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TABLE 3
Equivalent Length in Feet of Straight Pipe
for Valves and Fittings
Line
Size
Globe
Angle
O.D.
Valve
Valve
in.
3/8
7
4
1/2
9
5
5/8
12
6
3/4
14
7
7/8
15
8
1 1/8
22
12
1 3/8
28
15
1 5/8
35
17
2 1/8
45
22
2 5/8
51
26
Long radius elbow. Multiply factor by 1.5 for short radius elbow equiva
lent length.
Lennox equipment above five tons in capacity typically op
erates at a saturated condensing temperature of 125°F
(280psi per table 2). Lennox equipment is designed to hold
a charge allowing 10°F subcooling at 95°F ambient. The
condensing temperature and the subcooling are used to
calculate the maximum allowable pressure drop as de
tailed below.
NOTE - 95°F ambient is an arbitrary temperature chosen to
represent typical summer operating conditions used to cal
culate maximum allowable pressure drop. This tempera
ture (and the corresponding subcooling) may vary with re
gional climate.
Example -- Calculating maximum allowable pressure
drop: Find the maximum allowable liquid line pressure
drop of a unit operating at 10°F subcooling and 125°F (280
psi) condensing temperature. Subtract 10°F subcooling
temperature from 125°F condensing temperature to equal
115°F subcooled liquid temperature (245 psi / point at
which flash gas will begin to form). Subtract 245 psi sub
cooled pressure from 280 psi condensing pressure to find a
maximum allowable pressure drop of 35 psi.
To calculate actual pressure drop in the liquid line, calcu
late pressure drop due to friction and pressure drop due to
vertical lift and add the two.
Pressure drop due to friction in the pipe or other devices
must all be considered. Pressure drop ratings for different
pipe sizes are given in figure 10. Pressure drop ratings of
field installed devices are typically supplied by the
manufacturer.
Pressure drop due to vertical lift (1/2 pound per foot) is typ
ically high and can be a limiting factor in the design of the
system.
The liquid refrigerant pressure must be sufficient to produce
the required flow through the expansion device. Liquid re
frigerant (free of flash gas) should be delivered to the expan
sion valve at a minimum of 175psi to ensure the 100 psi nec
essary to produce full refrigerant flow at the rated capacity.
90°
45°
Long*
Long*
Tee
Tee
Radius
Radius
Line
Branch
Elbow
Elbow
0.8
0.3
0.5
1.5
0.9
0.4
0.6
2.0
1.0
0.5
0.8
2.5
1.3
0.6
0.9
3.0
1.5
0.7
1.0
3.5
1.8
0.9
1.5
4.5
2.4
1.2
1.8
6.0
2.8
1.4
2.0
7.0
3.9
1.8
3.0
10
4.6
2.2
3.5
12
Example -- Liquid Line Pipe Sizing
Given: 10 ton heat pump unit on ground level with a 10 ton
indoor coil on the third level above ground and a total of 96
linear feet of piping. Unit is charged with 10°F subcooling at
125°F condensing temperature (280 psi HCFC 22 liquid).
Refer to figure 11.
Find: Select tube size from figure 10.
LIQUID LINE SIZING EXAMPLE
3 FT.
10 TON
HEAT PUMP
UNIT
TWO 90° LONG RADIUS ELBOWS @ 5/8in. O.D. = 1 ft. EQUIV. FT. EA.
LINEAR LENGTH + EQUIVALENT LENGTH OF FITTINGS
TOTAL EQUIVALENT LENGTH = 98 ft.
TOTAL FRICTION LOSSES =
TOTAL FRICTION LOSSES + LIFT LOSSES + FILTER/DRIER
FILTER DROP = 1 psi (by manufacturer)
LIFT LOSSES = 40 ft. x 1/2psi per foot = 20psi
TOTAL PRESSURE DROP = 20 psi + 4.17 psi + 1 psi = 25.17 psi
ANSWER; 5/8 IN. O.D. COPPER TUBING CAN BE USED. PRESSURE LOSS DOES NOT
EXCEED MAXIMUM ALLOWABLE PRESSURE DROP (6°F TO 7°F SUBCOOLING WILL
BE AVAILABLE AT THE EXPANSION VALVE) AND VELOCITY IS ACCEPTABLE.
Figure 10 illustrates the relationship between liquid line siz
ing, pressure drop per 100 feet, velocity range and ton
nage. Enter figure 10 from the left and extend to the right to
the smallest tube size that will not exceed 300 fpm velocity.
Solution: For a 10 ton system, 5/8 inch O.D. line with 4.25
psi per 100 feet drop (per figure 10) is selected. Now, calcu
late pressure drop due to friction and liquid lift to determine if
this is a good selection.
The total friction drop for the application will include 96 feet
of 5/8 inch O.D. pipe plus 1 equivalent foot per elbow (two
elbows) to equal 98 equivalent feet.
In a 10 ton system, expect 4.25 psi drop per 100 feet of
5/8 inch O.D. copper (per figure 10). Multiply 4.25/100
by 98 equivalent feet to calculate the total friction loss of
4.17 psi.
Add the pressure drop caused by vertical lift. When
HCFC 22 refrigerant is used, there is 1/2 psi pressure drop
per foot of vertical lift. In this application, which has a
40 foot (12 m) vertical lift, add a 20 psi pressure drop
because of the lift.
Page 12
53 FT.
FILTER/DRIER
GIVEN: 10 TON EVAPORATOR
10 TON HEAT PUMP UNIT
WITH 10°F SUBCOOLING at 125°F
40 FT.
LENGTH OF LINE -- 96 FT.
FIND:
LIQUID LINE SIZE
SOLUTION: PRESSURE DROP
CANNOT EXCEED 35 psi.
SELECT A PROPOSED TUBING
SIZE: 5/8in. COPPER
TOTAL EQUIVALENT LENGTH =
4.25 psi
x 98 ft. = 4.17psi
100 ft.
TOTAL PRESSURE DROP=
FIGURE 11
10 TON
INDOOR
COIL
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