It is mainly from the economic point of view that the service life of fibrous layers is of interest.
Diagram 4.10.3.1
Assessment criteria are influenced largely by the interaction between the design and construction of the filter and the conditions of operation. If the content of solid particles in air can be ignored, the service life of the fibrous layer may amount to several years. In practice, however, these solid particles exercise a decisive influence on the deformation of the fibrous structure, and, therefore, on the service life of the fibrous layer. The displacement of liquid from the fibrous layer causes only a portion of the solid particles to be washed out of this layer. The remainder settles within the filter medium, causing an increase in pressure loss. The resulting strain on the fibres may, under certain circumstances, cause the functioning of the filter to be impaired. In order to reduce the solid matter pollution of depth filters, these are preceded by a surface filter in the circuit to separate coarse solid particles and liquid droplets
Figure 4.10.3.1
It is advisable to install a differential pressure gauge to enable a continuous optical check of filter element performance. Despite of all measures taken to obtain the most suitable filter for a given operating situation, the effective service life of filter elements cannot be predicted, as the differential pressure builds up only slowly in the course of operating time and the content of solid matter in the compressed air differs from case to case. The operator of a filtration system must determine the period of element replacement with optimum operating economy in mind. Knowledge of the relevant data enables such a period to be estimated. Differential pressure measurement indicates the pressure loss across the filter. This establishes two values, pressure before the filter p1 and pressure after the filter p2. These values offer means of estimating the performance loss along the lines of Pv = 1 - p22/p12. The installed motor power Pi of the compressor is a further basic indicator. The power loss through the temporary pressure loss of the filter can be deduced from the motor performance as a numerical value. Multiplied by the cost of electric current Cp, a mathematical relation between operating costs and pressure loss can be deduced. The investment costs CE for a new filter element must be compared with the operating costs. Formula 4.10.3.1 encompasses these specified criteria and determines the period of element replacement tEr. (Note : Anglicisation of some symbols)
Formula 4.10.3.1
Degrees of efficiency are not contained in this formula. These should, however, be taken into account. In the first instance, the relationship between installed motor power and effective shaft performance of the compressor should be observed. In addition, the characteristics of the service life of the filter elements in accordance with Diagram 4.10.3.1. Alternatively, one could lay down a maximum operational pressure loss linked to the use of the element and, when this is reached, element replacement should be effected.
