Length (ft)
20 50 80 100 125 150 175 200 225 250
18000
18000
18000
18000
5/8 13.6 2563 3 7 11 14 17 20 24 27 31 34
5/8 4.0 1539 1 2 3 4 5 6 7 8 9 10
3/4 1.5 1025 0 1 1 1 2 2 3 3 3 4
7/8 0.8 769 0 0 1 1 1 1 1 2 2 2
24000
24000
24000
5/8 6.7 2052 1 3 5 7 8 10 12 13 15 17
3/4 2.5 1367 1 1 2 3 3 4 4 5 6 6
7/8 1.3 1026 0 1 1 1 2 2 2 3 3 3
30000
30000
30000
5/8 10.1 2565 2 5 8 10 13 15 18 20 23 25
3/4 3.8 1708 1 2 3 4 5 6 7 8 9 9
7/8 1.9 1282 0 1 2 2 2 3 3 4 4 5
36000
36000
36000
3/4 5.3 2050 1 3 4 5 7 8 9 11 12 13
7/8 2.6 1538 1 1 2 3 3 4 5 5 6 7
11/8 0.7 902 0 0 1 1 1 1 1 1 2 2
42000
42000
42000
3/4 7.0 2392 1 3 6 7 9 10 12 14 16 17
7/8 3.5 1795 1 2 3 3 4 5 6 7 8 9
11/8 1.0 1053 0 0 1 1 1 1 2 2 2 2
48000
48000
48000
3/4 8.9 2733 2 4 7 9 11 13 16 18 20 22
7/8 4.4 2051 1 2 4 4 6 7 8 9 10 11
11/8 1.2 1203 0 1 1 1 2 2 2 2 3 3
60000
60000
60000
7/8 6.7 2564 1 3 5 7 8 10 12 13 15 17
11/8 1.8 1504 0 1 1 2 2 3 3 4 4 5
13/8 0.7 987 0 0 1 1 1 1 1 1 2 2
Line set application not recommended
Example2
Jason is servicing a 5--ton R--22 air conditioner with 7/8” suction
line. As part of his basic inspection he believes he has normal
airflow because the air filters are clean, ductwork appears to be
properly sized and in good shape and the evaporator coil is clean.
He then checks the superheat and subcooling at the outdoor unit
service valves. Taking pressures and temperatures he finds the
following:
S Liquid line pressure = 260 psig
S Liquid line temperature = 110°F
S Suction line pressure = 60 psig
S Suction line temperature = 65°F
Using an R--22 PT relationship, Jason calculates the subcooling to
be approximately 10°F with 30°F superheat. Because the
subcooling is correct but the superheat appears to be high, he is
concerned and decides to perform the Pseudo Evaporator
Superheat check.
Examining the lineset, Jason finds approximately 145 ft of suction
line with 4 long radius elbows. Per Fig. 33 and Table 7, each
fitting has an equivalent length of 1.4 ft. The total equivalent
length of the suction line is (145’ + (4 * 1.4’) ≈) 150 ft. Based on
Table 9, Jason determines there should be 10--psig pressure--drop
in the suction line.
Jason now takes the suction line temperature at the outlet of the
evaporator and obtains 51°F. Per the instructions, Jason adds the
10--psig pressure--drop to the 60--psig pressure at the outdoor unit
to get 70--psig at the evaporator. Saturated pressure of 70--psig
equates to approximately 41°F. Jason determines the Evaporator
superheat to be (51°F--41°F=)10°F. Jason concludes the TXV is
functioning properly.
NOTE: In this situation, both the pressure drop and the heat gain
in the suction line are significant contributions to the superheat at
the service valve. The pressure drop contributes approximately
7°F superheat and the heat gain in the suction line contributes
13°F.
Fig. 33 – Pseudo Evaporator Superheat Pressure and Temperature Measurement Locations