Lennox HS29-072 Installation Instructions Manual page 15

Example −− Suction Line Pipe Sizing
Given: 7 1/2 ton condensing unit with evaporator lower
than condenser. Application includes 82 linear feet of pip
ing and 4 ells. There is a 20 foot vertical lift and 62 feet of
horizontal run. Refer to figure 15.
Find: Select tube size from figure 14.
Solution: Select a 1 1/8 inch O.D. line with 6 psig per 100
feet pressure drop and 2900 fpm velocity. Now, calculate
pressure drop due to the friction to determine if this is a
good selection.
Example Indoor Coil Below Condenser
Suction Line
Suction Riser
Indoor Coil
2 FT.
Figure 15
From table 3, four ells at 1.8 equivalent feet each equals
7.2 equivalent feet. When added to the 82 feet of pipe, the
total equivalent feet becomes 89.2 feet (round up to 90
feet).
Multiply 6/100 by 90 equivalent feet to calculate total friction
loss of 5.4 psig.
Use figure 14 to calculate the pressure drop in 25 feet of 1 1/8
inch line. Multiply 6/100 by 25 feet to calculate friction loss of
1.5 psig. This loss has already been included in the capacity,
so it should be subtracted from the total.
The Btuh capacity lost in the total equivalent length" of the
refrigerant line (using figures 12 and 14) equals 1% X (5.4 −
1.5) X 90,000 = 3510.
Btuh lost = 0.01 x (3.9) x 90,000 = 3510
The capacity loss for the line selected is approximately
3.9%.
The preceding calculation shows that this is a workable
system, but the line will lose capacity and efficiency.
Alternative Sizing: Using the same (7 1/2 ton) example,
this time select 1 3/8 inch O.D. line. A 1 3/8 inch O.D. line
with 2 psig per 100 feet pressure drop has 1760 fpm veloc
ity. Now calculate the pressure drop due to friction loss to
determine if this is a better selection.
60 FT.
20 FT.
Oil Trap
From figure 3, four ells at 2.4 equivalent feet each equals
9.6 equivalent feet. When added to the 82 feet of pipe, the
total equivalent feet becomes 91.6 feet (round up to 92
feet).
Multiply 2/100 by 92 equivalent feet to calculate total fric
tion loss of 1.8 psig.
Use figure 14 to calculate the pressure drop in 25 feet of 1 3/8
inch line. Multiply 2/100 by 25 feet to calculate friction loss of
0.5 psig. This loss has already been included in the capacity,
so it should be subtracted from the total.
The Btuh capacity lost in the total equivalent length" of the
refrigerant line (using figures 12 and 14) equals 1% X (1.8 −
0.5) X 90,000 = 1170.
Btuh lost = 0.01 x (1.3) x 90,000 = 1170
The capacity loss for the line selected is approximately
1.3%.
The conditions in this example will allow either 1 1/8 inch or
1 3/8 inch suction line to be used, since capacity loss is
minimized and velocity is sufficient to return oil to the com
pressor.
Double Suction Risers
If the condensing unit is equipped to run with a capacity re
duction of less than 50 percent, suction lines can generally
be sized in accordance with the previous sections. If the
suction velocity is high enough to entrain oil when the unit is
operating at reduced capacity, double suction risers are
generally unnecessary.
However, suction−type hot gas bypass kits reduce the
unit's capacity and suction line refrigerant velocity, and
therefore, double suction risers are required in these
cases.
In general, double suction risers are necessary any time the
minimum load on the compressor does not create sufficient
velocity in vertical suction risers to return oil to the compres
sor. Double suction risers are also generally required any
time the pressure drop or velocity in a single suction riser is
excessive.
Figure 16 shows a simplified example of a double suction
riser installation. A trap is installed between the two risers
as shown. During partial load operation when gas velocity
is not sufficient to return oil through both risers, the trap
gradually fills with oil until the second riser is sealed off.
When this happens, the vapor travels up the first riser only.
With only the first riser being used, there is enough velocity
to carry the oil. This trap must be close coupled to limit the
oil holding capacity to a minimum. Otherwise, the trap
could accumulate enough oil on a partial load to seriously
lower the compressor crankcase oil level.
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