Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
367
STEAM SYSTEM HEAT CHARACTERISTICS
Figure 101 shows the characteristics of one kilogram of steam
as it travels through a steam heating system.
EXAMPLE:
The heat in 50m 3 of steam:
at 150 kPa is 43 kg x 2693 kJ/kg = 115 799 kJ
at 800 kPa is 208 kg x 2668 kJ/kg = 554 944 kJ
STEAM QUALITY
Steam tables generally show properties of dry-saturated
steam. Dry-saturated steam has no entrained moisture and is at
the boiling point for the given pressure. Dry-saturated steam is
said to have 100 percent quality. Steam produced in a boiler
usually has some water droplets entrained in the steam and is
called wet-saturated steam. Condensation collecting within
steam mains can also become entrained and lessen steam quality.
If 10 percent of the steam weight is liquid, the steam has
90 percent quality.
If steam has 85 percent quality and if 0.1 kg/s is needed
through a pipe or valve, then 0.1/0.85 = 0.118 kg/s of the
85 percent quality steam is required.
Superheated steam is steam at a temperature above the boiling
point for the indicated pressure. It can be produced by passing
the saturated steam through a superheater. A superheater
consists of steam tubes exposed to hot gases from the firebox.
This steam is hotter than the temperature listed in steam tables.
Superheat is expressed in degrees Fahrenheit or Celsius
(kelvins). Since superheated steam has a higher heat content
per kilogram, the steam quantity needed through a pipe or valve
is reduced.
Pressure reducing valves can also produce superheated steam.
For example: 800 kPa steam at 170
°
C passing through a pressure
reducing valve gives up no heat as it expands to 150 kPa, so the
150 kPa steam downstream will be at 170
°
C not 111
°
C. This is
59 kelvins of superheat and downstream valves and piping will
be exposed to the higher temperature. To correct for superheated
steam, 4 kJ/kg is added for each kelvin of superheat.
EXAMPLE:
300 kW of 150 kPa steam is required. From Table 6, 150 kPa
steam has a latent heat of 2226 kJ/kg. If condensate leaves at
80
°
C, the steam gives up 2356 kJ/kg (2226 kJ/kg condensing
the steam and 130 kJ/kg cooling the condensate from 111
°
C to
80
°
C). If the 150 kPa steam has 50 kelvins of superheat, the
added heat is 200 kJ/kg. Thus, heat available from 150 kPa
steam with 50 kelvins of superheat is:
2356 kJ/kg + 200 kJ/kg = 2256 kJ/kg.
Steam quantity needed is:
Fig. 101. One Kilogram of Water
in a Steam Heating System.
STEAM PRESSURE, TEMPERATURE, AND DENSITY
Steam temperature is directly related to pressure, see Table 6.
For a more extensive table refer to the General Engineering Data
section. Note that density, which is the reciprocal of specific
volume, increases sharply with pressure, while total heat per
kilogram remains relatively constant.
Table 6. Approximate Values
for Properties of Saturated Steam.
This table illustrates that in a given size pipe more than four
times as much steam can be carried with steam at 800 kPa as
with steam at 150 kPa. However, a 800 kPa steam main is 59
°
C
hotter than 150 kPa steam.
300 kW
2256 kJ/kg
= 0.117 kg/s = 117 g/s
STEAM
CONDENSING
IN RADIATOR
74 kJ TO HEAT WATER FROM 82 TO 100
°
C
2257 kJ TO VAPORIZE ONE KILOGRAM OF WATER
ELECTRICAL INPUT TO BOILER: 0.619 WATT-HOURS
VOLUME OF ONE KILOGRAM OF
WATER, ONE LITER (APPROXIMATE)
VOLUME OF ONE
POUND OF STEAM:
1.69 CUBIC METERS
STEAM
HEATER
WATER
82
°
c
CONDENSATE RETURN
2331 kJ
OUTPUT
C4621
Pressure
in kPa
(absolute)
Tempera-
ture
Saturated
ℑ
C
Density,
kilograms
per Cubic
Meter
Latent
Heat of
Vaporization,
kJ/kg
Total
Heat
kJ/kg
100
150
800
1600
99
111
170
201
0.59
0.86
4.16
8.08 2257.9
2226.2
2046.5
1933.2 2675.4
2693.4
2667.5
2791.7