Service Manual

33
Example
1
While on a service call, after checking for proper indoor and
outdoor airflow, Tom finds the following pressures and
temperatures at the service valves of a R- 410A air
conditioner:
S Liquid line pressure = 340 psig
S Liquid line temperature = 97F
S Suction line pressure = 125 psig
S Suction line temperature = 70F
Using a R- 410A PT chart, the subcooling is determined to
be 8F, which is within 3 of the 10F listed on the rating
plate. Tom believes the charge is correct. He calculates the
superheat to be approximately 27F superheat. The
apparently high superheat has Tom concerned.
Tom uses the Pseudo Evaporator Superheat method to
check the TXV performance. The system is a 3- ton R- 410A
air conditioner with 75 feet equivalent length of 3/4” suction
line. Based on Table 5, the system has approximately
3- psig pressure drop in the vapor line. Per the instructions,
he takes the suction line temperature at the outlet of the
evaporator and finds it to be 53F. Tom adds 3 psig to the
125- psig suction pressure at the outdoor unit to get 128
psig evaporator pressure. The saturated pressure of 128
equates to 44F. Tom calculates the evaporator superheat
to be (53F- 44F=)9F. The TXV appears to be
operating properly.
NOTE: The additional superheat at the compressor is due
principally to heat gain in the 75 feet of suction line with a
minor contribution by the pressure drop. Because the
suction line of the lineset was the same size as the vapor
service valve fitting and less than 80 feet, Tom could have
ignored the pressure drop in the suction line and obtained
the evaporator superheat by using the vapor service valve
pressure of 125 psig (saturated temperature = 43F) and the
evaporator outlet temperature of 53F. The evaporator
superheat is calculated to be (53F–43F=)10F.
Table 6—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
Example 2
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Fwith30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. 30 and Table
6, 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 5, 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.