Charging a system correctly

hey I was told something about charging a system today that I havnt come across Il give you the details of the system that I was working on, basically its a R404a system thats freezzing down a room to -10.c pretty straight forward well thats what I thought, I changed the compressor, drier and did the usally nitro leak check and vaced it out.

But once I had charged the system to a 90% sightglass my head pressure was up at 300 psi and suction at 80 psi room temp 15.c (was 2 machine working in the room for heat load).....so I was told to put a C.P.R on it (to help the compressor life)but once I got talking to the tech guy at the supplier he ask me what the problem was so I explained what had happened and straight away he said I had over charged it and that todays modern refrigerants shouldnt be charged by sight glass and that sight glasses are just for moisture indication.

he gave me a data sheet to regards of the running amps to suction saturation temp and on this instance the most amps I should be pulling when the room is 10.c or above is 8.99amps So once I got back at the unit I found the unit pullin 9.45 amps I then proceed to remove the refrigerant from the reciever in to my reclaim bottle to see the amps come flying down suction pressure coming down.He also told me when I was charging a new system up that I should only half charge the system wait til the room starts to pull down and keep squirting small amounts until am about 5.c off the set temperature then go back to the condensing unit take the ambient temperature add (12k as its a tropical condensing unit, this is the diff across the condenser) this should give me the discharge temperature if it high ave over charged it.

This is the first instance ave had this problem and I mite of not described the situation well or got my wires cross but it just to see if any one else could shed some light on it.
cheers

Basically, don't use a sight glass.
Grizzly

thanks ave already worked that out

Talk to Magoo.
Your problem has more questions than answers at present I am afraid.
Although my initial thoughts are what is the condenser like?
Grizzly

A compressor has working envelope, the motor is sized upon displacement and the type of refrigerant used. (plus power factor for current draw, but that is to deep at the moment)
So when charging you have to consider why and what the system was designed to do.
So if it is a once off pull down, then slowly, slowly with the charge until you get close to desired conditions, but if it is a system then load does vary massively then you look at a CPR or other methods to keep the system within the working envelope.

cheers fellars, the condenser spotless it was clean throughly, as for the heat load the system pulls down to -10.c but then the customer puts on another system that raises the temperature to +40.c!! Just found it interesting and wanted to see what other peoples thoughts where......the question is how many systems are there out there that are over charged!!

Hi NZ Pomme.
good advise from MF., another alternative is to use MOP TXV's, which limit TXV capacity with high pull down loads.

if you believe its overcharged just carry out a overcharge test. on start up and room warm the suction will be high thus high head like above says if no cpr then fit a mop tev or after defrost or start up that comp will soon knock itself out the chances of the suction staying that high that long will be slim when room starts recovering the suction will lower thus the head will drop i pressume it has a pump down proceedure? as far as sight glass goes in my eyes it is a indicator of liquid to metering device and secondly a moistue indicator but useually the moisture indicators read wrong anyway

Hi. As for me, I always charged under the date of **** on the unit. very difficult to determine the accurate charge, depending on the ambient temp and may even humydity.
The most correct and accurate charging - drained all the gas in the bottle and then charge on the scale it again.

thats fine until someone fits a larger/ smaller comp different receiver or whatever else, everything is a indicator .........run amps of comp /sight glass/ pressures and your temperatures we all have a different technique its what works for individuall engineer

charlie patt youve hit it square on the head.......very rare you get a sytem that hasnt been modified in some way other another, Int the u.k things where pretty uniformed well not always but in nz its just about getting it to work!!

Hi NZ Pomme.
your comment "in NZ its just about getting it to work "., isn't that what your are employed to do. Its not a guessing game.
magoo

haha . getting it to work and getting it to work correctly and for longer is a differnt thing . i cant talk though . i charge to the sight glass and pressures only . what else should i be checking . ?

sight glass has no bubbles , pressures are normal . cold air coming out . thats it ?

Quote:

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Originally Posted by Simeonx1

haha . getting it to work and getting it to work correctly and for longer is a differnt thing . i cant talk though . i charge to the sight glass and pressures only . what else should i be checking . ?

sight glass has no bubbles , pressures are normal . cold air coming out . thats it ?

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This reminds me of my first few weeks on this forum. How the members managed to put up with me :o. You will soon find out that there are a lot of experienced persons out there working in every corner of this industry, and so many way above my pay grade..;)

superheat/subcooling ,oil levels, td accross coils, operation of controls,safety circuits

Quote:

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Originally Posted by Simeonx1

haha . getting it to work and getting it to work correctly and for longer is a differnt thing . i cant talk though . i charge to the sight glass and pressures only . what else should i be checking . ?

sight glass has no bubbles , pressures are normal . cold air coming out . thats it ?

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Hi Install Monkey.
you are onto it, understanding system performance and interpruting information from gauges thermometers and amp meters. Called system balance and operational conditions, sometimes unfortunately sadly lacking in young techs these days.

yes magoo your correct, too many people fit systems and walk away without spending a few hrs comfirming its running at its optimal performance no just blowing cold air!

im sorry :( im use to install and welding copper pipe. ill try harder .
quick lesson on checking superheat please . i can remember most of it i think .

Hi Simeonx, I'll give you a quick lesson.

Suction super heat is the diff in temp from saturated suction temp to actual suction temp. Most DX eavps run 4-8K. Flooded evap 0-1k
Discharge SH is diff from Sat cond temp to actual discharge temp. Directly related to suction SH. Dx evap 25-40k,Flooded evap 10-18k
Subcooling: Sat cond temp versus actual liq temp. water cooled cond usually low SC say 2-5ish air cooled usually higher 6-12ish, depends on the system.

remember, you want super heat to ensure all the liq ref has boiled off and comp is only pumping vapor. You want subcooling to ensure all gas in condensed to 100% liq.

A note on bubbly sight glasses: 4 series of refrigerants are blends and usually have "glide" this means the condensing temp is different to the evaporating temp. (bubble point for evaporating and dew point for cond) R407c has 7k of glide. this makes a clear sight glass hard if you only have 3k of subcooling.
I hate r407c.

This is my experience only from an AC fridgy's point of view.

hope that helps. If you want more info PM me as i am only too happy to share what others have shared with me over the years.

to charge gas with right amount of gas you have to check your room temp. and your evaporator temp.
we say D.T. it should not be less than 6 for R 404a . if it is less than 6 you have to adjust expansion valve by closing it . when you adjust it you should check sight glass it should be full .
also you have to chose right size of condenser, and it must be clean to avoid of high pressure .
C.P.R. is very good and it protect you compressor from over pressure in suction side ( during start time and after defrosting and also in running time )