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Charging Residential Air Conditioning - by R.D. Holder

Different types of metering devices have different ways of charging. A Thermostatic Expansion Valve (TXV) is charged to the sub-cooling of the liquid line leaving the condenser. A fixed orifice is charged to the superheat of the suction line leaving the evaporator. To understand why this is, it requires an understanding of the physical properties of the refrigeration cycle. The four main components of the refrigeration cycle include:

Compressor
Condenser
Metering Devices
Evaporator
These four components are divided into sections and explained in depth as follows.

Compressor

The Compressor compresses a low-pressure superheated gas into a high-pressure superheated gas. If the suction gas is not superheated, the compressor can be damaged. The compressor pulls the refrigerant out of the evaporator and pushes it though a condenser. The act of compression is performed by any one of the following six types of compressors: a reciprocating piston, rotary, scroll, screw, centrifugal, and sonic compressors. Of the six, the reciprocating and scroll compressors are the two most frequently found in a residential air conditioning system.

The mass flow rate produced by a compressor is equal to the mass of the suction gas pulled in by the compressor. The compressor’s out put is equally only to its intake because the mass flow must be equal. The process of compression, through mass flow, raises the temperature and pressure of the refrigerant. The result of the temperature increase is superheat. Pressure and temperature of the refrigerant must be higher than the condensing temperature. The refrigerant temperature must be higher so heat can flow into the condensing medium. This process explains the necessary relationship between the increased pressure and the rise in temperature. If the pressure and temperature is not increased through compression, there is no heat transferred from the refrigerant to the condensing medium. The compressor has a maximum inlet temperature of about 70 degrees F and outlet of about 225 degrees F. Inlet refrigerant vapor cools the compressor motor.

De-superheating (heat leaving the refrigerant gas) of refrigerant begins as it is discharged. From a compressor and pushed into a condenser.

Condenser

The condenser removes heat and changes a high-pressure vapor into a high-pressure liquid. As the superheated (high-pressure) gas is pushed into the condenser, it is desuperheated, that is the temperature is reduced to saturated pressure-temperature.

The refrigerant does not start to change state until the temperature reaches saturated pressure-temperature. The only variable that can change the temperature is a pressure change. (See table 1) At saturation pressure-temperature point, the change of state becomes latent heat (invisible or hidden heat). Latent heat is a lack of rise or fall of temperature during a change of state (saturation). When the temperature does not rise or fall it is at saturation and the change of state process begins. Refrigerant continues to change state at one pressure-temperature. At the change of state the refrigerant liquid and vapor are at the same temperature. This is defined as equilibrium contact. The temperatures of the liquid and vapors will stay the same until the temperature of the refrigerant starts to drop. Temperature of the refrigerant start to drop once 98% to 99% of the refrigerant becomes a liquid. This is called sub-cooling. Sub-cooling is a temperature below saturated pressure-temperature. (See table 1) Sub-cooling is a measurement of how much liquid is in the condenser. In air conditioning, it is important to measure sub-cooling because the longer the liquid stays in the condenser, the greater the sensible (visible) heat loss. Low sub-cooling means that a condenser is empty. High sub-cooling means that a condenser is full. Over filling a system increases, pressure due to the liquid filling of a condenser that shows up as high sub-cooling. To move the refrigerant from condenser to the liquid line, it must be pushed down the liquid line to a metering device. If a pressure drop occurs in the liquid line and the refrigerant has no sub-cooling, the refrigerant will start to re-vaporize (change state from a liquid to a vapor) before reaching the metering devise.

Refrigerant –22 (Table 1)

Liquid line

Saturated temperate - Temperature = sub-cooling

200 psig = 101 degrees - 96 degrees = 5 degrees

210 psig = 105 degrees - 90 degrees = 15 degrees

240 psig = 114 degrees - 98 degrees = 16 degrees

Metering Devices

A metering device is a pressure drop point, which has two jobs:

(1) Holds refrigerant back in a condenser and

(2) Feeds refrigerant into the evaporator.

When high-pressure liquid enters a metering device, pressure starts to drop, as the temperature remains the same until it reaches saturation pressure-temperature. At this time both the pressure and temperature continues to drop to evaporator pressure-temperature. (See table 2) Low-pressure liquid that is leaving the metering device is boiling at saturated pressure-temperature. The process of a refrigerant changing its state (from a liquid to a vapor) in the metering device is called flash gas. Flash gas is what cools the refrigerant liquid in the metering device. A system with no sub-cooling has more gas that is flashed and less capacity.

Evaporators

The refrigerant enters the evaporator as a boiling low-pressure liquid at saturated pressure-temperature. It continues to boil at one temperature as long as the pressure remains the same. If there is not a pressure change in the evaporator, there will not be a temperature change in the refrigerant changing state. At saturation, refrigerant absorbs latent heat, which is a change of state heat. The refrigerant changes state at one temperature (for any one pressure) from the beginning of the evaporator until the entire liquid refrigerant has become a vapor. The only variable that can change a temperature is a pressure change. If a temperature change occurs a pressure change occurs. In latent heat, the liquid and vapor are at the same temperature due to equilibrium contact. When heat is added to the gas, past saturation pressure-temperature, it is called superheat. (See Table 2) Superheat is an indication of how full the evaporator is of liquid refrigerant. High superheat means the evaporator is empty. Low superheat means the evaporator is full. There have been reports that liquid refrigerant can still be boiling with 2 degrees of superheat. Superheat should never be observed below 4 degrees or a compressor failure may occur. The superheat gas is pulled into the compressor were it starts the cycle again.

Refrigerant –22 (Table 2)

Suction Line

Saturated temperate - Temperature =

58 psig = 32 degrees - 44 degrees = 12 degrees

64 psig = 37 degrees - 47 degrees = 10 degrees

70 psig = 41 degrees - 50 degrees = 9 degrees

Charging Methods

Before charging of a residential air conditioning system, two temperatures must be recorded:

(1) Condensing air inlet dry bulb temperature.

(2) Evaporator air inlet wet bulb temperature.

Wet bulb temperature is a measurement of the heat contained within air. Air may have many different wet bulb temperatures for one dry bulb temperature, depending on relative humidity of the air.

Different types of metering devices have different ways of charging.

Thermostatic Expansion Valve R-22

A/C with a Thermostatic Expansion Valve (TXV) is charged to the sub-cooling of the liquid line leaving the condenser because the superheat is fixed. The superheat is fixed at 8 to 12 degrees in most residential air conditioning systems. Sub-cooling is the amount of liquid held back in the condenser. This allows the liquid to give up more heat, below saturated pressure- temperature. For every one degree of sub-cooling at the same condensing pressure, capacity will increase 0.5 percent. Increasing sub-cooling with an increase of discharge pressure and compression ratio, decrease capacity. Add 5 degrees of sub-cooling for every 30 feet of liquid line lift.

To Measure Sub-cooling:

Obtain refrigerant saturation pressure-temperature. Take a pressure reading of the liquid line leaving the condenser. Refrigerant saturation temperature is the pressure-temperature when the refrigerant is turning from a high-pressure vapor into a high-pressure liquid (giving up heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature.

(1) Convert pressure to temperature with a pressure temperature chart.

(2) Take a temperature reading at the leaving liquid line of the condenser.

Compare both, the saturated temperature and leaving liquid line temperature. Subtracting one from the other, the difference is the amount the refrigerant has cooled past saturated temperature. This is sub-cooling. (See table 1)

This four-step procedure is known as sub-cooling. Manufacturers should be able to identify the amounts of sub-cooling they have designed into a system. A low charge will give a low sub-cooling. An overcharge will give a high sub-cooling along with a high compression ratio. Do not worry about a few bubbles in the sight glass. Sight glasses will not always be clear with a full charge. The zeotropes refrigerant group is known for their fractionation. It is possible to never have a clear sight glass. To determine what the sub-cooling should be in a system see table 3.

Sub-cooling for A/C with TXV R-22 (Table 3)
Evaporator Inlet Air Temperature Fahrenheit Wet Bulb
                 57  59  61  63  65  67  69  71  73
Outside Air	
Temperature DB
75               31  30  29  27  25  23  21  19  17
80               30  29  26  24  23  21  19  17  15   
85               28  27  24  22  21  19  18  16  14 
90               27  25  22  20  19  17  16  14  12 
95               25  23  20  19  17  15  13  11   9 
100              23  20  18  16  14  12  10   8   6 
105              20  18  16  14  12  10   8   6   4  
110              18  15  13  11   9   7   5   3   1    
115		 15  13  11   9   7   5   3   1   0
Fixed Orifice R-22

A/C with a fixed orifice is charged to the superheat of the suction line leaving the evaporator. Superheat is the gas temperature above the saturated temperature.

Superheat can be split into two types of heat:

(1) Superheat of the evaporators and

(2) Total superheat entering the compressor.

The evaporators superheat must be figured at the evaporator outlet not at the compressor inlet. Total superheat is figured at the compressor inlet.

To Measure Evaporator Superheat:

Take a pressure reading of the suction line-leaving evaporator to get refrigerant saturation pressure-temperature. Refrigerant saturation temperature is the pressure-temperature when the refrigerant is turning from a low-pressure liquid to a low-pressure vapor (absorbing heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature.

Convert pressure to temperature with a pressure temperature chart. If reading is obtained at the compressor, not at the evaporator leaving line, you may have to add a few pounds of pressure due to pressure drop in the suction line.

Take a temperature reading at the leaving suction line of the evaporator.

Compare both, the saturated temperature and the leaving suction line temperature. Subtracting one from the other, the difference is the amount the refrigerant gas has heated past saturated temperature. This is superheat. (See table 2)

This four-step procedure is known as superheat. Manufacturers should be able to identify the amounts of superheat they have designed into a system. A low charge will give a high superheat. An overcharge will give a low superheat along with a higher compression ratio. To determine what superheat in a system should be see table 4.

Superheat for A/C with fixed Orifice R-22 (Table 4)

Evaporator Inlet Air Temperature Fahrenheit Wet Bulb
Outside Air
Temperature DB
60             13  17  18  20  24  26  28  30  33  36  39
65             11  13  15  17  18  22  25  28  30  33  36 
70              8  11  12  14  16  18  22  25  28  30  33
75              5   7  10  12  14  16  18  23  26  28  30
80                  4   6   8  12  14  16  18  23  27  28   
85                      4   6   8  12  14  17  20  25  27 
90                          4   6   9  12  15  18  22  25         
95                              4   7  11  13  16  20  23 
100                                 5   8  11  14  18  20 
105                                 4   6   8  12  15  19  
110                                     5   7  11  14  18    
115		                            5   8  13  16
Refrigeration

The use of sight glass for charging is common in refrigeration. It is better to charge a system first by measuring the operating condition (discharge and suction pressures, suction line temperature, compressor amps, super heat, sub-cooling and coils temperature differential) before using the liquid line sight glass. If the sight glass is close to the exit of the condenser or if there is very little sub-cooling at the sight glass, bubbles may be present even when the system is properly charged. If a system is charged to full sight glass, overcharging may be the result, decreasing efficiency.

Note: Follow the manufacturer recommendation for superheat and sub-cooling.

A Thermostatic Expansion Valve (TXV) is designed to maintain a constant superheat. Over charging a TXV will rise sub-cooling, increases system pressures, and decreases system efficiency. Under charging a TXV will decrease sub-cooling, increases superheat, decrease system capacity, and lower refrigerant velocity leaving oil in the evaporator. An Automatic Expansion Valve (AXV) is a constant evaporator pressure valve and not normally used in A/C. A fixed orifices is the simplest metering devise made and the most critical to charge. Over charging fixed orifices will lower superheat, increase pressures, decrease efficiency, and flood the compressor with liquid refrigerant. Under charging the fixed orifices will raise superheat, lower pressure, lower capacity and lower refrigerant velocity leaving oil in the evaporator. Always refer to the manufacturer recommendations on charging fixed orifices.

The process of charging to superheat and sub-cooling improves an air conditioning systems' efficiency, capacity and lessens equipment failures. Always let system stabilize 10 to 20 minutes after adjusting the charge, this takes time but improves the accuracy.

Remember when charging refrigerants all superheat and sub-cooling adjustments must be checked and recorded. The procedure of recording adjustments is called Baselining. This procedure not only saves time, money and aggravation but it is a sign of a professional.

Roger D. Holder, CM, BSME, is a member of the Southern San Joaquin Chapter in Bakersfield, Calif. He also is a refrigeration and air conditioning specialist at National Technical Transfer Inc., Englewood, CO 80155