Modulating controls are a little different; the first thing you need to know is how the system works, then you can proceed to the adjustment. The modulating range of most compressors with this type of control is from 100 % down to 60 %. This means on a 100 /110 psig unit the inlet valve starts to close off the inlet at 100 psig and will continue to close until the capacity is near or at 60% which will be about 105 psig. At this point the amperage draw will become stable or stop rising and then if system capacity has been reached, the pressure will continue to rise (the capacity continues to drop from 60 % down to 40 %) and then reaches the unload set point of 110 psig and unload, and at that point the sump pressure will begin to fall, the amperage draw falls and you save energy. The system pressure will maintain a stable pressure during modulation and then slowly increase to the unload pressure. If you need the capacity of the compressor, then the pressure will stay at the same level, not up to 110 lbs, unload and drop to 100 lbs or lower and re-load. Lot of work on the inlet system, poppet valves seem to hold up better than the butterfly valve system.
In order for a modulating control system to work properly, it must be set up properly, there must not be any binds in the inlet valve or control linkage, no leaks on the tuning, no rust build up in the lines to reduce the CFM of the air that is being supplied to the controls (see controls system for explanation). The inline check valve must be clean and stop the back flow of air to the blowdown, therefore the blowdown valve must be in good working order, as well as the rest of the control system.
Modulating Control system
The purpose of the Compressor control system is to regulate the Compressor intake air to match the amount of compressed air being used. In our explanation of the control system we will use a Compressor modulating system with the primary pressure at 100 Psig, and the secondary pressure at 110 Psig. This pressure setting will cause the line pressure to maintain a constant pressure of 100 Psig until the capacity of the unit has been reached. If more air is required than the unit will produce, the line pressure will fall, if less air is being used, the line pressure will continue to rise until it reaches the secondary Setting and then, will unload. There are several types of controls system. Understand the type you are working on before attempting repairs.
To have a greater understanding of the control system we will discuss each part of the system separately.
*$$$$ money lost*
*$$$$ money lost*
Understanding the control circuit on air compressors is not complicated, unless you make it complicated. You must first understand and know how pressure and CFM work together. When you understand this, you will understand why control adjustment is quite easy. Pressure (PSI) controls the force of work being done with compressed air, while cubic feet per minute (CFM) controls the volume of work being done with compressed air.
We will start with how much CFM can be loss through an orifice at a given pressure and 70ºF.
A 1/64 hole will lose .406 CFM @100 Psig - .229 CFM @50 Psig.
A 1/32 hole will lose 1.62 CFM @100 Psig - 0.916 CFM @50 Psig.
A 1/16 hole will lose 6.49 CFM @100 Psig - 3.66 CFM @50 Psig.
A 1/8 hole will lose 26.0 CFM @100 Psig - 14.7 CFM @50 Psig.
A 1/4 hole will lose 104 CFM @100 Psig - 58.6 CFM @50 Psig.
After looking at these calculation you can see that we need more volume at a lower pressure to maintain the same work force at a higher pressure. With this in mind we will continue with the control adjustment on the Sullair compressors.
We will use a compressed air system of 100 Psig working pressure with a 110 Psig maximum cut out pressure.
There are several items that make up a modulating control system. A back pressure regulator (with bleed orifice), a 1/64 bleed hole in the piping (not on all systems) system for moisture drain, an air actuator, a pilot valve, check valve and pressure switch. The control system get its air from the dry side of the sump tank downstream of the separator, but upstream of the minimum pressure/check valve. This air is very hot and holds quite a bit of water vapor. This vapor will begin to turn to liquid water as it cools. This causes quite a problem in most all control systems.
The control actuator starts its travel as the pressure air reaches 100 Psig and the back pressure regulator begins to open and let air enter the control piping system. As the air enters the piping system that contains two (in most cases) bleed orifices (approximately 1/32 in size) it must first overcome the lose of air volume before it can begin to pressurize the chamber and force the actuator to start its travel. There are two objects that the air volume must overcome before it can begin its work, one is the lose of air through the orifices, the other is the resistance of the actuator control spring. It take volume and pressure to overcome the orifices and volume and pressure to overcome the spring resistance.
Pressure along will not do the job. We must have pressure and CFM (volume) in order to complete our task. If the supply line that is supplying our signal air is not restricted, we are in good shape, but what if our signal line is restricted. We may not be able to complete the task of supplying both pressure and CFM (volume) to the control system.
According to the chart we will need about (may vary on control systems) 0.916 CFM of air before we can start the task of controlling the compressors output pressure. We are going to need 1 CFM of air to start the control arm moving. In order to make the arm move the distance we need, we will need this volume of air (CFM} to have some force (PSI). This is where we will need the supply line to be unrestricted. Since a 1/4" line will supply us with 100 pounds of pressure at 104 CFM of air (even if the orifice was larger) we can be assured of enough CFM and pressure to perform our task.
The same holds true on the intake side as well. If we are going to produce enough CFM or the output capacity of the Compressor, we must open the inlet valve enough to permit this to happen. This is the other adjustment that is important on the control system. We will discuss it later.
In order to make the inlet control move, we know we will need at least 1 CFM of air, How much Psig will we need? On the standard system, assuming their is no restriction on the actuator, we will need at least 22 Psig just to make the actuator move. On most control setup, the supply pressure is 100 Psig or better, so if there is no restriction in the supply line to the pilot valve we will not have a problem moving the actuator. If we do have a restriction, the supply cannot overcome the lose of the bleed orifice and therefore will not move the arm to close the inlet valve. First order of diagnosing, check the supply line if the controls are not working properly.
The inlet valve opens to allow the Compressor unit to supply air according to the system demand. If the actuator linkage is adjusted to close, the valve may not open enough. In most cases you will not know this is happening. The minimum travel on the control rod travel is 1 1/4". You will have to be careful at this point not to open the valve to much. If the travel is to long, the spring will over extend and be subject to breaking. Set this adjustment with care.
The control adjustment is probably the most important, single factor in obtaining the maximum efficiency from a rotary screw air Compressor. Most servicemen don’t know they are having problems with this, others may not admit it.
Sooner or later someone will come along and adjust the controls properly and the customer will realize that his Compressor has never produced or performed this well. If you are having problems with this part, get help. A satisfied customer is the most important product you can ever produce.
Study the control system, completely understand it, know what to do and when to do it.
It takes about 22 Psig of pressure at 1 CFM to make the actuator control move. To make the lever move one inch, it’s takes 60 Psig. These measurement were taken on a control system that had no resistance. If your control system had some resistance it would take more pressure.
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