ASHRAE Journal - February 2023 - 42

TECHNICAL FEATURE
TABLE 2A Slip method pipe sizing for 15 psig supply, 0 psig return, (3.9% flash), 0.723 in./10 ft slope(1/4 psig/100 ft), Schedule 40
pipe.
NOMINAL PIPE
SIZE (in. ID)
0.5
0.75
1
1.25
1.5
2
2.5
3
4
6
8
TOTAL CONDENSATE FLOW
(pph)
196
413
782
1,614
2,423
4,680
7,469
13,217
26,962
78,837
161,575
FLASH STEAM FLOW
(pph)
8
16
30
63
94
182
291
514
1,049
3,067
6,285
MAX. FLASH STEAM
VELOCITY (fpm)
1,756
2,111
2,468
2,943
3,248
3,805
4,257
4,878
5,778
7,445
8,812
ASHRAE TABLE 37
FLOWS (pph)
210
450
860
1,800
2,720
5,320
8,520
15,200
31,300
-
-
TABLE 2B Slip method pipe sizing for 15 psig supply, 0 psig return, (3.9% flash), 0.723 in./10 ft slope
(1/4 psig/100 ft), extra heavy pipe.
NOMINAL PIPE
SIZE (in. ID)
0.5
0.75
1
1.25
1.5
2
2.5
3
4
6
8
TOTAL CONDENSATE FLOW
(pph)
138
312
613
1,317
2,011
3,955
6,363
11,400
23,590
68,914
143,418
(slip velocity ratio = 1), Schedule 40
pipe (not stated, but assumed) and
no safety factor. Table 37 could be
improved by:
* List table values by the percent
of flash steam instead of supply and
return pressures, adding more increments
of flash steam percentage.
* Use pressure drop rates consistent
with gravity flow per Table 1, and
use more common slopes.
* Consider updating table
values using the slip velocity
method, which would reduce the
risk of slug flow.
42
ASHRAE JOURNAL ashrae.o rg
FLASH STEAM FLOW
(pph)
5
12
24
51
78
154
248
443
918
2,681
5,579
MAX. FLASH STEAM
VELOCITY (fpm)
1,613
1,971
2,326
2,802
3,104
3,654
4,096
4,709
5,598
7,213
8,569
SCH. 80 VS SCH. 40
FLOWS
71%
76%
78%
82%
83%
85%
85%
86%
87%
87%
89%
* Don't limit table values to
less than 7,000 fpm (36 m/s) for
higher flash steam at higher steam
percentages. Flows should be based
on pressure drop rates.
* Add tables for Schedule 80 pipe.
Table 37 flow rates can be calculated
for the supply and return pressures
listed or for other slopes/pressure
differentials and flash steam
rates using the following steps. Steps
also apply to the slip velocity method
when using the void fraction ag in
lieu of the volume fraction Vc.
* Step 1. Determine the
F E B R U A RY 2023
SLIP METHOD VS.
TABLE 37
93%
92%
91%
90%
89%
88%
88%
87%
86%
-
-
maximum steam
velocity for pressure
drop rates consistent
with pipe slope from
2021 ASHRAE Handbook-
Fundamentals Fig. 18 or
other similar tables.
* Step 2. Determine
the percent of flash
steam based on the
table supply and return
pressures and equation
25 in Chapter 22 of the
2021 ASHRAE Handbook-
Fundamentals.
* Step 3. Determine
the volume fraction of flash steam
based on the table supply and return
pressures and equation 26 in Chapter
22 of the 2021 ASHRAE Handbook-
Fundamentals or use the void fraction
for the slip velocity method.
* Step 4. Determine the flash
steam flow based on the maximum
velocity in Step 1 and the steam flow
area based on actual inside pipe
diameter and steam volume/void
fraction in Step 3.
* Step 5. Calculate the total
liquid and flash steam flow rate by
dividing the steam flow in Step 4 by
the percent of flash steam in Step 2.
Designers will often use a heavier
walled pipe (Schedule 80) to
increase the useful life of the piping
system. Using Schedule 80 pipe
reduces flow areas and reduces
flow capacity between 11% and 29%.
All assumptions should be stated
in any table updates. Tables 2A and
2B list the changes in flow (pounds
per hour, pph) between Schedule
40 and Schedule 80 (extra heavy)
pipe. Note that Tables 2A and 2B use
the slip velocity method with a slip
velocity ratio of 7.07 and a void fraction
of 89.7%, which results in flow
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