Energy Saving in boiler blowdown

1. Boiler Blowdown
Engr. Sahadat Hussain
Sr. Consultant, Environmental & Process
Engineering Resources International (ERI)

2. Common Impurities in Raw Water
• Dissolved solids - scale forming substances. The principal
ones are the carbonates and sulphates of calcium and
magnesium. Not all dissolved solids will form scale.
• Suspended solids - sludge forming
organic
substances which are
inusually solid mineral or particles, carried
suspension. Not generally a problem with most waters.
• Dissolved gases - corrosive. Oxygen and carbon dioxide.
• Scum forming substances - mineral scum forming or foaming
impurities usually containing soda in the form of a carbonate,
chloride or sulphate.

3. Water quality needs to be such that we
operate under the following criteria:-
• Freedom from scale
• Freedom from corrosion
• Good quality steam
• Safety

4. ppm = mg/l
Units
•
of Concentration
Parts Per Million (ppm)
1 kg in 1,000,000 kg (1,000 Tonnes) of water
1 lb. in 100,000 lmp. gallons of water
• Milligramme Per Litre (mg/l)
1
1
1
mg
mg
mg
in
in
in
1,000,000 mg of water
1,000 g of water
1 litre of water
100 ppm
1,000 ppm
10,000 ppm
= 100 mg/l = 99.99% purity
= 1,000 mg/l = 99.9% purity
= 10,000 mg/l = 99% purity
ppm = mg/l

5. Total Dissolved Solids
Non
Hardness
Total
Dissolved
Total
Hardness
Salts Solids

6. Keeping the Steam Clean.
Pure steam
to plant
Build up ofFeed water
impurities inwith
impurities the boiler
Impurities
‘blown down’ to waste

7. Deciding on the Required Boiler Water TDS.
Maximum TDS tolerated Maximum TDS
by boiler type
Lancashire
(ppm)
10,000
4,5002-Pass Economic
3,000-3,500Package and 3-Pass Economic
2000-3000Low Pressure Water Tube
1,500Medium Pressure Water Tube
2,000Coil Boilers and Generators
•
•
The above figures are offered as a broad guide only
The boiler maker should always be consulted for
specific recommendations.

8. Calculating Blowdown Quantity.
Let:
F = Feed water TDS (ppm)
B = Boiler target TDS (ppm)
S = Boiler steaming rate (kg/h)
F x S
Amount to be blown down =
B - F
This equation is valid for any consistent
units eg lb/h, μS/cm

9. The effect of too high TDS in the boiler.
High TDS and/or Contamination of Fouling of Heat
ExchangersControl Valvessuspended solids.
Blockage of Steam Traps
Boiler

10. Automatic TDS Control

11. Project Boiler
10,000 kg/h
steam to
plant
Feed water
with 250
Maximum
allowable
ppm
impurities
boiler TDS =
2,500 ppm
Boiler operating pressure = 10 bar
oCSaturation temperature = 184

12. Project Boiler
Given Information
F = Feed water TDS (ppm) = 250 ppm
B = Boiler target TDS (ppm) = 2,500 ppm
S = Boiler steaming rate (kg/h) = 10,000 kg/h
F x SAmount to be blown down =
B - F
250 x 10,000
=
2,500 - 250
= 1,111 kg/h

13. actual blowdown?How much is the
Ba - F
=
=
Amount of excess blowdown = 1282 - 1111
= 171 kg/h
1,282 kg/h
250 x 10,000
2,200 - 250
Amount actually blown down = F x S
Given Information
Ba = Actual Boiler TDS (ppm) maintained = 2200 ppm

14. How much energy is there in the
blowdown?
Energy blown down (kW) = 1111kg/h x 782 kJ/kg
3600 seconds
= 241 kW
A typical house central heating system is rated
at 13 kW, so this is enough heat for
241
13
= nearly 19 houses.

15. Net saving
Saving in blowdown x Sensible heat lost x Cost of fuelNet Savings =
GCV of fuel x Efficiency of boiler x Sp. gravity of fuel (if oil)
=
= Rs. 695 per day
Total Saving = Rs. 20,850 per month
171 x 24 x (184 - 25) x 6 x 100
8000 x 80 x 0.88

16. Boiler Bottom Blowdown Rate
Flow
Rate
(kg/s)
Pressure (bar g)