Replacement of Dyno-drives with Variable Frequency Drives (VFD's) in Washer Drum Drives

Background

The contents of the digester, after cooking, are blown down to a blow tank. The blown pulp is then washed, to remove the dissolved lignin and chemicals.

Usually, washing is practised in counter current fashion, involving 3 or 4 stages of washing, using rotary drum washers. The washed pulp is then sent for bleaching and further processing. The rotary drum washers are operated under vacuum, utilising a barometric column. These drum washers are driven by a variable speed system, to achieve the desired speed variation, according to the throughput of the plant.

Previous status

In one of the old integrated paper plants, the washer drum drives were originally supplied with AC commutator motors. As these commutator motors had frequent maintenance problems, these were replaced with dyno-drives. The dyno-drives, though have lesser maintenance problems, are inefficient at lower speeds. As the washers were operating at 50 - 60% of the rated speed for majority of the time, the replacement of these drives with more efficient drives, such as, variable frequency drives (VFD) can result in substantial energy savings.

Energy saving project

The dyno-drives of the washers were replaced with variable frequency drives (VFD's).

Concept of the project

The dyno-drives are very inefficient at lower speeds. The dyno-drives also require special attention and maintenance, because of its semi-open construction. The variable frequency drives (VFD) are more efficient at lower/all speeds also and require lesser maintenance, in comparison to the dyno-drive.

Implementation status, problems faced and time frame

The dyno-drives in both the washer drums were replaced with 22.5 kW variable frequency drives (VFD's). A VFD can achieve the exact speed variation requirement energy efficiently depending on the process requirement. The problem faced during the implementation stage was the frequent tripping of the VFD's. The supplier studied this and suitable remedial action was taken, to solve the problem. The entire project was executed in 3 months time.

Benefits achieved

The replacement of dyno-drives with VFD's, resulted in a net reduction in power consumption. The net power saving achieved was 36,024 units/year (equivalent of 5.23 kW). The other major advantage is, the precise speed variation, which can be achieved.

Financial analysis

The annual energy saving achieved was Rs.1.08 lakhs. This required an investment of Rs.2.50 lakhs and had a simple payback period of 28 months.