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Handling sludge: gas or slurry mixing?

Supplier: Hurll Nu-Way
17 September, 2013

Hurll Nu-Way is a supplier of "continuous anaerobic digester gas mixing systems", manufactured in the UK by Utile Engineering, utilising sliding vane gas compressor technology.

This technology has proven to be the most efficient and cost effective method to mix, absorbing approximately one third of the power required for "pump" mixing.

Gas mixing requires a reliable gas compressor to work in conjunction with the most efficient and effective mixing systems. Overseas, for many years, operators have recognised that continuous unconfined gas mixing, using very efficient sliding vane gas compressors, provides the best results for both reliability and whole life cost.

With power usage coming under ever increasing scrutiny, it has been shown that slurry (pump) mixing absorbs almost three times as much power as a sliding vane machine for the same duty point. The introduction of a small, metered amount of oil to the gas compressors may therefore be seen to be a small price to pay for the longer life, reduced maintenance, lower power consumption, noise levels and less downtime that result. All of these factors contribute to considerably lower whole life cycle costs of a sliding vane mixing system.

With the increasing uptake of Co-generation, the reduced power absorbed by a sliding vane machine results in huge increases in available power that can be exported from the site to provide a significant income. The benefits to the Operator are:  possessing a very reliable system at low capital cost, with excellent power consumption thus reducing carbon footprint.

It is logical to select robust and reliable equipment which requires the least amount of maintenance. The sliding vane gas compressor uses a simple, well proven design to compress the gas inside the heavy duty Ductile Iron cylinder resulting in a quiet machine that delivers pulsation free gas.

In contrast, a "pump" type unit that relies on high velocity sludge flow through feed pipe work to mix accurately can result in bore erosion. This coupled with nozzle wear inside the tank, means high maintenance and additional expenses. The downtime comes with a reduction in overall mixing efficiency, and as a consequence, results in lower gas production.

Sludge pump seal failures could result in sludge on the floor, having Health & Safety issues for Operators to deal with as well as the obvious maintenance aspects. Sludge pumps are also intolerant and susceptible to a varying sludge depth. They tend to require a stable head (fixed sludge depth) to ensure suitable mixing.

Maintenance routines for the Utile sliding vane gas compressors on site typically require periodical topping up of the oil tank. Checking of blades for wear is annual and can be carried out without having to dismantle the machine; a simple removal of an eyebolt is all that is needed.

In conclusion, when investing in equipment, it is important to consider not only the short term capital costs, but both the direct and indirect whole life costs of it.