In Part II we discussed the main drivers in determining which refrigerant to use for the conversion from R-22 (HPAC December 2013, online at www.hpacmag.com). Once that decision has been made, a detailed plan can be assembled to ensure the conversion is completed in a minimum amount of time, and the result is a system that operates efficiently and trouble free.
1. Systems not operating properly with R-22 will not operate properly after a refrigerant conversion. A pre-conversion survey should be performed to ensure that system components are operating properly and a list of any necessary repairs should be assembled. Include a check of the system’s performance history in the survey. A system that has capacity issues on design high temperature ambient days might need an engineering review prior to the conversion. Repairs should be completed before or during the conversion.
If an oil change (mineral oil to POE) is required, do it about a week prior to the conversion. Field experience has shown that levels of remaining mineral oil higher than five per cent can be tolerated while still providing adequate oil return. But compressor manufacturers still recommend that the percentage of remaining mineral oil after the refrigerant conversion be less than five per cent.
3. Prior to the conversion do a complete system leak check and properly repair any leaks that are discovered. In addition, elastomer seals have a tendency to leak after a conversion is performed. This is not due to an incompatibility issue, it is because the elastomer seals will swell when in the presence of refrigerants. R-22 has the most aggressive swelling influence on elastomer seals, so after the conversion the seal will effectively shrink (the conversion refrigerant will result in a seal that swells less that it did with R-22). For example, lab testing revealed a Neoprene W O-ring in the presence of R-22 and mineral oil swelled about 4.1 per cent. The same O-ring was then exposed to R-404A and POE, and the swell was reduced by 2.6 per cent.
Every elastomer seal will take a compression set once it is compressed in the valve or component that it is providing a seal for. Over time the seal will lose some of its resilience, preventing it from providing an adequate seal after the “shrinkage” takes effect. To avoid any potential leak after the conversion it is recommended that all elastomer seals be replaced. These would include any O-rings (oil level controls, hand tight flare caps), tetra seals (used in some older model solenoid valves, which have been upgraded to wolverine seals), Schrader valves, pressure regulating valves, brass filter-drier shells, receiver level gauges, etc.
4. Older model ball valves may not have leak proof caps. If available, leak proof caps should be acquired and installed. If not available, replacement of ball valves might be necessary.
5. Recovery of used R-22 from system. An accurate estimate of the system charge should be made and the necessary amount of clean/evacuated empty recovery cylinders should be on hand to accommodate the removal of the R-22. Cylinders should not be filled to more than 80 per cent of capacity.
6. If this is a refrigeration system (particularly a supermarket with display cases) some product unloading might be necessary. Or adding dry ice to the display cases might negate that option. In either case, this is crunch time, a sufficient sized crew should be on hand to perform the necessary work in a minimum amount of time. If distributor nozzles or TEVs or TEV elements require replacement, this is the time to do it.
7. After replacing flow controls (if needed) and replacing all filter-driers and oil filters (if used) the system should be pressurized and leak checked. If no leaks are found evacuate the system down to 250 microns, with confirmation that the vacuum holds.
8. While the system is being evacuated it would be a good time to label the system with the new refrigerant. This might seem like an insignificant step, but imagine a technician taking a service call some weeks later and finding that the system was low on refrigerant. Without knowing that a conversion had taken place, a technician might add R-22 to a system that has the appearance of still being an R-22 system. This is a potential costly situation that should be avoided and can be by simple labeling. If a logbook is present, enter a summary of the work done during the conversion.
9. After the system has been properly evacuated the new refrigerant can be added and ultimately the system restarted. After the system has reached a stable operating condition, record and compare system operating conditions with those taken during the initial survey. Any pressure sensitive devices such as pressure controls, compressor unloaders, pressure regulating valves and system controllers should be reset for the correct pressure corresponding to the new refrigerant.
All system TEVs should be checked for proper superheat setting, and adjusted as required. If the refrigerant chosen for the conversion is a high glide refrigerant, then the technician will need to familiarize themself with the proper way to set superheat (and subcooling). Single component refrigerants such as R-22 will have a constant refrigerant saturation temperature at a given pressure.
Zeotrope refrigerant blends (such as R-407A/C, R-422A/B/D, R-438A) are multi-component. At a saturated condition in an evaporator each component of a Zeotropic blend will boil independently of the other components. The saturation temperature will be determined by the per cent of each component remaining and the temperature at which it is boiling. Where it gets complicated is that the lower boiling temperature component will boil at a faster rate than the higher boiling temperature components, causing a change or glide in the saturation temperature at a constant pressure.
For example, while R-22 boils at a constant 40F at 68.5 psi, R-407C at the same pressure will boil at 33.3F at the evaporator inlet. As the last molecule of liquid boils into a vapour it will be boiling at 44.6F. This is a glide of 11.3F.
R-407C consists of three components: R-32 (23 per cent - lowest boiling temperature), R-125 (25 per cent - intermediate boiling temperature) and R-134A (52 per cent - highest boiling temperature). At the inlet of the evaporator the R-32 is boiling at a faster rate than the other two components. Its effect is most noticeable at the evaporator inlet, as seen in the 33.3F boiling temperature. Because it is boiling at a faster rate it changes state into a vapour at a faster rate than the other two components, with the composition of the remaining Zeotropic blend changing–less R-32 in proportion to the remaining two components. The boiling temperature starts rising because the effect of the R-32 becomes diminished as its per cent of the total diminishes. To a lesser degree the R-125 is doing the same thing. At the point where the last drop of liquid boils into vapour there is a greater percentage of R-134A (in comparison to the R-32 and R-125), causing the 11.3 degree rise in boiling temperature.
This effect is known as “fractionatio.” It can cause issues with the integrity of the system charge if refrigerant leaks out of the vapour side of a system during a time when the compressor is off. The effect would be the same as venting vapour from a cylinder of refrigerant, with the lowest boiling point component vapourizing at a quicker rate than the other components thereby changing the percentages of the remaining components left in the cylinder. There are two problems with this occurring in a system: the refrigerant left in the system now has a different chemical formulation than the original blend that the system was charged to, resulting in different performance and efficiency. And because the formulation has changed you have no idea what the pressure-temperature relationship is any longer, meaning that it would be difficult to determine what the superheat and/or subcooling values would be.
The recommendation is that if a system with a Zeotropic blend experiences a leak in the vapour side of the system, and the leak occurs during the off cycle, then the system charge should be removed and recharged with virgin refrigerant. Certainly on a smaller system with a minimal charge the effect of fractionation would be more noticeable.
10. One of the drawbacks of R-22 is the high discharge temperatures it will operate at in low temperature applications. It is not uncommon for these systems to operate at temperatures that result in mineral oil decomposition, with the resulting contaminant being deposited all throughout the interior of the system. While in small amounts it might remain dormant in an R-22 system, the presence of POE will bring all of these contaminants back into circulation. A day after the conversion it would be appropriate to replace the system filter-driers. Close monitoring of the oil a week or two after the conversion will reveal if further action is required.
That concludes the refrigerant conversion series. R-22 we will miss you. <>
Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at firstname.lastname@example.org.