Economiser Boiler

INTRODUCTION

ECONOMISER

The flue gasses, having passed through the main boiler and the super heater, will still be hot. The energy in these gases can be used to improve the thermal efficiency of the boiler. To achieve this, the flue gases are passed through an economizer.

The energy in the flue gases can be used to improve the thermal efficiency of the boiler.

The economizer is a heat exchanger through which the feed water is pumped. The feed water thus arrives in the boiler at a higher temperature than would be the case if no economizer was fitted. Less energy is then required to raise the steam. Alternatively, if the same quantity of energy is supplied, then more steam is raised. This results in a higher efficiency. In broad terms a 10ºC in crease in feed water temperature will give an efficiency improvement of 2%.

Note:

Because the economizer is on the high-pressure side of the feed pump, feed water temperature in excess of 100ºC are possible. The boiler water level controls should be of the “modulating” type, (i.e not “on-off”) to ensure a continuous flow of feed water through the heat exchanger.

The heat exchanger should not be so large that:

• The flue gases are cooled below their dew point, as the resulting liquor may be acidic and corrosive.

• The feed water boilers in the heat exchanger.

2) PAYBACK CALCULATION

            TYPE OF BOILER    : 3 PASS FIRE TUBE

           

            CAPACITY                : 6 TONNES PER HOUR

            AUTHORISED SAFE WORKING PRESSURE  :  17.5 Bar

Before Installing the Economiser

Flue gas temperature        : 240ºC

Water inlet temperature   : 100ºC

Load (average)                 :  80%

Efficiency                        :  85%

Fuel Consumption

Fuel Consumption  = Steam Generating Capacity x Latent Heat Evaporation X Present Load

                                                                        Efficiency of Boiler x Caloric Value

                               =   6000 x 540 x 0.8

                                       0.85 x 9350

                               =   326.14 sm³ per hour

After installing the Economiser

Flue gas temperature               : 140ºC

Water inlet temperature          : 160ºC

Load (average)                        : 80%

Efficiency                               : 90%

Fuel Consumption

Fuel Consumption       = Steam Generating Capacity x Latent Heat Evaporation x Present Load

                                                            Efficiency of Boiler x Caloric Value

                                   

                                    = 6000 x 540 x 0.8

                                            0.90 x 9350

                                    = 308 sm³ per hour

Total running hours = 8000 hour per year

Net saving on fuel cost with 8000 hours/year working time & 0.49 cents / sm³ as fuel cost

=          (326.15 – 308) x 8000 x 0.49

=          RM71, 108.00

Total Investment Cost

1. Economiser c/w fittings                      RM      96,000.00
2. Modification and Erection                  RM      36,000.00
3. Instrumentation & Electrical works    RM      30,000.00
4. Civil Works                                      RM        6,000.00

Total                                                    RM    168,000.00

Payback Period           =                                  Total Investment / Net Saving

                                    =                                  RM 168,000.00

                                                                        RM   71,000.00

                                    =                                  2.4 Years

ADVANTAGES OF INSTALLING ECONOMISER

1.     Low fuel bills.

2.     Increment in actual steam output.

3.     Improved boiler response to variable process demand.

4.     Equipment can be used even with 100% condensate return.

5.     Easy to operate & maintain.

6.     Better efficiency.

7.     Reduction in thermal shock

8.     Reduction in carbon dioxide (environmental friendly).