- 366 -

SIZI

NG

CO

ND

ENSA

TER

ETU

RN

LIN

ES

NO

MIN

AL

PIP

E SI

ZE (I

NC

H)

UP

STR

EAM

PR

ESSU

RE

(PSI

G)

EXAMPLE

PIV

OT

LIN

E

END

PRES

SURE

(PSI

G)

CONDENSATE FLO

W (LB/HR)

600500410350275220170130100

60

30

180100

50

3020

10

5

0

60K

40K

30K

20K

15K

10K

5K

1K

10

8

6

5

4

3

1

SIZING CONDENSATE RETURNLINES

When condensate passes through asteam trap orifice, it drops from the up-stream pressure in the heat exchangerto the downstream pressure in the con-densate return line. The energy in theupstream condensate is greater than theenergy in the downstream condensate.As the condensate passes through thesteam trap, the additional energy fromthe upstream condensate forms a per-centage of flash steam that changes

based upon the upstream and down-stream pressures (this percentage canbe seen in Table 5 in the CondensateCommander section).

When sizing condensate return linesafter the steam trap, it is important totake into account the amount of flashsteam created when hot, saturated con-densate undergoes a pressure drop.The flash steam has very large volumeand can cause very high velocities if thereturn line is not sized properly. Thesehigh velocities can create high backpres-

sure in the return line that often leadsto poor steam trap performance.

We will size the condensate return linebased upon flash steam velocities, Thepercentage of flash steam versus con-densate (water) is usually on the orderof 20 to 1, so the effect of the water inthe system sizing is usually small.Choosing a velocity of flash steam isoften subjective and different manufac-turers will suggest different values. Thenomograph below sizes return linesbased upon 50 feet/second.

EXAMPLE: Inlet Trap Pressure = 100 psigOutlet Pressure (return) = 0 psig (atmospheric)Actual condensate flow rate = 5,000 lb/hr.

Start at the Upstream Pressure line at 100 psig. Make astraight line through the End (Downstream) Pressure of 0psig and stop at the pivot line. From that point, make astraight line through the Condensate Flow Rate of 5,000and stop at the Nominal Pipe size line. It intersectsslightly higher than 4”. You may select the 4” line sizewithout concern for undersizing the line because a lowvelocity of 50 ft/sec was used.

Note: If design requirements dictate using a velocity otherthan the 50 ft/sec value in the Nomograph, a ratio can bemade of the pipe size because the velocity is proportionalto the Pipe Diameter squared. For example, if you requirea Pipe Diameter for 80 ft/sec, use the following equation:

50 FT/SECNomograph Diameter x ____________________√New Velocity (FT/SEC)

Example: The Nomograph Diameter determined in the previousexample is 4.2". Using the above formula, the Pipe Diameter for80 ft/sec is 3.3".

SIZING CONDENSATE RETURN LINES