LogixPro Index

Dual Compressor
Student Exercises



Exercise #1 --- Single Compressor Operation

In this first exercise, the pressure switch PE1 (I:1/02) is to be utilized alone to control the operation of motor (O:2/0) and maintain the compressor's storage tank pressure. The pressure range will be dictated by the settings shown for PE1. Using your mouse, adjust both the limit (make 120PSI), and the adjustable span setting (20PSI) of PE1 to match the settings shown below.

Allow the user to start and stop the air system using the appropriate panel mounted switches, and ensure that the "Run" lamp is illuminated whenever the system is enabled. Lamp "C1" should be illuminated only when compressor #1 is actually running.

Prior to testing your program, adjust the system's discharge flow rate to 50% as shown. This setting should prove to be low enough that a single compressor will then be able to supply the needs of this particular pneumatic system.

Once you have created your program, download it to the PLC and test out it's operation. When the start button is pressed, the Compressor should start and begin to build up pressure within the storage tank. Once the pressure reaches 120PSI, the compressor should stop, and remain idle until the pressure in the storage tank drops below 100PSI.


Exercise #2 --- Alternating Compressors when Loading is Light

In this exercise, each compressor is to take it's turn bringing the storage tank pressure back up to the selected pressure setting. Pneumatic/Electric switch PE1 will continue to be utilized for this purpose, and the settings will remain the same as those used in the previous exercise.

The task of alternating back and forth between loads is sometimes referred to as a load toggle function, and there are numerous methods to accomplish this in relay logic. In this exercise however, you are asked to limit yourself to using only basic relay type instructions when creating your solution.

Prior to testing your program, adjust the system's discharge flow rate again to 50% as shown. As we have already determined, this flow rate can be readily maintained by a single compressor. Adding a second compressor however, will share the loading and allow for an extended cooling period between cycles.

Once you have created your program, download it to the PLC and thoroughly test out it's operation.

Finally, adjust the flow rate which controls the amount of air leaving the storage tank to 80% and then 100% and note the effect. At higher rates of flow, a single compressor will not have the capacity to supply this system's peak needs on it's own. Obviously we will need a bit of help at times from the second compressor.


Exercise #3 --- Coping with Large Demands for Plant Air

Your current program should be suitable for maintaining the desired pressure range as long as the plant air consumption remains relatively modest. As the plant air consumption approaches 100% capacity however, it becomes obvious that we will need to have both compressors running in order to satisfy this increased loading.

Modify your program so that the second pressure switch PE2 will detect when the storage tank pressure drops below our current minimum setting of 100PSI. This situation will occur if a single compressor is unable to keep up to the load and the tank pressure continues to drop. If and when the pressure drops to 98PSI, the idle compressor should be started, and both compressors will then continue to run until the tank is up to full pressure.

It is likely that your modifications will also result in both compressors being run when the system is first started and the tank pressure is initially being brought to within range of the pressure switches. This action will reduce the time it takes to bring the plant air system up to pressure, and is therefore considered desirable. Please ensure that your system does actually operate in this fashion.

Once you have created your program, download it to the PLC and thoroughly test out it's operation at both 50% and 100% rates of flow. When at 50% loading, the compressors should alternate with each taking a turn. At 100% loading, both compressors should engage once it is detected that the pressure is continuing to drop.

Your program should now be able to handle both light and heavy demands for air quite effectively. While this current solution likely performs as well as most systems employing relay logic, with just a bit more effort you should still be able to even improve upon this. Before continuing to the next exercise, run your system with the flow rate adjusted to 78%, 80%, and then 82%, and carefully note the result. You should now have a good idea of where improvements might be made.


Exercise #4 --- Detecting When 1 Compressor is not Enough

When the plant's requirement for air closely matches the maximum volume that can be supplied by a single compressor, our current control solution simply falls apart. It's possible that this single compressor could run for hours without ever being noticed, nor obtaining any down time in which to cool. How long one of these compressors might be allowed to run continuously would normally be specified by the manufacturer, but for our purposes we will just ensure that excessive run-on can simply not occur.

When attempting a solution for this run-on problem, a couple of ideas may come to mind. If a single compressor runs beyond the time that would normally suffice to attain full pressure at 50% or 60% flow, the idle compressor could be started to assist in the effort. A second method might be to track the time it takes for the pressure tank to drop from it's maximum to minimum settings, and from this determine beforehand whether 1 or 2 compressors need to be employed.

If given some thought, you may detect minor faults with either of the above methods. The first method could permit repeated short cycling of the back up compressor when operating at certain continuous flow rates. The second method attempts to anticipate load demand, but may not accommodate rapid changes in plant air utilization. It may be that your solution should combine both methods, or possibly a new one not even discussed. Your goal should be to analyse the system and come up with the best solution possible using the available equipment.

Modify your program so that continuous run-on of a single compressor will not occur. Your solution should continue to alternate between compressors when plant flow rates are typically 50% to 60% or lower. In addition, try to equalize wear on each compressor, and avoid short cycling as much as practically possible.

Continue to utilize PE2 to detect if the system pressure descends below the prescribed minimum, but feel free to tune or adjust both this pressure switch and any timing mechanisms employed in order to attain the best possible performance.

In order to monitor how well your system is working, provide a means to display the accumulated run-time for each compressor. Utilize both the panel mounted selector switch, and LEDs for this purpose.

Once you have your program tuned to perfection, you should have a good grounding in the basics of controlling many types of similar systems. It's fairly common to find fans, or pumps etc. being controlled in a similar fashion, and the techniques you've developed here should closely apply.






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