2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
32
Makeup Air Heating Coils
The steam trap must handle
lots of air and drain condensate
at saturated steam temperature
continuously while the load and
pressure are changing and thus a
Float-Thermostatic type is recommended
for all air heating coils.
The trap is mounted below the
condensate outlet from the coil
with a vertical drop giving enough
hydraulic head to enable a suitable
size to be selected. A 14"
head should be the minimum and
represents about 1/2 psi, a 28"
head about 1 psi, and to reduce
possibility of freeze-up, a drop of
3 ft. to the trap is recommended.
Preheat/Reheat Coils
The preheat/reheat coil hookup
shown in Fig. II-26 (page 96) may
employ a direct-acting temperature
control or with larger coils, a quicker
responding pilot-operated type
with a closer control band is recommended.
This arrangement
allows filtration and perhaps humidification
of the air to be carried out
at the controlled preheat temperature,
and the reheat coil brings the
dry bulb temperature of the conditioned
air to the required value for
distribution. The preheat coil is
used to heat outside air up to the
intermediate temperature but as
outside temperature increases, the
temperature control lowers the
steam pressure in the preheat coil
and condensate drainage tends to
slow down. If the coil is being used
where design loads occur at subzero
temperatures, there can
sometimes be only atmospheric
pressure in the coil, although the air
passing over it is still cold enough
to lead to freeze-up problems.
This difficulty is greatly
reduced if the temperature sensor
controlling the steam supply to the
preheat coil is set to the needed
distribution temperature. Part load
conditions would then lead firstly to
lowering the steam pressure in the
reheat coil, where freezing will not
occur, but pressure is maintained
in the preheat coil until outside air
temperatures are above the danger
point. Such an arrangement
reduces freeze-up problems in
many instances on existing installations,
at minimal cost.
Corrosion And Waterhammer
Problems
Condensate mixed with air
becomes corrosive and assuming
the boiler water treatment is satis-
Case in Action: Air Handling System Steam Coil Drainage
Typical storage buildings are extremely large and difficult to
heat. This example in specific has three floors with approximately
486,000 ft 2 of floor space and heated with 150 air
handling units. These units are comprised of bay heaters,
overhead door heaters and administrative office area
heaters. The minimum steam supply pressure to all of them
is 20 psig and are pneumatically controlled.
In the preceding 12 month period, $201,000 was
spent on labor and materials to repair damaged coils. The
common problem was condensate standing in the coils,
unable to drain, causing erosion due to presence of carbonic
acid and bulging/splitting as a result of freezing.
Solution
Starting with a training session at the facility that addressed
this problem and typical solutions, Spirax Sarco’s local
sales office implemented a “Cooperative Research and
Development Agreement” (CRDA). The purpose of the
agreement was to test a proposed solution including
Pressure Powered Pumps and Pump/Trap combinations
to eliminate system stall, thereby assuring thorough condensate
drainage, regardless of supply air temperature,
control valve turn-down or over-sized heaters.
A test was conducted on four air handling units. One
unit was hooked up as usual, without Pressure Powered
factory, coil corrosion problems
are usually due to condensate
regularly backing up or lying stagnant
on the bottom of the tubes
during shutdown. If the coil is
trapped correctly, the most likely
cause is an overhead return
which prevents the coil from draining.
One remedy for this is to fit a
liquid expansion steam trap at the
lowest piping level, as shown in
Fig. II-26 (page 96), set to open
when the temperature drops
below 90°F. The coil then drains
only cold condensate to a sewer.
In high pressure systems
where waterhammer on startup
remains troublesome, a “safety
drain” trap is sometimes used. This
consists of a stock 15 psi rated
inverted bucket trap fitted above
the main trap which discharges to
drain whenever coil pressure is
low, but due to its design locks shut
at higher pressure. While this is
useful on pressurized mains, the
safety trap may require a pressure
considerably higher than its nominal
rating to lock shut and on
modulating service a considerable
amount of condensate may be
wasted. This makes the combination
pump/trap a more viable
solution to this problem.
Pump drainage systems. The other three were drained
by either open or closed loop PPP systems. Four days into
the test and the unit without a PPP drainage system had
three frozen coils. It was found that as outside supply air
temperature dropped below 36˚F, it was necessary to close
outside dampers and use 100% recirculated air, or the
coils would freeze. The three units drained by PPP systems
continued operating trouble-free.
Benefits
Employee Safety
• Improved indoor air quality through the use of a higher
percentage of outside air supply.
• Reduced chance of injury by eliminating water leakage on
the floor from broken coils and subsequent slippage.
• Fewer burns because there are fewer steam leaks.
• Greater employee awareness of hazards because of
training.
Cost Savings
• Reduced steam and condensate losses resulting in
energy savings.
• Reduced cost for management support (paper-work).
• Cost savings of up to 30% above the initial installation
cost in a 12 month period.