Help making sense of measurements

I'm trying a different approach and trying to work from only temperature measurements and not pressure. This is something i wanted to try after reading a lot of Gary's posts and an approach i think most 'old skool' engineers use.

The system i'm working on is a feeding an AHU that cools a production area in a food processing factory. AHU has two other compressors feeding it, all three are separate with their own condensers and TEV. All sharing the common AHU.

The problem i'm having is the sightglass is still flashing constanly, not bubbling. It appears as the refrigerant is hitting the moisture indicator pin in the middle if the sightglass. All my measurements are telling me the system is charged correctly.

System was vacuumed to regs and filled with virgin R404a

Here are my measurements all in degree C
SST: -24
SCT: 24
Ambient: 3 With one condenser fan on constant and the other cycling in and out. Other two condenser fans not needed.
TD @ condenser: 21
Subcool: 4-6k This is steady with the head pressure holding between 11-13 bar.

Liquid line @ TEV: 7
Evaporator outlet: 9-11

Going by my subcool measure results the condenser is working efficiently.

Starting to get tunnel vision so if it's something glaringly obviously please point it out ;)

if it works,correctly dont worry, sight glasses are the work of the devil!!!

I'm starting to learn this. Although i've worked to the sightglass many times before and never had a problem. There is not temp difference between the inlet and outlet of the drier so im happy a blockage is causing a disruption to the flow

search the forum for this thread Lazy sightglass/Over condensing...Started by monkey spanners‎, 12-12-2011 11:29 PM

Thanks for the link.

Ah the infamous over condensing. I did suspect this given the low ambient. The day before i took the measurements i gave earlier the ambient was 8 and still the same problem. I even stopped the 2nd fan coming in, in an attempt to increase the head pressure.

At what point do we decide, especially at this time of year, that the head/liquid pressure is too low

i had a fridge pack the other day where as a few cases alarmed out, on arrival found there was a wide superheat on a few of the cases off the pack.
having a wide superheat generally some people instantly think the system is short of gas, found the discharge pressure at 140 psi and found the radford 8 relay board for the condensor fan motors faulty and all the condensor fan motors running with a low ambient temp.
so the system was over condensing hence the wide superheat due lack of liquid pressure on the shop floor, replaced the relay card for the condensor fan motors and gained control of the fans.
got my discharge pressure to between 180 and 200 psi approx 29 to 32 degC
it shouldnt really matter much about the ambient temp if you have a system with fan speed control it should in reality keep your discharge pressure at the setting for the system.
obviously in some cases of abnormal weather conditions even with control the system discharge pressure may vary more than usual.

Bear in mind also that you are using a 400 series blend.

These suffer from glide to some degree and this can be seen in the sight glass, maybe due to the pressure change going through the sight glass itself.

Brian i had taken this in to consideration. I had another play around with the condenser fans yesterday, bring the head pressure up to see if it helped with subcooling given the low ambient temp

Being that i now have my subcool steady at 5k should i disregard the sightglass?