ASHRAE Journal - February 2023 - 38

TECHNICAL FEATURE
the heat exchanger. The drainage system from P11 to
P7 needs to be evaluated for peak flow with P11 being
at a positive pressure and at part load when P11 is
sub-atmospheric.
When the heat exchanger is sub-atmospheric, air
vents/vacuum breakers open to let air in and increase
pressure P11 close to atmospheric pressure. Vacuum
breakers are usually spring-loaded check valves that
require a minimum pressure differential to open
(0.3 psi to 1 psi [2.1 kPa to 6.9 kPa]). Thermostatic air
vents will open when the steam temperature drops
below its bellows operator setpoint (usually below
200°F [93°C]). When evaluating drainage from heat
exchangers, the elevation H3 needs to be sufficient
height to offset any negative pressure at P11 due to the
vacuum breaker setpoint, plus the friction loss DPP11-P7
including the steam trap pressure drop at each load
condition. Ideally heat exchangers with modulating
control valves should be drained to a separate vented
receiver to reduce the probability of condensate
drainage issues associated with backpressure at P7. If
modulated heat exchangers are connected to a receiver
inlet line with flash steam, the connection point should
be as close to the receiver as possible to reduce the
backpressure from other lines with flash steam.
Lifting condensate in a drainage system uses
backpressure to push a plug of water up in elevation.
The best place to lift condensate is the trap discharge
line (P4 to P5) where plug flow already exists. The
pressure available to lift condensate is the pressure
difference between P4 and P5 minus friction loss DPP4-P5
or less than 2 ft (<0.6 m) for low pressure systems and
between 2 ftand10 ft (0.6
m
and3 m) for high pressure
systems. Lifting condensate between P5 and P6 should
be avoided to prevent slug flow.
Flow in a vertical drop is a mixture of steam vapor and
liquid droplets. Pressure drop in vertical drops is equal
to the steam vapor pressure drop. Pipe sizes in vertical
drops could be reduced based on pressure drop, but I
would recommend keeping the same size to reduce the
hydraulic jump at the bottom of the riser.
Condensate from high pressure traps can be combined
with low pressure traps when the receiver pressure
is near atmospheric pressure provided that the extra
percentage of flash steam is accounted for in the pipe
sizing. This is because the condensate line pressure P5
to P6 is only a function of the receiver pressure and line
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ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2023
pressure drop. If flash tank steam is being recovered to
a low pressure steam line, high pressure traps should be
connected to a separate high pressure condensate line
that drains to the higher pressure flash tank.
When a condensate pump fails, liquid condensate
continues to flow and can back up into Vent 5. To
prevent excessive pressure on the condensate pump
receiver, an overflow drain with a p-trap should be
provided. The height of the p-trap H5 should be greater
than the pressure loss of Vent 5 under normal operating
conditions. It is always a good idea to vent condensate
pump receivers to the outdoors to prevent any flash
steam entering mechanical spaces.
Air Venting and Vacuum Release
During start-up of a cold piping system, steam pushes
air to the air vents (Air Vents 1, 2, and 3 in Figure 3) or to
open drain valves used for supervised start-up. When
steam is shut off for maintenance or by control valves
or when control valves reduce the heat exchanger pressure
below atmospheric pressure, the remaining steam
condenses and creates a vacuum. Vacuum breakers
are required for proper drainage in both steam lines
and equipment when the static head is not enough to
overcome the increased pressure differential created by
the vacuum. Air vents will also act as vacuum breakers
when the steam temperature drops below their setpoint.
The best locations for air vents and vacuum breakers
are at the high points and at the end of mains and
at heat exchanger inlets and outlets. Air venting does
occur through steam traps but at a relatively slow rate.
Steam mains and equipment should have air vents and
vacuum breakers for faster air venting, faster warm-up,
and gravity flow during cooldown.
Design Flow Rates
Design flow rates for drainage systems are based on
the sum of peak trap flows occurring at system start-up.
Current guidelines regarding trap drainage rates are
based on either safety factors applied to steady-state
full-capacity loads or the thermal energy change of
pipe materials during warm-up over a fixed period of
time (usually 60 minutes). Safety factors appear to be
based on historical experience. A summary of safety
factors based on application are listed in Table 6-1 of
the ASHRAE Fundamentals of Steam System Design3 and in
publications from major steam trap manufacturers.
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ASHRAE Journal - February 2023

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