Variable ref flow system-correct ref charge

[PRM83]

Hi everyone

Please help me determine the correct refrigerant charge for this brand new system.

Description
ODU-VRF mini air con of 15.5Kw cooling capacity (inverter driven)
IDU1- VRF DUCTED CONCEALED UNIT 7.2kW (24000 BTU/HR)
IDU2- VRF DUCTED CONCEALED UNIT 5.6kW (18000 BTU/HR)
IDU3- VRF DUCTED CONCEALED UNIT 5.6kW (18000 BTU/HR)
REFRIGERANT - R410a
STANDARD CURRENT OF ODU - 23.6A SINGLE PHASE
MEASURED RUNNING CURRENT - 18A

Running with factory refrigerant charge on name plate is 4kg. Pipeline is not very long its less than 15m one way. Did not additionally charge for pipelines.

Question no 1) system is running at 493PSI high side & 101PSI. discharge pressure looks higher than standard but suction pressure looks low. Did not want to charge more refrigerant for additional pipes as discharge pressure looks higher. Any clues? Any fault related to air flow side of the indoor units?? Any more data required please let me know.

Question no 02) Can I use subcool method to determine the correct refrigerant charge in this system? If yes please tell me the steps to do.

Thanks

[Brian_UK]

What does the manufacturer recommend for your installation ?

They should give details of additional pipeline length refrigerant quantities.

What are you current ambient conditions?

Are the rooms served down to temperature or are you test figures taken during pull down?

Have you taken diversity into account during your test run?

[PRM83]

What does the manufacturer recommend for your installation ?

They should give details of additional pipeline length refrigerant quantities.

What are you current ambient conditions?

Are the rooms served down to temperature or are you test figures taken during pull down?

Have you taken diversity into account during your test run?

Hi Brian

Thanks for the reply

I can calculate the additional charge. But already discharge pressure is higher. Hence afraid of charging more gas as the discharge might go even higher which is above 500.

Ambient temp is 32-33 centigrade as read by the ODU inlet sensor.

These readings are not on test mode. It’s after 2Hrs of normal operation. But I suspect the return air flow for the ducted indoor units are somewhat restricted. Will it cause for the discharge pressure to go above 490PSI like this?

My other problem is why only discharge pressure gets higher, without increasing the suction too? Is the factory charge is too much because the pipe length is short?

Can you explain me the diversity while test run please

If you want I can get you a computer generated full reading set of 1Hr or more for your better study

[Brian_UK]

Going back to basics....

Was the system correctly strength and pressure tested with OFN?
Was the ODU pressure monitored while the pressure test etc was undertaken? Any chance of valve let by during the test?

After the pressure test was the correct procedure also followed for evacuation of the system? E.G. all eevs open to ensure no OFN trapped anywhere? and all air removed from the system?

I'm trying to establish whether there is any contamination of the refrigerant.

[RANGER1]

PRM83,
Discharge pressure very high.

Is there good airflow over condenser?
Check hot air not recirculating over condenser.
Check air on/off temperature of condenser.
Can you measure condenser return bend temperature to see if you can determine at what point it stops condensing(may be 100% liquid , then subcooling, indicating overcharged).

Any chance if was not evacuated correctly.
What is room temperature?
Are your liquid & suction lines correct as per instructions

[PRM83]

Hi Brian

All above evacuation steps were followed correctly. Im in to VRF for more than 10 years in the installation. But since you say so, we shall follow the same process once again by recovering the refrigerant and charge the gas with weight.

[PRM83]

Hi Ranger

1) Air flow is not bad actually. Ambient is around 32 as identified by ODU ambient sensor at condenser coil inlet.

2) Hot air is not recirculated. enough space to go in.

3) I dont quite get your point. can I take the saturation temperature according to the discharge pressure and liquid line physical temperature to calculate the liquid subcool then identify the overcharge??

4) it was evacuated properly. but it is worth trying it since it is only 4Kg of refrigerant to recover.

5) room temperature is 23c

6) yes of course suction and liquid lines are correct. it is a small pipe path. no more than 15m.

[RANGER1]

You might be able to identify where subcooling starts to occur, ossibly easier said than done, as need to get to return bends using accurate touch probe.
From my understanding you usually only add refrigerant with longer pipe runs, but in your case maybe mistake in factory.
You should also have some suction super heat or compressor damage could occur.

[PRM83]

we evacuated the refrigerant once again.. vaccumed... charged with weight in method. still the discharge pressure is 490 & suction is 120 psi at this moment.

1) Please explain what the return bend is. Ill try to get the information tommorow as you instructed.

2) Shall I get the saturation temperature with respect to the discharge pressure from PT chart & physical temperature from the liquid line near stop valve, so we can calculate the subcool??

[RANGER1]

https://m.youtube.com/watch?v=B8udgG8q2XQ

Petty basic video, but gives general idea.
On smaller units they can have a pre expansion device before it comes out of condensing unit, bigger ones probably haveEEV or TX valve.
Video in deg F

I think you could also contact manufacturer or supplier for advice, as not your fault new unit has problems.

[Brian_UK]

Just out of interest, what make and model is the unit you are working on?

[PRM83]

Hitachi
ras6.0hnskq odu
rpiz2.5hnatnq idu1
rpiz2.0hnatnq idu2
rpiz2.0hnatnq idu3