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Freezon
14-07-2015, 01:16 PM
I am working on a very old Jackson and Church 20 ton RTU. It has 3 unloaders sitting on the top of the compressor. We are currently experiencing near 100 degree ambients and this unit is tripping high head control. I have instructed the resident maintenance crew to clean the 2 condenser coils extensively which I am assuming they have done. These coils are close to 6" thick and require a lot of cleaning to make any difference.
Over the years this unit has been sliced and diced. It was originally a multizone, hot and cold deck. Now it is a straight cool or heat, minus the zones.
I realize the unloaders should be working on 1st or 2nd stage cooling calls. The original compressor I am pretty sure only had two unloaders, this one has three. Either way, I am thinking if I drop out one of the unloaders at times of high head I might be able to stop the manual trip control and if that is the best answer would I go about installing a auto reset psi controller with a adjustable time delay so it doesn't short cycle? any suggestions would be greatly appreciated. I will start with the cleaning and fan check out first, just getting armed before I make the journey. I know it is not overcharged. Thanks in advance!

Freezon
14-07-2015, 02:08 PM
I guess another way to do it(assuming that is the way to do it) is to use an ambient stat and drop an unloadr

Glenn Moore
14-07-2015, 11:03 PM
Hi Freezon
Yes I would start with the condenser clean using one of the foaming coil cleaners, that are on the market. They get into the fin coil and push out the dirt from between the fins. You may have to do this a couple of times but it will be worth it.
If that fails you can use a High Pressure switch with Auto reset and set it to switch an unloader at approx 20 psi below the HP safety switch cut out point, with a differential of something like 30psi, and a second switch for the 2nd unloader set for 10 psi below below the HP safety switch. These settings can be altered to suit the way the discharge pressures change after being unloaded. Ie just tune the system to suit your requirements, but if the condenser is cleaned on a regular basis this should not be necessary
A 3rd way could be by spraying water over the condenser, controlled via an ambient stat with a water solenoid valve. Just use a copper tube with holes on it to spray water onto the condenser during high ambients, similar to what they do in Disneyland to keep the punters cool while waiting to get on the rides, Kind Regards Glenn

Glenn Moore
14-07-2015, 11:06 PM
Hi Freezon
Sorry if using the last method use soft water otherwise you will cake the condenser with calcium, and magnesium (limescale) and youll be in more trouble

Freezon
15-07-2015, 12:45 AM
Hi Freezon
Sorry if using the last method use soft water otherwise you will cake the condenser with calcium, and magnesium (limescale) and youll be in more trouble



Thanks. yes I have done the water thing before and yes if it isn't softened it gets pretty messy. I ran a series of tests and it turns out one of the three fan blades was a greater pitch which made the 1 hp motor over amp. The torched looking label was the first tell tale. Cut out was 400 psi. Tested it and it was dead on. Started covering condenser and had to cover 75 to 80 % w/ambient of 96 to get it to trip. Then I took one fan away with card board removed and it ran at 375 psi in the morning. So I zero'd in on the fan motors and found the one over amping. Was probably going to trip when it got a little warmer. Put the correct blade on and its been running all day. Looks like we got it. Thanks so much for your input. I really appreciate it greatly.

Glenn Moore
15-07-2015, 06:32 PM
Glad it's OK now kind regards Glenn