Steam Distribution Systems And Control; Introduction; Advantages Of Steam Systems Vs Hot Water Systems; Steam System Objectives - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
PUMP
PUMP
TERMINAL
V5
UNITS
V3
Fig. 100. Two-Pipe Multiple-Zone Dual-Temperature
System Using Zone Pumps.
STEAM DISTRIBUTION SYSTEMS
AND CONTROL

INTRODUCTION

Steam distribution systems are classified as either low
pressure (200 kPa and less) or high pressure (above 200 kPa).
Low pressure systems have many subclasses such as one-pipe,
two-pipe, gravity, vacuum, and variable vacuum. See HOT
WATER DISTRIBUTION SYSTEMS for steam-to-hot water
converter configurations and control.
ADVANTAGES OF STEAM SYSTEMS VS HOT
WATER SYSTEMS
The principle reasons for the use of steam to distribute heat
in commercial buildings or in groups of buildings separated
from the heating plant are:
— Steam is light weight (1.69 cubic meters per kilogram).
— Steam has high heat content (2325 kilojoules per
kilogram).
— Steam flows through pipes unaided by pumps.
— Steam does not create excessive static pressure on piping
in tall buildings.
— Terminal units can be added or removed without basic
design changes.
— Draining and filling are not necessary to make repairs
as with hot water systems.
— Steam can be distributed through a large system with
little change in heating capacity due to heat loss.

STEAM SYSTEM OBJECTIVES

Carefully consider distribution system objectives when
applying controls either to the boiler or to the distribution system
itself. Control and/or piping of the boiler or steam generator
are not considered in this section. Not all of Objectives 1 through
7 may apply to any given distribution system.
PUMP
PUMP
V1 V2
TERMINAL
UNITS
V4
C2921
1. Steam mains must provide adequate capacity so steam
velocity is between 40 and 60 meters per second.
2. Water must not be allowed to accumulate in the mains.
Provisions must be made for the use of traps or
superheated steam to reduce or eliminate water in mains.
3. Pockets of water must not be allowed to accumulate.
Steam traveling at 60 meters per second (216 kph) can
propel the water causing water hammer, which can
damage or destroy piping.
4. Condensate must be returned to the boiler at the same
rate as steam leaves the boiler. Otherwise, the boiler will
be shut down by low water cutoff control or be damaged
from lack of water covering heated metal.
5. Provision must be made to expel the air when steam is
again supplied. When any part of the system is not
supplied with steam, that part of the system fills up with
air from the atmosphere. If air is present with the steam,
the air plates the heat exchanger surfaces and reduces
capacity. The oxygen in the air causes pitting of iron and
steel surfaces and the carbon dioxide (CO
forms an extremely corrosive carbonic acid solution.
6. The return condensate piping system must be sized for a
low-pressure loss to eliminate flashing. For example, if
200 kPa steam condenses in a heating coil, the condensate
is still near the boiling point, say 115 C; and if the return
main is at atmospheric pressure, the condensate can flash
into steam. This wastes heat and can block the return of
condensate to the boiler.
7. If necessary, the flow of steam must be accurately
measured to account for steam usage. When steam is used
in a closed system (none is vented to atmosphere), the
steam flow to a building or zone can be measured by
measuring condensate flow.

PROPERTIES OF STEAM

Adding 4.2 kilojoules to one kilogram of water raises the
water temperature one kelvin. When water temperature reaches
100 C at sea level (101.325 kPa) it contains (100 – 0) x 4.2 =
420 kJ/kg. However, it takes another 2275 kJ to convert the
one kilogram of water to a vapor (steam). The total heat of the
vapor is: 420 kJ/kg + 2257 kJ/kg = 2677 kJ/kg. The 2257 kJ/kg
is the latent heat required to vaporize water.
One kilogram of water in the liquid state occupies about 0.001
cubic meters at 0 C. When converted to vapor at 100 C, it
occupies 1.673 cubic meters or 1673 times as much space as
the liquid.
One kilogram of steam (water vapor) when cooled and
condensed in a radiator or other heating device gives up 2257 kJ
to the device and returns to its liquid state. If the liquid (water)
leaves the radiator at 82 C, it gives up another 74 kJ, so the total
heating value of low pressure steam is said to be 2325 kJ/kg per
kilogram (actually 74 + 2257 or 2331 kJ/kg).
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) in the air
2