Long Lineset Instructions
Page 15
EXAMPLE 5
Given: HCFC-22, 10-ton (single stage) condensing unit on
ground level with a 10 ton evaporator on the third level
above ground and a total of 96 feet (linear) of piping. Unit is
charged with 10F subcooling at 125F condensing
temperature (280 psi HCFC-22 liquid). See figure 8.
10 TON
CONDENSING
UNIT
10 TON
EVAPORATOR
53 FEET
40 FEET
FILTER/DRIER
3 FEET
Given: 10−Ton Evaporator
Find: Liquid Line Size
Solution: Pressure drop
cannot exceed 35 psi.
Tubing Size: 5/8 inch copper
Two 90º long radius elbows @ 5/8 inch O.D. = 1 foot equivalent feet
each.
Total equivalent length = linear length + equivalent length of fittings.
Total equivalent length = 98 feet.
Total friction losses =
4.25 psi
100 feet
x 98 feet = 4.17 psi.
Total pressure drop = Total friction losses + lift losses + filter/drier.
Filter drop = 1 psi (by manufacturer)
Lift losses = 40 feet x ½ psi per foot = 20 psi.
Total pressure drop – 20 psi + 4.17 psi + 1 psi = 25.17 psi.
Answer: 5/8 inch O. D. copper tubing can be used. Pressure loss
does not exceed maximum allowable pressure drop (6ºF to 7ºF
subcooling will be available at the expansion valve and velocity is
acceptable.
Figure 8. Liquid Line Sizing Example
Find: Select line size from figure 4.
Figure 4 illustrates the relationship between liquid line
sizing, pressure drop per 100 feet, velocity range and
tonnage. When using liquid line solenoid valves, velocities
should not exceed 300 fpm to avoid liquid hammer when
closing. Enter figure 4 from 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 outside diameter
line with 4.25 psi per 100 feet drop is selected. Now,
calculate pressure drop due to friction and liquid lift to
determine if this is a good selection.
The pressure lost to two elbows must be added to the
equation. The total friction drop for 96 feet of 5/8 inch
outside diameter. pipe plus (from table 8) 1 equivalent foot
per elbow = 98 equivalent feet.
Figure 4 shows that, in a 10 ton system, we can expect 4.25
psi drop per 100 feet of 5/8 inch outside diameter. copper.
When we multiply 4.25/100 by 98 equivalent feet, we see
that the total friction loss is 4.17 psi.
Now, we must add the pressure drop for vertical lift.
HCFC-22 pressure drop is ½ psi per foot of vertical lift.
When multiplied by 40 feet vertical lift we find that pressure
drop due to lift = 20 psi.
Finally, we have added a filter drier to the liquid line which
has 1 psi drop (this number provided by manufacturer).
Add the three components of pressure drop together to find
that the total pressure drop in this 5/8 inch line = 25.17 psi.
Now, by comparing 25.17 psi to our maximum allowable
pressure drop we find that this setup falls well within the
acceptable range. The 5/8 inch line, therefore, is a good
selection because it is well below the maximum allowable
pressure drop, is in a satisfactory velocity range, uses
minimum refrigerant and provides sufficient pressure at the
expansion valve.
ALTERNATIVE PIPE SIZE
Suppose ¾ inch outside diameter. line with 1.6 psi drop per
100 feet had been selected. The total equivalent length is
computed by adding the linear length (96 feet.) plus the
equivalent length of the fittings (two 90 elbows at 1.25 feet
each). The total equivalent length is 98.5 feet. The total
friction drop would have been 1.6/100 multiplied by 98.5
feet = 1.57 psi. When the pressure drop due to lift (20 psi)
and the filter drier (one psi) are added we find that the total
pressure drop for ¾ inch line = 22.57 psi.
Yet, ¾ inch line is a less desirable choice. Why?
The difference in pressure drop between 5/8 inch line and
3/4 inch line is only 2.35 psi. But, the larger line adds 5.5
pounds. more refrigerant into the system (see table 10 on
page 9). The risk of refrigerant slugging is increased and
the smaller line will be less costly. The smaller line should
be used.