View Full Version : Twin Scroll Staging Setpoints

07-08-2016, 09:14 PM
Im currently commissioning x2 114kW bespoke air cooled AC units and I'm trying to work out the correct staging setpoints for the twin scrolls. The setup is that the unit has x2 circuits and each circuit has x2 twin scrolls Model: ZR144KCE-TFD-455 with common oil balancing lines between the two compressors. They are staging the compressors and the main setpoint off supply air temperature which is causing it to short cycle.

Configuration is:
Circuit 1 Compressor 1
Circuit 1 Compressor 2
Circuit 2 Compressor 1
Circuit 2 Compressor 2

So I have a few questions about the setup:
1. How should the compressors stage up? I would usually expect them to be C1,C2,C3,C4 but after trending the temperature today we found it to be more stable as C1,C3,C2,C4. Is it ok to have this configuration and run x1 compressor on each circuit. Will this cause any issues with oil return because of the reduction in mass flow rate of the refrigerant?

2. What is the recommended staging ON/OFF setpoints for the compressors?

If we cannot stabilise the cycling of compressors then I will probably recommend changing it to return air temperature control.

Can you recommend any documentation I can read on this?

Thanks in advance

18-08-2016, 07:25 PM
If i get this correct you have two AC units connected to one evaporator coil in a ventilation system?

It shouldnt matter how they stage up as long as the oil return line is good.

Is it the same capacity at compressor C1 and C2?

The minimum on time should be around 5-10 minutes and minimum off time should be about 5 mins or something like that. Depends on compressor model, but usual its max 6 starts an hour.

If it's cooling an office i prefer to regulate at the return air, but have a sensor in the inlet air so its possible to stop cooling if inlet air gets too cold.

25-08-2016, 11:31 AM
Are you thinking of 2 different A/R ratios on one compressor housing? If so, I imagine this would cause the lower backpressure path on the exducer to be favoured, so you'd end up with most of the air flow going through that. If both paths were subject to equal back pressure, then I imagine the smaller A/R (with it's higher outlet pressure) would try to overcome the larger A/R (with it's increased boost response) and you'd end up with a net, errr, something not great. Also keep in mind that A/R has much less of an effect on a compressor than on a turbine, and that the twin scroll turbine is typically fed per cylinder pairing (I don't think feeding a cylinder pairing with a twin scroll compressor would be a good idea (different boost levels in different cylinder pairs)).