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ASHRAE
Method for
Sizing Return
Lines
FLOW RATE (lbs./hr.) FOR DRY RETURN LINES
Flow Rate
Supply Pressure= 5 psig
(lbs./hr.)
Return Pressure=0 psig
p/L, psi/100 ft.
Pipe
Size
1
1
⁄
⁄
1
16
4
(in.)
1
⁄
240
520
1100
2
3
⁄
510
1120
2400
4
1
1000
2150
4540
1
1
⁄
2100
4500
9500
4
1
1
⁄
3170
6780
14,200
2
2
6240
13,300
2
1
⁄
10,000
21,300
2
3
18,000
38,000
4
37,200
78,000
6
110,500
a
8
228,600
a
Flow Rate
Supply Pressure= 100 psig
(lbs./hr.)
Return Pressure=0 psig
p/L, psi/100 ft.
Pipe
Size
1
1
⁄
⁄
1
16
4
(in.)
1
⁄
28
62
133
2
3
⁄
62
134
290
4
1
120
260
544
1
1
⁄
250
540
1130
4
1
1
⁄
380
810
1700
2
2
750
1590
2
1
⁄
1200
2550
2
3
2160
4550
4
4460
9340
6
13,200
a
8
27,400
a
a
For these sizes and pressure losses, the velocity is above 7000 fpm.
Select another combination of sizes and pressure loss.
42
Condensate that collects ahead of a steam
trap is approximately at saturation
temperature and corresponds to the
operating pressure. As the condensate
(normally above 212°F.) drains into the
return line, it must flash to reach saturation
temperature at atmospheric pressure. The
excess Btu's are released in the form of
flash steam in the return lines. The return
lines must be sized to handle the volume of
steam and condensate at reasonable
velocities to minimize any backpressure.
The volume of steam is normally several
times the volume of condensate and is
generally maintained at less than 7,000 feet
per minute.
The following tables are for horizontal return
lines draining to a return system. Return
1
⁄
in. per 10 ft. of
lines should pitch 1
2
horizontal run. Select the return line size
Flow Rate
Supply Pressure= 15 psig
(lbs./hr.)
Return Pressure=0 psig
p/L, psi/100 ft.
Pipe
Size
1
1
⁄
⁄
16
4
(in.)
1
⁄
95
210
2
3
⁄
210
450
4
1
400
860
1
1
⁄
840
1800
4
1
1
⁄
1270
2720
2
a
2
2500
5320
a
2
1
⁄
4030
8520
2
a
3
7200
15,200
a
4
14,900
31,300
a
6
44,300
a
8
91,700
Flow Rate
Supply Pressure= 150 psig
(lbs./hr.)
Return Pressure=0 psig
p/L, psi/100 ft.
Pipe
Size
1
1
⁄
⁄
16
4
(in.)
1
⁄
23
51
2
3
⁄
50
110
4
1
100
210
1
1
⁄
200
440
4
1
1
⁄
310
660
2
a
2
610
1300
a
2
1
⁄
980
2100
2
a
3
1760
3710
a
4
3640
7630
a
6
10,800
a
8
22,400
based on the steam operating pressure and
the allowable
100 and 150 psig steam for either a vented
return system or a 15 psig pressurized return
system such as a flash tank, deaerator or closed
return system.
Example: A condensate return system has a
steam supply at 100 psig and the return line is
at 0 psig and not vented. The return line is
horizontal and must have a capacity of 2500
lbs./hr. What size pipe is required?
Solution: Since the system will be throttling
non-subcooled condensate from 100 psig to 0
psig there will be flash steam and the system
will be a dry-closed return with horizontal pipe.
Select a pressure drop of
use a 2
Flow Rate
Supply Pressure= 30 psig
(lbs./hr.)
Return Pressure=0 psig
Pipe
Size
1
1
⁄
16
(in.)
450
1
⁄
60
2
950
3
⁄
130
4
1820
1
250
3800
1
1
⁄
520
4
5700
1
1
⁄
780
2
a
2
1540
a
2
1
⁄
2480
2
a
3
4440
a
4
9180
a
a
6
27,300
a
a
8
56,400
Flow Rate
Supply Pressure= 100 psig
(lbs./hr.)
Return Pressure=15 psig
Pipe
Size
1
1
⁄
16
(in.)
109
1
⁄
56
2
230
3
⁄
120
4
450
1
240
930
1
1
⁄
500
4
1400
1
1
⁄
750
2
a
2
1470
a
2
1
⁄
2370
2
a
3
4230
a
4
8730
a
a
6
25,900
a
a
8
53,400 110,300
Reprinted by permission of the American Society of Heating, Refrigeration and Air-Conditioning
Engineers, Atlanta, Georgia, from the 1989 ASHRAE Handbook-Fundamentals
p/L, psi/100 ft. Selections for
1
⁄
4
1
⁄
in. pipe for this system.
2
Flow Rate
(lbs./hr.)
p/L, psi/100 ft.
Pipe
Size
1
⁄
1
4
(in.)
130
274
1
⁄
2
280
590
3
⁄
4
530
1120
1
1110
2340
1
1
⁄
4
1670
3510
1
1
⁄
2
3270
a
2
5250
a
2
1
⁄
2
9360
a
3
19,200
a
4
a
a
6
a
a
8
Flow Rate
(lbs./hr.)
p/L, psi/100 ft.
Pipe
Size
1
⁄
1
4
(in.)
120
1100
1
⁄
2
260
2400
3
⁄
4
500
4540
1
1060
9500
1
1
⁄
4
1600
14,200
1
1
⁄
2
3100
a
2
5000
a
2
1
⁄
2
8860
a
3
18,200
a
4
53,600
a
6
a
8
psi/100 ft. and
Supply Pressure= 50 psig
Return Pressure=0 psig
p/L, psi/100 ft.
1
1
⁄
⁄
1
16
4
42
92
200
91
200
420
180
380
800
370
800
1680
560
1200
2520
1110
2350
a
1780
3780
a
3190
6730
a
6660
13,800
a
19,600
a
a
40,500
a
a
Supply Pressure= 150 psig
Return Pressure=15 psig
p/L, psi/100 ft.
1
1
⁄
⁄
1
16
4
43
93
200
93
200
420
180
390
800
380
800
1680
570
1210
2500
1120
2350
4900
1800
3780
7800
3200
6710
a
6620
13,800
a
19,600
40,600
a
40,500
83,600
a
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