How to mitigate the impact of heat on valves is something that chemists and electronic design engineers must consider, reports Gary Stevens.
There are a multitude of solenoid valves used in lab instruments and process systems. What they have in common is that heat can be detrimental to their performance and reliability.
Most solenoid valves for analytical and scientific use are designed to be small, inert and have low internal volumes. However, many do not have a 100 per cent duty rated coil - something that is often overlooked when the valves are being used and as a result, the performance can be compromised. Because of this, applications that may well have worked using manual valves during the development phase can have unwelcome results when solenoid valves are energised for long periods.
Why the duty rating is so important comes down to basic physics: put power in and heat will come out somewhere along the line.
The coil in the solenoid is designed to develop sufficient force to operate the valve at a specified voltage by attracting the armature and completing the magnetic circuit. Once the armature has moved and completes the magnet circuit between itself and the core, the power required to keep the valve energised is reduced. However, the coil is still drawing the same current as it did to energise the valve initially and the excess power is dissipated as heat. That heat creates a number of problems, from unpredictable control of the media to cold flow and distortion of the body materials and even total valve failure. Heat can also be transferred to the media going through the valve, which can affect the chemistry of analytical applications - not a good idea.
Solenoid valves: What can be done to reduce the heat and power requirements?
What can be done to reduce the heat and power requirements of solenoid valves? There are a number of options available to the system designer, the two most common being latching valves and strike-and-hold drivers.
Latching valves are valves that are bi-stable, being energised to change state but then held in position by mechanical means, more often than not magnetic. Their advantages include excellent power saving (as no power is required once the desired state is achieved) and the fact they maintain position even when the power is switched off. Their holding is limited only by the magnetic force designed into the device and they are ideal for use on battery-operated portable equipment. The disadvantages of latching valves include the fact that they are more expensive than standard valves and can sometimes be larger than standard products.
With regard to strike-and-hold drivers, this name refers to energising valves using an electronic driver to first apply a pull-in voltage and then a lower voltage to hold the valve in the energised state. This type of operating system is in general use but the methods used to attain the function vary widely. Three of the methods generally used are voltage control, current control and pulse width modulation (PWM). The advantages of strike-and-hold drivers include the fact that they use standard valves at no extra cost and they also work on pinch valves. Power saving can typically be 66 per cent (but will vary between manufacturers) and they provide improved battery life when used on portable equipment. There are only two main disadvantages associated with strike-and-hold drivers: they need a driver for switching and some pinch valves need increased holding currents as they need to maintain compression, so less power savings are attained.
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Gary Stevens is with NResearch, based in Northampton, UK.