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Feeze
30-09-2004, 07:35 PM
I have come across delay on timers been used in series with low pressure switches. I have read somewhere that delay on timers are used to prevent chattering of contactors should the Lp go faulty or if a power failure occurs and instaneously returns creating an unwanted short cycling of the equipment/compressor.
The point that comes to mind is: If a delay timer is used this way, what should the duration of the delay time be set to in order to prevent floodback or any other potential problems?
What are your thoughts on this guys?

chemi-cool
30-09-2004, 08:38 PM
Hi Feeze.

If the LP is for protection only and doe's not cut out on pump down then by adding a small relay it should cut the SV as well.

Rest time should be about 5 minutes but it would be better to use a LP with manual reset so a tech will be called to find out why did it trip and get the problem fixed instead of letting the unit go on and off endlessly.

Chemi

Feeze
30-09-2004, 09:03 PM
Thanks for your reply ChemiCool.
However when a Lp switch is used to stop the compressor when the thermostat or defrost timer switches the solenoid off
(allowing the compressor to first pump down before switching off) then a manual reset Lp switch would, in my opinion, not be appropriate as it would trip after every defrost or every time it reaches temperature and then has to me manually reset.

chemi-cool
30-09-2004, 09:16 PM
Hi Feeze.
That's why asked you if it is used with the pump down.

There was a post here somewhere with an electric diagram how to protect the compressor from a floodback.

when the power is shot off, the SV is off too so no problems then.

If there is a suction line accumulator before the compressor then set the timer for 3 minutes.

Chemi

Feeze
30-09-2004, 09:34 PM
Thanks Chemi-Cool,
that answers my question

RogGoetsch
01-10-2004, 04:51 AM
I usually only see delay timers on air conditioning equipment to prevent rapid compressor cycling due to thermostat abuse, a heart-breaking disorder.

I always see, and prefer to install, delay-on-break timers which start counting when the system shuts off and prevent a restart within five minutes or so. If the system is off longer, the relay allows immediate restart upon demand.

If I understand correctly, the timer you are referring to delays restart every time?

The delay-on-break timers take a bit longer to wire, but allow more rapid system response.

Rog

Feeze
01-10-2004, 06:04 AM
Hi Rog,
Now this seems like a much better design.
Thanks,
Feeze ;)

chemi-cool
01-10-2004, 02:24 PM
Hi Rog,

I totally agree with you.
For the past few years, I don't use any timers cause everything is already in the controllers so I usually set a minimum working time and minimum off time which works just like off delay.

With the new controllers where digital inputs are used, it is very simple to program them and connect them straight to all the pressure controls and control the unit very good.

With AC I use on delay timers for short cycle protection.

Chemi :)

Peter_1
01-10-2004, 06:06 PM
Haven't read all the previous posts but Refac installed standard on some units I remember a timer over the LP switch ( a temporary shortcut of the LP switch) so that the system can build up his LP during start-up after a long stand-still period.
Otherwise it never could start up due to too slow HP pressure build up.

Feeze
01-10-2004, 10:24 PM
Hi Marc,
This topic is beginning to get very interesting.
How would you wire this up?
Do you perhaps have a schematic diagram that you could post?
Thanks,
Feeze.

Feeze
06-10-2004, 08:10 AM
To Brian, UK
I have received an email notification of your post that reads as follows:

Feeze, this is a diagram and description that I copied from a 'fridge book some many moons ago but is still quite useful. I hope it helps.

However your post does not appear in the forum.
Would you kindly repost.

Thanks,
Feeze

Brian_UK
06-10-2004, 12:29 PM
Sorry Feeze, the attachment was too large to post so I cancelled it.

I am trying to reduce the size of the item and will repost.

Cheers

Feeze
06-10-2004, 06:54 PM
Brian_UK,
If the file is to large for the forum email it to me directly.

Cheers,
Feeze

shogun7
06-10-2004, 11:53 PM
Delay time on anti cycle controls have a limited safety factor and can be set up to 10/15 min if need be, however they will not protect the compressor for any length of time if other conditions exist, for example:
In time, all mechanical parts will fail, for example, liquid line solenoid valves can leak, causing compressors to short cycle on their low-pressure control while in pump down. Discharge valves can also leak and cause refrigerant pressure to build in the crankcase, which can short cycle compressors on their low-pressure controls while in pump down. Cylinder unloading devices are also known to leak and short cycle compressors while in pump down.
Serious compressor motor damage and contactor damage can result if the short cycling continues unchecked for any period of time. A remedy for the problem of short cycling is designing a non-short-cycling pump down circuit using a latching relay. For example, on a call for cooling, the thermostat closes and energizes the liquid line solenoid. This allows the low side of the system to experience refrigerant pressure. This will allow the low-pressure switch to close. The latching relay coil (LRC) will now be energized through the normally closed contacts A and B. Once the latching relay coil (LRC) is energized, it will close contacts B and C and contacts C and D, while opening contacts A and B. With the LRC energized, the compressor contactor (CC) will be energized through contacts C and D and the low-pressure switch. This will start the compressor.
The system will be in a normal cooling cycle. When the thermostat is satisfied and opens, the liquid line solenoid is de-energized. This physically closes off the liquid line and initiates an automatic pump down. The low-pressure switch will soon open, de-energizing the CC, which will shut off the compressor. The system will now be pumped down. The compressor will remain off until there is another call for cooling from the thermostat.
If, however, the low-pressure control tries to close from a leaking component while the thermostat is still open and not calling for cooling, the CC will not be energized because of the normally open contacts C and D of the latching relay. This will prevent short cycling. The LRC will also not be allowed to be energized because of the normally open contacts B and C of the latching relay. These latching relay contacts prevent compressor short cycling during pump down when components leak, which causes low-side pressures to increase and low-pressure switches to close prematurely.
;)

Feeze
07-10-2004, 07:45 AM
Hi Shogun7,

My initial concern regarding delay timers is the maximum duration of delay that these timers should be set to in order to prevent flood back or flooded starts due to the faults you just described. A once off pumpdown, which is what you are recommending, would increase the danger of this happening as the compressor would receive alot of refrigerant which would sit in the sump and create havoc when the compressor restarts after a long off period such as for example during the entire defrost period. A continuous pump down would prevent this from happening.
I prefer that should a fault which creates short cycling exist, that it cause the overload or compressor circuit breaker to trip (this will happen if they are correctly selected and rated for for the specific compressor ). This will draw attention to the fact that a fault exists which needs to be rectified asap.
Does anybody else agree/disagree, please feel free to chip in.

Regards
Feeze.

Feeze
07-10-2004, 08:12 PM
Thanks Brian_UK,
Your effort is appreciated.
Feeze

Brian_UK
07-10-2004, 11:14 PM
Feeze, further to your floodback concerns I believe it has been suggested in the past, somewhere, and if your are trying to protect a 'large' compressor. How about installing a small compressor to the system that would only be active during normal pumpdown periods to maintain the pumpdown state.

Obviously this may be classed as overkill but if you are worried about expensive kit suffering due to the failure of another component then it may be cost effective.

rbartlett
08-10-2004, 04:37 AM
jogged is not a phrase i'm familuar with..

cheers

richard

Feeze
08-10-2004, 07:58 AM
This is the what I love about this forum, the spontaneous and unselfish sharing of knowledge so that all may learn from the experience and knowledge of those in the know.
In a nutshell jogging can be explained as follows:
Jogging of a compressor this is the procedure of starting a compressor for a few seconds (2 or 3 seconds ) and then switching it off and then waiting for maybe 10 seconds before the next restart of 3 seconds. This is repeated until the foaming in the oil sight glass subsides. The foaming is as a result of refrigeration migration to the compressor crankcase when a compressor has tripped for a long time period and then when restarted the refrigerant spontaneously explodes washing the bottom bearing of the the conrods and also thinning the oil.By jogging the compressor we hope minimise this situation.
O, and if you believe in God pray, hopefully the compressor survives.
Cheers,
Feeze

chemi-cool
08-10-2004, 11:21 AM
O, and if you believe in God pray, hopefully the compressor survives.

Hi Feeze.

I was looking for that line,

If the system is a pumpdown one, then I would set the LP to 5psi to avoid such situation. Restart at 30psi and you keep the comressor "alive" for a much longer time.

Where do this practice comes from?

Chemi :)

Feeze
08-10-2004, 04:29 PM
Hi Chemi,
Which practice?
The practice of praying or the practice of jogging?

Regards,
Feeze :D

chemi-cool
08-10-2004, 06:05 PM
Hi Feeze.

If praying would have helped, I would have done it for 24/7.
I got to eat though!

The jogging practice is an interesting one.

Chemi :)

Feeze
08-10-2004, 06:38 PM
Chemi,
Some things are out of your control so in those cases "hope" for the best or "hope" lady luck is with you or "God"
favoured you today.Take your pick. I personally always adopt a posative attitude and try to make the best out of most situations.My statement was a figure of speech but I am glad I made your day.
Cheers Mate,
Feeze.

ps "I got to eat though!" I missed this comment on my first read. I guess pray then if the compressor is under guarantee by you.

Moris
09-10-2004, 11:44 AM
On larger plant, compressors are designed for a limited number of starts per hour. The Hallscrew comes to mind with a max of 4 starts per hour ie 15min delay before starting.

You will see a time delay switch across a differential oil pressure switch to allow time for the oil pressure to build to working pressure, these timers usually have a delay of 90 seconds.

Not exactly the answers you were looking for but may be useful in the future if you deal with larger plant.


Moris

arcticair
23-09-2009, 02:02 PM
do you have a schematic for this latching relay to stop short cycling and is it a single or a dual coil relay?

goshen
25-09-2009, 11:43 AM
hi u all. on all units in cold rooms using pump down we usually install a delay on break timer model TD73 by supco these work great for years and are usualy set for 5 minutes good luck

Sledge
14-10-2009, 01:10 AM
I agree with almost everything posted by Shogun7. I am a big fan of a pump down cycle.

I would not wire it so that the compressor cannot run, if there is a build up of pressure in the crank during the off cycle. I think that the compressor should be able to run to keep the system in a pump down status and prevent high crank pressure from building up.

I would use a time delay to prevent short cycling any time that the compressor is to start.

If you want to get elaborate, install a latching relay circuit so that an alarm is created if the compressor is trying to run due to a build up of crank pressure when there is no call for cooling.

goshen
14-10-2009, 09:20 PM
I would not wire it so that the compressor cannot run, if there is a build up of pressure in the crank during the off cycle. I think that the compressor should be able to run to keep the system in a pump down status and prevent high crank pressure from building up.

hi there ,the way we usually wire, is even if pressure builds up during off cycle, after the time delay the comp will indeed kick in and pumpdown again till stopped by low !!