Energy Efficient Steam Systems
Homework
1. The surface temperature of 200 ft of 8-inch diameter un-insulated pipe carrying
steam at 335 F is 250 F. The pipe is located in a room with air and surroundings
at 50 F. The surface emissivity of the pipe is 0.70. Calculate convection,
radiation and total heat loss from the pipe (Btu/hr). The pipe is insulated with 3
inches on insulation with thermal resistance R = 2 hr-ft2-F/Btu per inch. The
surface emissivity of the insulation is 0.70. Calculate convection, radiation and
total heat loss from the insulated pipe (Btu/hr). Calculate the heat loss and fuel
savings from insulating the pipe (Btu/hr) if the efficiency of the steam system is
65%.
2. The surface temperature of a steam-heated, un-insulated rectangular tank with
four walls with height 6 ft and length 10 ft is 170 F. The temperature of fluid in
the tank is 190 F, and the temperature of the air and surroundings is 70 F. The
surface emissivity of the tank is 0.70. Calculate convection, radiation and total
heat loss from the tank walls (Btu/hr). The tank walls are insulated with 1 inch
on insulation with thermal resistance R = 5 hr-ft2-F/Btu per inch. The surface
emissivity of the insulation is 0.70. Calculate convection, radiation and total heat
loss from the insulated tank walls (Btu/hr). Calculate the heat loss and fuel
savings from insulating the tank walls (Btu/hr) if the efficiency of the steam
system is 80%.
3. A 1.0-inch inverted bucket trap rated at 125 psi fails 100% open. The actual
steam pressure is 100 psig, and 100% of the condensate is returned at 180 F.
The overall efficiency of the boiler is 75%. The boiler operates 5,000 hours per
year and the cost of natural gas is $10 /mmBtu. Calculate the annual natural gas
cost savings ($/yr), from fixing the steam trap.
4. A boiler burns 10,000 mmBtu of natural gas per year at a cost of $10 /mmBtu.
The average temperature of the incoming combustion air is 50 F and the average
temperature of the exhaust is 420 F. The fraction excess air in the exhaust is
measured to be 70%, but is reduced to 10% by adjusting the inlet combustion air
dampers. The specific heat of exhaust is 0.30 Btu/lb-F. Calculate a) the
projected annual cost savings ($/yr) and b) the projected savings as a percent of
current annual natural gas use.
5. A boiler burns 2,500 mmBtu of natural gas per year at a cost of $10 /mmBtu.
The average temperature of the incoming combustion air is 60 F. The fraction
excess air in the exhaust is measured to be 15%. The exhaust temperature from
the boiler increases from 390 F to 480 F over a 15 month period due to scale
buildup. Calculate a) the projected annual cost savings ($/yr) and b) the
projected savings as a percent of current annual natural gas use from descaling
the boiler.
6. A boiler burns 5.0 mmBtu/hr of natural gas at a cost of $10 /mmBtu, while
generating 5,000 lb/hr of steam. The boiler operates 6,000 hours per year. The
fraction excess air in the exhaust is measured to be 50%. The average
temperature of the exhaust is 420 F and the average temperature of the boiler
feedwater is 100 F. In stoichiometric combustion, the mass ratio of exhaust gas
to natural gas fuel is 17.2 lba/lbng. The density of air is 0.075 lb/ft3 and the high
heating value of natural gas is 23,900 Btu/lb. The specific heat of exhaust is 0.30
Btu/lb-F and the specific heat of water is 1 Btu/lb-F. The overall efficiency of the
steam system is 75%. If a 60% effective counter-flow economizer were installed,
calculate a) the projected annual cost savings ($/yr) and b) the projected savings
as a percent of current annual natural gas use.
7. A traditional hot-water boiler burns 10,000 mmBtu of natural gas per year at a
cost of $10 /mmBtu. The average temperature of the incoming combustion air is
50 F. The average temperature of the exhaust is 320 F. The fraction excess air in
the exhaust is measured to be 15%. It is proposed to 1) install a larger process
heat exchanger that reduces the temperature of the return water from 150 F to
110 F, and 2) install a new condensing boiler. The average temperature of the
exhaust from the condensing boiler is 130 F. Calculate a) the projected annual
cost savings ($/yr) and b) the projected savings as a percent of current annual
natural gas use.
8. Use SteamSim to model the following system:
Tca = 70 "F" "temp of combustion air entering burner"
Tex = 500 "F" "temp of exhaust"
EA = .3 "fraction excess combustion air"
Effecon = 0.0 "effectiveness of economizer"
P2 = 150 psig
P3 = 150 psig
Dsp = 1
Lsp = 1000
Rsp = 3
Mstl = 0
Qprocess = 10,000,000 Btu/hr
Fcl = 0
Fbd = .06
Db = 8 ft
Rb = 3
Lb = 20 ft
Dcp = .2
Lsp = 1000
Rsp = 3
Dda = 5 ft
Rda = 3
Lda = 10 ft
To = 60
Pda = 18 psia
a) Starting from the base case, calculate savings (Btu/hr) from reducing P3 to
100 psig.
b) Starting from the base case, calculate savings (Btu/hr) from adding a 50%
effective economizer.
c) Starting from the base case, calculate savings (Btu/hr) from decreasing
blowdown to 4%.
d) Calculate savings (Btu/hr) from doing all of the above simultaneously. Why
isn’t the savings the same as the total of the individual savings?