Long Lineset Instructions

Page 19
Btuh lost = 0.01 x (2.55) x 180,000
Btuh lost = 4590
Capacity loss for the line selected is approximately 2.55%.
LOW STAGE CAPACITY
1-5/8 inch line appears to be appropriate for this system
operating at full (15 ton) capacity. Now we must check the
low stage (9 ton) capacity to determine if 1-5/8 inch line is
appropriate.
OEM two stage units operate at approximately 60%
capacity when operating on low stage:
15 tons x 0.6 = 9 tons (if an engineering data sheet is
available use the actual capacities rather than this
approximation).
Nine tons capacity when used with 1-5/8 inch pipe (figure 8)
indicates a velocity of 1500 fpm. This is sufficient to return
oil to the compressor and meets the requirement of
maintaining at least 1200 fpm in vertical risers.
When comparing high stage to low stage performance in
this case we find that a single 1-5/8 inch suction riser may
be used and double suction risers are not required.
EXAMPLE 1: SUCTION SIZING VARIABLE CAPACITY
– HOT GAS BYPASS
There are two basic types of hot gas bypass kits. The most
desirable is the type that feeds the hot gas from the
compressor discharge line to a side tap on the distributor in
the evaporator coil. When installed in this manner, full flow
of suction gas is maintained in the suction line and suction
piping should follow standard procedures as outlined in the
previous sections.
The second type of hot gas bypass is installed and
connected within the condensing unit. This is known as a
run-around hot gas bypass in that hot compressor
discharge gas and liquid from the liquid line are circuited to
the hot gas bypass valve and directly into the suction line.
This method reduces flow through the evaporator and
suction line. Special handling of suction risers is required.
Refer to OEM instructions for proper installation of the hot
gas bypass kit.
When to Use Double Suction Risers
If a condensing unit can unload more than 50% either by a
hot gas bypass (run-around cycle) or other mechanical
means, double suction risers may be required.
If the condensing unit unloads less than 50%, suction lines
can be generally 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 not required.
In general, double suction risers are required any time the
minimum load on the compressor does not create sufficient
velocity in vertical suction risers to return oil to the
compressor. Double suction risers are also generally
required any time the pressure drop or velocity in a single
suction riser is excessive.
How Double Suction Risers Work
Figure 14 shows a typical double suction riser installation. A
trap is installed between the two risers as shown. During
partial load operation (figure 15) 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
occurs, 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.
The second suction riser must enter the main suction line
from the top to avoid oil draining down the second riser
during a partial load. See figure 15.
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 CAPACITY
Figure 14. Typical Double Suction Riser
INDOOR
COIL
TYPICAL SIZING
DOUBLE SUCTION RISERS
5 TON
2 STAGE
OUTDOOR
UNIT
3 TON (3/4IN.)
2 TON (5/8IN.)
3/4IN. - SIZING BASED ON MINIMUM LOAD.
5/8IN. - SIZING BASED ON REMAINING LOAD.
Based on Figure 5
Figure 15. Typical Sizing - Double Suction Riser
Sample Calculation
Given: Ten ton condensing unit with hot gas bypass
(runaround type) or mechanical unloaders capable of 65%
unloading. Matched evaporator is located below
condensing unit. Piping will require 57 (linear) feet of pipe
(figure 16). Construction without double suction risers will
only require 2 ells.
Find:
1. Select tube sizes for horizontal runs and risers (figure
6).
2. Determine if double suction risers are needed.
3. Size double suction riser for proper system
performance.
Solution: Size each segment based on the tons of
refrigerant 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 part load capacity is 6.5 tons.