Long Lineset Instructions

Page 18
EXAMPLE 3: SUCTION LINE SIZING PROCEDURE
Given: 7-1/2 ton condensing unit with evaporator lower
than condenser, with 112 feet of piping and four elbows.
The piping includes 20 feet of vertical lift and 92 feet of
horizontal run.
Figure 12. Indoor Coil Below Condenser
Find: Select tube size from figure 6.
Solution: 1-1/8 inch outside diameter. line with 6 psi per
100 feet pressure drop and 2900 fpm velocity is selected.
Now, calculate pressure drop due to friction to determine if
this is a good selection.
From table 8, four ells at 1.8 equivalent feet each = 7.2
equivalent feet. When added to the 112 feet of pipe, the
total equivalent feet becomes 119.2 feet (round up to 120
feet).
When we multiply 6/100 by 120 equivalent feet, we see that
the total friction loss is 7.2 psi.
Use figure 6 to calculate the pressure drop in 25 feet of
1-1/8 inch line. When we multiply 6/100 by 25 feet, we see
that the friction loss is 1.5 psi.
The capacity lost in the total equivalent length of the
refrigerant line (using figures 8 and 10) = 1% x (7.2 – 1.5) x
90,000.
Btuh lost = 0.01 x (5.7) x 90,000
Btuh lost = 5130
Capacity loss for the line selected is approximately 5.7%.
The preceding calculation shows that this is a workable
system but will result in a loss of capacity and efficiency.
EXAMPLE 4: ALTERNATIVE PIPE SIZE
Using the same (7-1/2 ton) example, this time select 1-3/8
inch outside diameter. line. 1-3/8 inch outside diameter. line
with 2 psi per 100 feet pressure drop has 1760 fpm velocity.
Now calculate pressure drop due to friction loss to
determine if this is a better selection.
From figure 5, four elbows at 2.4 equivalent feet each = 9.6
equivalent feet. When added to the 112 feet of pipe, the
total equivalent feet becomes 121.6 feet (round up to 122
feet).
When we multiply 2/100 by 122 equivalent feet, we see that
the total friction loss is 2.4 psi.
Use figure 6 to calculate the pressure drop in 25 feet of
1-3/8 inch line. When we multiply 2/100 by 25 feet, we see
that the friction loss is 0.5 psi.
The capacity lost in the total equivalent length of the
refrigerant line (using figures 8 and 10) = 1% x (2.4 – 0.5) x
90,000.
Btuh lost = 0.01 x (1.9) x 90,000
Btuh lost = 1710
Capacity loss for the line selected is approximately 1.9%.
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
compressor.
EXAMPLE 5: SUCTION SIZING WITH VARIABLE
CAPACITY — TWO STAGE CONDENSING UNIT
Some variable capacity installations may use a single
suction riser for minimum load conditions without serious
penalty at design load. OEM units with two-stage
compressors have approximately 67% capacity at low
stage and normally do not require double suction risers
Given: 15 ton two-stage condensing unit with a single 15
ton (dual circuit) evaporator.
High Stage Capacity = 15 tons and,
Low Stage Capacity = 9 tons.
The system is plumbed with the evaporator 60 feet below
the condensing unit and 40 feet horizontally away from the
condensing unit. A trap is plumbed at the bottom of the
riser. The trap is composed of 90 ells.
Find: Determine if single suction riser is suitable or if double
suction riser must be used.
TWO STAGE SIZING EXAMPLE
Figure 13. Two Speed Sizing Example
Solution: Select the line size based on full unit capacity (15
tons) 1-5/8 inch outside diameter. line with 3 psi per 100
feet pressure drop and 2600 fpm velocity (at full capacity) is
selected. Then determine the equivalent length of the
segment to calculate the pressure drop.
60 feet of pipe (vertical), plus 40 feet of pipe (horizontal),
plus four 90 elbows (2.8 equivalent feet each) = 111.2
equivalent feet length (round to 111).
From figure 6, 1-5/8 inch outside diameter. suction line with
15 tons capacity has three psi drop per 100 feet. When we
multiply 3/100 by 111 equivalent feet, we see that the total
friction loss is 3.3 psi.
Use table 8 to calculate the pressure drop in 25 feet of 1-5/8
inch line. When we multiply 3/100 by 25 feet, we see that the
friction loss is 0.75 psi.
The capacity lost in the total equivalent length of the
refrigerant line (using figures 8 and 10) = 1% x (3.3 – 0.75) x
180,000.