low charge or cap tube blockage?

[NC joe]

Hello everyone,

I have never used this before so hopefully I wont screw it up.
i have been messing with a refrigerated compressed air dryer. the system has 134A in it. I have about 30 to 35 degrees f superheat. My head pressure is about 165 psi suction is about 28 and stable no matter how much I load the evaporator. this dryer has a fan switch that turns the fans on at 175PSI and off at 105psi. I had thought that if my high superheat was due to a low charge I would see lower than normal head pressure with increased head temperature. I have the increased temp but not low head pressure. Even when i load the evaporator way beyond its design point the hot gas valve is still assisting the suction pressure (which explains my overheating problem) without droping the head pressure. To me this means I have plenty of regrigerant there is just not enough goin through the cap tube. I am obviously not a refrigeration expert but am I wrong in thinking that the cap tube must have blockage? How could I be sure if it is cap tube blockage or low charge without evacuating the dryer and weighing the charge? thanks in advance for any input.

[Bones74]

Hello everyone,

I have never used this before so hopefully I wont screw it up.
i have been messing with a refrigerated compressed air dryer. the system has 134A in it. I have about 30 to 35 degrees f superheat. My head pressure is about 165 psi suction is about 28 and stable no matter how much I load the evaporator. this dryer has a fan switch that turns the fans on at 175PSI and off at 105psi. I had thought that if my high superheat was due to a low charge I would see lower than normal head pressure with increased head temperature. I have the increased temp but not low head pressure. Even when i load the evaporator way beyond its design point the hot gas valve is still assisting the suction pressure (which explains my overheating problem) without droping the head pressure. To me this means I have plenty of regrigerant there is just not enough goin through the cap tube. I am obviously not a refrigeration expert but am I wrong in thinking that the cap tube must have blockage? How could I be sure if it is cap tube blockage or low charge without evacuating the dryer and weighing the charge? thanks in advance for any input.

Interesting. Well here is the bad news first. It's probably restricted and you over charged it. Probaby wax or moisture in the inlet of the cap tube. The pressure that you see is the refrigerant backing up inside the condensor Rule of thumb if you suspect a restriction, Put only half the name plate charge in the unit and if the pressures stay the same, replace the cap tube. I would change it and the filter, put in fresh gas and test it from there. Just a thought.

[Peter_1]

The hotgass bypass valve only tries to maintain a suction temperature above the freezing point of water.
This valve doesn't know if there's enough refrigerant in the system orr if the evaporator is heavy loaded.
Reclaim the gas, remove drier, blow through the caps in opposite direction, replace filter drier, vacuum the system the proper way and refill.

[Frudge it]

Hi NCJoe ,
I reckon the 2 previous posts to this on have your problem(s) nailed nicely - But I do want to add a fine point with the cap tube.Make sure the cap tube entry is nice & straight on reduction from 1/4 " & ensure that the tube where its been cut has been restored to original diameter by removing internal ridging at the original cut point.A fine set of gas tip cleaners for oxy / acetylene does wonders here , in conjuction with the both ways dry nitro blowout.
Cheers

[paulsref]

Hi Everyone,

I am currently working on a true 4 door cooler tpp119. I am having trouble with charging the cap tube system. My original call was to replace a thermostat on warranty, that was replaced by another service tech but he put on an air sensing stat. anyway I changed the t stat and was called back the next day unit to hot.
When I went back unit seemed to be runnimg properly with the exception of the drier I had 108 going into drier and 88 coming out. The company recommended I change the compressor and drier. I did, same thing owner calls and says to hot I go back check unit again true says I need to change the cap tube so I take unit apart change cap tube called the next day again unit up to 70. I go check unit and it starts right up pressures seem good. I check evap per true and get 22 degrees on evap which should be 18 degrees. I add charge and subtract charge trying to get this unit to go lower t stat seems to be working properly. when their I put my temp probe back in evap and turned unit back on but it wouldn't start no power to compressor so I went to check t stat to see if it was closed when I pulled connectors off one broke so i suspected it was a problem I jumped t stat wires and could hear relay kicking in and t stat was closed. Now I am suspicious of relay or could it be something else. Compressor was not off due to overload protector I just didnt have any power to compressor. I read the previous post and will check the superheat and subcooloing. If anyone has expierenced this, your input would be greatly appreciated. I suspected low air flow over evap but now I feel either electrical or I still have a restiction problem maybe I didnt get cap tube in right I did make sure when I cut tube it was opened up to correct size. I use nitro soldering and blew out with nitro leak checked with nitro evacuated and added dry nitro again and evacuated. Thanks Paul

[nike123]

The strainer on a restricted cap tube should be about room temperature because of the sub cooled liquid that has backed up, and running your hand down slowly from the top of the condenser the temperature should be warm then suddenly cool to room temperature to the level where the liquid refrigerant has backed up.
In a system that is short of refrigerant the condenser would have a gradual change in temperature from the top all the way down to the strainer. In fact the strainer will be slightly above room temperature.

[Deniver45]

I agree with Bones;

and the reason is, I went through EXACTLY what you NC Joe is going through and I did remove the cap tube and it was cloged; badly cloged.

But with my customer, he ran the darn thing with blocks of ICE in his 134a cooler and has had it charge OFF a leak for many years now. IF the previous tech put a can of that AUTOMOBILE 134a in there; then the thing is REAL contaminated!

I ONLY FOUND OUT ABOUT what he had done THE 3rd trip out@!, and man was I PI$$ED! I handed him a bill for $750.00 (including parts) and HE was pi$$ed! Go figure......... a AC tech gets 4 HOURs to replace a access valve!

Anyway, I found the leak in the evaporator and brazed in a new dryer too.................that is when I had the problem NC Joe had.

I did everything right here; I am super meticulous and do it by the book, anyway I FORGOT to bring the lid to the condensing unit with me after I took it from there. After I brazed the NIB Evap and dryer; I disconected the evap fan and she iced up nice; then I get the unit back to the customer, back on top of his reach in cooler and it dropped to 49, then 45, then SHOT UP FAST to 55 and hung at 55.

I removed it and threw it back into my truck. this time, I had the click on top to the whole unit,(Hozishaki RH2-AAC = never have problems, so the tech @ Hozishaki says) I obviously would never had brought it back to the customer in the first place because I could have seen the proper subcoolant and superheat = what it was really doing............

I could NOT blow through the cap tube after I removed it, I had to braze a fitting to it and blow 150lbs of nitrogen through it........that is when I decided to throw a new one in for $10.

I KNOW it is full of moisture, wax and the contaminents that 134a is FAMOUS for!

[mcunliffe4]

recover all refrigerant from cabinet, replace drier with new, blow pipe through with ofn to test for any restriction in cappilary, replace if blocked, tripe vac with ofn, weigh in refrigerent, make sure cond/evap coils are clean and all fan motors working to spec.

[paulsref]

okay OFN is that hitrogen? Thanks Paul