The performance of air handling units

The majority of fans used in building services today are fitted in air handling units. Under such circumstances the performance of a fan is quite different from that established by BS 848. Air handling units should be tested according to BS 6583 (1985).

If a centrifugal fan is installed in a cabinet, as is the case with the fan section of an air handling unit, the proximity of the cabinet sidewall to the fan inlet eye imparts a swirl to the air stream entering the impeller and affects its performance. This must be taken into account. ASHRAE (1993) and AMCA (1973) give a relationship between the distance from the fan inlet eye to the cabinet wall and the loss of total pressure incurred. See Figure 15.24. It is recommended that a cabinet wall should not be closer than 0.75 D to the fan inlet and that adjacent fans in a cabinet should not be closer to each other than 1.5 Z), where D is the diameter of the fan inlet. Similar considerations apply to the roof and floor of the cabinet and the centre-line of the inlet eyes should be equidistantly located between them.

Fig. 15.24 The relationship between total pressure loss and the proximity of a cabinet wall or an adjacent fan. A cooler coil section is attached when testing a fan cabinet to BS 6583: 1985. Separate tests are done for the cases of draw-through units (as shown) and blow-through units. If, as is common, multiple fans are used, they are driven from a common impeller shaft.

In order to test and specify the performance of the fan section of an air handling unit to BS 6583 (1985) the fan cabinet is attached to a cooler coil section. This is to simulate actual operating conditions. The combination is then regarded as a fan and tested to BS 848 (1980). Thus the characteristics of the fan in the cabinet and the influence of a cooler coil section in close proximity are taken into account. Separate tests are carried out when the cooler coil section is located on the discharge side of a fan cabinet.

The difference between the total pressure at inlet to the cooler coil cabinet and the static pressure at the outlet from the fan cabinet, is measured against volumetric airflow rate for each fan speed considered, and the results plotted for the fan and cooler coil cabinet combination. The total pressure loss through the cooler coil cabinet is established according to BS 5141: 1975 for each airflow rate and is added to the curve for the fan and coil cabinet combination. This then gives a curve for the fan cabinet alone. By subtracting the pressure drop (determined according to the appropriate BS) for any other plant items to be included in the air handling unit, the manufacturer can establish a curve for the performance of the unit as a whole. Such a characteristic curve expresses the static pressure offered by the unit to offset external total pressure losses. Figure 15.25 illustrates this. Some of the velocity pressure at fan discharge can be profitably converted to static pressure if a smooth expansion piece, of adequate length, is fitted. This is then also available for offsetting part of the external total pressure loss. See section 15.10 and Figure 15.22.

The external total pressure loss is usually termed the external resistance and is the sum of the total pressure losses on both the suction and discharge sides of the air handling unit. It should include the extra pressure loss incurred by the presence of the dirt on the filter, just before the filter is to be replaced.

Posted in Air Conditioning Engineering