Effect of Ambient Conditions on Boiler Efficiency and Design

Ambient conditions, namely temperature, humidity, altitude have an important role in determining the efficiency of a boiler and the sizing of auxiliaries.

• Stack loss, which constitutes —70-80% of the total loss, is measured with ambient temperature as the datum. The efficiency would therefore appear to be lower by —0.25% for each 5°C increase in ambient temperature.

• It is important to ascertain the range of temperature and humidity variation to ensure proper sizing of the fans and other volumetric devices. It is not unusual to encounter an ambient temperature variation of -5°C to +50°C in the tropics and -20°C to +35°C in temperate climates. This translates to a variation of nearly 20% in the specific volume of air.

• The fan must be sized by the maximum temperature and the motor rating by the minimum temperature.

• All motors are required to withstand the highest ambient temperature and the insulation has to be suitable.

• Gas turbine outputs vary dramatically as depicted in the figures of Chapter 14.

• Thermal insulation is another area affected by the ambient temperature. Economi­cal insulation results when there is a difference of 20-25°C between the ambient and the lagging temperatures. This can result in a surface temperature of 70-75°C in summer in the tropics. There can be a doubling of insulation thickness if a flat 60°C is to be met at all times under such conditions!

The effect of humidity is rather small in both boiler efficiency and auxiliary sizing. Alti­tude has a major impact on the sizing of fans as the specific volume of air increases with altitude.

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