July 2015 Archives

#212 Heat-Saving Measures - Boiler Management -1

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(1) Load rate and thermal efficiency of a boiler

The thermal efficiency of a boiler is defined as the percentage of heat energy obtained from fuel that is effectively utilized to generate steam.
The thermal energy possessed by various types of fuel can be calculated from its chemical composition. The actual measurement values are also available from the suppliers.The fuel usage can be determined by an oil gauge or a gas flowmeter.

However, it is quite difficult to measure the amount of steam generated.It may be easier for big corporations as they usually have a steam flowmeter, but it is not the case for most small and mid-sized companies.For those without a steam flowmeter, it is still possible to estimate thermal efficiency using the amount of water supplied instead of the amount of steam generated.In this case, it is not an easy task to calculate the precise thermal efficiency because the thermal energy of steam generated varies according to the steam pressure.

To make this task simpler, calculate the evaporation ratio, which is the amount of water supplied (amount of steam generated) in kg per kilogram of fuel based on the fuel usage amount and the amount of water supplied (excluding the discharge amount) for a month.Use the steam table to calculate the thermal energy of steam based on the pressure (gauge pressure) of a boiler under normal operation. Then, calculate the thermal energy of the water supply at the feed-water temperature along with the thermal energy of fuel before combustion takes place. The approximate thermal efficiency can be calculated by using these values.

As the feed-water temperature and the fuel temperature vary season to season, these values fall within a certain range of the seasonal variation coefficient.

Recent boilers are more efficient than the conventional ones. You may see "thermal efficiency 98%" in the product catalog. However, this efficiency value can only be achieved at the optimum operating state during continuous operation.Such efficiency cannot be achieved during actual operation.This is because the load rate of a boiler fluctuates depending on the operating condition.
For example, when a surface treatment plant starts its operation, steam needs to be supplied continually to heat a degreasing bath or a plating bath. Once the bath temperature has risen to a certain temperature, the bath no longer needs that much steam since the high specific heat of these solutions minimizes the temperature drop.
In this case, the boiler operates at an extremely high efficiency with the load rate of 100% around the time of starting daily operations. Once the surface treatment work enters into a steady operation cycle, the boiler operates at low load with decreased efficiency as the burner intermittently burns fuel.In other words, the boiler releases more heat into exhaust gas during low-load operations.

[Fig.1] shows the relationship between the efficiency and load rate of a boiler.

Figure

#211 Heat-Saving Measures - Boilers

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While many types of boilers are available, the ones adopted for projects involving surface treatments are usually once-through or water-tube boilers in smaller sizes.
A boiler is a device used to create steam by heating up water using fuel and to transport the steam to a heating object. From its function of conveying the heat energy from fuel to a heating object through water, you could say that a boiler works as a heat exchanger.

[Fig.1] shows the principle of a one-through boiler. The exhaust heat recovery area called the 'economizer' preheats water pumped through boiler water tubes. The water starts boiling in the tubes while it circulates from the lowest-temperature to the highest-temperature area in the combustion chamber. At this time, water and steam coexist. The water and steam mixture is sent to the steam separator where water droplets are removed from the steam by gravity. The water droplets are then transported back to the water-supply pump.
Heating up the steam separated from the water at a temperature higher than its vaporization point generates "superheated steam" to be supplied to a heating object through the main steam valve.
As shown in [Table 1], the boiler water quantity is relatively low for the once-through boiler. This means that it produces high-quality steam (extremely dry and high-energy steam) in a remarkably short length of time.

Figure

[Table 1] Example of boiler water quantity and concentration factor for a once-through boiler
Equivalent evaporation (kg/hr) 500 600 750 1000 1200 1500
Boiler water quantity (L) 80 115 115 118 120 125
Concentration factor 6.25 5.22 6.52 8.47 10.0 12.0

The important factors in applying heat-saving measures for a boiler include proper management of fuel, boiler feed-water, boiler water (water in the boiler chamber), steam, exhaust gas, and drain as well as the appropriate combustion control. The concentration factor shown in [Table 1] means that impurities in the boiler water become concentrated by this factor when operating the boiler for one hour. Failure to eliminate impurities (Ca, Mg, silicate, hydrochloric acid, oxygen gas, etc.) completely from the boiler water or feed water may cause the water tube to clog or corrode. To prevent these problems, it is necessary to implement the proper management of feed water and boiler water. The feed water management includes demineralization using ion-exchange resin and degassing. As for the boiler water management, boiler water is blown off (discharged) on a periodic basis.

When choosing fuel, important characteristics to look out for are high calorific value per unit weight, ease of combustion control, no air contaminants, and high thermal efficiency of the boiler, in addition to the purchase price.

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