Alternative switching sequence Grasso RC4211

[krunny]

We are currently useing a Grasso RC4211 compound compressor in an industrial cooling and freezing installation. The RC4211 has six cylinders, four working in the low pressure stage and two in the high pressure stage. During normal operation, we need the two high pressure cylinders to be switched on and one or two low pressure cylinders. However, if we use the normal switching sequence recommended by the manufacturer, we can only switch on both high pressure cylinders if we also switch on all four low pressure ones. This gives too much cooling capacity in the low pressure stage.

Has anyone experience with switching of two low pressure cylinders on the RC4211 while both high pressure cylinders are under load? What could be the adverse effects if we do this?

[RANGER1]

The only thing that would happen is the interstage pressure would find a new balance point. Usually a compound machine has no side load except a liquid line subcooling H/X
The interstage pressure would decrease, with no ill effects of compressor operation .

[krunny]

The new interstage pressure is no problem. That is actually wat we want to achieve. The system is running with NH3 at -40C with an interstage temperature of -10C. Because we have extra evaporaters in the -10C circuit we need relatively more high pressure cooling power than low power. We can currently only achieve this by sucking a high vacuum in the -40 stage.We don't like that situation because of the possibility that air will leak in the system. Hence our plan to shut down some LD cylinders to keep the -40 pressure just above the vacuum point.

The problem I have heard of when switching off too much LD cylinders on the Grasso RC11 series is that the cylinder heads can become too hot because they are not filled with new gas but just pumping the same gas around.