PART LOAD OPERATION

PART LOAD OPERATION ISSUES:

One of the very strong features of the Turbocor compressor is its ability to operate at part load conditions. This is particularly true in multiple compressor units where, as the load reduces, one or more compressors can be staged off.

It is always preferable for the Turbocor compressors to run part loaded as much as possible as opposed to cycling on and off. When reviewing operating logs, anytime it is noticed that one or more compressors are cycling more than once per hour, attention should be given to what is causing this cycling.

Also, in multiple compressor units, attention should be given as to the part load operating characteristics of the various compressors in the system such that they all operate the same under part load conditions..

The most difficult situation for part load operation is when the system has high discharge pressures. Normally, part load operation in strict A/C applications will be during night time hours when the discharge pressure will be lower, due to lower outside ambient temperatures.

In cases where a low load condition exists on a single compressor unit at the same time that discharge ´pressures are high, the compressor will be typically near the surge limit of the compressor and, particularly if cycling occurs, may enter into surge while cycling off.

Their is also the circumstance where load on the chilled water system is suddenly reduced and the chiller controller holds the compressor on at very low load/ chilled water temperature due to delays built into the chiller control algorithm. During this time, the compressor may enter surge.

The parameters of the chiller control algorithm are defaulted to values that serve for the majority of installations. However, in the case of an installations subject to sudden changes in the chilled water loop and/or part load operation combined with high discharge pressure attention must be given to the modifying these parameters to prevent the compressor from going into surge during these conditions. There also may the possibility to put an offset into the controls such that the minimum rpm of the compressor is set at a fixed offset from the calculated minimum rpm (surge limit) for the compressors

In multiple compressor units, particularly an older unit where changes have been made to one or more compressors, it is necessary to look at the BMCC program revision level. Different BMCC programs have different part load operating characteristics.

One compressor may run fine at part load conditions for that particular installation, while another tends to cycle.

In the case of multiple compressor units the optimal solution is to make sure all compressors have the same BMCC program revision. If this is not practical, it may be possible to make the compressor that operates well at part load conditions, the permanent lead compressor. This means that when the chiller unloads, this particular compressor will always be the last compressor running.

Given that, mechanically, the Turbocor compressors do not experience ¨wear¨, there is no need to be overly concerned about equalizing run hours between compressors. In fact, it would be better to run the compressor that does not cycle, always as the last compressor running rather than put a different compressor having lower run hours online when that compressor is going to cycle.

In the case of multiple compressor units with all compressors having the same BMCC revision level and one of the compressors runs differently under part load conditions, investigation must be made as to why this difference exists.