Time off. Everybody likes it. Everybody needs it. So do air conditioning compressors. When cycling compressors to maintain comfort, off-time is essential to preventing an early demise of the compressor.
For this project, EES engineers took full advantage of the architecture given to us when designing the cycling logic. This particular system, shown below, has four compressors.
Compressors 1 and 3 are the “top” half of the coil area, and 2 and 4 are the “bottom”. I chose to make the system work in six stages of cooling. Stage 1 is one compressor (1/2 the opening) running at “low” (these are two-stage compressors). Setting up an alternator: if the demand is for one compressor, not all the time, we alternate from #1 to #2 to #1, and so on. As the outside temperature increases, and thus the load on the air handler, we go to stage 2. That is the second half of the face coil. So we run #1 and #2. If that demand point is satisfied and stage 2 shuts off, we use the alternator to start #4 the next time, increasing the time off for compressor 2. If we hang on stage 2, we go from #2 to #4 to #2 to #4.
Again, utilizing the hardware gifted to us, we make the best use of it while saving the equipment from premature demise. Stage 3 is one of the running compressors on “high”. We alternate that as well, again to provide greater off time, as the balance of on and off is somewhat tight to maintain supply temperature.
Stages four, five, and six don’t get to alternate. Four is running both compressors in “high”. Five is the standby compressor in “low”. Six is all four in “high”.
Watching the system run in varying outside-air temperatures shows that the alternator logic works and minimizes supply temperature variation while saving the hardware from brutal service.
To even out the runtimes of pairs 1-2 and 3-4, the final alternator happens on Sunday. We alternate which pair is in the lead so that each pair gets time as the lead. Remember, unless you are the lead dog, your view never changes, so we give each compressor a chance for a view.
There is also an economizer that works when conditions are right to use outside air to do the cooling. In New Jersey, this period is not all that long, but we maximize its ability when possible.
As is normal, we also do a supply-temperature reset based on outside air conditions, asking the system to work harder when it is hot outside and easing back when the outside air is less uncomfortable.
When we were brought in to the project, the hardware was already delivered. We evaluated the supplied hardware, identified operational issues, and recommended VFD units for the fans (which were not in the original design) to address the varying static pressure needs. We were rewarded with the owner agreeing and amending the design to include VFDs. Several other items were missed, and we were able to add them back to get a working system, ensuring that the final system would meet the owner’s needs.
In conclusion, ensure your systems integrator has a comprehensive understanding of the equipment being controlled and maximizes its capabilities not just for the moment but a strategy and operational understanding that protects the investment and preserves the longevity of the assets while getting the most out of the equipment provided.