Condenser fan control

[get the gauges]

I have come across a problem with some HP pressure switches that control condenser fan contactors. The first pressure switch makes pretty quick as the pressure rises but when the next 3 switches come in as the pressure slowly rises there is a lot of flashing of contacts and chattering as the contacts try and make.
The switches are standard danfoss auto reset type and have been in for years but have very clean contacts. Its a bit of a coincidence that these switches have all started playing up now.The HP pressure comes off the head and i have checked the pressure at the discharge service valve. Has anyone seen this happening when the switches are operating at the top end of the HP range coming in on slow creeping pressure rise. Also does anyone know of any other good reasonable cost HP digi contollers to control my 3 phase fan contactors. Thanks GTG's

[SteinarN]

I've seen this several times on Danfoss HP pressostats. I've seen HP safety try to stop the compressor at high discharge. The contacts try to open, the compressor contactor chatter out and in in a frenzy, it was crazy to see, i felt realy sorry for the compressor.

One idea can be to connect the pressostat at the receiver. You will alvays have some pulsation at compressor discharge. Pulsations make it harder for the pressostat to switch without that chattering.

[philfridge]

Have You Checked The Differential Is Correct On The Hp Switches As They May Be Cutting In And Out In Very Quickly

[get the gauges]

I've seen this several times on Danfoss HP pressostats. I've seen HP safety try to stop the compressor at high discharge. The contacts try to open, the compressor contactor chatter out and in in a frenzy, it was crazy to see, i felt realy sorry for the compressor.

One idea can be to connect the pressostat at the receiver. You will alvays have some pulsation at compressor discharge. Pulsations make it harder for the pressostat to switch without that chattering.


Yes i thought exactly that too,and i thought of taking the pressure from after the oil separator, my problem is that my compressor and pressostats are remote of the condenser and cooler pipework. I'll have to try rolling out some 1/4 tube to rule it out.

[get the gauges]

Have You Checked The Differential Is Correct On The Hp Switches As They May Be Cutting In And Out In Very Quickly

Checked, All diffs are in the region of 30psi.

[nike123]

What is diameter of conection tube from port to switch?
Do you mean that you control 4 fans with 4 diferent HP switch on same discharge line?

[Grizzly]

GTG.

A good controller that we have used with great success.
Is the Dixcell XC440C which will control up to 4 comps or fans.
follow the link for more details....
http://www.dixell.com/controllore-digitale-con-gestione-fino-a-4-compressori-o-ventilatoric-32x74mm/&m58LangNew=ENG

Cheers Grizzly

[get the gauges]

What is diameter of conection tube from port to switch?
Do you mean that you control 4 fans with 4 diferent HP switch on same discharge line?

Yes,one 1/4 tube from comp (40hp comp) Tee'd up to four danfoss auto reset switches. The worst one is the last and slowest to make no.4

[nike123]

Yes,one 1/4 tube from comp (40hp comp) Tee'd up to four danfoss auto reset switches. The worst one is the last and slowest to make no.4
I once have situation that high pressure switch was placed right at discharge line near the compressor outlet.
After every defrost I found tripped high pressure switch.
Then I installed Refco capillary for pressure switches of 3m length and that problem is solved, and I still have function of pressure switch.
Capillary has acted as choke. You should try that as quick fix solution.
But I think that something has changed in your system what causes this behavior that was not present before.

Also, if you have 4 fans, why are they separately switched by corresponding HP switch? That is big pressure oscillation.
You should switch only one or maybe two fans in pair with only one HP switch. If that is no good for your conditions, than you should install fan speed control for all fans.

[get the gauges]

GTG.

A good controller that we have used with great success.
Is the Dixcell XC440C which will control up to 4 comps or fans.
follow the link for more details....
http://www.dixell.com/controllore-digitale-con-gestione-fino-a-4-compressori-o-ventilatoric-32x74mm/&m58LangNew=ENG

Cheers Grizzly

Looks good, How much are they with a pressure sensor then ? And what the suction reading for, or is it just an added bonus? Thanks GTG's

[get the gauges]

I once have situation that high pressure switch was placed right at discharge line near the compressor outlet.
After every defrost I found tripped high pressure switch.
Then I installed Refco capillary for pressure switches of 3m length and that problem is solved, and I still have function of pressure switch.
Capillary has acted as choke. You should try that as quick fix solution.
But I think that something has changed in your system what causes this behavior that was not present before.

OK, sounds good ,i'll have a look at that.

[get the gauges]

Changed all h/p switches and fed all h/p switches from the discharge line after the oil sep as opposed to the head connection. Ran the system and before the problem 3rd and fourth fans came in i noticed that all the condenser fans were running backwards. Swapped all the fans round and run the job again this time only fan no.1 came in then no.2 occassionaly. No.3 and No.4 never run in this weather now that all the fans are of corrrect rotation.

Conclusion
Resident maintenance changed fans in the winter and set rotation wrong causing ' weird stuff to occur ' . Although it could always of been the other probs solved by parts fitted that day.