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NC joe
20-10-2007, 01:03 PM
Hello everyone,
i have never used a discussion forum before so please be patient as I figure out how to use this. i have posted this question before but I think in the wrong forum so here we go. I have a compressed air dryer that is shutting down on high refrigerent discharge temperature. This system is very simple as far as refrigeration circuits go. It is a 134A system.It is simply a compressor, condenser, then a cap tube to the evaperator out of the evaperator there is about a foot of suction line before the hot gas valve tees in. This hot gas valve is really nothing more than a suction pressure regulator. When this thing is under a normal load, I have about 35 degrees F superheat. Head pressure is at a normal pressure of about 165 psi, suction is stable at 28 psi no matter what load you throw at it which tells me the hot gas valve must be working well. in past experience with similar machines that are designed like this, I have observed that when the charge is low my head pressure usually drops while the temperature of the discharge refrigerent from the compressor climbs to higher than normal levels and superheat increases. Since my head pressure seems normal at lower loads and seems to climb as expected with increased ambient temperature I am thinking the cap tube must have blockage. even under an extreme ambient temp and extreme load on the evaperator the hot gas valve is still assisting the suction pressure. my theory is that I have enough refrigerent but since the cap tube is restricted I am not sending enough refrigerent into the evaporator so the hot gas valve(or suction pressure regulator as I like to call it) has to open and let some of the high side refrigerent in to keep the suction up. At the same time I still seem to have plenty of head pressure. I am working with someone who likes to add refrigerent no matter what the symptoms are and his answer to every problem is "Add more! It has to be low!" If this system were low wouldnt the result of the hot gas valve opening to assist the suction side pressure be that my head pressure would be low? Is there any good way to be absolutely sure whether or not my problem is cap tube blockage or low charge without evacuating the charge and weighing it? I like to do things based on facts and not just add refrigerant and change parts untill the thing starts working. Any help in understanding this better is much appreciated. Thanks in advance for any input.

Regards,
joe

philjd26
20-10-2007, 03:09 PM
hi.briefly run system without the back pressure reg and watch pressure if it countinues to drop and the liquid pressure is up too normal,your cap tube is blocked....also watch the subcooling wit reg disconnected if larger than normal there is no move ment of refrigerant

NC joe
29-10-2007, 03:05 PM
hi.briefly run system without the back pressure reg and watch pressure if it countinues to drop and the liquid pressure is up too normal,your cap tube is blocked....also watch the subcooling wit reg disconnected if larger than normal there is no move ment of refrigerant

Many thanks for the help. I was wondering if that would tell me anything. the only thing i was worried about is that if i do that and my suction side pressure drops too low which I think it will, could I damage the compressor by slugging it with liquid??
Right now when the evaporator is being subjected to a high thermal load the back pressure regulator is still bypassing so I would guess that if I block that off, all of the refrigerant that was bypassing over to the suction side will now be forced to stay on the high pressure side and I would think, cause the head pressure to be even higher?????? My ambient is 90 degrees f the temperature of the refrigerent as it leaves the condenser is 109 degrees f and the head pressure is about 165 psi. How do I determine subcooling?? I am not familiar with what that term means.

thanks again your help is appreciated

philjd26
31-10-2007, 08:19 PM
hi joe,

subcooling; the temp of a liquid below the condensing temp at the same pressure....

take a look at you head pressure and go to refrigerant cond temp of r134a....take a temp reading at the outlet of the condenser subtract the two readings... thats your subcooling, should be around 8-10 c...if excessive there is no movement of refrigerant

if the back pressure drops off then your only problem would be no cooling back to compressor causing to go out internal thermo...no refrigerant no slugging, this would be the case if were overcharge wit small superheat indicating this...my guess is you might have a blockage on a drier or your cap...

i would reclaim gas, vac to 400 microns,change driers etc and tip a fresh charge in, with a good load on evap...

regds phil

PoodleHeadMikey
02-11-2007, 02:19 AM
Is the condenser coil dirty? Can you shine a flashlight though and see the light? Maybe you have more than one problem.

PHM

Gary
10-11-2007, 04:15 PM
From a troubleshooting standpoint, all of the useful indicators in a refrigeration system involve subtracting one temperature from another temperature.

Subtracting condenser entering air temperature from condenser leaving air temperature (condenser delta-T or dT) gives us an indication of condenser airflow. Unfortunately, you have not provided that information.

Subtracting liquid line temperature from saturated condensing temperature (pressure converted to temperature on a P/T chart) tells us the condenser outlet subcooling which is an indicator of refrigerant charge.

165psi converts to about 118F.

You state that the leaving refrigerant temperature is 109F. (Is this a liquid line measurement?)

118 - 109 = 9F subcooling, which is a little low, particularly when coupled with the high (35F) (evaporator outlet?) superheat. Add refrigerant until the SC is about 15F.

There are many more indicators, telling us pretty much everything we need to know about the system and all of them involve subtracting one temperature from another temperature.

paul_h
10-11-2007, 04:47 PM
A blockage in the cap tube or TX device doesn't cause high head pressure, the vapour in the condenser has more time to stay in the condenser, because it's not flowing out to the evap. Hence it loses more heat and the pressure stays low, filling it up with liquid like a reciever until the compressor can't add any more refrigerant to the condenser because it's not getting any back from the evap.

I've worked on a few of those driers, they normally have a fan controller on the condenser.
Is the condenser clean, is the fan running full speed?


edit: Sorry, I realise now that you say the HP is good but the DT is high. It's really hard to tell with these systems that artifcially keep the LP up with hot gas. You're right about high DTs are mainly caused by lack of suction vapour return, but it's hard to measure the suction pressure or temperature with these systems. Does this one have a pressure regulator on the suction line to set the evap temp? Some of the ones I've work on have.

Gary
10-11-2007, 05:05 PM
If we subtract ambient temperature from SCT we get a condenser TD of 118 - 90 = 28F TD, which is borderline high, particularly on newer systems with larger condenser surface area. After adding refrigerant to raise SC to 15F, the TD will no doubt be considerably higher, so there is indeed a high head problem, most likely caused by insufficient condenser airflow. Condenser dT (air out minus air in) would confirm this, but that information was not provided.

Temprite
12-11-2007, 07:25 AM
A blockage in the cap tube or TX device doesn't cause high head pressure,

Unless it is a cap tube system that has a partial restriction that someone has overcharged trying to get a good low side pressure to allow refrigeration to occur.

Then you will get high pressure and lots of subcooling.

Happened to me a couple of weeks ago.