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Discussion Starter · #1 ·
Troubleshooting a Trane TTA090A3 that continues to freeze up. This is the only unit I have seen that has two compressor units (Both Trane A3s) feeding a common evaporator (different refrigerant lines) and common blower and duct work. The unit is feeding an electrical/electronic equipment space that heats up quickly when the unit freezes up. I checked the evaporator coils and extremely clean with new filters. The charge on the system is different, one seems about right for the low load on the system this time of year, the other is 190/48 but the low charge has good SC (6 degrees) and the SH seems a bit high at 33 degrees. The other unit has a charge of 235/62, but the SC is high and SH is about the same on both units. Funny thing is, when either unit is off, the other works fine alone - problem only occurs when both units are running together. Also, the unit with the low charge will not take a charge - I was thinking that for the units to work together, they might need to be balanced load wise, so attempted to charge the lower unit to the same pressure as the higher unit. Any help would be appreciated!!
 

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High superheat and low suction pressure usually means undercharged. What's your delta t on the problem unit? Is it a txv metering device? That suction should be above 60psig, right now the saturation temp is below 32F, which is why it's freezing.

Because your problem only exists when both units are running may indicate insuffcient air flow across the coil. See if increasing the cfm raises your section pressure. That should also increase your delta t.
 

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Discussion Starter · #3 ·
Thanks for the reply. A bit more history. The unit was installed just over a year ago and has been running fine until this first freeze up about 10 days ago. This unit is on an offshore rig in Mexico and before I was sent, they had a local company work on it three times. The locals convinced everyone that the other unit was overcharged and that was the problem, and then they installed suction line filters (?) and of course, the problem didn't go away. They had spent all of their time on the higher pressure unit and didn't do anything with the lower pressure unit. I believed that the unit was undercharged and causing the freezing problem, however, still fumbling (and showing my ignorance) as to why I can't get the unit to charge.

The evaporator blower is a standard belt driven blower on a single speed motor. I am going to take your recommendation and check for belt slippage or something that may have caused the air flow to decrease. The delta T across the room when the problem unit is running is about 8 degrees, this unit cycles about every 5 minutes when running by itself. This is a TXV metering device. I didn't mention in the first post, but the compressor currents also are low, compared to FLA. Problem unit is 17.5 amps and the other is 21 amps, FLA is 25. I contribute some of it to the low load while the outside temp is lower and the heat load of the space is down.
 

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I think you got a wrong model number posted.

The model number posted TTA090A3 is a single compressor condenser.

Also. is the indoor blower coil actually a matching Trane unit. or just the old unit before this new condenser was installed.

Do you have the service facts sheets from Trane for each unit.
 

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Discussion Starter · #7 ·
Actually, the units are both the same single compressor models installed side by side. The evaporator unit is hard to identify as it is a locally manufactured "box" with the evaporator coils installed into the box. The fan and motor are just installed into the "box" and there is no identifying labels on this part of the installation. Checked the fan belts and all is good. Have lots of airflow. Yesterday I removed the TXV bulb and tapped on the TXV. Pressure came up some, so I may have a bad or blocked TXV, will chase that out today.
 

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Ok, so you have 2 condensers connected to 1 indoor unit.
And it is a MISMATCHED system.

And you also have no idea if its moving enough air, or you would have posted how much air its moving in CFM instead of saying "have lots of air flow".

Considering that at 4400CFM you would have lots of air flow. But still have low air flow.

Is your indoor coil an upper lower config. Or is it a font back config.

A front back needs max air flow or the first coil will freeze almost every time the second condenser comes on.

Your unit that won't take a charge, is probably already over charged.

And that system has probably had low air flow from the time the 2 new condensers were installed.

You said the indoor unit was made by a local company. Contact them and find out how much air its suppose to be able to move. At what static pressures. And then measure the static pressures its working at, so you know how much air its moving.

Wouldn't be the first time a 12 indoor ton unit had 15 tons of condenser installed on it.
 

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Discussion Starter · #9 ·
Thanks for info. Yes, I do not have any way of measuring air flow here. The units are front to back. Have tried to check for local contractor but, this is Mexico and we have no drawings and no contract info here, I am having the office check for the contracts, but my experience is that it will give a minimum requirement and have to assume that is what they went with or whatever was sitting around the shop at the time of installation.

Is it normal for the system to operate fine for a period of time and then start developing problems if the airflow is inadequate? I have checked for something that might have changed air flow, but can find nothing as far as the motor or fan belt slipping. Motor rpm is right at rated, belts are tight and in good condition and pulleys are tightly installed on shafts. The fan is wired to run continuously, regardless of the thermostat setting so the good thing is that when it freezes, it thaws quickly and shifting to either unit individually works fine but load is low and outside temps are mild right now, could change drastically in a few months.
 

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Probably part of what is causing the problem. is that its using 2 thermostats.
Good chance that the upstream coils condenser is starting first, and causing the downstream to flood with both refrigerant and oil.

Or the downstream is stopping first.

They should have been set up to run off of 1 stage cooling thermostat and each condenser set up as a pump down system, using solenoid valves.

What is the blower motors FLA, and what is it drawing?
 

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Discussion Starter · #11 ·
Hey, what do you know, I went to check the blower motor current and being that the blower is on continuous run, when I turn off the power I am not watching the unit. Well when I went to remove the covers, which are heavy as hell, I got a head start on the electrician and when he turned off the power I was already watching the motor - which start slipping really bad. Not noticeable when it was running but when it started slowing it was obvious. I replaced the belt and tightened it, turned on both units and they have been working great for several hours. Will continue to watch them tonight and if all is well, I will be heading back to land tomorrow and having a cool one. Thanks again for all of the help, it made me look at things differently and I probably would not have opened the evaporator up again if not for the reply today.
 

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Computer/electronics rooms generally have a SHR of .9 or more.
So they often need 450+ CFM per ton.

On 15 tons, thats 6,750CFM.


Air flow is one of the most over looked problems with HVAC equipment.

If you work on a lot of equipment that you can't find the CFM rating of the indoor equipment. And need to know if its moving enough air. You might want to invest in a mini vane anemometer. That would allow you to atleast verify how much air its moving. And could save lots of hours of diagnostics.
 

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Discussion Starter · #13 ·
Thanks for the info, I have already added to my book. Definitely will get the anenometer and also found that a dual temperature probe is a necessity. Trying to get air temps with a lollypop doesn't provide reliable information and the infra red thermometer is great for the refrigerant side but pretty much useless on the air flow side. Much learning accomplished on the process. Definitely different than the package units and reefers that I usually deal with. Again thanks for the help - this sight will be part of my toolbox from now on.
 

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IR's aren't good for checking refrigerant line temps either.

I use them for checking air temp on supply or return grilles that are out of reach without bringing in a 6' or better ladder.

IR's are good for a reference check, but thats about all.
 
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