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Finally, consider the impact of the filter drier to the liquid,
line which has a 1 psi pressure 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 equals
25.17 psi which is well within our 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 Sizing: Suppose 3/4 inch O.D. line with 1.6
psi drop per 100 feet had been selected. The total equiva
lent length is equal to the linear length (96 feet) plus the
equivalent length of the fittings (from table 3, two 90° ells
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 = 1.57 psi. When the pressure drop due to lift (20
psi) and the filter drier (1 psi) are added we find that the
total pressure drop for 3/4 inch line equals 22.57 psi.
Though the 3/4 inch line provides a lower pressure drop,
the larger diameter pipe will require more refrigerant
which will increase the risk of refrigerant slugging. In addi
tion, the smaller line will be less costly. The smaller line
should be used.
Vapor Line Function and Design
The vapor line returns refrigerant vapor and oil from the
evaporator to the compressor. Vapor line design is critical.
The design must minimize pressure loss in order to
achieve maximum unit efficiency and provide adequate oil
return to the compressor under all conditions.
Because oil separates from refrigerant in the evaporator,
the vapor velocity must be adequate to sweep the oil along
the pipe. Horizontal vapor lines require a minimum of 800
fpm velocity for oil entrainment. In order to ensure oil en
trainment, vapor line risers require a minimum velocity of
1200 fpm (1500 fpm is preferred) regardless of the length
of the riser.
Figure 14 illustrates the relationship between vapor line
sizing, pressure drop, velocity and cooling capacity. Use
this chart to determine vapor line pressure drop and veloc
ity. As the pipe size increases, so does the capacity re
quired to ensure oil entrainment.
Vertical lift has no significant effect on system capacity.
However, systems lose approximately 1 percent of capac
ity for every pound of pressure drop due to friction in the va
por line. In order to calculate capacity loss, first estimate
pressure drop in the total equivalent length of the piping run
(refer to figure 14). Capacity ratings given in the Lennox
Engineering Handbook include the loss for a 25 feet refrig
erant line. Therefore, subtract the pressure loss of 25 feet
of piping from the total calculated for your particular ap
plication.
DETERMINING VAPOR LINE CAPACITY LOSS
IF PRESSURE DROP IS KNOWN
25 ft.
Line
OUTDOOR
UNIT
Once Pressure Drop Is Found:
Btuh lost = 1% x (Total Press. Drop minus 25 ft.) x rated capacity
When an indoor coil is located above or on the same level
as the heat pump, the vapor line must rise to the top of the
evaporator. See figure 13. This helps prevent liquid from
migrating to the compressor during the off cycle. Traps
should also be installed at the bottom of all vertical risers for
migration protection in the off cycle.
VAPOR LINE PIPING
Indoor Coil Above or On Same Level with Outdoor Unit
TO TOP LEVEL
OUTDOOR
UNIT
OUTDOOR
VAPOR LINE
UNIT
If equipment is on same level, inverted trap should still be used
in order to prevent liquid migration to compressor during off cycle.
Horizontal vapor lines should be level or slightly sloped to
ward the heat pump unit. Pipe must avoid dips or low spots
that can collect oil. For this reason, hard copper should be
used, 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 linear length of each line and determine the number of
ells, tees, valves, driers etc. Add equivalent length of fit
tings (table 3) to linear length of pipe to get total equivalent
length used in determining friction loss. Again, refer to
manufacturer's data for pressure drop information on ac
cessory components. The resultant pressure drop must be
considered.
Page 13
Total Pressure Drop
For Equivalent Length
Total Pressure Drop
Minus Press. Drop in
25 ft. of Line
INDOOR
FIGURE 12
VAPOR LINE
INDOOR
COIL
RAISE PIPE
OF COIL
INDOOR
COIL
INSTALL TRAPS AT
BOTTOM OF
EACH RISER
FIGURE 13
UNIT
TRAP
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