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Nobby
16-09-2002, 01:08 AM
Hi Guys and gals
I have an small under the counter fridge that has been retro-fitted from R12 to R409a. My only indication of this is someone scratched out the R12 and wrote 409a. Fair enough, but the quantity to put in after a leak is obviously different from R12. The label says 1.5LB or R12 and I need to know the correct weight for R409a. There is a mathmatic formula, Not sure what it is however. I'm not certain on the methos of accessing what refrigerent is in the fridge. As you may have guessed I'm pretty new at this and hardly do any fridges. I am certified and have taken a nine month course, but that was some 2 years ago and if you don't do something on a daily basis the knowledge fades.

superheat
16-09-2002, 01:40 PM
There is no set weight of 409. 409 has more capacity than R12. You will put in about 80% then use: amp draw, subcool, superheat, split or your favorite method to fine tune the charge. With a domestic frig you have the heat exchanger to prevent liquid from entering compressor. Don't be suprized if you have no subcool at the drier when you get it charged. Pay attention to all the stuff you know about the refrigeration cycle and good luck.

Nobby
16-09-2002, 02:04 PM
Thanks Superheat :)

Prof Sporlan
16-09-2002, 10:51 PM
One can use the difference in liquid densities between the refrigerants to estimate the change in refrigerant required.

For example, at a 105F liquid temperature, R-12 has a density of 78.2 lb/ft3, and R-409A has a density of 71.5 lb/ft3, per the RefProp program. The liquid density of R-409A is about 91 percent of R-12, so a ballpark number for the amount of R-409A required is 91 percent of the weight of R-12 in the system. Taking subcool and superheat readings are always advisable whenever switching refrigerants... :)

superheat
17-09-2002, 01:46 PM
Prof with all due respect, is liquid density a large factor in detirmining charge? I quess it does add something, but it seems like 409 having higher heat of vaporization would be a larger effect. I do not have a table in front of me, but I recall 409 has 20% more heat of vaporization than R12. If you circulate 90% of the 409 by weight trough the system, it would be overcharged. 20% extra capacity with 90% total weight would be 108% capacity compared to an R12 system charged by nameplate weight.

These systems can be confusing. I have charged systems that appeared to have next to no subcool and next to no superheat as well. I preffer liquid line heat exchangers or TEVs on retrofits. Sometimes I remove all armaflex to get my superheat up.

Prof Sporlan
18-09-2002, 01:10 AM
Prof with all due respect, is liquid density a large factor in detirmining charge? I quess it does add something, but it seems like 409 having higher heat of vaporization would be a larger effect.
Obviously, there will be a number of factors that will affect required refrigerant charge. But consider the following: When you charge a refrigeration system, you are essentially filling up a volume. The majority of that volume can be expected to be vapor, but a significant amount will be liquid... the liquid line, a portion of the receiver (if present), or a small amount of the condenser if a receiver is not present. A small amount of liquid will be also present in the evaporator.

Now some numbers: 105F R-12 has a density of 78.2 lb/ft3. R-12 saturated vapor at 20F and 105F have densities of 0.90 lb/ft3 and 3.44 lb/ft3 respectively. Vapor in the discharge and suction lines will be superheated so actual densities will even be less than these values. So the Prof is making the assumption that there is enough volume in a typical refrigeration unit filled with liquid that figuring remaining vapor weight is not going to add much insight when converting from one refrigerant to another.

The higher heat of vaporization, a.k.a. the net refrigerating effect, only gives you the cooling effect per unit mass of refrigerant flow, and switching refrigerants will affect the refrigerant flow rate the compressor is able to deliver and ulimately the system's cooling capacity. But does it provide any insight as to proper refrigerant charge? If you think about it, one should be able to take a snapshot of the system in operation, and figure out the volumes occupied by liquid and vapor... this eliminates flow rate from the picture, and it gets us back to the question: is there enough vapor refrigerant weight in the system to be concerned about?

Nobby
18-09-2002, 02:31 AM
Thanks Prof for the in depth reply, Most of it over my head so far. I hardly do any refrigeration while out doing what I do, I therefore don't get into the flow of things (No pun intended). But the odd fridge crops up and I have to fix it. Although legaly able to repair and 9 months book work, Its no substitute for "Hands On" experience. Previously in the UK we were thrown into the lions den with the curt instruction "Fix It" Which was great for me as I loved the challenge. With refrigeration this is a no no. Anyway, I'm rambling here, thanks for the reply Prof.

Nobby

Gary
18-09-2002, 04:02 AM
Personally, I don't think it's a good idea to become dependent upon factory weigh-in charging. It's fine as a time saving option, but should not become a crutch.

A service tech should be able to tell if a system is charged properly without physically removing and weighing the refrigerant. By the same token, and using the same techniques, he should be able to charge the system properly even if he lacks the weigh-in information.

zolar1
20-01-2003, 08:27 PM
Why not simply look at the frost pattern on the evaporator and adjust the charge accordingly?

Prof Sporlan
21-01-2003, 01:51 AM
Eyeballing a frost pattern? Surely using superheat and subcool measurements will give better results, when weighing in a charge is not feasible. Though the Prof is not against anyone using their visual skills as a final check of proper system operation... :)

zolar1
21-01-2003, 04:51 AM
LOL...I didn't mean frost pattern as THE method, but as a general indicator to gauge when to check with other measures.

LOLOL

superheat
21-01-2003, 04:23 PM
Frost pattern can tell alot about a restriction in one circuit.

zolar1
31-03-2003, 07:00 AM
An old timer taught me to go by suction side pressure versus freezer compartment temperature.

Whirlpool products are the odd ball one's. The others are almost exactly the same. (<5psig @ 0 deg F or below freezer compartment temperature with no frost back to the compressor. Ambient has to be 60 deg F or above)

R12, R414B, R406A, R134A behave almost the same at the above parameters with R134A being about 3 to 5 deg F warmer.

You could use the frost-back method - initially charge the system (best guess) and if the freezer compartment was at or below 0 deg F and you had frost back to the compressor, remove a very small amount of charge until the frost on the suction line just disappears. This USED to be the prefered charging method before all the regulations.

There are several other 'tricks' of the trade. The above procedure works most of the time.

superheat
03-04-2003, 07:09 PM
When I cut corners on tight wade PMAs, I touch the compressor for charge check. If the bottom of the compressor is cool, it is overcharged. Simalar to the old frostback method, but more accurate.

Brian
30-06-2003, 12:06 AM
prof why did you go on about volume when some refrigerants pick up Energy/temp diff differently?

Brian
30-06-2003, 12:11 AM
no i know!!!! sorry!