Page 16
The second suction riser must enter the main suction line
from the top to avoid oil draining down the second riser dur
ing a partial load.
Figure 16
Evaporator
Condensing
Unit With
Hot Gas
Bypass
Horizontal Suction Line Is Sized
To Handle Total Load.
In The Vertical Portion Of The Line,
A Smaller Line Is Sized To Handle
The Reduced Capacity.
Typical Double Suction Riser
Double Suction
Riser
The Remaining Line Is Sized To
Handle The Remaining Capacit
y
Figure 17 illustrates a typical double−suction riser
construction.
Figure 17
Double Suction Riser Constructio
n
Method A"
90
_
Street Ell
Method
Method B"
UBEND
METHOD
A"
B"
Vapor Line
From Air Handler
(Slope to Compressor)
Reducing Tee
Ubend
Vapor Line
To Compressor
A"
B"
Vapor Line
From Air Handler
(Slope to Compressor)
Reducing
Tee
90° Street
Elbow
90° Street Elbow
Reducing Tee
Street Ell
Vapor Line
To Compressor
Figure 18
Evaporator
10 Ton
Condensing Unit
With Hot Gas
Bypass
15 FT.
40 FT.
2 FT.
Double Suction Riser Exampl
e
Double Suction
Riser
A
B
Example −− Double Suction Riser Sizing
Given:
10ton condensing unit with hot gas bypass (run
around type). Matched evaporator is located below the
condensing unit. Piping will require 57 (linear) feet of pipe
(figure 18). Construction without double suction risers will
only require 2 ells.
Find:
Select tube sizes for horizontal runs and risers (fig
ure 14). Determine if double suction risers are necessary.
Size the double suction riser for proper system perfor
mance.
Solution:
Size each segment based on the tons of refrig
erant that will flow in the segment.
Full load capacity = 10 tons. Minimum load capacity is 35%
of 10 tons = 3.5 tons. The difference between full capacity
and partial load capacity is 6.5 tons.
From figure 14, select a pipe size for full load capacity. A
13/8 inch O.D. pipe with 3.3 psig drop per 100 feet and
2400 fpm velocity has been selected. Now, by using figure
14, 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.
If you try to size this system by simply reducing the riser
size to 11/8 inch, you 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 the construction of double suction risers. Construc
tion 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. Se
lect from figure 14 a 7/8 inch O.D. line (smallest line with
acceptable velocity). When operating at 3.5 tons capacity,
this line will operate at 2500 fpm and will produce 6 psig
drop per 100 feet.
