This is the continuation from the refrigeration topic on previous article where we have touch a bit on recharging the system. Note: Information here may be a little technical but if you are a trained aircon servicing technician, then it should be no problem for you. So let’s get started.
You need to understand what metering device your system uses, there are several different types of devices used to control the flow of refrigerant but for our purposes we need only be concerned with two, 1. Capillary tube or fixed orifice, 2. TXV or thermal expansion valve. The CT or fixed orifice device is basically a small hole that does not change and is not adjustable and the pressure on the low side is controlled by the size of the hole and the pressure pushing on the high pressure side. The TXV is a self adjusting device that controls the flow of refrigerant by taking a temperature reading on the suction line exiting the evaporator and adjusting refrigerant flow to keep the proper level of liquid refrigerant is the evaporator. Before we move on, you might also want to check out our article on refrigerant charging.
After you determine what type of metering device you have then you need to take a pressure reading to start the process of evaluating the present condition of your system, as a general rule if your metering device is fixed you will be using super heat to charge the system and if you have a TXV you will be using sub cooling. Your manufacturer will have a data sheet to tell you the recommended charging procedure for your piece of equipment, this data should be in the installation instructions or service literature that came with your unit if you do not have this data you will have to call the manufacturer they all have 1-800 numbers and they will be glad to send you anything you need and with faxes and email you should be able to get it very quickly.
With an outdoor temperature of 95 degrees, an indoor temp of 82 you put your gauges on the system (low side gauge on the left hose, should be connected to either the port marked low or a tap on the larger cooler line) you get a low side reading of 48psi (the outermost ring of numbers on the gauge) the temperature of the refrigerant at that pressure is on the gauges also r-22 is noted by the green numbers. Your high pressure reading 190psi you have already determined that you have a fixed orifice so you take a reading of the suction line temperature as close to the evaporator exit as possible your suction line temperature is 85 degrees your gauges tell you that at 48psi the temperature of your refrigerant vapor is 24 degrees this would mean that you would have 61 degrees super heat 85-24=61. Your manufacturer info will tell you under these temperature conditions you will need somewhere between 15 and 25 degrees super heat this means you are undercharged.
Same outdoor conditions with a TXV your liquid line temperature 150 degrees your high pressure gauge is reading 155psi and your gauges tell you that at 155psi r-22 temperature is 82 degrees, your manufacturers info tells you that at that outdoor temperature (95 degrees) your high pressure should be about 275psi and you should have at least 12-14 degrees sub-cooling which means your liquid line temperature should be about 111 degrees because the temperature of r-22 at 275psi is 125 degrees so 125-111=14 degrees sub-cooling, this system is undercharged.
Information courtesy from Mike Newberry from the Aircon Repair