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Process control systems

Johno12345
Hi,


I'm looking at some plant in the facility where I work, which uses a significant amount of energy. Some of it needs replacing because its worn out and I think its a good opportunity to look at the process and potentially upgrade the controls with a view to saving energy. 


I spoke to our energy advisor, which didn't give me a great amount of confidence.


It basically consists of systems that cool a liquid (cooling towers), move the product hydraulically, and stir the liquid. In total, there are 15 motors up to about 25kW and it all runs star-delta, manually controlled, part has a thermostat but that brings on the whole plant where only a small part could be on. 


I think a lot could be run with inverters, based on the process temperature and outside temperature, along with other inputs.


This is where it gets beyond me, I can describe how I want it to operate, and what I want to achieve, but all the people I speak to seem to fail to grasp the system as a whole, and can only focus on a tiny part, like 2 or 3 motors or pumps.


I'm obviously looking for a specialist but don't know what they would be called, or where to look. Does anyone know where or who i could speak to? 


Thanks
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  • 0
    Hi Johno

    You need to produce a flow diagram, with each stage marked, and each input to the stage, such as temperature or flow. Where there is a motor involved you need to define the degree of control that can be applied, for example, a high-speed mixer has a speed^4 characteristic, so much speed reduction would turn out a different product, particularly if the idea is to mix in air. Really you need to talk to the plant designer because there are usually a lot of variables to take into account, many of which may not be obvious. From that point, a PLC and a re-designed control panel are all that is necessary along with drives where significant speed reduction is sensible, and extra measuring sensors as necessary. This is all likely to cost quite a lot of money and a good deal of your effort, but the overall system design should not be too difficult, given the assistance suggested above. The payback period may be long, and you may be able to save quite a lot of money simply by changing the motors for modern high-efficiency ones (with a gain in reliability for a period), and perhaps switching them on and off when required with a relay controller and a couple of sensors. Process control is not really like building controls, it is usually quite complex, and needs to be exactly right to make the correct product specification. Often things work very "closed loop" with the exact result, something like the exact colour of biscuits coming from an oven varying the temperature of sections, for example. The EU is terribly keen on VFDs, but in some cases they have little or no advantage in energy saving where power reductions are not possible. Your best bet may well be to try the cooling tower fans first, which can easily vary with the ambient and product temperatures. You will need to make a lot of measurments to correlate the two, to design the control algorithm. Good luck.
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  • 0
    Hi Johno

    You need to produce a flow diagram, with each stage marked, and each input to the stage, such as temperature or flow. Where there is a motor involved you need to define the degree of control that can be applied, for example, a high-speed mixer has a speed^4 characteristic, so much speed reduction would turn out a different product, particularly if the idea is to mix in air. Really you need to talk to the plant designer because there are usually a lot of variables to take into account, many of which may not be obvious. From that point, a PLC and a re-designed control panel are all that is necessary along with drives where significant speed reduction is sensible, and extra measuring sensors as necessary. This is all likely to cost quite a lot of money and a good deal of your effort, but the overall system design should not be too difficult, given the assistance suggested above. The payback period may be long, and you may be able to save quite a lot of money simply by changing the motors for modern high-efficiency ones (with a gain in reliability for a period), and perhaps switching them on and off when required with a relay controller and a couple of sensors. Process control is not really like building controls, it is usually quite complex, and needs to be exactly right to make the correct product specification. Often things work very "closed loop" with the exact result, something like the exact colour of biscuits coming from an oven varying the temperature of sections, for example. The EU is terribly keen on VFDs, but in some cases they have little or no advantage in energy saving where power reductions are not possible. Your best bet may well be to try the cooling tower fans first, which can easily vary with the ambient and product temperatures. You will need to make a lot of measurments to correlate the two, to design the control algorithm. Good luck.
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