Boiler Rating

Many people doesn't know about boiler ratings. Sometimes they mistake boiler pressure as its efficiency. So in this short article we are gonna learn about boiler ratings, its efficiency.

'From and at' rating

The boiler manufacturer gives us rating of 6.3 ton/h steam or 13800 lbs/h steam. Most of the time we mistake that this boiler will give us this 6.3 ton or 6300 kg steam per hour. But that is wrong. Because this rating is only from and at 100 degree C of feed water temperature. If you are not maintaining 100 Degree C in your feed tank then you will not get this 6.3 ton steam per hour. Most of the time the feed tank temperature is 80 degree C. So less the steam production than rating.

 

 

he graph in Figure 3.5.1 shows feedwater temperatures plotted against the percentage of the 'from and at' figure for operation at pressures of 0, 5, 10 and 15 bar g.

Fig. 3.5.1
'From and at' graph

The application of the 'from and at' rating graph (Figure 3.5.1) is shown in Example 3.5.1, as well as a demonstration of how the values are determined.

Example 3.5.1

A boiler has a 'from and at' rating of 2 000 kg/h and operates at 15 bar g. The feedwater temperature is 68°C.

Using the graph:

The percentage 'from and at' rating ≈ 90%

Therefore actual output = 2 000 kg/h x 90%

Boiler evaporation rate = 1 800 kg/h

The use of Equation 3.5.1 will determine a factor to produce the same result:

Equation 3.5.1

Where:

A	=	Specific enthalpy of evaporation at atmospheric pressure.
B	=	Specific enthalpy of steam at operating pressure.
C	=	Specific enthalpy of water at feedwater temperature.

Note: These values are all from steam tables.

Using the information from Example 3.5.1 and the Equation 3.5.1 the evaporation factor can be calculated:

Therefore: boiler evaporation rate = 2 000 kg/h x 0.9

Boiler evaporation rate = 1 800 kg/h


Some manufacturers will give a boiler rating in kW. This is not an evaporation rate, and is subject to the same 'from and at' factor.

To establish the actual evaporation by mass, it is first necessary to know the temperature of the feedwater and the pressure of the steam produced, in order to establish how much energy is added to each kg of water. Equation 3.5.2 can then be used to calculate the steam output:

Equation 3.5.2

Example 3.5.2

A boiler is rated at 3 000 kW (kJ/s) and operates at 10 bar g with a feedwater temperature of 50°C. How much steam can be generated?






Boiler Horsepower (BoHP) and Heating Surface


Sometimes you can see in the name plate that the boiler Model No. is 400-250.  This means the boiler horsepower is 400 and the maximum pressure that the boiler can operate is 250 psi(14.5 psi = 1 bar)

Some manufacturer also provides heating surface such as 113 m2(square meter). If you wants convert it to BoHP than just multiply it by 3.54. After gating BoHP you can also convert it to lbs/hour by multiplying it by 34.5.


Heating Surface = 113 m2
                            = 113 X 3.54
                            = 400 BoHP
                            = 400 X 34.5
                            = 13800 lbs/hour
                            = 13800/2.204
                            = 6300 kg/h