ASHRAE Journal - February 2023 - 40

TECHNICAL FEATURE
The safety factors range from 1.5 to 3 based on trap
application.
My own studies using time-based heat transfer
equations determined that many of these safety factors
and tables are conservative. My studies also show that
in some cases the safety factors and table values may
underestimate flows, especially for drip traps on steam
mains smaller than 8 in. (200 DN). Peak condensate
flows during warm-up appear to be influenced by the
surface area exposed to steam, in addition to pipe mass
and temperature differences. Modeling should also
consider the rate of air removal in determining steam
temperatures with respect to time. Additional research
using time-based heat transfer would improve the
accuracy of condensate flow estimates.
Pipe Sizing Criteria
For years designers have depended on the tables in
the ASHRAE Handbook-Fundamentals and in ASHRAE's
Fundamentals of Steam System Design to size drainage
piping. In looking into the history of these tables it
appears that they have not changed since the 1950s.
Since that time, the assumptions behind these tables
may have been lost. During my review on how the
tables in the ASHRAE Handbook-Fundamentals Chapter
22 were generated, I have concluded the following:
Table 34: Return Main and Riser Capacities for Low Pressure Systems
" Low pressure " is not clearly defined. The table
values do not appear to correspond to other tables.
Table 34 also lists smaller pipe sizes for risers. Per my
previous discussion, reducing vertical risers should
not be considered. My conclusion is that this table is
not useful and should be dropped from the handbook.
ASHRAE Fundamentals of Steam System Design does not
include this table.
Table 35: Vented Dry Condensate Return for Gravity Flow Based on
Manning Equation
Notes indicate that a safety factor of 3, Schedule 40
pipe, and 180°F (82.2°C) condensate were used in
generating the table. This table could be more useful
and improved by:
* Using no safety factor. Too many safety factors can
oversize piping. Designers need to understand where
safety factors are applied.
* Use 50% liquid height to pipe diameter (y/di)
40
ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2023
ratio and the correct terms for the Manning equation.
Correct equation values are:
* Hydraulic radius rh (ft) = Wetted flow area A
(ft2)/Wetted perimeter P (ft)
* A (ft2) = 1/8 * (q -sin q) * di
2
* P (ft) = q * di/2
* q = angle in radians formed by the pipe radius
(r= di/2) and the liquid height y or q = 2 * arccos((r - y)/r)
* Velocity V (fps) = C / n * rh
0.667 * S0.5 (Manning
equation)
* C = 1.49 (for I-P units)
* n = pipe roughness factor, usually 0.012. Consider
using 0.018 for corroded pipe.
* S = slope percentage, vertical drop/horizontal
run (ft/ft)
* Flow Q (pph) = rf * V * A * 3,600 s/h
* rf = density of the liquid condensate (lb/ft3)
* Use the density of condensate at 212°F (100°C),
which is 2% lower than at 180°F (82°C).
* Use slopes that are more common today (1/4 in. per
10 ft (0.21%), 1/2 in./10 ft (0.42%), and 1 in. /10 ft, 0.83%).
Slopes greater than 1% may not be as useful as more
table values for slopes less than 1%.
* Add tables for Schedule 80 pipe. Schedule 80 pipe
flows are less due to smaller inside pipe diameter.
Table 36: Vented Wet Condensate Return for Gravity Flow Based on
Darcy-Weisbach Equation
Notes state that 180°F (82°C) condensate, and Schedule
40 pipe were used. Based on table values it appears that
a safety factor of 3 was also used. Note that fully flooded
lines only occur in limited locations. In using Table 36,
designers will need to first determine the available feet
of head per 100 ft, which is equal to the Table 1 values
using the pipe pitch of the upstream horizontal piping,
and then adjust for the actual equivalent length of the
flooded line. Table 36 could be improved by:
* Using no safety factor.
* Use the density of condensate at 212°F (100°C).
* Use pressure drops in units of psi/100 ft. Values
should correspond to actual pipe slopes used (less than
1 psi/100 ft).
* Add tables for Schedule 80 pipe.
Table 37: Flow Rate for Dry-Closed Returns
Flow rates appear to be based on the liquid condensate
and flash steam occurring at the same velocity
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