A cyclone pressure drop results from the following factors (Shepherd and Lapple 1939):
1. Loss of pressure due to gas expansion as it enters the cyclone
2. Loss of pressure due to vortex formation
3. Loss of pressure due to wall friction
4. Regain of rotational kinetic energy as pressure energy
Factors 1, 2, and 3 are probably the most important. Iinoya (1953) showed that increasing wall roughness decreases the pressure drop across the cyclone probably by inhibiting vortex formation. If this assumption is true, then energy consumption due to vortex formation is more important than wall friction. First (1950) found that wall friction is not an important contributor to pressure drop.
Devices such as an inlet vane, an inner wall extension of the tangential gas entry within the cyclone body to a position close to the gas exit duct, and a cross baffle in the gas outlet lower the pressure drop (Leith 1984). However, such devices work by suppressing vortex formation and decrease collection efficiency as well. Designing cyclones for low pressure drop without attaching devices that also lower collection efficiency is possible.
Cyclone pressure drop has traditionally been expressed as the number of inlet velocity (v^) heads AH. The following equation converts velocity heads to pressure drop AP:
The value of AH is constant for a cyclone design (i.e., cyclone dimension ratios), while AP varies with operating conditions.
Many analytical expressions for determining AH from cyclone geometry are available (Barth 1956; First 1950; Shepherd and Lapple 1940; Stairmand 1949; Alexander 1949). None of these expressions predicts pressure drop accurately for a range of cyclone designs; predictions differ from measured values by more than a factor of two (Dirgo 1988). Further, evaluations of these models by different investigators produced conflicting conclusions as to which models work best.
Dirgo (1988) and Ramachandran et al. (1991) developed an empirical model for predicting pressure drop, which was developed through statistical analysis of pressure drop data for ninety-eight cyclone designs. They used stepwise and backward regression to develop the following expression for AH based on cyclone dimension ratios:
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