The pressure loss, delta p = p1 - p2, occurring downstream from the filter, is the second most significant basic parameter of the filter, after separation efficiency E. The pressure loss is proportional to the gas flow velocity through the filter. Should delta p be known for a certain velocity, it can, without difficulty, be recalculated for any other velocity. Pressure loss delta p is likewise proportional to filtering layer thickness. When using a multi-layer filter, the total pressure loss is given by the sum of the partial pressure losses at the individual layers. Furthermore, delta p depends on the value of the packing density coefficient. This increases with decreasing distance between the filter fibres. Lastly, delta p always depends on the temperature of the gas flowing through, as the viscosity of the gas increases with temperature. In reality, the differential pressure of fibre filters are always smaller than those calculated theoretically. This is because of the random filter structure. The differential pressure represents a pressure loss measured for the total filter and is of considerable significance as a source of performance loss. This pressure loss consists of:
- Pressure loss of the filter housing
- Pressure loss through the filter element
- Pressure loss through loading with liquid aerosols
- Pressure loss due to collected solid particles
Pressure loss rises more than proportionally with the throughflow quantity, mathematically described through Bernoulli’s energy theorem as:
Formula 4.6.2.1
If p = constant, the continuity equation runs
Formula 4.6.2.2
The pressure loss thus being described by the expression
Formula 4.6.2.3
Because density as well as cross-section are constant entities, the expression is supplemented with regard to the operational state by means of formula 4.6.2.4.
Formula 4.6.2.4
This formula also determines the mutual dependence of throughflow quantity V and absolute pressure p in relation to pressure loss delta p.
