Boiler, Boiler Losses, Boiler Feedtank



Boiler

The job of a boiler is to supply good quality dry steam at the correct pressure at the right time. Boilers and the associated fire equipment should be designed for efficient operation. They should also be properly sized. A boiler, which has to cope with a peak load above its maximum continuous rating, will operate at reduced efficiency. Pressure may drop and the resultant priming and carry-over will mean that the boiler is unable to do its job of providing good quality steam.

If a boiler has to work at a small percentage of its rating, radiation losses become significant and, again there is a drop in overall efficiency. Clearly, it is not easy to match boiler plant to what is normally a variable steam load. Two or more boilers are more flexible than a single unit which explains the common arrangement of a large boiler for the winter load with a smaller boiler for the summer load.

Boiler losses

                                      

There are four different types of boiler losses:

  •     radiation losses
  •     flue gas losses (stack losses)
  •     blowdown losses
  •     losses due to low rate of condensate return from the production plant

The radiation losses are determined through the thermal insulation of the boiler. It should be tried to return as much condensate as possible back to the boiler, so the used heat energy in the boiler can be minimized.

Flue Gas Losses

To achieve a high thermal efficiency, thereby minimizing fuel costs, the amount of combustion air should be limited to that necessary to achieve complete combustion of the fuel.

Excess air is the extra air supplied to the burner beyond the air required for complete or stoichometric combustion. If the supplied air is less, not only will this result in a smoking stack (black smoke indicates the presence of unburned fuel i.e. combustibles in the flue gas), but it will significantly reduce the energy released per unit of fuel. If a burner is operated with a deficiency of air, carbon monoxide and hydrogen will appear in the products of combustion. These combustibles are fuel, and anything in excess of a few hundred parts per million in the flue gas indicates inefficient burner operation.

Too little excess air is inefficient because it permits unburned fuel, in the form of combustibles, to escape up the stack. But too much excess air is also inefficient because it enters the burner at ambient temperature and leaves the stack hot, thus stealing useful heat from the process.

Getting the right mix of fuel and air in your burner is the most critical and difficult step in achieving boiler efficiency. The Combustion Analyser makes sure you always get the right mix of air and fuel.

Blowdown Losses

Boiler blowdown is a very essential function which – if not carried out in the most energy efficient manner – can be an unnecessary source of loss.

Raw water will contain impurities. As the boiler water is evaporated into steam and replaced by make up water, the concentration of the solids in the boiler water will obviously increase. If they are allowed to increase much beyond the recommended level then the functioning of the boiler will be affected. "Foaming" will take place within the boiler resulting in carry-over of water into the steam distribution system, and in the worst situation, malfunctioning of the boiler water level controls can occur.

The original method of "blowing down" a boiler in order to maintain the total dissolved solids (TDS) at an acceptable level was to manually operate a blowdown valve fitted to a discharge pipe at the lowest point of the boiler shell, at the same time hoping that the frequency of operation of the blowdown valve and the length of time for which it is open is sufficient for the needs of the boiler.

Best engineering practice now in the furtherance of energy efficient plant, is to apply the use of a Boiler Blowdown Control System which is not necessarily high in capital expenses compared with the payback period in terms of energy saving.

The losses from blowdown may be also be minimized by using the heat in the blowdown water. A certain percentage of the blowdown water will flash off into steam when it enters a region of pressure lower than that which existed within the boiler. This flash steam can be recovered and used by the application of a Blowdown Heat Recovery Unit which consists of Blowdown Flash Vessel and Heat Exchanger system to recover the flash steam and use the heat from blowdown water to preheat the Boiler Feed Water. Flash steam is sent to feed water tank for direct heating of water.

The EffiMax 2000 package provides a complete monitoring and data acquisition solution for your boiler performance. EffiMax calculates the efficiency of the boiler based on indirect efficiency computation and computes individually the total amount of losses like stack loss, enthalpy loss, radiation loss and blowdown loss in your boiler. Using the data generated on the system losses, on-line suggestions can then be used to fine tune the system to generate more steam with the lesser quantity of fuel.

Water for the Boiler

The boiler feed tank is the heart of any steam system. It provides a reservoir of returned condensate and fresh make-up water with which the boiler feed pump can replenish the boilers.

The feed tank must be properly sized and allowance made for fluctuations and possible interruptions in supply; it is normal to hold enough water to provide one hour of steam at maximum rating. However, there should be enough free space to cope with the relatively massive return at start-up. Significant quantities of condensate can be lost if this is not provided.

In order to prevent corrosion in boilers and ancillary equipment it is necessary to eliminate both dissolved oxygen and dissolved carbon dioxide from the boiler feed water. Oxygen is the main cause of corrosion and the presence of carbon dioxide in the water presents a pH value of something less than neutral (pH 7) causing the water to be acidic. The ideal pH value for boiler feed water is around pH 9. A higher pH value can cause what is called "caustic embrittlement" in the boiler, particularly a boiler with riveted joints. A Feed Water Tank System provides complete solution to manage the boiler feed water for any capacity boilers.

A Deaerator Head ensures vigorous mixing of steam and feedwater to reduce dissolved oxygen content. This action reduces the need for oxygen scavenging chemicals to a minimum.

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