Long Lineset Instructions

Page 20
From figure 6, select a pipe size for full load capacity. 1-3/8
inch outside diameter. pipe with 3.3 psi drop per 100 feet
and 2400 fpm velocity is selected. Now, by using figure 6,
find the velocity for the selected pipe size at part load
capacity. The part load velocity is approximately 850 fpm.
850 fpm is sufficient to return oil in horizontal runs but not in
vertical risers.
EVAPORATOR
10 TON
CONDENSING
UNIT WITH HOT
GAS
BYPASS
15 FT.
40 FT.
2 FT.
DOUBLE SUCTION RISER EXAMPLE
DOUBLE SUCTION
RISER
A
B
Figure 16. Double Suction Riser
If we tried to size this system by simply reducing the riser
size to 1-1/8 inch, we would find the velocity in the riser to be
excessive (3800 fpm) when the system is operating at full
capacity. As a result of these obstacles, this system will
require construction of double suction risers. Construction
of double suction riser will require five ells and two tees total
for a system.
Size small riser — Riser carrying smallest part of
load
The unit produces 3.5 tons capacity at minimum load.
Select from figure 6 a 7/8 inch outside diameter. line
(smallest line with acceptable velocity). When operating at
3.5 tons capacity, this line will operate at 2500 fpm and will
produce 6 psi drop per 100 feet.
Size larger riser — Riser carrying largest part of load
The larger line carries 6.5 tons capacity at full load. Select
from figure 6 a 1-1/8 inch outside diameter. line (smallest
line with acceptable velocity). When operating at 6.5 tons
capacity, this line will operate at 2500 fpm and will produce
4.5 psi drop per 100 feet.
Putting the Segments Together
Next, we must determine if the line sizes we selected will
result in satisfactory pressure drop between the
condensing unit and the evaporator.
Start by finding the total equivalent feet of the large (B) riser.
15 feet of pipe, plus two tees (branch side of tee at 4.5
equivalent feet each), plus four elbows (1.8 equivalent
feet), plus one tee (line side of tee at 1.0 equivalent feet) =
21.0 equivalent feet length.
Slope
Figure 17. Double Suction Riser Construction
Use the total equivalent length of each riser to compute the
pressure drop of each riser. For the large (B) riser, 1-1/8
inch outside diameter. suction line with 6.5 tons capacity
has 4.5 psi drop per 100 feet. When we multiply 4.5/100 by
31.2 equivalent feet, we see that the total friction loss is 1.4
psi.
For the small (A) riser, 7/8 inch outside diameter. suction
line with 3.5 tons capacity has six psi drop per 100 feet.
When we multiply 6/100 by 21 equivalent feet, we see that
the total friction loss is 1.26 psi.
The total pressure drop for the riser is equal to the average
of the pressure drop in both risers:
1.4 (B riser pressure drop) + 1.26 (A riser pressure drop) =
2.66
2.66  2 = 1.33 (average pressure drop through A and B
risers).
Find the pressure drop for the horizontal run of pipe. 1-3/8
inch pipe at 10 tons of capacity has 3.3 psi drop per 100
feet. When we multiply 3.3/100 by 61 equivalent feet, we
see that the total friction loss is 2.01 psi.
The pressure drop through the risers is added to the
pressure drop through the horizontal run to find the total
pressure drop for the system:
2.01 psi (horizontal run) plus 1.33 psi (avg. riser) = 3.34 psi.
Use figure 6 to calculate the pressure drop in 25 feet of
1-3/8 inch line. When we multiply 3.3/100 by 25 feet, we see
that the friction loss is 0.825 psi.