Selecting Traps For Heat Exchangers - Xylem Bell & Gossett Hoffman Speciality Series Engineering Data Manual

Chapter 3
Selecting
Steam Traps
for Heat
Exchangers
Steam to Fluid Heat Exchanger
HEATED FLUID
OUT
FLUID TO BE
HEATED IN
24
Steam heating devices using a modulating
temperature regulator must operate over a
wide range of conditions. As the temperature
regulator controls the flow of steam, condesa-
tion causes a change in pressure. Thus, the
steam trap must be capable of handling a
wide range of capacities at varying pressures.
Selection of the trap for these conditions is
more involved than it would be for drip traps
or steam equipment operating at constant
pressure.
A heat exchanger is sized to heat a maximum
expected flow rate through the tubes, over the
maximum expected temperature rise with a
predetermined maximum steam operating
pressure.
When the tube side flow rate is reduced, or
the incoming fluid being heated requires less
of a temperature rise, the steam control valve
partially closes, reducing the flow of steam to
maintain a constant set temperature of the
fluid being heated. The condensing of the
steam under reduced load conditions results
in a lower steam pressure in the shell of the
heat exchanger.
During very low load conditions the condens-
ing of the steam can create an induced vacu-
um in the shell of the heat exchanger. This
condition requires that a vacuum breaker be
installed to allow air to enter and relieve the
induced vacuum. Without a vacuum breaker,
the induced vacuum would cause a negative
pressure differential across the trap and the
condensate would not be drained from the
heat exchanger shell.
TEMPERATURE
REGULATOR
TUBE SUPPORTS
HEAT EXCHANGER
TUBE BUNDLE
VACUUM BREAKER
HEAT EXCHANGER
SHELL
Complete condensate drainage under all vary-
ing pressure and condensing loads is essen-
tial to prevent tube damage due to water
hammer. The steam flow in the heat exchang-
er shell must pass around the tube support
sheets. If condensate builds up in the heat
exchanger shell, it will condense rapidly as
steam is mixed with it, causing water ham-
mer. The water hammer is often evident by
indentations in the tubes and collapsed
tubes.
Thus, it becomes evident that the design con-
densing rate at design pressure is not the
only load the trap must handle. The condens-
ing load of a heat exchanger designed for 15
psi may in fact be in excess of 90% at 0 psig.
When the heat exchanger is selected, a foul-
ing factor is added to assure adequate tube
area as scale builds up on the tube walls.
Before this scale develops, the heat exchang-
er is in fact oversized which results in a lower
steam operating pressure.
A steam trap must then be selected to handle
the full condensing load with the heat
exchanger operating at 0 psig. The heat
exchanger may operate at a slight vacuum
due to the condensing of steam. A vacuum
breaker is required on a heat exchanger to
prevent induced vacuum. The differential
required to open the vacuum breaker is usual-
ly less than 0.25 psi.
Recommended practice is to install the trap
as far below the heat exchanger as possible.
The minimum distance should be 15 inches.
A 15 inch static head will develop approxi-
mately 0.5 psig at the trap inlet, less the dif-
ferential required to open the vacuum
breaker. Assuming 0.25 psi to open the vacu-
um breaker, a properly sized trap must be
capable of draining the full rated condensing
load with 0.25 psi differential across the trap,
draining into an atmospheric gravity return
line. A static head of 2.4 feet will provide 1
psig. The differential required to open the vac-
uum breaker must be subtracted from the
static pressure to determine the differential
STEAM LINE
across the trap.
Selecting the type of trap becomes the next
step. Traps that operate in response to tem-
perature should be avoided for heat exchang-
er operation. This eliminates thermostatic
traps from our selection. As described above,
F&T TRAP the trap must be capable of responding to
varying condensing rates at various differen-
tial pressures. The two types of traps that can
meet these requirements are Float and
Thermostatic Traps and Bucket Traps. The
Float and Thermostatic Trap has the ability to