2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
34
Draining Temperature Controlled Steam Equipment
An example plot is shown on
Fig. 46 for a coil where air is heated
to 80°F and the trap must
discharge against back pressure.
Step 1. The system is designed for
100% load when air enters at 0°F
(T 1) and there is 0% load when air
enters at 80°F (T 2). Draw line
(T 1/T2) connecting these points.
Step 2. At maximum load, the
arithmetic mean air temperature
(MT) is 40°F. Locate (MT) on line
(T 1/T 2), extend horizontally to 0%
load, and identify as (MT 1).
Step 3. Allowing for pressure
drop, the control valve has been
sized to supply 25 psig steam to
the coil at 100% load. This pressure
is (P 1) and has a steam
temperature of 267°F. Mark (P 1)
and draw line (P 1/MT 1).
Line (P 1/MT 1) approximates the
steam supply at any load condition
and the coil pressure is below
atmospheric when it drops below
the heavy line at 212°F. In a gravity
system with sub-atmospheric
Design MTD
Stall MTD
Temperature °F
R 2
conditions, a vacuum breaker and
hydraulic pressure due to condensate
will prevent stall and allow
the trap to drain the coil.
Step 4. In many systems, the trap
does not discharge freely to
atmosphere and in our example,
total back pressure on the trap is
15 psig, drawn as horizontal dotted
line (P 2). Coil pressure equals
back pressure at the intersection
of (P 2) with (P 1/MT 1) which when
dropped vertically downward to
(R 1) occurs at 93% load. At less
than this load, the required trap
differential is eliminated, the system
“stalls,” and the coil begins to
waterlog. In our air heating coil
the air flows at a constant rate
and extending the air temperature
intersection horizontally to (R 2),
stall occurs when the incoming air
is 6°F or more.
The same procedure applies
to a heat exchanger although the
example temperature is not a
common one. If the stall chart
Figure 46: Air Make-up Coil Stall Chart
400
380
360
340
320
300
280
260
240
220
200
180
160
140
120
100
80
60
40
20
P 1
MT
0
T1 100 90 80 70 60 50 40 30 20 10 0
Percentage Load
R1 P 2
235
180
140
105
75
55
34
20
10
3
0
5"
10"
15"
20"
25"
T 2
MT 1
Inches Vacuum Pressure psig
example represented a heat
exchanger where the liquid was to
be heated through a constant temperature
rise from 0 to 80°F, but at
a flow rate that varies, stall would
still occur below 93% load. In this
instance, if 100% load represents
a 50 GPM exchanger, the system
would stall when the demand was
46.5 GPM (50 x .93) or less.
Draining Equipment Under
“Stall” Conditions
“System stall” is lack of positive
differential across the steam trap
and temperature controlled
equipment will always be subject
to this problem when the trap
must operate against back pressure.
Under these conditions, a
vacuum breaker is ineffective
because “stall” always occurs
above atmospheric pressure.
Even when steam is supplied at a
constant pressure or flow to
“batch” type equipment, stall can
occur for some period of time on
startup when the steam condenses
quickly and the pressure drops
below the required differential.
What happens when the system
stalls is that the effective coil
area (“UA” in the formula) drops as
the steam chamber floods and
heat transfer is reduced until the
control valve responds to deliver an
excessive supply of steam to the
coil. This results in a “hunting system”
with fluctuating temperatures
and hammering coils as the relatively
cooler condensate
alternately backs up, then at least
some portion is forced through the
trap.
The solution to all system stall
problems is to make condensate
drain by gravity. Atmospheric systems
tend to operate more
predictably and are generally easier
to control but major heating
equipment is usually not drained
into an atmospheric return
because of the large amount of
energy that is lost from the vent. In
many process plants, venting
vapors of any type is discouraged
and a “closed loop” system is not
only required but is less subject to
oxygen corrosion problems.