How do you calculate the efficiency of Air pre heater (APH) in Boilers??

 

APH is an auxiliary used in steam generators or boilers to increase the Boiler efficiency and to reduce the fuel consumption.

There are different types of air heaters. Major types are tubular & regenerative air preheater.

These may be installed horizontally or vertically as per requirement.

In some design of APH air flows outside the tubes and flue gas inside the tubes where as in some cases air flows inside the tubes and flue gas outside of the tubes.

The main functions of APH is to reduce the flue gas temperature to allowable limit that is coming out from Boiler and to increase combustion air temperature.

The efficiency of the APH are calculated from two ways one is from air side other from gas side.

APH gas side efficiency

ηAPHg = (Flue gas inlet temp.Tfi-Flue gas outlet temp.Tfo) X 100 / (Flue gas inlet temperature tfi-Air inlet temperature Tai)

 

APH air side efficiency

ηAPHa = (Air outlet temp.Tao-Air inlet temp.Tao)) X 100 / (Flue gas inlet temperature tfi-Air inlet temperature Tai)

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Calculate the APH gas side & air efficiency if its flue gas inlet and out let temperature are 230 deg C and 145 deg C and air inlet and out let temperatures are 28 deg C & 180 deg C respectively.

ηAPHg = (Flue gas inlet temp.Tfi-Flue gas outlet temp.Tfo) X 100 / (Flue gas inlet temperature tfi-Air inlet temperature Tai)

 

ηAPHg =(230-145) X 100 / (230-28)

ηAPHg = 42.1%

ηAPHa = (Air outlet temp.Tao-Air inlet temp.Tao)) X 100 / (Flue gas inlet temperature tfi-Air inlet temperature Tai)

 

ηAPHa = (180-28) X 100 / (230-28)

ηAPHa = 75.24%

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A Tubular type APH is having air side efficiency 79%, its flue gas and air inlet temperature are 225 deg and 30 deg C respectively, calculate the APH outlet air temperature

We have,

ηAPHa = (Air outlet temp.Tao-Air inlet temp.Tao)) X 100 / (Flue gas inlet temperature tfi-Air inlet temperature Tai)

 

79% = (Tao-30) X 100 / (225 – 30)

Tao-30 = 0.79 X 195

Tao = 184.05 deg C

A Tubular type APH is having gas side efficiency 39%, its flue gas and air inlet temperature are 225 deg and 30 deg C respectively, calculate the APH outlet flue gas temperature

We have,

ηAPHg = (Flue gas inlet temp.Tfi-Flue gas outlet temp.Tfo) X 100 / (Flue gas inlet temperature tfi-Air inlet temperature Tai)

 

39% = (225-Tfo) X 100 / (225-30)

0.39 X 195 = 225-Tfo

Tfo = 148.95 deg C

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