When maintaining / operating an industrial refrigeration system, understanding the data that you write down is extremely important.
Just knowing where to find the data to write down can be challenging itself, but understanding exactly what the data is telling you can be even more of a challenge.
Modern-day compressor control panels come in a variety of color graphic displays for enabling the operator to interact with the controller. When performing rounds on screw compressor(s), writing down the slide valve percentage can be easily misunderstood.
Capacity percentage & slide valve percentage are relative but have two different meanings.
Slide valve percentage (capacity control) can be compared to an accelerator pedal on a combustion engine vehicle. At idle a vehicle would be at 0% and 100% when the pedal is to the floor. The pedal has an infinite number of positions ranging from 0% - 100% and that is just like a screw compressor slide valve percentage, it also has an infinite number of positions for controlling capacity “capacity control”.
Capacity percentage can also be compared to a combustion engine vehicle. When a vehicle is idling, it is actually still producing a small amount of horsepower (5-10hp). When the gas pedal is to the floor, that would be the max rated horsepower (e.g., 300hp). A screw compressor at 0% slide valve position will still have a capacity percentage of around 10% (just like a car idling has 5-10hp). When a screw compressor is completely unloaded at 0%, there will still be around 10% of capacity through the machine. The following is an example of how slide valve percentage & capacity percentage are relative to each other:
•SV 80% or more, capacity 0-5% less
•SV 80% - 40%, capacity 10-15% less
•SV 40% - 30%, capacity 2-4% less
•SV 30% - 20%, capacity 5-10% more
•SV 20% or less, capacity 10-15% more
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