Lennox HS29-072 Installation Instructions Manual page 13

The total friction drop for the application will include 96 feet
(29 m) 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 a 4.25 psig (29 kPa) 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 psig (28 kPa).
Add the pressure drop for vertical lift. HCFC 22 pressure
drop is 1/2 psig per foot of vertical lift. In this application
which has a 40 foot (12 m) vertical lift, we find that the pres
sure drop due to lift equals 20 psig (138 kPa).
Finally, add a filter drier to the liquid line which has a 1 psig
pressure drop (this number provided by manufacturer).
Add the three components of the pressure drop together to
find that the total pressure drop in this 5/8 inch line equals
25.17 psig (172 kPa) which is well within the acceptable
range. The 5/8 inch line, therefore, is a good selection be
cause it is well below the maximum allowable pressure
drop, is in a satisfactory velocity range, uses minimum re
frigerant, and provides sufficient pressure at the expansion
valve.
Alternative Sizing: Suppose you selected a 3/4 inch O.D.
line with 1.6 psig drop per 100 feet. Compute the total
equivalent length by adding the length [96 feet (29 m)], plus
the equivalent length of the fittings [from table 3, two 90°
ells at 1.25 feet .381 m) each]. The total equivalent length is
98.5 (30 M) feet. The total friction drop would have
been 1.6/100 multiplied by 98.5 = 1.57 psig. When you
add the pressure drop due to lift (20 psig) and the filter
drier (1 psig) the total pressure drop for 3/4 inch line
equals 22.57 psig.
Though the 3/4 inch line provides a lower pressure drop,
the larger diameter pipe will require more refrigerant; this
larger diameter will increase the risk of refrigerant slugging.
In addition, because the smaller line will be less costly, use
it instead of the larger line.
Suction Line Function and Design
The suction line returns refrigerant vapor and oil from the
evaporator to the compressor. Therefore, the design of the
suction line is critical. The design must minimize the pres
sure loss in order for the unit to operate at maximum effi
ciency. The design must also provide adequate oil return to
the compressor under any conditions.
Because the oil separates from the refrigerant in the evap
orator, the suction velocity must be adequate to sweep the
oil along the pipe. Horizontal suction lines require a mini
mum of 800 fpm velocity for oil entrainment. In order to
ensure oil entrainment, suction risers require a mini
mum velocity of 1200 fpm (1500 fpm is preferred) re
gardless of the length of the riser.
Figure 14 illustrates the relationship between the suction
line sizing, pressure drop, velocity, and cooling capacity.
Use this chart to determine suction line pressure drop and
velocity. As the pipe size increases, so does the velocity re
quired to ensure oil entrainment.
Vertical lift has no significant effect on system capacity.
However, systems lose approximately 1% of capacity for
every pound of pressure drop due to friction in the suction
line. In order to calculate capacity loss, you must first esti
mate pressure drop in the total equivalent length of the pip
ing run (refer to figure 14). Capacity ratings include the loss
for a 25 foot refrigerant line. Therefore, subtract the pres
sure loss of 25 feet of piping from the total that you calcu
lated for your particular application. See figure 12.
Determining Suction Line Capacity Loss
If Pressure Drop Is Know
Outdoor
Unit
Once Pressure Drop Is Found:
Btuh lost = 1% x (Total Press. Drop minus 25 ft.) x rated capacity
When an evaporator is located above or on the same level
as the condensing unit, the suction line must rise to the top
of the evaporator. This helps prevent liquid from migrating
to the compressor during the off cycle. Install traps at the
bottom of all vertical risers for migration protection during
the off cycle. See figure 13.
Indoor Coil Above or On Same Level with Outdoor Unit
Outdoor
Unit
Outdoor
Unit
If equipment is on same level, the inverted trap should still be used
in order to prevent liquid migration to compressor during off cycle.
Horizontal suction lines should be level or slightly sloped
toward the condensing unit. The pipe must avoid any dips
or low spots that can collect oil. For this reason, use hard
copper, especially on long horizontal runs.
As with liquid line sizing, begin by making a sketch of the
layout complete with fittings, driers, valves etc. Measure
the length of each line and determine the number of ells,
tees, valves, driers etc. Add the equivalent length of fittings
(table 3) to length of pipe to get the total equivalent length
which is used to determine friction loss. Again, refer to
manufacturer's data for pressure drop information on ac
cessory components. You must consider the resulting
pressure drop.
Page 13
n
Total Pressure Drop
For Equivalent Length
25 ft.
Line
Total Pressure Drop
Minus Press. Drop in
25 ft. of Line
Figure 12
Suction Pipin
g
Vapor Line
Raise Pipe
To Top Level Of
Coil
Install Traps At
Bottom Of
Vapor Line
Each Rise
Figure 13
Indoor
Unit
Indoor
Coil
Trap
Indoor
Coil
r