Steam System Heat Characteristics; Steam Pressure, Temperature, And Density; Steam Quality - Honeywell AUTOMATIC CONTROL Engineering Manual

STEAM SYSTEM HEAT CHARACTERISTICS

Figure 101 shows the characteristics of one pound of steam
as it travels through a steam heating system.
STEAM
CONDENSING
IN RADIATOR
VOLUME OF ONE
POUND OF STEAM:
27 CUBIC FEET
STEAM
VOLUME OF ONE POUND OF
WATER, ONE PINT (APPROXIMATE)
WATER
32 BTU TO HEAT WATER FROM 180 TO 212°F
970 BTU TO VAPORIZE ONE POUND OF WATER
ELECTRICAL INPUT TO BOILER: 293.7 WATT-HOURS
HEATER
Fig. 101. One Pound 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 pound
remains relatively constant.
Table 6. Approximate Values for Properties
of Saturated Steam.
Density,
Pounds
per 1000
Pressure Temperature
in PSIG Saturated
1 215F
10 239F
100 338F
200 406F
This table illustrates that in a given size pipe more than four
times as much steam can be carried with steam at 100 psig as
with steam at 10 psig. However, a 100 psi steam main is 99F
hotter than 10 psi steam.
EXAMPLE:
3
The heat in 1000 ft of steam:
at 10 psi is 61 lb x 1,160 Btu/lb = 70,760 Btu
at 100 psi is 257 lb x 1,190 Btu/lb = 305,830 Btu
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
1002 BTU
OUTPUT
180°F
CONDENSATE RETURN
C2922
Latent Heat of Total
Cubic Vaporization, Heat
Feet Btu/Lb Btu/Lb
40 968 1152
61 953 1160
257 881 1190
469 838 1199

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 1000 lb/hr is needed
through a pipe or valve, then 1000/0.85 = 1176 lb/hr 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. Since
superheated steam has a higher heat content per pound, the
steam quantity needed through a pipe or valve is reduced.
Pressure reducing valves can also produce superheated steam.
For example: 100 psi steam at 338F passing through a pressure
reducing valve gives up no heat as it expands to 10 psi, so the
10 psi steam downstream will be at 338F not 239F. This is 99
degrees Fahrenheit of superheat and downstream valves and
piping will be exposed to the higher temperature. To correct
for superheated steam, 1 Btu/lb is added for each Fahrenheit
degree of superheat.
EXAMPLE:
1000 MBtuh of 10 psi steam is required. From Table 6, 10
psi steam has a latent heat of 953 Btu/lb. If condensate leaves
at 180F, the steam gives up 1012 Btu/lb (953 Btu/lb condensing
the steam and 59 Btu/lb cooling the condensate from 239F to
180F). If the 10 psi steam has 90 degrees of superheat, the
added heat is 90 Btu/lb. Thus, heat available from 10 psi steam
with 90F of superheat is:
1012 Btu/lb + 90 Btu/lb = 1102 Btu/lb.
Steam quantity needed is:
1,000,000 Btuh
1102 Btuh
369 ENGINEERING MANUAL OF AUTOMATIC CONTROL
= 907.4 lb/hr