FEATURE HYDRAULICS & PNEUMATICS
Designing under
pressure I
nfectious diseases are a constant threat to society – as evidenced by the
current Coronavirus outbreak. To protect the human population, laboratories are increasingly reliant on automated liquid handling systems to develop and produce new vaccines. The advantages of these systems are well known, but they represent a significant investment for a laboratory and often come with a host of features that are surplus to requirements. In these situations, pressure-
controlled liquid dispensing systems present a cost-effective alternative by providing a simple, quick and reliable way to dose nanolitre- to millilitre- scale volumes of fluid with precision. Pressure-controlled liquid dispensing
systems have the flexibility to overcome wide variants in fluid properties and composition and are suitable for substances as thin as alcohol or as thick as honey. They can also cope with corrosive and acidic substances. Mechanically simple, these dispensing
systems require a minimum number of components, which makes cleaning simple. A pressure regulator, paired with a safety valve, pressurises the reservoir containing the liquid. This pressure then drives the liquid through the tubing and solenoid dispense valve. Finally, the nozzle dispenses the liquid into the vessel via a needle tip with a calibrated orifice (see image, above right). It is important to consider the following
factors when designing a pressure-based system, as they all affect the quantity and quality of the doses: 1. Flow resistance The system’s flow rate is defined by the pressure, as well as the overall resistance of the fluid path. This resistance depends on the tube’s inner diameter (ID), length, dispense valve geometry and fittings. Dispense tip size directly affects the velocity of liquids. Unlike pressure and time – which can be controlled and modified via software – the right tip should be selected from the outset. 2. Pressure Pressure has a significant effect on dosing volumes and – more importantly – helps control the velocity of the fluid as it passes through the dispense tip. It can also be used as a control for fluid viscosities, achieving clean, splash-free
24 JULY/AUGUST 2020 | DESIGN SOLUTIONS
dispenses. Pressure can be generated in several ways – for example, by using an external gas source such as nitrogen, or a compressor to pump air into the closed liquid reservoir. The pressure ranges found in automated dispense systems are generally low – only 100–250 millibars. Nevertheless, it is important to have safety measures in place that can relieve the pressure in case of a leak or other technical failure. 3. Dispense time The dispense quantities are controlled by solenoid valve controls, with the volume of fluid dispensed primarily influenced by the valve’s cycle time. Valves with short, highly repeatable, response times offer much better dispense precision at higher speeds. It is important to note that solenoids generate energy when open, and their performance often changes as they warm up. Operating a higher-flow valve for shorter durations reduces this heat but requires a valve with excellent repeatability, such as Festo’s VTOE (see boxout, right).
SCALING UP: MULTI-HEAD DISPENSE SYSTEMS One of the biggest benefits to pressure- based dispense systems is their scalability. Individual dispense heads can be combined easily to create multi-channel dispense heads that handle different aliquot volumes, fluids and pressures. Fluids with different classes can even be dispensed by mounting multi- channel dispense heads on an electric
The components in a pressure-operated liquid dispensing system
Paul Kendall, industry sector manager – Lifetech at Festo GB,
explains how pressure-controlled liquid dispensing systems are an ideal solution for laboratories developing and producing new
vaccines, and looks into the factors to consider when designing a pressure-based system
axis – creating a gantry system. When it comes to multi-channel
systems, however, small differences among the inlets, valves and nozzles can cause some channels to dispense higher volumes than others – with intrinsic tip- to-tip variability of the order of 4%. At resolutions of 1ms or higher, it is difficult to compensate for these small variations using the processor that is controlling the solenoid dose time. A better course of action is to calibrate the separate channels by varying the pull-in and hold- in solenoid current so that all channels dispense the same quantity using the same dispense times. Festo makes this easy with the VAEM valve control module, which utilises user-friendly software to reduce the tip-to-tip variability to less than 1% in most cases.
DELIVERING RESULTS Improvements in the scale, speed and quality of pharmaceutical and biotechnology processes enabled by automated liquid handling have helped to reduce timescales and deliver faster results, while also making drug discovery more
efficient. Pressurised liquid dispensing systems can help optimise throughput in lab automation applications at a fraction of the cost of complex robotics.
Festo
www.festo.co.uk/liquidhandling
Pressurised liquid dispensing systems can help optimise throughput in lab automation applications at a fraction of the cost of complex robotics
VTOE DISPENSE HEADS ACHIEVE HIGH LEVELS OF PRECISION
Festo VTOE pre-assembled dispense heads consist of a manifold, 2/2 valve, dispenser nozzle and tubing connector. This family is available with a transparent
polycarbonate (PC) channel plate, as well as a media resistant channel plate made from polyether ether ketone (PEEK) for applications involving aggressive media. Up to 12 dispense heads can be mounted on a single rail— enabling different fluids and aliquot volumes to be dosed in parallel. Dosing needle IDs are available in three sizes: 0.32, 0.6 and 1.0 mm.
/ DESIGNSOLUTIONS
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