apprentice need help getting my head around.

[Wallbank]

Hey, so im an apprentice fridgie.. And just found this forum to help get my head around things.
I'm really enjoying tafe and the company I work for.. But I tend not to ask too many questions to not look stupid.
So I work on more cool rooms and installation of split systems.. But lately I have been in service and diagnosing faults on systems. I have got my head around pressures on cool rooms and how the gauges show the temperature of the coils and what not. Like reading the head presure and suction.

But what really gets me is inverters.. I was at a system the other day, it wasn't blowing cool air. So I went to the outdoor unit and put the gauges on it. I couldn't get my head around the pressures! It was 407 and it would ramp up and down like an inverted does.. But my supervised said don't take too much notice of the games and pressures? What the? He said because of the expansion valve there isn't really a suction or discharge pressure? How am I meant to know the coil temp? How am o meant to know if the pressures are low? Like they were at different pressures I put the high side manifold on the smaller pipe, and lower pressure on the larger pipe (usually suction?)
He said he could tell it was low by still looking?
I just don't understand.. Is there a low pressure and high? What pressures should you expect. And is it worth putting both hoses on the ports to look at the pressures

I'm just really struggling to understand the pressures of a split system.
I believe this thread and reading all these will help me become a better fridgie.

Thanks

[The Viking]

Hi Wallbank and welcome to the forum.

First of all, the only stupid question is the one not asked.

Pressures / temperatures and their relationship in modern, inverter driven, AC systems...
In the good old days, the normal AC system worked just like your walk in cold room. Fixed speed compressor, solid refrigerant and the expansion device by the evaporator.
Sub cooling was sub cooling and super heat was super heat.

However, a lot of developments have happened since the good ol' days.
Changes of refrigerants, use of invertered compressors and electronic expansion valves means that the relationship between pressures and temperatures isn't as clean cut as they once used to be.

There is still a high pressure and a low pressure side of the refrigeration cycle, there need to be otherwise the system wouldn't work. They are just not neccessary where you expect them to be.
As the modern AC got the expansion vale in the outdoor unit, both pipes between the outdoor and indoor units are on the low side of the system when the unit is cooling. The whole of the high pressure side is inside the outdoor unit. The smaller pipe between the units is therefore referred to as an expansion line but the larger one is still your normal suction.
Assuming your system is a heatpump then the opposite is true in heating mode, the larger pipe is your discharge line and the smaller your liquid line.

All things considered, the best way to know if a modern AC system got the correct charge is to reclaim the refrigerant and weigh it. Quite often when I get to go to site to look at splits they are actually over charged, not short of refrigerant. It is to easy to say it doesn't perform and add some refrigerant to the system.
If in doubt, always reclaim and weigh. (Regardless of what the ladies say, scales never lie)


Happy learning.

:cool:

[frank]

It might help you if you review this thread http://www.refrigeration-engineer.com/forums/showthread.php?19701-Refrigeration-101&highlight=refrigeration+101

[Wallbank]

Thanks so much mate, I have been going through so many threads on here. I have started to get an understanding of it.. Definetly different compared to the old ones hey.

Thanks again mate, helps to know I can atleast get access to the low side pressures easy enough.
Say if I have it in heating.. It would the. Be the high side?

Thanks again mate

[Wallbank]

Thanks frank.. At work now. I will have a read through it on my lunch break. Thankyou