Steam offers the following advantages:
• Steam flows through the system unaided by external energy
sources such as pumps.
• Because of its low density, steam can be used in tall
buildings where water systems create excessive pressure.
• Terminal units can be added or removed without making
basic changes to the design.
• Steam components can be repaired or replaced by closing
the steam supply, without the difficulties associated with draining
and refilling a water system. • Steam is pressure-temperature
dependent; therefore, the system temperature can be controlled
by varying either steam pressure or temperature.
• Steam can be distributed throughout a heating system with
little change in temperature In view of these advantages, steam
is applicable to the following facilities:
• Where heat is required for process and comfort heating,
such as in industrial plants, hospitals, restaurants, dry-cleaning
plants, laundries, and commercial buildings
• Where the heating medium must travel great distances,
such as in facilities with scattered building locations, or where
the building height would result in excessive pressure in a water
system • Where intermittent changes in heat load occur
Steam is the vapor phase of water and is generated by adding
more heat than required to maintain its liquid phase at a given
The preparation of this chapter is assigned to TC 6.1, Hydronic
and Steam Equipment and Systems. sure, causing the liquid to change
to vapor without any further increase in temperature. Table 1
illustrates the pressure-temperature relationship and various
other properties of steam.
Temperature is the thermal state of both liquid and vapor at
any given pressure. The values shown in Table 1 are for dry saturated
steam. The vapor temperature can be raised by adding more heat,
resulting in superheated steam, which is used (1) where higher
temperatures are required, (2) in large distribution systems to
compensate for heat losses and to ensure that steam is delivered
at the desired saturated pressure and temperature, and (3) to
ensure that the steam is dry and contains no entrained liquid
that could damage some turbine-driven equipment.
Enthalpy of the liquid hf (sensible heat) is the amount of heat
in Btu required to raise the temperature of a pound of water from
32°F to the boiling point at the pressure indicated.
Enthalpy of evaporation hfg (latent heat of vaporization) is
the amount of heat required to change a pound of boiling water
at a given pressure to a pound of steam at the same pressure.
This same amount of heat is released when the vapor is condensed
back to a liquid.
Enthalpy of the steam hg (total heat) is the combined enthalpy
of liquid and vapor and represents the total heat above 32°F
in the steam.
Specific volume, the reciprocal of density, is the volume of
unit mass and indicates the volumetric space that 1 lb of steam
or water occupies.
An understanding of the above helps explain some of the following
unique properties and advantages of steam:
• Most of the heat content of steam is stored as latent
heat, which permits large quantities of heat to be transmitted
efficiently with little change in temperature. Because the temperature
of saturated steam is pressure-dependent, a negligible temperature
reduction occurs from the reduction in pressure caused by pipe
friction losses as steam flows through the system. This occurs
regardless of the insulation efficiency, as long as the boiler
maintains the initial pressure and the steam traps remove the
condensate. Conversely, in a hydronic system, inadequate insulation
can significantly reduce fluid temperature. • Steam, as
all fluids, flows from areas of high pressure to areas of low
pressure and is able to move throughout a system without an
external energy source. Heat dissipation causes the vapor to condense,
which creates a reduction in pressure caused by the dramatic change
in specific volume (1600:1 at atmospheric pressure).
• As steam gives up its latent heat at the terminal equipment,
the condensate that forms is initially at the same pressure and
temperature as the steam. When this condensate is discharged to