Compressor Setpoints

[Eeram]

Hi Guys and Girls, I need some help on a recently installed ammonia system.

The 3 compressors were running for the last 7 months all the time, eating up some 9000Kw per 24 hour period!

We were asked to step the compressors according to size, so not all run at the same time at only 10% slide valve capacity, in order to save energy.

We change the compressor setpoints with the help of the Installation Company, but now, every night they trip on motor failure. The Danfoss soft starters either show phase imbalance fault or under frequency fault. Danfoss soft starters are set to motor information.

This never happens when they were running all the time.

All three compressors were set to the following set points:

Cut in - 215 Kpa
Control set point - 205 Kpa
Cut out - 150 Kpa
Minimum trim slide valve position - 10%
Minimum pre trim slide - 90%
Pre trim Control setpoint offset - 20 Kpa
Discharge pressure - average 1060 Kpa
Surge Drum pressure - 250 Kpa
Liquid pump pressure - 500 Kpa (2 of 3 pumps are running)
8 Rooms running at 2 deg. Celsius.

We left the Mycom 250 VSD settings as it was and change the other 2.

200 VLD Settings now:
Cut in - 220 Kpa
Control Set Point - 210 Kpa
Cut Out - 160 Kpa

160 VLD Settings now:
Cut in - 225 Kpa
Control Set Point - 215 Kpa
Cut Out - 170 Kpa

The compressors are cycling now, with the 250 VSD doing most of the work. The only problem now is the tripping only at night time. Electricity supply is very reliable.

Anybody with ideas please? I would appreciate it.

[717NH3]

Eeram,
Do you have a recycle delay time in place? Are the pressures equalizing before trying to start back up? Are the compressors cycling off at night due to low suction pressure & then trying to restart too soon? If this is the case in a low load condition you may want to lower you cycle off set point or if there is a minimum slide valve set point – lower the set point. You may also want to look at the cycling back on set point. Maybe raise the set point to allow the compressor to stay off longer. Another thing to look at is where does the suction transducer get its reading from – before or after the suction check valve. If before, when the compressor cycles off the suction pressure will go to the high side pressure unit the package bleeds down to suction pressure. Also when using soft start starters what configuration are they setup as – constant torque? Is there a control system in place that cycles equipment in the plant off at night?

[Eeram]

All 3 compressors has a 17 minute anti recycling. The 3 compressors are in parallel. The suction transducer is before the check valve and normally reads the suction what the other 2 compressors are running on.

I have changed the phase imbalance sensitivity and under frequency on the soft starter in the mean time and it run very well through the night. Will have a look if the soft starter is running in constant torgue.

The coldrooms run all the time as there are 3 loading/unloading shifts. The day and night operation stays the same as well as the load on the compressors.

[Segei]

As far as I understand, the refrigerant is ammonia. Can you give more information about the rooms? What kind of production do you have? Evaporator fans run nonstop or they cycle. What is the ambient temperature(day, night)?

[Magoo]

Sounds more like the VSD settings at start-up. The ramp up time too slow, minimum hertz too low. Both will report a fault as phase imbalance and over current and lock out.
Is the compressor/ s starting fully unloaded?.
magoo

[Eeram]

Hi Segei and Magoo

The refrigerant is Ammonia. There are 8 rooms, 45meter X 30meter X 8meter high. Oranges are kept and waiting for export to Europe, Middle East and Japan.

Evaporator fans are running non stop, except for 2 X 20minute defrosts/24hours. Ambient at this time of year is 10 deg. C night time and 23 deg. C daytime.

All 3 compressors starts fully unloaded at 0%.

The Danfoss softstarters were set up very basic and since we change the phase imbalance and under frequency settings, the system behaves so far.

No alarm sms's for 3 days!