System stability collapses quickly, bringing a second 100-hp trim unit online to help the 400-hp base unit, overdriving the system again before the controls can catch up. The system continues to run out of control with too many units running at part load and short cycling. None reach a sustained idle and shut off.
We see far too much of this type of problem, which some careful planning can so easily avoid. The fix for this system was simply reconnecting the local five-step variable-displacement control on the 400-hp unit and allowing it to run base load outside the control of the central air management system. If you look, you’ll find modern well-thought-out central air management systems that can support two-step controls on both small and large units. Usually, these have the ability to control five-step units, modulation units and, perhaps, even centrifugal compressors without requiring that you convert all units to two-step control.
If you’re investigating a central air management system, be sure that its specific capabilities fit your situation. If it allows you to keep the five-step control operating as a five-step, your control will be a lot more responsive and reliable. When in doubt, observe candidate systems in operation. Even those with limited flexibility are described as having the same features and, of course, they all have so-called smart electronics. The real secret is in the knowledge and experience of the software developers and their understanding of a capacity control system and its effect on various types of compressors under various conditions.
The data from the 24-hour trend curves for the central air management system study show:
- Three units online simultaneously, each operating at part load
- 2,948 cfm online, 1,720 cfm average demand (base unit capacity is 1,768 cfm)
- Demand peaks between 1,900 cfm and 2,000 cfm
- Pressure swings from 103 psig to 117 psig
The relevant data is summarized in Table 1. The two-step controller has a slower response time than either variable-displacement or modulation control. Therefore, it can’t respond as effectively to system dynamics. But, of course, you can do better than that in your own plant.