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Steam Engineering Principles and Heat Transfer

Thermal Rati​ng

Design ratings for items of plant can be both helpful and misleading, as changing any factor can alter the predicted heat output. Also, how to calculate steam load requirements from the kW rating.

Some items of manufactured plant are supplied with information on thermal output. These design ratings can be both helpful and misleading. Ratings will usually involve raising a stated amount of air, water or other fluid through a given temperature rise, using steam at a specified pressure.
They are generally published in good faith with a reasonable allowance for fouling of the heat transfer surface.

It must be clear that changing any factor at all will alter the predicted heat output and thereby the steam consumption. A secondary fluid which is colder than specified will increase the demand, while steam at less than the specified pressure will reduce the ability to transfer heat.

Temperature and pressure can often be measured easily so that corrections can be applied. However, flowrates of air, water and other fluids may be far more difficult to measure. Undetected fanbelt slip or pump impeller wear can also lead to discrepancies, while lower than expected resistances applied to pumps and fans can cause flowrates to be higher than the design values.

A more common source of error arises from the assumption that the manufacturer’s rating equates to actual load. A heat exchanger may be capable of meeting or exceeding a given duty, but the connected load may often only be a fraction of this. Clearly it is useful to have information on the thermal rating of plant, but care must be taken when relating this to an actual heat load.

If the load is quoted in kW, and the steam pressure is given, then steam flowrate may be determined as shown in Equation 2.8.1:

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