real head scratcher
For me anyways. I'm new to the field, Started in October of last year down here in South Florida (for those who require a basic introduction before soliciting advice). Last December I worked on a 404a walk in freezer that had blown the start and run caps from drawing high amps. the Filter drier was installed backwards as well. After recovering charge and replacing caps and repairing burnt off compressor wires, installing new Liquid Line Filter drier (in the proper direction), pulling a vacuum on the system, I went to charge the unit up again and found that my head pressure was through the roof and the sight glass was bubbling. Suction pressure was sitting steadily at 25-30psig and Head was at 325 or more. Compressor was rated at 8.2 amps and was drawing around 9. I figured it was the TXV or Liquid line solenoid clogged by some rougue filter drier dessicant, but it was getting late so I removed enough charge to get the unit limping along without drawing over the full load amps. Yesterday got a call (yes I know the problem was never actually solved the first time but the customer never compained again until yesterday.) the freezer was down. Caps blew up again. did some more diagnosing and with the high superheat at the evaporator and high head pressure I was certain the valve needed to be replaced. Today I replaced the valve and the same problem came about. I purged the liquid line before I brazed the valve in (opened up the liquid line solenoid as well to do so), pulled a vacuum on the entire system, put virgin refrigerant in and got it running 35psi/325psi. Bubbles in the sight glass, any addition or reduction in charge did not change the suction pressure, just head pressure. Checked the liquid line temperature at the condenser all the way to the TXV inlet with little temperature change, essentially ruling out a clog. The box was hot when I started it but the head pressure never wavered from 325. Liquid line temperature was sitting at 115-112 the entire line from before the filter drier to the inlet of the TXV, much too high for my liking! Subcooling was around 8-9 degrees F. TXV appeared to hunt. Evaporator outlet was running a steady 15 degrees even after about 5 turns counter clockwise (not consecutive and as a last resort) of the superheat adjusting valve to lower the superheat. My only theory would be that I have a giant bubble of nitrogen stuck in the system to create the high head, but I pulled a vacuum and the liquid line port is on the top of the receiver! My only other theory is that the condensing unit is terrible undersized for the operation. Compressor is a Tecumseh model number AWA2460ZXD and evaporator is an American Panel model number QALH1557. Any wisdom would be greatly appreciated.
i would work on high head first. verify for sure coil is completly clean, fans working, head pressure control working, etc... lpm is high also. but what box temp and ambient temps are you talking about? bubbling sight glass should nrule out over charge, so figure why head is high and correct, suction will then come down, box should also then come down.
Is start cap pulling out?
Last edited by Core_d; 03-20-2013 at 12:50 AM.
In my experience with American Panel, they DO typically undersize condensing units. And in my experience, they typically use fan cycle switch instead of headmaster for low ambient control. Based on that alone, I'm guessing that your problem is not a headmaster leaking by when it shouldn't be (though that could explain your symptoms,) since I'm assuming the unit you are working on does not have a headmaster.
"but I pulled a vacuum and the liquid line port is on the top of the receiver!"
Although the port is often at the top of receiver on Tecumseh condensing units, all receivers are designed to pull liquid from the bottom of the receiver tank. If the port is on top, then there is an internal dip tube attached to the outlet of the receiver (imagine sticking a straw in a drink so you suck from the bottom, it is the same concept.) This ensures that there is a liquid seal on the receiver outlet. Now if the "straw" were cracked you might get liquid mixed with bubbles, and it isn't very common, but it can happen. If you recovered the charge, pulled a vacuum, and charged with virgin refrigerant, I seriously doubt there is a nitrogen "pocket" in the system.
Hate to ask the simple questions, but is the condenser clean? No hot air recirculation through condenser coil? Condenser fan blade turning the right direction?, etc., etc? These could cause high head pressure. A clogged expansion valve would not typically cause a high head pressure on a remote system like this; the excess refrigerant would just stack up in the receiver...unless of course the receiver is 100% full and then the liquid refrigerant backs up into the condenser, and THEN it could cause an increase in head pressure.
Another problem that isn't really common, but can happen, is that you may have some current flowing through the defrost heaters during refrigeration. This may explain high superheat and amp draw. I would confirm no amp draw through heaters. Then, I would charge up the unit until the sightglass was clear. If the condensing unit is indeed undersized, which leads to higher than normal amp draw, one possible solution would be to install a CPR valve on the suction line at the condensing unit. You can limit the maximum load on the condensing unit so that you don't overload the compressor during hot pull-down. Good luck!
well, to answer a few questions.....
-When the start cap blew up this time around it ended up damaging the potential relay so I replaced it and verified that the new one does take the capacitor out of the circuit.
-The unit has no head master, the condenser coil has some dirt on it, and I was going to clean it just to see if it helped any but I was urged to just let it pull down for the time being since it started with a box full of thawing product.
-I first cycled the unit to get to 30 degrees inside the box and then set the thermostat to around 0 degrees to see how it did. I wasn't able to stay for the entire pull down but it was on pace for getting down to temp.
-Outdoor temp was likely hovering around 70-75 degrees, though I didn't take a temperature reading.
If I'm sent back soon the first thing I do will be to clean that coil. The fan was certainly running and although I wasn't really paying too much attention to the direction of the airflow, I'm quite confident that the subcooling I was getting indicated cool air flowing over the coil. I will of course verify this if I go to look at this unit again.
Thank you for all the responses
According to Tecumseh's compressor performance data, the AWA2460ZXD running on R404A at -5ºF SST/125ºF SCT (28#/335#), it should draw 8.6 amps. You stated that it was drawing 9 amps at about this condition, so I'd say that would be acceptable. Also, their condensing unit data for units with this compressor indicate an RLA for the compressor as 12.6 amps.
Changing out the relay may very well solve the cap failure issues, but I'd want to look for reasons why it failed in the first place...like a short cycling condition or lack of a start cap resistor.
As for the apparent high head pressures, a lot depends on which condensing unit model you have. Tecumseh makes three basic low temp R404A models with this compressor.
AWA2464ZXDXC rated at 1.5 HP with a single condenser fan
AWA2479ZXDXC rated at 2.0 HP with a single fan & larger condenser
AWA2490ZXDXC rated at 2.25 HP with dual fans & still larger condenser
Here's the performance data for the 1.5 HP 1 phase model:
If this is the one you have, the head pressure doesn't surprise me, especially if it could use a good cleaning.
TEV is wide open under high load conditions showing some bubbles. The compressor is working very hard under this condition too. Customer likely loads a lot of warm product or leaves door open for long periods of time forcing the compressor to work to max and creating additional heat in wiring over time along with the FL weather. I'd be sure my new wiring is flawless ( from last Dec ), adj SH once box is at 10-15F, and be sure condensing fan blade & motor correct, coil clean and sign off on it. My simple take on this issue.
I'd also check to see if the evaporator & condensing unit are matched correctly if you see any indication either unit could of been changed if all else fails.
Check the small diameter feeder tubes between the expansion valve and the evaporator. There could be a clog in one or more of them or the manifold/distribution block they feed from. Maybe you are not using the entire evaporator coil.
Couple things jump out at me.
Originally Posted by shellkamp
#1. Your pressures are extremely high. This is causing the high amperage. Try closing down the suction valve to get your suction pressure in line (20ish) and see what happens to the head pressure and the amperage.
#2. Quit adjusting the TEV. This must ONLY be done when the box is at temperature.
High suction pressure does NOT equal clogged tubes.
Originally Posted by eric1589
JP, would this be a good application for a CROT, with the door being left open, etc, making the compressor work hard all the time?
[Avatar photo from a Florida training accident. Everyone walked away.]
2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
AOP Forum Rules:
As I mentioned earlier, I wouldn't too concerned about the pressures given here if the condensing unit model is the one I suspect it is (AWA2464ZXDXC) because those units have grossly undersized condensers (IMHO). If you read the performance data sheet I attached, you'll see that its operating envelope tops out at 43#/372#, which I agree is quite high, but it's still within the design capability of the compressor.
If the OP could verify the condensing unit model, we could save a lot of guessing.
Since the OP's initial concerns with this system's operation was the higher than RLA amp draw, I'm questioning that 8.2 amp RLA rating he stated. From the info I have on this compressor, it should have an RLA of 12.6 amps.
Again, the unit model would clear this up.
Another thing which need to be done is a thorough cleaning of that condenser coil. The OP has already mentioned it was dirty and these units will tend to look OK, but still have a lot of impacted dust buildup.
It would also be helpful to know what this system has for a TXV. From the suction pressures of always around 30-35#, I'll bet it has a MOP pressure limiting powerhead. The inability to affect a superheat change at this pressure also points to that, although the flash gas as indicated by the bubbling SG could cause that too.
The size of the TXV can also be a factor, so it would be good to get that info as well. I've seen standard, non-MOP valves act like pressure limiting valves during a hot pulldown simply because they don't have the flow capacity to handle the load at that condition. A unit like this may be sized for 6000 Btuh at -20ºF SST, but during pulldown the capacity is nearly double that.
Finally, as JP stated, quit messing around with the TXV...at least until you get the box temp down near design. Set it back to about the midpoint on the adjustment stem and leave it there until later. This is especially important if the valve is indeed an MOP type valve. I've seen guys open them up to try to get them to feed during pulldown only to have them flooding like a SOB once the box temps drop.
Must clean cond coil and verify fan operation before you do anything else. Walk in condenser coils can be deceiving. May not look very dirty till you put the foam to it. Do not chase high head problems, ever, until coil and fans are verified clean and operational. Good luck