Page 265 - Boiler plant and distribution system optimization manual

250 Boiler Plant and Distribution System Optimization Manual
Heat Transfer Efficiency
Accumulations of air and noncondensible
gases in the steam system can also limit steam
flow, steam temperature and heat energy release.
Air is present in the system on start up and
is also introduced by vacuum breakers on heat
exchangers and process equipment. Nonconden-
sible gases are liberated in the boiler by bicarbon- equipment. Air and noncondensibles in the sys-
ates which forms CO . Oxygen is carried through
2
the system. These noncondensible gases, when
released, flow with the steam and can create heat
transfer problems.
The gases cause a temperature reduction by
contributing to total system pressure.
Dalton’s Law of Partial Pressure states that a
mixture of steam and other gases is equal to the
sum of the partial pressures. This effectively re-
duces steam pressure, temperature and energy
transfer.
Table 13.4 shows the effect of air on the tem-
perature of a steam-air mixture.
Table 13.4—Air-steam temperatures.
Insulating Barriers
There is a second phenomena involved with
noncondensible gases in the steam. When steam
condenses on the heat exchange surface, the non-
condensible gases also accumulate on the surface
forming an insulation barrier.
The first one percent of air barrier has the
most effect on reducing heat transfer (Figure 13.3).
In this figure T is the steam temperature and T
S
W
is the water temperature of the heat exchanger.
These gases take up volume and don’t con-
dense into a liquid as readily as steam, hence the
term noncondensible. If allowed to accumulate
for long periods, they take up enough volume to
effectively block steam flow and energy transfer.
Bellows type thermostatic steam traps can
be used as automatic air vents on heat exchange
tem tend to be lighter than the steam and accumu-
late in quiet zones. If installed at these locations,
the thermostatic device can sense the temperature
reduction caused by the air.
Batch process cookers, large shell and tube
heat exchangers and large steam coils should in-
corporate automatic air vents to eliminate air ac-
cumulations.
STEAM DISTRIBUTION SYSTEM LOSSES
Insulation
Steam is distributed through hot pipes which
must be kept insulated to prevent excessive loss of
heat and for safety. The range of surface tempera-
ture can vary from 200-500°F. The bare surface loss-
es can vary from 300 Btu/Hr to 1700 Btu/Hr. The
losses from uninsulated surfaces can be impressive,
Figure 13.4 shows the losses from an uninsulated
four inch gate valve for one year for steam costing
$5.00, $8.00 and $11.00 per million Btus.
Steam users seem to be fully aware of the
need to insulate hot surfaces. to prevent heat
loss. Insulation pays for itself quite quickly, but
insulating steam piping means not only the main
piping, but also the unions, flanges, valve bodies,
steam traps and everything else that is hot.
Removable (lace-up, foam in place and mold-
ed etc.), insulating covers are available for valve
bodies and other hard to fit shapes. Mechanical
steam traps such as float and thermostatic (F&T),
inverted bucket (IB) should also be insulated as
well as the bodies of thermodynamic disc traps
(the covers should be left bare). Only Thermostat-
ic (TS) traps and their cooling legs should be left
uninsulated.
Condensate return lines are often not insu-
lated because their heat losses are not considered