Distribution Energy

Distribution energy is most commonly electrical energy consumed to operate fans and pumps, with fan energy typically being far greater than pump energy except in all-water distribution systems. The performance of similar fans is related by three fan laws which relate fan power, airflow, pressure and efficiency to fan size, speed and air density. The reader is referred to the ASHRAE Handbook: HVAC Systems and Equipment for additional information on fans and the application of the fan laws.3

Fan energy is a function of the quantity of airflow moved by the fan, the distance over which it is moved, and the velocity of the moving air (which influences the pressure required of the fan). Most HVAC systems, whether central or distributed packaged systems, allair, all-water, or a combination are typically oversized for the thermal loads that actually occur. Thus the fan is constantly required to move more air than necessary, creating inherent system inefficiency.

One application of the third fan law describes the relationship between fan horsepower (energy consumed) and the airflow produced by the fan:

where

W = fan power required, hp

Q = volumetric flow rate, cfm

Because fan horsepower is proportional to the cube of airflow, reducing airflow to 75 percent of existing will result in a reduction in the fan horsepower by the cube of 75 percent, or about 42 percent: [(0.75)3 = 0.422] Even small increases in airflow result in disproportional increases in fan energy. A ten percent increase in airflow requires 33 percent more horsepower [1.103 = 1.33], which suggests that airflow supplied solely for ventilation purposes should be kept to a minimum.

All-air systems which must move air over great distances likewise require disproportionate increases in energy as the second fan law defines the relationship between fan horsepower [W] and pressure [p], which may be considered roughly proportional to the length of ducts connected to the fan:

The use of supply air at temperatures of less than 55°F (13°C) for primary cooling air permits the use of smaller ducts and fans, reducing space requirements at the same time. This technique requires a complex analysis to determine the economic benefit and is seldom advantageous unless there is an economic benefit associated with space savings.

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