Page 62 - Boiler plant and distribution system optimization manual
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Distribution System Efficiency 47
FLASH STEAM This is why high pressures are used in dis-
tribution systems and pressure reducing valves
The largest energy loss in steam systems is (PRV) are used close to the point of use of the
flash steam in open condensate recovery systems. steam.
When the hot condensate passes through a trap Let’s look at an open system, in this case
from a high energy level to a lower energy level the 175 psi [12 Bar] steam system in our example
and percentage of the water flashes into steam. which is delivering steam to the points of use.
Figure 5.3 illustrates this energy balance across a Assuming there is a load on this system of 75,000
steam trap. pounds of steam per hour passing through steam
traps, the flash rate is about 17%. This will pro-
duce 12,000 pounds of flash steam per hour cost-
ing over $480,000 a year. This is an 11% system
loss.
This is a basic problem of these systems,
flash steam losses as great as total boiler plant
losses. High temperature hot water systems do
not have a number of problems associated with
steam systems.
Figure 5.3—Flash steam forms on the low pressure
HIGH TEMPERATURE HOT WATER SYSTEMS
side of the steam trap, in this case with almost 3
times as much energy as the condensate.
High temperature water distribution sys-
tems are divided into three categories:
The formula for flash steam is simple; the
difference in enthalpy [Btu/lb] of the water on 1. High Temperature Hot water systems have a
either side of the steam trap is divided by the Btus temperature range from 350 F [177C] to 450
required to form a pound of steam at the lower F [232C).
pressure. 2. Medium Temperature Hot water systems
have a temperature between 250 F [121C]
475 – 180 and 350F [l77C].
———— = 30.4% Flash Steam
970 3. Low Temperature Hot water systems are
usually below 250F [121C].
All steam used by the system must pass
through steam traps, so the whole system is It is interesting to note that one cubic foot of
subject to this loss if atmospheric vents are used. hot water at 350F contains 64 times as much heat
Closed pressurized condensate recovery systems as a cubic foot of saturated steam at 350F [177C].
are used in well designed plants to avoid this When high temperature hot water cools from
problem. 350F to 25°F, it releases 40 times more heat than a
Flash steam percentages vary with pressure cubic foot of steam.
drop across steam traps, for example exhausting Other features of high temperature water
to an open (atmospheric) system: systems are:
1. High Temperature Water systems are closed
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systems and should only use one half of one
600 psi trap has a 30% flash percent make up water compared to boilers
175 psi trap has a 17% flash which use quite a lot; in some cases 100%.
50 psi trap has a 4% flash
2. They do not have the usual losses associat-
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