Condensor Fans Control

[Feeze]

What would be the best settings for the ambient control pressure switches on an air cooled condensor with 3 condensor fans and 2 condensor(ambinent) pressure control switches. The 1st condensor fan runs whenever the compressor is on while the other 2 run on increase of pressure.
At what stage should the 2nd fan and 3rd fan cycle on/off and
should they overlap?
The system was designed for 507 gas and the condensing
temperature of 41 deg C was selected.
Should you require any other info please let me know.
Thanks
Feeze

[rbartlett]

go here and download M O'B's fan calculator

http://fridgetech.com/calculators/

cheers

richard

[Feeze]

Hi Richard,
This is quite an informative and interesting site but I still have some burning questions.
Please advise me if my thinking is correct here.
In order to maintain a condensing temperature of approximatelly 41 deg C I should allow the 2nd fan to cycle on at 43 deg C and off at 39 deg C and the 3rd fan on at 47 deg C and off at 41 deg C. (Obviously I have to set the pressure switches at the equivalent pressures according to the pressure temperature chart for 507 gas)
Another note I would like to add is that I have been loosing the contactors that drive these fans (probably cycling too often). Twice the coils burned out, once the contacts welded together and the fan motor that was connected to this particular contactor burned out.These are all 3 phase 400v 500mm diameter Ziehl fans.The running amperage is not more than 1 amp per fan.The contact rating on the contactors are 12 amps. No power failure problems reported by the customer
and none found my me either.The pressure switches installed are Danfoss KP5.
Any sugestions?
Regards,
Feese (ie Freeze without the 'r' )

[chemi-cool]

Hi Feeze.

About your contactors.
Check the coil voltage. Is it the same as your control voltage?

Check is it AC or DC?
The coil has to fit the control cercuit voltage and polarity.


Chemi

[Mark]

Hi Feeze

Are there any overloads fitted on the condenser fan contactors,if so what is the operating tolerance?.

Best regards.Mark

[chemi-cool]

Hi Mark.

Ziehl fans are equipped with thermistor to protect the motor windings from over heating.

The thermistor should be connected in series with the contactor so no overload is needed.

Chemi

[Feeze]

Hi All,
Yes Chemi, the coils are all 240v AC and the control voltage is the same.The coils for all the other contactor ie eveporator,
and compressor have not failed.
The plant was installed 6 years ago by me and this is the first time that this problem surfaced.I have lost these 3 contactors and the fan motor over the last 4 weeks.
Mark,
There are no overloads connected to the contactor outputs
however the thermocontact which is built into the motor are wired in series with the control voltage of the coil.
Mark,(sorry I meant Marc)
I am inclined to agree with your diagnosis of short cycling. I have a great respect for your knowledge but your statement is a bit over my head.Would you kindly elaborate.
I really appreciate the quick response from all.
Thanks Guys,
Feeze.