Charging Refrigeration Systems | HVAC Know It All

Refrigerant Charging

Adding Refrigerant to a Refrigeration and Air Conditioning System is known as Charging. There are many ways to do this depending on the system type, status of the system, and refrigerant type. This article will detail best practices for Refrigerant Charging, and supply information on equipment, refrigerants, and processes relating to Charging. This is the third of three articles in our Commissioning Series on Pressure Testing, Evacuation, and Charging.

Essential Charging Equipment

Refrigerant Scales

No matter which charging method is used and what system type is worked on, a Refrigerant Scale will be used for charging. Scales may be the tool that determines the Charge by weight, or if you are charging to another metric such as Superheat, the Scale will still record your charge. For the latter purpose, a Scale will record the refrigerant quantity installed in the system for future reference. We will look at different scales below divided by weight capacity.

Small Critical Charge Systems

The Yellow Jacket Hydrocarbon Charging Kit can be used to accurately charge small quantities of refrigerants such as Propane (R290) into Small Critically Charged Systems. A system of this type may require very accurate quantities of refrigerant to operate properly, so a kit like this is most helpful.

Medium Capacity Scales (30-330 Pounds)

To begin with, in this weight category, a more traditional scale is the CPS CC220. I have used this scale personally and appreciate the robust case and scale, the clear digital display with its hook/magnet for mounting, and the option to have the unit maintain power for longer than 30 minutes. Note: some refrigerant scales will auto-power off after 30 minutes if you do not press a button. This can be tedious if you’re charging for longer than this period, as you can lose your weight measurement if the scale turns off.

A newer style and more automatic option is the testo 560i Kit. This type of scale has been gaining popularity in recent years, as it allows target metrics to be set which automatically ends charging when they’re achieved. This allows you to focus on other tasks while the charge is being weighed in. The scale can be controlled by testo’s phone application and/or their Digital Manifold which I will detail below.

Large Capacity Floor Scales

In factories that employ very large refrigerant bottles, Large Capacity Floor Scales can be used. A scale of this type can additionally be used to weigh other items (possibly for shipping weights) required around the shop. The practicality of this type of scale may fall short as they are quite large and not the most portable if required.

Crane Scales

Crane Scales are a great method of weighing heavy refrigerant bottles in a shop, or in the field. Their high capacity and portability make them great, so long as the bottle has a Rigging Point to hook onto.

Refrigerant Manifolds

The Yellow Jacket Titan is a classic Manifold, which employs a newer 4-handle arrangement including a sight glass. I have a lot of experience using this manifold and have found it very comfortable and free of issues.

A newer style of Manifold is the testo 550s. This has some great features such as on-board Pressure Temperature Charts for 90+ Refrigerants, use with the above-mentioned testo Scale, as well as use with testo “Smart Probes” (below), and their smartphone application.

Temperature and Pressure Measurement

Temperature Probes

Temperature Probes are important tools used to find measured temperature, and/or to assist in calculating Superheat and Subcooling. A popular kit I have used for temperature measurement is the Fluke HVAC/R Kit. It utilizes Type K Thermocouples which can be attached to the meter for accurate temperature measurement. It is also a very good Electrical Multimeter, and I reference this kit as a more “old school” method of taking temperature readings.

Pressure Gauges

I have had great success using the Elitech PGW-800 with high-pressure refrigerants. It will also display negative pressure within reasonable accuracy, has good battery life, has a good case with accessory fittings, and has a backlit display that is easy to read.

Temperature and Pressure “Smart Probes”

A more modern way to take, share, and store both temperature and pressure is testo’s Smart Probes. Again, compatible with testo’s Manifold and smartphone application, these probes integrate nicely with their product line. The advent of using probes for pressure measurement also has a huge benefit of reducing refrigerant loss where you’d traditionally hook up Manifold Gauges with hoses to the system.

Recovery Equipment for Charging

In some cases, you can get your full charge in without a Recovery Machine by leveraging pressure differential: suck liquid from a bottle into a system that is in a vacuum, then (if required) pull the remainder into compressor suction as vapor while the compressor is running.

However, in many cases, a Recovery Machine is required for charging. Recovery Machines (also used for Refrigerant Recovery) are a large topic that I will briefly cover here. Some machines can transfer vapor only, liquid only, or both. The CPS TRS600 (image above) can move vapor (using its Compressor) or liquid (bypassing its Compressor). Its physical size/appearance and function match similar machines for Domestic/Commercial Applications in the HVAC/R Industry. Machines of this type can be 120-volt power supply or higher and are commonly battery-powered as well.

I have had RefTec quote me before for equipment for Large Commercial/Industrial Applications. From this website link, the first chart shows different transfer rates for their machines, and whether they can handle liquid, vapor, or both. Below this under “Products” you can see different equipment for large transfers of vapor state or liquid state refrigerant. Machines of this type will be 120-volt power supply or higher.

Refrigerant Bottles

Ranging from 30-pound (or smaller) Bottles to Tanker Trucks that deliver refrigerant for large systems, there is quite a range of different options in size when purchasing refrigerant.

Very commonly, bottles from 30-125 pounds are used. They may employ a single handle, or two handles: one for vapor, and one for liquid with a dip-tube. Bottles can have a threaded bottle cap to prevent Valve Shearing, or a protective ring permanently welded to the bottle’s top around the valve handle(s).

Sometimes purchasing more refrigerant/a larger bottle can save on price per pound, but deciding which size bottle to purchase primarily comes down to convenience in its use.

Bottle Heaters

To increase Differential Pressure between the Refrigerant Charging Bottle and the System, Bottle Heaters are used. They are strapped to the refrigerant bottle and plugged into 120-volt power to turn on and warm the bottle.

Methods of Charging Refrigerant

Refrigerant can be charged into an operating system in the vapor state through the Compressor’s Suction. When using Refrigerant Blends with a considerable Glide, transferring liquid into the system requires slowly Metering/Flashing liquid into the Compressor’s Suction so that Evaporation occurs as refrigerant enters the system. For more information on refrigerant blends and glide, see our article on azeotropic vs zeotropic refrigerants.

In a system that is empty/in a vacuum, refrigerant can be charged mainly in the liquid state wherever there is access. Usually, an access point is selected which has a large volume component adjacent to it, such as a Receiver or Condenser. This allows a space for the refrigerant to easily fill up for minimum resistance to the lessening Differential Pressure from bottle to system as charging continues.

Charging by Weight (Scales)

As mentioned above, Scales can be used when weight is the charging metric you are charging to. In this case, you would have a weight listed on the equipment’s manufacturer nameplate and weigh this total refrigerant charge into the system. If you do not have a weight listed on a nameplate, you may calculate the system’s refrigerant charge based on components and line sizes/lengths. Note: sometimes this calculated charge is only an estimate, and refrigerant may need to be added or removed after operational checks.

If not charging by weight, scales will still record what is put into the system for future reference.

Charging Charts

In the above image, a Charging Chart is shown. These are sometimes used in Domestic applications to add an appropriate refrigerant charge to an Air Conditioner in varying outdoor/indoor conditions due to Seasonal Conditions. The chart references Outdoor Air Dry Bulb Temperature (OA DB) as it applies to your Condenser operation and Indoor Air Wet Bulb Temperature (IA WB) for Evaporator operation.

Note: Dry Bulb Temperature is a “normal” temperature reading with no consideration for moisture, while Wet Bulb Temperature considers the moisture content of the air.

A Psychrometer (digital or analog) is first used to take indoor and outdoor air conditions. For example (referencing the above chart), if you read an OA DB of 100°F and an IA WB of 68°F, you would charge until reaching a Superheat of 12°F at your Evaporator Outlet.

Charging by Subcooling

When a Thermostatic Expansion Valve (TXV) is used as the system’s Metering Device, the system will be charged based on Subcooling at the Metering Device Inlet. This will ensure a full column of liquid is supplied to the TXV so that it operates properly. For more information about TXVs and metering devices, see our article on adaptive vs fixed expansion valves. The subcooling value required can be gleaned from the system’s IOM (Installation, Operation, and Maintenance Manual).

Charging by Superheat

With a Fixed-Orifice or Capillary Tube Metering Device, Evaporator Superheat is the metric used for charging. This value is obtained by reading the Superheat value at the outlet of the Evaporator. This method ensures the compressor will only pull vapor state refrigerant from the Suction Line. The required Superheat can be based on the system’s Saturated Suction Temperature (SST), or again the IOM can provide a required Superheat value.

Step-by-Step Charging Procedure

In this section, I will cover an example of charging a system. If you are at this stage of commissioning, you would have completed Evacuation including a Decay Test.

This scenario is a simplified version of charging a Compressor Test Stand with Refrigerant R1234ze(E). The unique point of this example focuses on charging a system that has/has had Water (H2O) in its Water-Cooled Condenser. This necessitates practices that will avoid causing ice to form in the water side of the condenser, which would cause freezing and bursting of the Heat Exchanger. This is like charging a Flooded Chiller – even when new, you should assume it has come from the factory with some water remaining in the Chiller Barrels from testing.

The diagram below shows the system in a P&ID (Piping and Instrumentation Diagram) style drawing with charging equipment represented. A required charge of 80 Pounds (lbs) of R1234ze(E) has been calculated. We will use the earlier mentioned CPS TRS600 Recovery Machine, which is compatible with the A2L Refrigerant. “1234” is being used and tested in Chillers and Refrigeration, and is also the refrigerant in my 2022 truck (R1234yf).

Besides the Recovery Machine, we will utilize a Refrigerant Scale, Bottle Heater, two hoses, and a Digital Pressure Gauge. The method of charging we will use is Direct Liquid Charging, but we must begin with Direct Vapor Charging. All equipment is located inside at 70°F.

The system employs a Brazed Plate Heat Exchanger (BPHX) for the Water-Cooled Condenser. This system’s water side has been pressure tested with water, so we must avoid freezing the heat exchanger while charging our refrigerant.

Pre-Charging Checklist

1. The system is in a vacuum of 200 Microns. This is read on the Digital Pressure Gauge on the Condenser Inlet: marked “PSIG” (Pounds per Square Inch Gauge) in the diagram. This gauge is also capable of handling positive refrigerant pressure. We now toggle its increment used from Microns to PSIG. Note: the EXV (Electronic Expansion Valve) should already be driven fully open from evacuation. After filling our condenser, refrigerant will be free to flow into the system’s Low Side.
2. The “System or Hose Valve” is named to indicate that it can be an access valve on the system or an isolation valve attached to the end of the hose. The System or Hose Valve (colored green) and the “Bottle Valve” (blue outlined in red) are both currently closed. The red/blue hoses (colored lines in the diagram) and Recovery Machine are connected and are full of air. The refrigerant Bottle Valve has two separate handles: one for vapor off the bottle’s top, and one for liquid with a Dip-Tube to its bottom. The vapor handle is now opened to purge the hoses and recovery machine of air and fill them with Refrigerant up to the “System or Hose Valve”. Additionally, open both the suction and discharge valve on the recovery machine. You may then “Crack” the fitting immediately before the System or Hose Valve, until the refrigerant vapor has pushed all the air out. Note: by avoiding Manifold Gauges we have a simpler arrangement, and less refrigerant will be wasted when charging is complete.
3. Strap the Bottle Heater to the bottle. The Scale can now be “Zeroed”. We can now record this full 125lb bottle of 1234ze(E) being charged into the system until our scale is reading “-80lbs”: as the bottle loses refrigerant to the system, it loses weight and becomes lighter.
4. We will now Flow Water by turning on the Hydronic System’s water pump. Besides carefully charging the refrigerant, the circulation of water through the heat exchanger adds another level of security by further reducing the possibility of water freezing in the heat exchanger.
5. Turn on the bottle heater. This could be done later and is not required yet as we’ll have a good Pressure Differential from the pressurized bottle to the vacuumed system. However, I like to do this at the start of charging out of simplicity.
6. Using a Pressure Temperature Chart (Danfoss Ref Tools) as a reference, we will charge vapor into the system until reaching a Saturated Pressure corresponding to a Saturated Temperature above the freezing point of water (32°F). To account for a small Safety Factor and any gauge inaccuracy, we will aim for a pressure associated with 40°F: 22.2 PSIG, rounded to 22 PSIG. We will charge vapor until the system pressure reaches 22 PSIG, which will minimize the chance of freeze-up. Compared to liquid, refrigerant vapor is far less dense and is unlikely to cause water to freeze through a heat exchanger, especially while water is circulated.

Note: depending on the heat exchanger type you are charging into, some techs will forego vapor charging and start with liquid while flowing water. A BPHX, however, is a good candidate to begin by vapor charging, since its channels are so small and likely to freeze.

Steps to Charge the Refrigerant

1. Open the “System or Hose Valve” to begin charging. Due to pressure differential, a considerable amount of vapor will be pushed through the recovery machine without being turned on (at 70°F “1234” has a Standing Pressure of 49.5 PSIG, flowing into a vacuum). Keep an eye on the scale to monitor the refrigerant being added.
2. Once the refrigerant flow slows down (this is subjective), turn on the recovery machine. If the pump begins to make slugging/hammering sounds, partially close off/throttle the machine’s inlet valve. You can then slowly open the valve more until achieving the maximum open valve position the recovery machine can handle. Charge vapor until the gauge reaches 22 PSIG. The temperature of the refrigerant in the heat exchanger is now 40°F, and the chance of freezing water has been avoided.
3. Open the liquid handle on the Bottle Valve and close the vapor handle. Again, adjust your recovery machine’s inlet valve if you hear slugging/knocking sounds.
4. Continue charging until you get within a few ounces (there are 16 ounces in 1 pound) of “-80lbs” on the scale, then close the Bottle Valve’s liquid handle to try to time your charge’s weight perfectly. If you undershoot, you can open the valve briefly and try again. If you overshoot, a couple of ounces extra on a charge of this size is likely nominal. Once you have closed off the refrigerant supply, the recovery machine will continue running to pump/push out what remains in the recovery machine, and the hoses.
5. Let the recovery machine start to pump the hoses and machine out: the CPS TRS600 will keep running and go into a “Purge” cycle when its refrigerant supply is closed off. Other machines have more complex settings in this regard, but this CPS Machine is simple.

From TRS600 Owners Manual Page 6: “8. Recovery Unit will run continuously. When 0 PSIG level is observed on LOW Side Manifold Gauge, close both LOW & HIGH Side Manifold Valves. CAUTION: For Class A2, A2L and A3 recovery, Recovery Unit must be turned off when 0 PSIG to prevent possible ingestion of air during recovery process.”

6. As R1234ze(E) is an A2L, once 0 PSIG is reached, turn off the recovery machine and quickly close the System or Hose Valve. If you will not adjust the refrigerant charge after system start-up, you are now done charging. You may purge the slight refrigerant pressure in your recovery equipment by slowly loosening the hoses from the machine’s inlet and outlet, and then allowing all pressure to come out. You can now disconnect all recovery equipment and hoses from the system.
7. Perform a Refrigerant Leak test with a Refrigerant Leak Detector. This is additional insurance to confirm there are now no refrigerant leaks: rarely, systems that pass nitrogen/vacuum tests may immediately leak refrigerant. Once operating, the system should again be leak-checked. Note: Thermal Cycling components/piping may cause leaks over time, so additional leak checks should be performed periodically.

Conclusion

Methods for efficiency and accuracy are paramount when performing Refrigerant Charging. As simple as the concept is in premise, there are many considerations regarding equipment and processes utilized while getting the refrigerant into the system.