Staging condenser fans, at what pressure?

[ChillTechnican]

I have a condenser with 5 fans and a controller with 4 stages, the last 2 fans come on at the same time. The system is a freezer running on r404a and there is a few issues we are having . My boss is telling me to set the first fan to come on at 250 psi or about 40 degrees , the other fans are set to come on at 15 psi intervals after that, he seems to think that it might be over condensing with the cooler nights ,( its getting down to 0c), if it is running at around 220-230psi . i thought that if the pressures were 12 to 20 psi above ambient then we are in the right zone. Is there any point of having a high set point ?? dont we lose efficency with the compression ratio being higher?
Also when the second fan cuts in and the pressure drops the liquid in the sight glass disappears until fan has cycled off and pressure raises again . Is the differential between fans to high?
thank you for any answers or advice you may have

[cool_tech]

hey mate
try and drag the intervals out to 20 psi and see how that goes. i know how cold it is getting i am in melbourne. you can also replicate by blocking some of the condensor and see what that does

[Yuri B.]

I think you should find out the reason for liquid in the sight glass disappearing. With condeser fans continuing receiving signals to drop the condensation pressure you probably have incondensables in the system.

[ChillTechnican]

I think that when the pressure drops the liquid drops below condensing temperature and gas appears , maybe due to the little subcooling that is happening, this happens no matter what set point that i have the fans set at. Is this a chareristic of r404a , do we need a subcooler?

[ChillTechnican]

The system is only 4 months since a new compressor and a complete vac down to 200 microns.

[Yuri B.]

How comes low subcooling with all fans driving the condensinig pressure down to no matter setpoint ?

[Yuri B.]

Then, please describe the positions of the condenser, line receiver, the sight glass, and the evaporator in terms of the distances both horizontally and vertically.

[lowcool]

what pressures are you getting?

[littleyapper]

is the sensor for the fans on discharge or liquid line?

[ChillTechnican]

Yuri b , the condenser is 10 metres above the compressor. Evaporator is 6 meters horizontal away from compressor . Liquid reciever is mounted under compressor and sight glass is after filter drier along side reciever.

The transducer is connected to a port in the block of the compressor.

[ChillTechnican]

Pressures at the moment are Sst 22 psi
Sct varies between 250 - 275 psi

[Yuri B.]

With 10 m height difference note, by the way, that actual press at receiver is some 10 psi lower than at cond outlet.

You still did not say clearly what for issues you were having. Too often LP tripping (at cool nights) holding back normal freezing process?

[Yuri B.]

That is, pressure higher at the receiver side.
And the receiver seems to be starving on refrigerant.

[Yuri B.]

It also seems you have charged the system at a somewhat warmer part of the year - check one more time again whether the charge was correct.

[Yuri B.]

Maybe just filter is clogged?

[ChillTechnican]

Question 1 . What is the optimum pressure to set the fans to control to? and why?

Question 2 . How close should the differential be between fans?

I am trying to keep the liquid level up as it drops when second fan cuts in and the pressure drops.

[lowcool]

gday chilltech
i usually start at 225 psi and stage as close as possible.5-7 psi stepping up,glass would flash on stage down,dont know if this is from lack of surface tension of refrigerant.top up charge check superheats on evaps,would not be overly concerned about achieving a full glass all the time.is first fan on at manifold end.

[littleyapper]

you are running your fans on discharge pressure(comp block)move transducer to the outlet of condensor then you will see a vast difference smoother running etc

[Entropie]

The lower the condensing temperature is the more efficient the system works. This is due the fact that condenser fan motors usually draw much less current then the compressor this gives you the benefit.The critical item in the whole thing is the minimum differential pressure required from the expansion valve (the design engineer should know). This point indicates where the 1st stage has to switch off (no fans running). It is highly recommended to set the differential between ON/OFF of the fans not too small because this causes unstable operation and excessive fan cycling (motor/contactors). I do recommend about 5 barg as differential on each stage. This allows the unit to stabilize with minimum fan cycling. The following stages may come about 1 barg later each (but with identical differential of 5 barg). Following these instructions, the first stage ON should be set to minimum dp (XV) + 5 barg.

[ChillTechnican]

Ok thanks all for your replies , i will try some of those settings and suggestions.
cheers CT