Energy Efficient Steam Systems

Steam System

Deaerator tank

FeedWaterpump

HX

Boiler

Steam trap

Qfuel Qprocess

1

2 4

al

av

Throttling valve4P

3

Economizer

7

Qex

haus

t

1e

0

Qb

Qda

Qcp

Qsp

56

Tex

SteamTrapleak

Blo

wdo

wn

Con

d lo

ss

Fla

sh v

apor

Mak

eup

wat

er

Pum

p po

wer

Com

b ai

r

Steam System

0.0 0.5 1.0 1.5 2.00

200

400

600

800

s [Btu/lbm-R]

T [

°F]

200 psia

100 psia

20 psia

0.2 0.4 0.6 0.8

Steam

2

3, 4

5, 6, 7

0

aval, 1

1e

Energy Flows

• Energy enters a steam system as:– Fuel and combustion air– Makeup water– Pump work

• Energy leaves a steam system as:– Heat to the process– Exhaust air– Blowdown– Condensate loss– Flash vapor– Heat loss from the boiler, steam pipes, condensate pipes and deaerator tank.

Fuel Use Reduced

• Fuel use reduced by reducing:– Heat to the process– Heat in exhaust air– Blowdown– Condensate loss– Flash vapor– Heat loss from the boiler, steam pipes, condensate pipes and

deaerator tank.

Energy Saving Opportunities

1) Reduce Steam Demand

• Insulate hot surfaces

• Cover uninsulated tanks

2) Fix Steam Traps

• Steam traps are automatic valves that discharge condensate from a steam line without discharging steam.

• If the trap fails open, steam escapes into the condensate return pipe without being utilized in the process.

• If trap fails closed, condensate fills the heat exchanger and chokes-off heat to process.

• Fixing failed steam traps is highly cost-effective.

3) Insulate Pipes and Tanks

• Insulate– steam pipes– condensate return pipes– condensate return tanks– deaerator tank– valves

4) Preheat Boiler Feed-water Using Exhaust Air

• Economizer is heat exchanger that preheats feed-water to the boiler using heat from the exhaust gasses.

• Economizers are most cost effective in process boilers that operate all year.

5) Minimize Steam Pressure

• Generating steam at excess pressure:– decreases boiler efficiency– increases heat loss– increases flash loss.

• Reducing boiler pressure to match the highest required process temperature decreases these losses.

• Reducing steam pressure to match local required process temperature reduces flash loss.

• Thus, always produce and supply steam at the minimum pressure required to meet the process temperature requirement.

6) Install Automatic Blow Down Controls

• Blow down is the practice of expelling steam to reduce contaminant build ups.

• Typical blowdown rates range from 4% to 8% of boiler feed-water.

• Manual blowdown relies on intuition or periodic testing. Always results in excess blow down that wastes energy or insufficient blow down that creates excess scale on heat transfer surfaces and reduces efficiency.

• Automatic blow down reduces energy, water and water treatment costs.

7) Operate Boiler in Modulation Mode

• Most boilers are designed for peak load, but operate at part load most of the time.

• Efficiency increases as firing rate modulated to part load.

• Each time a boiler cycles on and off, it purges natural gas with fan and loses heat.

• Efficiency decreases at part load in on/off control.

• Installing a burner with a smaller minimum firing rate can eliminate the on/off cycling and reduce fuel use.

8) Adjust Fuel/Air Ratio

• Most boilers use linkages that connect natural gas supply valves with combustion air inlet dampers.

• Unfortunately, the linkages do not function perfectly, and the air/fuel ratio is seldom held constant over the firing range.

• The linkages should be adjusted to maintain 10% excess air high fire. 70%

75%

80%

85%

90%

0 10 20 30 40 50 60 70 80 90 100

Excess Air (%)

Eff

icie

ncy Ts=300F

Ts=400F

Ts=500F

9) Install O2 Trim

• Most boilers linkages do not function perfectly, and the air/fuel ratio is seldom held constant over the firing range.

• O2 trim combustion controls

regulate combustion intake air to maintain 10% excess air across the entire firing range.

• O2 trim most cost-effective for

boilers that operate all year long.

Original

9) Install O2 Trim

9) Install O2 Trim

With

O2

Trim

9) Install O2 Trim

Annual Savings: 1,227 mmBtu, $14,724

Annual Maintenance Cost: $2,000

Cost of O2 Trim System: $30,000

Payback: 28 months

Low Fire Medium FireBaseline Excess Air 113% 35%Proposed Excess Air 10% 10%

Baseline Comb. Efficiency 83.6% 84.4%Proposed Comb. Efficiency 86.6% 85.3%

10) Switch to Hot-water System for Low-Temperature Applications

• When Treturn < 120 F, condensing boiler efficiency > 90%

• When Tinlet ~ 70 F, direct- contact water heaters capitalize on low water temperature, counter-flow design, and large surface areas for efficiency > 98%