Page 264 - Boiler plant and distribution system optimization manual

Steam System Optimization 249

Figure 13.2—Flash Steam Energy Balance.

the rest is lost through the trap. If we were able Table 13.2 shows why steam pressure should be
to drop the working pressure on a piece of equip- reduced at the point of use. It conserves the loss of
ment from 125 psig to 5 psig, the latent heat would energy through the steam trap.
be 83.5% of the total energy instead of 73.5%. ———————————————————————
Latent Percent
Significant steam savings can be realized by
Pressure Heat of Total
reducing steam pressure as low as possible using
psig Btu/lb Heat
pressure reducing valves in new as well as exist-
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ing installations.
125 868 73.5
As shown in Table 13.3, the percent heat sav-
ings available from reducing pressure is signifi- 50 912 77.5
cant. For example, there will be a 13% savings by
reducing the pressure from 150 to 20 psig. 5 961 83.5
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A Rule of Thumb for Optimizing Steam Table 13.3—Heat savings available from reducing
System Pressure pressure at the point of use. (To calculate these
———————————————————————— numbers, flash steam reduction and the latent heat of
a. System steam pressure should be distribut- the steam and the heat in the condensate at the old and
new pressure is considered)
ed at high enough pressure as is practical to
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overcome line losses and satisfy the highest
Original Pressure
pressure user.
(psig)
Reduced 125 150 200
b. Steam pressure should be reduced at the
Pressure (Steam Savings %)
point of use to as low a pressure as is practi- ———————————————————————
cal.
———————————————————————— 100 psig 2.1 3.6 6.8
75 psig 4.4 5.9 9.0
Two general points to consider in the steam
distribution system is, design requirements limit 50 psig 7.2 8.6 11.6
steam velocities to less than 6,000 to 12,000 feet
per minute and the system pressure drop should 20 psig 11.5 13.0 15.8
not exceed 20 percent of the total maximum pres-
10 psig 13.7 15.0 17.8
sure of the boiler.
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