| Editor's Note: Following is the
conclusion of a three-part article on steam heat exchanger application and
troubleshooting. Part one appeared in the September 1996 edition of Tech Talk, and part
two appeared in the February 1997 edition. The two previous options we discussed,
Electric Pump and Pressure Powered Pump (PPP), are effective methods of eliminating back
pressure on the steam trap. These methods enable the condensate to discharge freely from
the steam space in the heat exchanger (see figure 1). However, both systems utilize a
vacuum breaker as part of the system. The vacuum breaker will introduce air into the
exchanger any time the control valve closes. The steam inside the steam space collapses to
a much smaller volume of condensate. Vacuum conditions may be more prevalent in exchangers
operating at high turndown, or if the exchanger is designed with a large log mean
temperature difference (LMTD). Whenever the vacuum breaker opens, oxygen-laden air enters
the steam space in the exchanger. Although there is no condensate inside the exchanger to
mix with this newly introduced air and cause corrosion, air itself can create other
potential problems inside the steam space.
Barriers to heat transfer inside the heat exchanger include air films that surround the
tubes inside the exchanger. A layer of air only .04" thick can offer the same
resistance to heat transfer as a layer of copper 43 feet thick. This comparison
illustrates the serious consequences of air film development in the exchanger.
Air must be evacuated from the exchanger before steam can enter. This is usually not a
problem if a Float & Thermostatic steam trap is used. This type of trap discharges air
very quickly.
CLOSED PPP PACKAGE
Another option available to the designer of the system is a closed loop PPP package.
This system is designed to eliminate air entering the exchanger, since a vacuum breaker is
no longer needed.
This design allows the exchanger to work in vacuum conditions while preventing
condensate build-up and oxygen-laden air from entering the steam space.
The steam trap (as shown in Fig. 2) is now moved from between the exchanger and
receiver to the discharge of the PPP. The vent from the pump is connected to the vent of
the receiver which, in turn, is connected to the steam inlet pipe (after the control
valve). These connections make it possible to have the receiver, pump, and exchanger
experience the same pressure or vacuum at all times.
The components are now balanced.
Under high load, steam fills the exchanger, receiver, and pump. Steam is trapped at the
pump's outlet. At this point there is sufficient positive steam pressure for the steam
trap to operate. As the modulating valve closes, steam pressure decreases to the point
where steam pressure at the trap inlet is equal to or less than back pressure in the
return system. The trap cannot open and condensate backs up into the pump cavity.
Condensate will back up until the pump fills and activates the stroke cycle. Motive steam
will drive condensate through the trap and into the return system. A receiver is still
required to store condensate as the pump strokes. Even if the system falls into a vacuum
condition, the condensate still flows by gravity to the lowest point (i.e., the pump).
We conclude with the following two examples that illustrate sizing techniques for both
types of PPP return systems.
Heat load = 7500 pph
Steam pressure = 15 psi
Condensate lift = 20 ft.
Return line length = 150 ft.
Return line size = 2-1/2"
Dearator pressure = 3 psi
Motive available = 100 psi
PPP discharge rate = 90 GPM |
|
The first system is set up as
shown in Fig. 2. Here, the steam trap is under the heat exchanger, and our receiver and
PPP are vented to atmosphere. This system will utilize a vacuum breaker allowing air into
the heat exchanger during low load and shut-off conditions. |
Back pressure = lift @ 20'
2.3 = 9 psi
Dearator = 3 psi
Friction loss @ 90 GPM through 150 feet of 2-1/2" pipe = 9 psi
Total Back Pressure = 21 psi |
|
| Motive pressure equals 100 psi and back
pressure equals 21 psi with a 7500 pph load. A 3" x 2" PPP unit is recommended. |
| The Float &
Thermostatic steam trap used for this job should be sized for 7500 pph @ 1/2 psi, as the
trap will always discharge against zero back pressure. Sizing the trap at full load at 1/2
psi will offer an acceptable safety factor. The sizing tables show that a 2" FTO15X
will do the job properly. |
|
Example 2: With a closed
loop PPP system, the vacuum breaker is no longer needed since the system will operate
under vacuum. This situation is possible because the vent lines from the receiver and PPP
connect to the steam inlet line after the control valve, and the steam trap is now located
after the PPP. This allows a hydraulic connection between the heat exchanger, receiver,
and PPP. All information is the same as in Example 1, except consideration must be given
to the effect of installing a steam trap after the PPP.
 The steam
trap must now discharge against a back pressure instead of into a vented receiver. This
will happen when the steam inlet pressure is higher than the back pressure on the trap.
The valve
modulates down and delivers a lower steam pressure, eventually creating a
"stall" condition where steam inlet pressure is less than or equal to system
back pressure. At this point the PPP fills and will begin to cycle. At the point where the
PPP discharges, the steam trap must be able to handle the discha~ge load (i.e., 90 GPM).
As the PPP
discharges through the steam trap, the AP across the trap must also be factored into the
back pressure equation. The steam trap must now be sized for the largest load, which will
be 90 8pm or 90 x (60 min/hr) x (8.34 lbs./gallon) = 45,036 pph. Working backwards from
the steam trap sizing tables, we must use an FT125 Series of trap to handle the 100 psi
motive steam pressure. A single trap is not available to use as 125 psi differential is
needed to pass 45,000 pph. Using two steam traps in parallel is our best option. An FT125C
2-1/2" steam trap has a capacity of 24,500 pph at 25 psi differential. Doubling the
load would be 49,000 pph capacity at 25 psi differential.
The total back
pressure of the system when we use the two FT125C traps in parallel is:
Condensate lift = 9 psi
Dearator pressure = 3 psi
Friction loss through piping = 9 psi
Pressure loss through steam traps = 25 psi
TOTAL 46 psi Back Pressure |
|
The PPP must handle 7500 pph with
100 psi motive pressure and 46 psi motive pressure and 46 psi back pressure. From the
sizing charts, a 3" x 2" is large enough to handle the load. A pump/trap
combination in a closed loop system offers the advantage of eliminating air from entering
the heat exchanger. However, we are forced to use two large steam traps which increases
our original cost. |
SYNOPSIS
Heat exchangers that experience "stall" conditions can suffer with problems
caused by control hunting; temperature shock; and corrosion possibly from flooding, steam
collapse, and freezing. Heat exchange packages should include a condensate pump if the
condensate cannot drain by gravity after discharging from the steam trap. Condensate pumps
can either be electric or pressure driven - each has advantages to be considered when
specifying the pump. Using a Pressure Powered Pump (PPP) in a closed loop system
eliminates the need for a vacuum breaker which would allow air into the heat exchanger.
Air has considerable negative impact on steam heat exchangers, which makes a pump/trap
combination in a closed loop system very attractive. Equipment sizing must be done
carefully when designing any of the above heat exchange packages. There are advantages and
disadvantages to using each pipe-in method, and these must be considered when deciding
which one to use.
 |
Reprinted from TechTalk October 1997
|
Article
Morton Grove
Tel 847 966-3700
Troubleshooting Steam Heat Exchangers - Part 3
