Refrigerant charging-step by step procedure

“Refrigerant charging”

1. Why do we need to evacuate an air conditioning system or a refrigeration system?

All the refrigeration system are designed to run without moisture and non-condensable gases else it may not work as designed and is likely to fail prematurely.

Presence of moisture in the air conditioning system may lead to the formation of acidic sludge in the oil and may cause restrictions on various places like expansion valve and evaporator coils thereby reducing the overall cooling effect.

In case of a hermetic compressor, it may cause deterioration of the insulation winding which may lead to short circuit and compressor motor burn out.

Non-condensable gases are just air which can enter during refrigerant charging, repair or maintenance. These gases take up space in the condenser and limit the amount of liquid refrigerant leading to reduced cooling efficiency.

Evacuation is carried out by using a vacuum pump and a recovery bottle, ones the system evacuates we need to wait for about 20-25 min to check the pressure gauge is holding the negative pressure (vacuum) as shown in the image.

This process may ensure that there are no leakages in the system as chances are air or moisture could enter via pipe cracks or leaky gasket or joints while performing evacuation procedure.

In the above image, we can see there are two devices fitted one is an analog pressure gauge showing the final negative reading and another one is a digital vacuum gauge showing 500 microns as final reading, connected to the same line to ensure the system is 100% evacuated.

2. Liquid refrigerant charging

Liquid refrigerants are normally added in the liquid line following proper conditions. If any heating method is used to inject vapour refrigerant into the suction line then 100% vapour must enter the compressor.

Liquid refrigerants are incompressible and can cause severe damage to any compressor.

3. Refrigerant Gas charging

Always charge the vapor refrigerant from the point where the system pressure is lower than the pressure in the charging cylinder.

If we try to charge from the discharge side of the compressor then instead of refrigerant going from the charging cylinder to the system it may start reversing, filling the charging cylinder itself.

Remember that high pressure flows towards the lower side.

Only during refrigeration system evacuation or when the system is out of refrigerant, the refrigerant can be inserted to both the high and low-pressure side of the HVAC unit.

Mostly large charging cylinder has two valves. The red one is connected to the dip tube and is for the liquid charging. Blue one is connected from the top with no dip tube and is for gas charging. In both, the conditions cylinder is kept in an upright position while charging.

If the refrigerant charging cylinder has only one valve, then we don’t have a dip tube and in this case, It’s necessary to invert the cylinder for liquid charging.

4. What is Copper Plating process in a refrigerating system?

When moisture is present in the system, it combines with the refrigerant to form an acidic solution.

This acidic solution dissolves copper tubings and extracts copper from copper-based alloys like brass or bronze present in different parts of an air conditioning system.

This copper gets deposited into the compressor bearings and suction/discharge valves as a copper plating which may lead to refrigerant system leaks, improper evacuation or vacuuming of the system, malfunction of filter/drier, contamination of refrigerant and oil.

Now again continuing with our vapor charging process, the charging cylinder must be weighed before charging by using a weighing scale.

Refrigerant Charging Methods:

Case 1: Refrigerant gas charging in ac | air conditioner

Considering that we have are very low in refrigerant and need to replenish whole of the system with a fresh charge.

  1. Connect hose 2 and 3 connection to C and D respectively.
  2. Both the gauges must be zeroed.
  3. Attach the temperature probe at the correct location near the sensing bulb to sense the degree of superheat.
  4. Evacuate the system into a recovery bottle by connecting the vacuum pump to the charging hose 1, open both the valves A and valve B with HVAC system(air conditioner or split ac) in stop condition.
  5. Stop the vacuum pump after the system has evacuated at the desired negative pressure, close the valve A and B, check the pressure drop for around 20 min to know if there has been any leakage in the system.
  6. Disconnect the charging hose 1 from the vacuum pump and connect it to the blue connection of the charging cylinder for a gas charge.
  7. Open the valve A, slightly open the connection at C and purge the whole line by from 1 to 2 by crack opening the gas valve of the refrigerant cylinder, try not to avoid any excess leak directly into the environment.
  8. Same way purge the high-pressure liquid side from 1 to 3 to remove any air inside the hose.
  9. Now tight both the low and high-pressure side hose at position C & D respectively as shown in the image.
  10. Start charging the gas refrigerant to the low-pressure side of the HVAC system at connection C by opening the valve A and the refrigerant valve until the vacuum comes to zero.
  11. Now start the compressor as the suction pressure goes above zero.
  12. After about 30 – 40 seconds of charging close the valve A and check the suction gauge pressure for a rise in pressure.
  13. Keep monitoring the degree of superheat by the temperature calibrator to ensure liquid is not entering into compressor suction also we can calculate how much charge present in the system by the pressure chart.
  14. If the superheat temperature is high, then it means the system is running low in refrigerant whereas lower superheat temperature than the desired value means the system has overcharged.
  15. Repeat the procedure 10 to 11 until the suction pressure reaches to 60 psi (as per maker) this avoids the risk of overcharging of the system also keep measuring the refrigerant by the weighing scale.
  16. Once the refrigerant has charged, disconnect all the hoses and secure everything.

CASE 2: Air Conditioning system: Low refrigerant charge

Low refrigerant charge is harmful to the environment, if we have not checked or repaired the leak then it may release the refrigerant again into the atmosphere.

So it’s better to find out the cause as many refrigerants are falling into the category of ozone layer depletion and global warming potential.

HFC refrigerant contains chlorine which reacts with ozone (O3) to convert it into oxygen (O2) hence leads to depletion of ozone layer.

Major HFC refrigerants:

  • R23
  • R32
  • R134a
  • R404A
  • R407A
  • R407C

The ozone layer protects the life on earth by absorbing U.V rays from the sun, U.V rays are responsible for skin cancer, genetic damage and immune suppression in humans.

HFC refrigerants have a high global warming potential which can lead to increased sea level, rising temperature and damage to habitats.

Now, coming back to charging up an HVAC system running short of refrigerant.

  1. Connect all the hose 1, 2 and 3 with the HVAC kit.
  2. Purge air from the hose from 2 to 1 by tightening it at point C, valve A open, valve B closed and connection to the charging cylinder slightly open for air purging, later close after purging.
  3. Likewise, air purge the high-pressure side hose 3.
  4. Keeping the HVAC system running we will open the charging cylinder gas valve, valve B closed and open the valve A, allowing the refrigerant charge into the system
  5. Start charging the gas refrigerant to the low-pressure side of the HVAC system at connection C by opening the valve A and the refrigerant valve until the vacuum comes to zero.
  6. Now start the compressor as the suction pressure goes above zero.
  7. After about 30 – 40 seconds of charging close the valve A and check the suction gauge pressure for a rise in pressure.
  8. Keep monitoring the degree of superheat by the temperature calibrator to ensure liquid refrigerant is not entering into compressor suction also we can calculate how much charge present in the system by the pressure chart.
  9. If the superheat temperature is high, then it means the system is running low in refrigerant whereas lower superheat temperature than the desired value means the system has overcharged.
  10. Repeat the procedure 10 to 11 until the suction pressure reaches to 60 psi (as per maker) this avoids the risk of overcharging of the system also keep measuring the refrigerant by the weighing scale.
  11. Once the refrigerant has charged, disconnect all the hoses and secure everything.

5. Liquid refrigerant charging procedure for an air conditioner, window a.c (HVAC system)

    1. Evacuate the system by pumping down the refrigerant into a recovery bottle using a vacuum pump.
    2. Connect the liquid and gas line hose and purge it.
    3. Connect the charging hose to the red connection of the charging cylinder for a liquid charge, and if only one connection provided, then we need to invert the cylinder.
    4. Attach the temperature probe at the correct location near the sensing bulb to sense the degree of superheat and subcooling at the condenser outlet.
    5. Allow the liquid refrigerant to flow by opening the valve A and keeping the valve B closed.
    6. Since the system has a vacuum, the liquid refrigerant itself enters the system and move towards the evaporator and condenser until the pressure comes down to the charging cylinder pressure.
    7. Start the system, the liquid refrigerant gets equally divided between the evaporator and the condenser, and no danger of liquid flooding exists in the compressor.
    8. As the refrigerant leaves the charging cylinder, the charging cylinder pressure decreases, so to maintain the flow create a negative pressure by partially closing the receiver outlet throttle valve and allow the refrigerant to flow from the cylinder.
    9. In a refrigeration system, receiver outlet is provided with a “king valve” used for liquid charging also known as a throttle valve and can trap the entire system refrigerant inside receiver.
    10. This way liquid refrigerant gets admitted safely, but since the throttle valve reduces the pressure across the expansion valve, the system doesn’t run normally.
    11. In between close the refrigerant charging valve A and open the throttle valve fully and check the temperature and pressure readings.
    12. When fully charged the sight glass get’s clear also weigh the charging cylinder to know how much charge required as per maker for a full recharge.

6. Liquid charging on large commercial plants:

Liquid charging is on a liquid line, and it must get vaporized before it reaches to the compressor suction.

There is always a risk of liquid refrigerant entering compressor suction which can cause severe damage to the compressor as liquid is non-compressible.

To avoid liquid going into the compressor we usually charge before the thermostatic expansion valve (TXV) or metering device which ensure the degree of superheat gets achieved (i.e., liquid converted to 100% vapour) before reaching to compressor suction.

For a large commercial refrigeration HVAC plant or industrial plant are provided with a large receiver tank attached to the refrigeration plant if provided to collect all the liquid refrigerant.

The receiver tank has a gauge glass where we can see the amount of refrigerant during operation and stop condition. Gauge glass helps to give an idea not to overcharge the refrigerant while filling.

Procedure for a smaller quantity refrigerant charge in large refrigerating plants are performed by pump down method.

receiver sight glass

Pump down means collecting refrigerant temporarily inside receiver

  1. Start the cooling medium to the condenser (fan or water).
  2. Close the receiver liquid outlet valve.
  3. Start the compressor to suck the lower pressure side of the refrigerant into the condenser.
  4. The compressor stops automatically as the low pressure cut out switch gets activated (the set point at around 0.02Mpa).
  5. Connect the liquid line charging hose at the liquid side charging point usually at the filter/drier and purge it.
  6. Attach the temperature probe to measure the degree of superheat.
  7. Open the liquid charging valve of the charging cylinder and start the compressor.
  8. Keep measuring the degree of superheat, if the degree of superheat temperature is going down; start throttling the liquid refrigerant from the charging bottle.
  9. Keep an eye on the receiver gauge glass for an increase in level.
  10. Check the suction gauge and ones the desired pressure has reached as per maker, stop the compressor.
  11. Remove the charging connection, open the condenser outlet valve and run the system normally.
  12. It’s very important to weigh the charging bottle before charging and log down the amount of refrigerant charged.

For small HVAC systems like split air conditioner, window a.c we do not have a refrigerant gauge glass for a small sized receiver to check the refrigerant level.

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