I have read that a good rule of tumb is the max high side pressure should be 250 PSI with an ambient temperature of 85F; while charging R134a. I was wondering if there is a good rule of thumb for the max high side pressures for higher ambient temps such as 90F and 100F ? Thanks.
If you optimize charge by vent temp at the mean temp of 85F and use the high pressure max rule of thumb of 250PSI. If pressures are taken on a hotter day of around 100F ambient what should be expected as the max high pressure that should be seen ?
Edited: Thu June 12, 2008 at 4:17 PM by y2k600f4
A system should never get above 325 PSI in my opinion. It's best to charge by the OEM specifications.
I wouldn't use that 250psi rule of thumb. It's pretty crude. The head (high) pressure determines on several different factors including how well the air is flowing over the condenser (how dirty the fins are), how much air is flowing over the condenser and how humid it is outside. It also depends on how efficient the condenser is (ie it's design). Humidity can also have a significant effect on evaporator heat load.
If you're charging a system with an expansion valve (TXV) then best to measure the amount of subcooling of the refrigerant liquid in the condenser and receiver-drier. Measure the head pressure and then from the gauges (or look up table) get the temperature of the refrigerant at that pressure. Then measure, using a thermometer, the tube temperature out of the condenser and/or out of the drier. The liquid refrigerant down stream from the condenser should be cooled 5'F (5'F to 15'F) below that which you read off the gauge. This tells you that enough liquid has pooled in the bottom of the condenser that it has cooled a little below the dew point temperature of the refrigerant. That way, if there's a pressure drop in the line from condenser to expansion valve, the refrigerant won't flash back to a vapor. Refrigerant vapor in the line leading to the expansion valve will choke the flow of refrigerant to the evaporator and will reduce cooling capacity.
If you're charging a system with an orfice tube (OT) then best to measure the amount of superheat of the refrigerant vapor coming out of the evaporator. Measure the suction pressure and then from the gauges (or look up table) get the temperature of the refrigerant at that pressure. Then measure, using a thermometer, the tube temperature out of the evaporator. The vaporized refrigerant (the tube temperature) coming out of the evaporator should be hotter than 5'F (5'F to 15'F is a good range) above that which you read off the gauge. This tells you that the evaporator is sufficiently full with refrigerant to give efficient cooling performance, without being flooded or starved. It also ensures that no liquid is coming out into the suction line that can slug the compressor. It also helps ensure that refrigerant vapor flow out of the evaporator is sufficient (volume flow) to help carry any oil that is no longer miscible in the refrigerant.
In doing these tests you want to make sure the system has been running for 15 min or longer and is fully settled and fully performing.
Edited: Thu June 12, 2008 at 7:49 PM by bearing01
Here is the thing with the 2x whatever. It's a guess and should only be used a guide if the system is working perfectly. Let's say you have a R12 system the requires 32 ounces of refrigerant. You follow the evacuate and charge procedure while using the 2x theory. You may end up only adding 20 ounces of R134a refrigerant. This is not enough to fill the voids and carry enough lubricant back to the compressor. Compressor fails and one may think it was defective as you follow these procedures. Bottom line is you need to charge a system to as close as the R12 specifications. Some conversions can handle this some can't. The ones that can't need an upgraded PFHE style condenser or need to be kept using R12.
This is why no one can answer the question that comes up all the time. What should my pressures be and what is the charge level after one has converted.
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