中央空调设计英文文献和中文翻译(2)
Plant enclosures should allow for future equipment replacement or
addition, with wall openings and possibly roof sections which can be
removed and replaced.
3. Electrical Service. Many plants, particularly those with chillers
or electric boilers, comprise a major electrical load for the facility.
Proximity to the primary electrical service is a cost concern. The elec-
trical service should be well thought through, and should allow for
any projected plant expansion, if not in present gear, at least in space
and concept. Since the plant environment may be coarse (althoughcleanliness is a virtue), electrical equipment is often housed in a sep-
arate room with filtered, fan-forced ventilation. Some electronic gear
needs to be in an air conditioned space.
Where many motors are involved in a plant, motor control centers
(MCCs) are preferred to inpidual combination starters. Large plants
may have several MCCs to reduce the length of wiring runs.
The electrical service should have a degree of redundancy. Hospitals
and other critical-care facilities require access to at least two inde-
pendent utility substations. This carries into the large plant in the
form of multiple transformers and segmented switch groups with tie
breakers. Standby power generation may be included in plant design
in addition to backup power for life safety issues.
4. Valving. In central plants there is no substitute for isolation
valves for every piece of equipment. Multiple high-pressure steam
boilers require double valving with intermediate vent valves to protect
workers inside a unit that is down for maintenance. Valves should be
installed in accessible locations.
7.3 Central Steam Plants
Some general concepts of steam distribution were presented in Chap.
6. Steam plants require considerations of siting, structure, and elec-
trical service, as described in this chapter. Boilers are the primary
component of steam plants and are supported by a host of auxiliary
components such as boiler feed pumps, deaerating feedwater heaters,
condensate holding tanks, water softeners, blowdown heat recovery
systems, water treatment systems, flue gas economizers, fuel-handling
equipment, etc. See Fig. 7.1.
Each component of the steam system is available in a range of qual-
ity and performance characteristics. Selection depends on duty and on
the sophistication of the plant operation. Equipment for a smaller
school will be of a different character than for a campus or an indus-
trial plant. With all the subjective differences, the technical calcula-
tions are similar.
Because condensate originates in heat exchange devices as a fluid
without pressure, it must drain by gravity to a collection point. If a
steam plant can be located at the low point of the served system, the
entire condensate return line may flow by gravity. Otherwise, inter-
mediate collection points and booster pumps may be required.
An important aspect of a steam plant is the condensate storage ves-
sel. When a boiler fires up after a time of setback or at the onset of a
peak heating load, a significant amount of feedwater will be evapo-
rated and sent out into the system with a time lag before any of the
condensate will get back to the plant. The storage tank must holdenough water to sustain the initial demand, and then it must have
enough ‘‘freeboard’’ or residual capacity to accept the returning con-
densate after an evening load shutdown. Failure to provide adequate
storage is observed through storage tank overflow, with high makeup
water rates and high treatment costs.
Small plants often use the feedwater heating tank as a combination
storage-and-preheat vessel.
Most steam plants use a version of a feedwater heater to remove
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