ASHRAE Journal - February 2023 - 35

TECHNICAL FEATURE
FIGURE 1 Flow patterns found in two-phase
flow.4
FIGURE 2 A comparison of two-phase velocity ratio with mixture quality.
Vg
VL
Gas Mass Flow Rate →
Fundamentals of Steam System Design3 characterize
condensate drainage systems as " vented wet, " " vented
dry, " or " dry-closed " systems. " Vented wet " and " vented
dry " systems are systems with no fl ash steam present,
and liquid condensate is moved by gravity only. " Vented
wet " systems apply to sections of drain piping that
are fully fl ooded, which occurs between two vented
receivers when the drain line is below the normal level
of the downstream receiver . A " vented dry " system only
occurs between two receivers both vented to the same
reference pressure (usually atmospheric pressure) and
in heat exchangers operating at or below the pressure
of the receiver. A " closed wet " system, not discussed in
Fundamentals or Fundamentals of Steam System Design, occurs
at the inlet of steam traps that have suffi cient head
pressure to keep condensate from fl ashing. All other
non-vented (closed) drainage systems are " dry-closed "
systems with a mixture of liquid condensate and fl ash
steam and other gases.
ASHRAE Research Project RP-1674 further characterizes
dry-closed systems in seven states. These systems are
bubble fl ow, plug fl ow, stratifi ed fl ow, wavy fl ow, slug
fl ow, annular fl ow and mist fl ow. The character of twophase
fl ow illustrated in Figure 1 is a function of the ratio
of the liquid and gas mass fl ow rates.
Bubble fl ow occurs when steam bubbles initially form
and are carried by the liquid condensate. The modeling
of pressure drop in this regime can be characterized
by the Darcy-Weisbach equation when the pipe is fully
fl ooded. This usually occurs when the pressure of the
saturated liquid condensate
changes pressure less than
1 psi (6.9 kPa), either before
or after the steam trap. When
the pressure changes more
than 1 psi (6.9 kPa), the steam
bubbles grow larger and become
Benjamin & Miller
McGee
Sutor
Isbin, Sher, Eddy
Quality, %
a large portion of volume in the pipe. Depending on
the pipe size and the fl ash steam fraction, the twophase
mixture will transition to one of the other states
shown in Figure 1. As fl ash steam fl ow/velocity increases
relative to the liquid condensate velocity, the fl ow
regime transitions from:
Bubble → Plug → Stratifi ed → Wavy → Slug → Annular
→ Mist
TAKEAWAY PO I NTS
Water hammer can be very destructive to objects in
the path of the water slug.
Liquid accumulation in piping and/or heat
exchangers can result in multiple problems.
Due to the density and viscosity differences between
water vapor, other gases and liquid water, the velocities
of each fl uid can be different. When the pressure
drop rate of the vapor/gas is close to the pressure drop
of the liquid, the interface between the gas and liquid
is non-turbulent and results in stratifi ed fl ow. As pressure
drop differences between the vapor/gas and liquid
increases, the surface of the liquid transitions to waves
inside the pipe (wavy regime). When the wave height
reaches to the top of the pipe, liquid seals off the downstream
steam from the upstream steam forming water
slug that is accelerated by the
upstream steam. The slug continues
at a high velocity until its
energy is dissipated by striking
an elbow fi tting or other obstacle
in the fl ow path resulting in
water hammer.
F E B R U A RY 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
35
Liquid Mass Flow Rate →
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ASHRAE Journal - February 2023

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