When the nameplate charge is not specified — as with larger field-assembled refrigeration systems — there are guidelines and techniques all technicians should follow when charging a system. The following techniques do not apply to automotive or any transportation air conditioning system charging.

CHARGING TXV/RECEIVER/SIGHT GLASS SYSTEMS

Most large commercial refrigeration systems utilize thermostatic expansion valves (TXVs) as metering devices. Because the throttling action of the TXV satisfies changing evaporator loads, a liquid receiver after the condenser is usually employed to act as a liquid refrigerant reservoir and surge tank.

Liquid receivers also act as a storage vessel to store the condenser charge variations between summer and winter operations on some systems where floating head pressures are not applicable. When the TXV is throttled down at lower evaporator loads, the receiver will be fuller with liquid. At higher heat loadings, the TXV will throttle more open and draw liquid refrigerant from the receiver. The remote bulb will send more of an opening pressure to the TXV’s diaphragm, thus opening the valve more.

This is why it is of utmost importance to put the refrigeration equipment under a high evaporator heat load when charging with refrigerant. High loads will make sure that the TXV metering device is fully open and delivering the maximum amount of refrigerant to the evaporator. At these higher evaporator heat loads, the receiver will be at its lowest level. High loads can be achieved simply by opening the case door or putting a false heat load on the evaporator. It is at these higher loading times that a sight glass located in the liquid line will bubble if undercharged.

A system can be undercharged and — at a low evaporator heat loading — not bubble the sight glass located in the liquid line. This is because the TXV is throttled partly closed, and the receiver has plenty of liquid stored. Unless severely undercharged, only under higher evaporator heat loads will a sight glass bubble. Service technicians must make sure that the sight glass does not bubble under high and low loads in order for the system to be properly charged.

Service technicians will often confuse a bubbling sight glass with a low-flow-rate sight glass. If undercharged and at high load, the receiver will be at its lowest level, and the sight glass will have entrained refrigerant gas bubbles within the liquid. This is referred to as a bubbling sight glass. If, however, the system is charged correctly but experiences a very low heat load on the evaporator, a low-flow-rate sight glass — not a bubbling sight glass — can be seen. Low-flow-rate sight glasses will not have refrigerant gas bubbles entrained in the liquid flow. A low-flow-rate sight glass will only partially fill the volume of the sight glass with liquid refrigerant. The top portion of the sight glass will be occupied with vapor, which does not indicate an undercharge.

The low flow rate of refrigerant through the system is caused from the lower heat loads on the evaporator. Lower evaporator heat loads mean lower suction pressures, causing higher compression ratios and lower volumetric efficiencies. Now, the density of the return gas to the compressor is also decreased at these lower evaporator heat loads, causing low refrigerant mass flow rates.

Horizontally run liquid lines with sight glasses encounter this phenomenon much more often than vertically run liquid lines with sight glasses. With vertical liquid lines, gravity will force the liquid in the liquid line to settle to the lowest point, and this can entirely fill a sight glass volume, even at low flow rates. In either case, technicians must make sure that the liquid line after the sight glass, and especially just before the TXV, is full of liquid. This can be done with the use of an electronic sight glass.

An electronic sight glass will tell techs both visually (lights) and audibly (beeping sounds) whether or not there is 100 percent liquid. This will assist them in charging the system and ensure 100 percent liquid at the metering device. Electronic sight glasses are especially helpful when the refrigeration system has no sight glass. Simply clamp the sensors about 1½ inches apart on the liquid line and listen for beeping sounds or look for flashing lights. This will indicate the presence of vapor and liquid in the liquid line. Electronic sight glasses can also be used on the evaporator outlet or start of the suction line to let technicians know if there is liquid entrained with the vapors. Electronic sight glasses are also used extensively in automotive and commercial/residential air conditioning system charging.

FOLLOW THE RULES

When charging a refrigeration system, there are five main rules to follow:

Rule 1. Always charge a receiver/sight glass/TXV system under a high evaporator heat load. This could mean simply opening the doors or putting an artificial or false heat load on the evaporator.

Rule 2. For systems under a vacuum, once the desired vacuum level has been reached, the vacuum pump has been isolated from the system, and no leaks exist, always charge liquid refrigerant in the high side of the system until high- and low-side pressures equalize and liquid stops flowing. It is preferable for service technicians to charge liquid refrigerant into the receiver if valving permits.

Once liquid has been charged into the high side of the system and system pressures have equalized, let the system sit idle for a minimum of 15 minutes. This will vaporize any liquid that the TXV has released into the evaporator while charging. After a 15-minute minimum wait, start the system and monitor the amp draw. Keep the doors open and the system under high evaporator heat loading to allow the TXV to throttle open fully. This will draw a maximum amount of liquid from the receiver. Let the system run for a while at this high heat loading to reach equilibrium. If the sight glass is bubbling, charge refrigerant vapor into the low side of the system until the sight glass stops bubbling.

It is important to note that if the refrigerant is a 400 Series refrigerant blend that has a temperature glide associated with it, liquid refrigerant (not vapor) must be throttled through the low side to avoid fractionation of the blend. The proper technique is to employ a combination sight glass/throttling valve, which allows liquid to be removed from the cylinder while manually adjusting the valve, causing enough restriction to flash the liquid before it enters the suction valve. This flashing will be visible through the glass or plastic shroud.

Rule 3. With the system under high evaporator heat loading, charge refrigerant vapor into the low side of the refrigeration system until the sight glass stops bubbling. If the refrigerant has a temperature glide associated with it (400 Series blends), throttle liquid refrigerant in the low side of the system to avoid fractionation until the sight glass stops bubbling. (When properly charged, the sight glass should not bubble under high or low loads. A slight bubbling may be allowed for some 400 Series refrigerant blends with a higher temperature glide. Consult with the refrigerant manufacturer if this phenomenon occurs.)

Take evaporator superheat, compressor superheat, and condenser subcooling readings. Evaporator superheat reading guidelines are listed in Figure 1 (if the manufacturer specifications are available, use them instead).

If evaporator superheat settings are not within the guidelines, adjust the TXV’s superheat spring. Turning the spring clockwise will close the valve more, giving the evaporator more superheat. Always give the TXV enough time (minimum 15 to 30 minutes) to react to the change before taking the next reading. Note: It is of utmost importance that when setting evaporator superheat, the system should be under the lowest evaporator heat load possible; otherwise, false (higher) superheat readings will result.

Make sure that the condenser subcooling is at least 6°F. This is just a starting point, because actual needed subcooling amounts are system-dependent. Subcooling amounts will depend on what refrigerant is being used and how much pressure drop the subcooled liquid will see from the condenser outlet to the inlet of the TXV. This ensures that the condenser is producing enough liquid to deliver a solid column of 100 percent liquid to the TXV.

The compressor should have some compressor superheat. This amount varies and depends on the size of the system, length of suction line, and the existence of a suction line accumulator and/or heat exchanger. The old rule of thumb used to be that systems need at least 20° of compressor superheat, and this is no longer true. I recommend having at least 10° of compressor superheat to ensure that the compressor will not see any liquid refrigerant at the lower evaporator heat loads. However, always consult with the compressor manufacturer for suggested compressor superheat levels. Compressor superheat will also prevent compressor overheating because a certain density and mass flow of refrigerant is needed to cool some compressors. It will also prevent high amp draws.

Rule 4. Once the desired box temperature has been reached, measure evaporator superheat, compressor superheat, and condenser subcooling readings and compare them to the suggested guidelines. Record both the condensing and evaporating pressure. Monitor the amp draw of the compressor with an ammeter. Take a voltage reading at the compressor’s terminals, and check measured amps with the manufacturer’s amp curve for the pressure readings and voltage taken. Make sure it is within the range (±5 percent) for the pressures and voltage recorded.

Rule 5. Record the condensing and evaporating pressure of the system. Compare the amp reading of the compressor to the manufacturer’s amp curves for the pressures recorded. Make sure the amp draw is within range (±5 percent) on the curves for the two pressures and voltage recorded.

After applying these basic rules, a TXV/receiver/sight glass refrigeration system can be charged to the proper amount of refrigerant.

Publication date: 6/10/2019

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