ANALYTICAL AND LABORATORY EQUIPMENT 13
Effects of heat on solenoid valves
How to mitigate the impact of heat on valves is something that chemists and electronic design engineers must consider, reports Gary Stevens.
T
here 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.
Te 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. Tat heat creates a number of problems, from unpredictable control of the media to cold flow and distortion of the
Fig. 2. A PWM valve driver.
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.
What can be done to reduce the heat and power requirements of solenoid valves? Tere are a number of options available to the system designer, the two most common being latching valves and strike-and-hold drivers.
Fig .1. Example of a latching valve from NResearch.
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. Teir 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. Teir holding is limited only by the magnetic force designed into the device and they are ideal for use on battery-operated portable equipment. Te 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. Tis type of operating system is in general use but the methods used to attain the function vary widely. Tree of the methods generally used are voltage control, current control and pulse width modulation (PWM). Te 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. Tere 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.
For more information ✔ at
www.scientistlive.com/eurolab
Gary Stevens is with NResearch, based in Northampton, UK.
www.nresearch.com
www.scientistlive.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92