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samiam
24-07-2007, 12:11 AM
I am currently busy with a 2 stage casacde system (LT & MT) that utilises Ammonia as the main refrigerant. A swing screw compressor is piped between the two system.

What protect do I require for the swing machine to the wrong flow of discharge gas? (i.e. specila valves, piping, ?)
Any other swing compressor protection required?

Key information:
LT = Low Temperature with -36/-13
MT = Medium Temperature with -13/35

US Iceman
24-07-2007, 02:56 AM
I'm assuming you mean a two-stage compression system and not a cascade system. Or is it really a cacade system?

A swing compressor has the valves it needs already (check valves & service valves). If you want to have secondary valves for the cross-over piping then you need to decide if you want this as a manual or automatic operation.

The important thing to know is; is the oil separator and motor sized correctly for a swing compressor operation?

For the LT operation you need a larger oil separator and smaller motor, and for MT operation you need a smaller separator and a larger motor. Also, make sure the oil cooling is selected and installed for the worse case operation.

TXiceman
24-07-2007, 08:28 PM
I much prefer a manual valving for a swing compressor. Designing te swing compressor can be a challenge as you have to design the oil system for both high and low duty and the motor has to be sizd for the higher power cas.

I would not recommend trying to get a swing compressor on a cascade system with two independent refrigerant circuits.

Ken

samiam
25-07-2007, 10:19 PM
Cascade system it is.

The swing compressor has pipes to both the LT and MT stage. The swing would be only be activated when either a LT compressure or a MT compressor has mulfunction.

Motor size, oil sizing, discharge/suction pipes have been sized to the large requirement.

The change/activation will be a manual one - i.e. seetings have to change, etc.

I though maybe there was a more automatic change over...

Thanks Guys,

US Iceman
25-07-2007, 10:33 PM
I would not recommend swinging a compressor from one refrigerant system to another. Only if the same gas is used.

If your intent is to swing the compressor form the MT to the LT system where both of them are ammonia, you should be OK.

I don't see any reason why this could not be done automatically if you use the right automated valves and operating procedures though.

A manual change is probably the safest as this would require someone to be there during the change-over.

samiam
25-07-2007, 10:42 PM
Sorry - same gas for MT/LT which is Ammonia.
I have not heard of a swing being used on a system with different gases.

There is a manual required on the compressor - limitation on the compressor control wrt to settings.

US Iceman
25-07-2007, 10:49 PM
Sorry - same gas for MT/LT which is Ammonia.
I have not heard of a swing being used on a system with different gases.


No problem. I just wanted to be sure to say that as a matter of completeness in my reply.;)

If the control system had the capability, the different setpoints could be used within. This might present a little bit more risk involved as the compressor controller would have to have a wider range on the limit settings.

TXiceman
26-07-2007, 02:51 PM
I would not recommend that the change be made automatically. Most micro based control systems do have the capability to have two sets of operating conditions (with the required safety set point) and switch between them based on a digital input to the panel.

Automatically starting a large compressor tat is already set for service in a given condition is fairly safe, but I would not trust a series of large valves to make all of the required changes without some human interface.

If you are intent n doing this automatically, you need to have a positive confirmation that the valves have sequenced properly. The best way to accomplish this is have a hard-wired digital switch input to confirm each valve status as open or closed. Make sure the proper set of operating controls and safety are transfered to the compressor. I would still want an operator to check the machine and confirm a permissive to start.

Ken

US Iceman
26-07-2007, 03:51 PM
As usual TXiceman adds some good comments. I forgot to mention the valve position limit switches. Without these providing confirmation of the valve position starting the compressor might be VERY interesting.

These limit switches are little micro-switches mounted on the valve operator.

It does of course make sense to have this attended to by an operator for overview of the changeover and start-up of the swing compressor.