Page 169 - Boiler plant and distribution system optimization manual

154 Boiler Plant and Distribution System Optimization Manual

sume steam but send it on to do additional work
in the plant as opposed to an electrical generating
plant where the steam is dumped into a condens-
er to turn it back into water wasting a great deal
of heat present in the final steam stages.
This is generally practical on 50 KW units
on up and with steam flows of 3,000 pounds of
steam per hour significantly reducing local elec-
trical costs.

Table 10.5
Why We Reduce Steam Pressure
Pressure Latent Heat % of Total
PSIG Btu/lb Heat
———————————————————————
125 868 73.5
50 912 77.5
5 961 83.5
———————————————————————

Figure 10.49—The Pressure Reducing valves (PRv)
When steam pressure is reduced, more heat
are the (hour glass shaped symbols) which can be
becomes available and the steam is used more ef-
augmented by turbogenerators (cone shaped) which
ficiently. This is why pressure reducing valves are perform the same function with the added benefit of
used wherever possible. producing electrical power at high efficiency.

11. HIGH TEMPERATURE HOT Advantages/Disadvantages
WATER SYSTEMS Losses with a high temperature hot water
system are much less than steam because:
Cost 1. It is a closed system and leaks are usually
High less than one half of one percent whereas a
steam system can require up to 100% make
Potential Savings
up feedwater. The actual requirement for
High
makeup water can vary widely.
Description: 2. It minimizes losses associated with steam
1. High temperature hot water has a system traps, valve stem and packing gland leakage,
temperature from 350°F to 450°F. Medium blowdown and flash losses associated with
temperature hot water is between 250°F and open condensate receivers.
350°F and Low temperature hot water is be- 3. It can pack more usable heat into a given vol-
low 250°F. ume than a steam system and needs smaller
2. One cubic foot of water at 350°F contains 50 diameter piping.
times as much heat as a cubic foot of saturat- 4. Return water temperature must be main-
ed steam at 350°F. tained above a certain limit to avoid thermal
3. When a high temperature hot water system stress and piping system damage.
cools a cubic foot of water from 350°F to
250°F, it releases approximately 27.6 times A recent study has shown that in a 20 Mil-
more heat than a cubic foot of saturated lion Btu/hr system, comparing HTW at 350°F to
steam at 350°F. saturated steam, showed that there is a $300,000