This is the third part of a three-part series of posts on high-glide refrigerants for refrigeration system retrofits. Part 1 covered calculating superheat and subcooling with glide, and control of super-heat with the glide. Part 2 deals with the selection of proper thermostatic charges, capacities of several refrigerant blends relative to both R404A and R22, and examines how existing components on the liquid side of a system may be affected by a retrofit to a newer, high-glide blend. Part 3 deals with select components on the suction side of the system, and considerations for filter-driers, oil, and external seals. An overall guide summarizing retrofit considerations is also provided.
In Part 2, we noted the large differences in mass flow between R404A and R22. This coupled with R22’s higher liquid density means that on the liquid side of the system, a valve has to flow a much greater volume of R404A than R22 to deliver a given amount of refrigeration capacity. However, the density relationship between R22 and R404A reverses for saturated vapor. R404A vapor is much denser than R22 vapor. Because of this, the volume of vapor that a valve on the suction side of the system must flow to deliver a given capacity is quite similar for both. Here, we will examine how existing mechanical evaporator pressure regulators, electric evaporator pressure regulators, and suction solenoid valves will perform when a system is retrofitted from R22 or R404A to R407A, R407F, R448A, or R449A.
An evaporator pressure regulator (EPR) valve is installed between the outlet of the evaporator and the suction header of a supermarket refrigeration system. Its function is to maintain the desired pressure in the evaporator while suction pressure downstream of it is lower, so it is a “hold-back” or opens on rising of inlet pressure device. Most evaporators in a system operate best when set at a design pressure, and therefore operate at the proper temperature. The evaporator will then deliver the proper temperature air to the case or walk-in cooler, and not frost up too quickly. Systems typically have loads that require different evaporator temperatures, but the lowest evaporator temperature on a system dictates the common pressure at which the suction header must be maintained. The EPR allows evaporators designed to operate at higher temperatures to do so and maintains this pressure within a small range to buffer the evaporator from suction pressure fluctuations.
EPRs are sometimes used to control individual loads but are more often used to control suction pressure on a system branch or circuit with multiple loads. For this reason, we will size for a two ton (24,000 Btu/hr) load at low-temperature conditions (-20°F evaporator, 100°F condensing) and a four ton (48,000 Btu/hr) load at medium temperature conditions (20°F evaporator, 100°F condensing). We will assume the suction header is running at -25° saturated for the low-temperature system, and at 17°F for the medium temperature system. In an actual system, it would be typical to have 5°F difference on a low-temperature header and 3°F on a medium temperature header. Sporlan offers EPRs with three different modes of operation: direct acting, internally piloted and externally piloted. Our comparison will consider the externally piloted type because it is the most common in this application.
The suction vapor volumetric flow of R404A and R22 is about the same for a given capacity under like conditions. Does that result in the selection of the same EPR? When using the low-temperature system parameters already discussed, the Sporlan selection program gives the following results for R404A and R22:
While percent loading varies some, the same SORIT-12 is chosen for both. For medium temperature conditions:
A SORIT-15 is chosen for both, and loading is again within a few percent difference. Let’s examine how these EPRs may function with the alternate refrigerants, starting with the low-temperature example from above:
Sizing for all the alternates is acceptable, with percent loading falling in between that of R404A and R22 for most, and R407A being slightly higher. For the medium temperature condition and load:
Sizing for the alternates is acceptable at medium temperature conditions as well, with little variation from that of R404A or R22. When retrofitting an existing low or medium temperature R404A or R22 system to R407A, R407F, R448A, or R449A, existing evaporator pressure regulator valve sizing will likely be acceptable. It is good practice to check EPR sizing for confirmation.
While externally piloted EPRs were used in this example, there are many internally piloted EPRs in use today, particularly on loop-piped systems. The function of internally piloted EPRs can be more dependent on proper sizing than externally piloted ones. Thus, they may be more sensitive to a refrigerant retrofit. If a retrofit is to be performed on a system with internally piloted EPRs, sizing must be checked with the alternate refrigerant to ensure that sizing guidelines will not be exceeded. The manufacturer of the valve will be able to provide guidance on how the existing valve may function with an alternate refrigerant, and whether replacement is recommended.
Electric evaporator pressure regulators are also widely used in supermarket systems. EEPRs perform the same function as described in Section 4.1 for an EPR, but are operated by an electric stepper motor, and controlled via a pressure transducer and electronic controller. We also expect EEPRs for R22 and R404A systems to be sized similarly. We will perform the same comparison for our two-ton low and four-ton medium temperature systems with EEPRs sized for R22 and R404A, then look at how the selected valves might perform with the alternates:
As with mechanical EPRs, EEPRs for R404A and R22 are acceptable for the alternates. When retrofitting an existing low or medium temperature R404A or R22 system to R407A, R407F, R448A, or R449A, existing electric evaporator pressure regulator valve sizing will likely be acceptable. It is good practice to check EEPR sizing for confirmation.
The purpose of a suction solenoid valve is to provide electrically actuated on/off refrigerant flow control. Most are piloted valves, as discussed in Section 3.3. Suction solenoid valves are opened by the coil applying electromagnetic pull on the stem but require a small amount of pressure drop across the valve to close it. At sizing conditions, this pressure drop should be 1 psi or greater. If a solenoid valve is too large, there may not be enough ΔP across it to initially close it, particularly at low load conditions.
If a solenoid valve is too small, there will be more pressure drop across it than is desired. In a suction line, a pressure drop is a system inefficiency – it causes the compressor to operate at a lower suction pressure than it would otherwise. For the refrigerants in this exercise, 1.7 – 2.0 psi of drop is 3°F of saturation at -20°F and 2.1 – 2.6 psi of drop is 2°F saturation at 20°F. Because of this, suction solenoid valves with no greater than 3°F saturation drop at low-temperature conditions, and no greater than 2°F saturation drop at medium temperature conditions should be selected. Within this range, the suction solenoid valve will function reliably without causing excessive loss of system efficiency. These valves are then applied to the alternate refrigerants.
An E25 solenoid valve is chosen for both the two-ton low-temperature load, and the four-ton medium temperature load with R22 or R404A. When applied to the alternate refrigerants, the goal of maintaining at least 1 psi ΔP to ensure operation of the solenoid valve was met, while not excessively reducing system efficiency. Saturation drop was acceptable for all refrigerants at both medium and low-temperature conditions.
When retrofitting an existing low or medium temperature R404A or R22 system to R407A, R407F, R448A, or R449A, existing suction solenoid valve sizing will likely be acceptable. It is good practice to check suction solenoid valve sizing for confirmation.
There are several other areas that must be considered when performing a refrigerant retrofit. Among the most important of these are filter changes, oil changes, and replacement of elastomeric seals. This section will touch on these but not cover them exhaustively. There are many good refrigerant retrofit guides that cover these topics in detail.
Any system refrigerant retrofit should include a change of all system filters, filter-driers or filter-drier cores, regardless of refrigerant. Clean filters minimize pressure drop, reducing startup difficulty and maximizing efficiency. It is best practice to check for significant pressure drop due to contamination after a few hours of system operation, and change filters again if significant contamination is found. Retrofits from R22/mineral oil to HFC/POE oil are known to cause contamination deposits to dislodge and move through the system, so this is a very important step in any retrofit of an R22 system. Filter manufacturers provide guidelines on filter changes for system cleanup and for normal operation.
New filter-driers or cores will also help ensure the system is dry as possible, benefiting system startup and operation. When the system is re-started following a retrofit, liquid line moisture indicators should be reviewed after 24 hours of operation. If moisture is still indicated in the system, change the filter-driers or cores again. This process should be repeated until the system is indicated dry.
There will normally be little concern with the size of liquid or suction filters when retrofitting from either R404A or R22 to R407A, R407F, R448A, or R449A. Filters are usually selected with some amount of additional capacity. It is good practice check the size of filter-driers, to confirm there will be both adequate flow capacity and moisture removal capability in relation to the size of the system. All Sporlan filter-driers, cores, and suction filters are fully compatible with these alternate refrigerants.
When retrofitting an R22 system to R407A, R407F, R448A, or R449A, the existing mineral oil in the system must be changed to a synthetic POE oil. Some R22 systems may have POE oil instead of mineral oil. Recommended procedures for HCFC to HFC retrofits apply to the blends with HFO content as well. Established guidelines call for reducing mineral oil content to 5% or less through successive oil changes. Specific oil type and viscosity recommendations from the system or compressor manufacturer should be followed.
An R404A system will already have POE oil instead of mineral oil. When retrofitting to any of these alternates, a change of oil types is not necessary. It is still good practice to change oil in conjunction with a retrofit, to remove as many contaminants from the system as possible. The cleaner the system is, the more efficiently and reliably it will operate. In this situation of a POE-for-POE change, multiple oil changes (as may be necessary with a mineral-to-POE change) are likely unnecessary, unless there is severe oil contamination.
When retrofitting a system to R407A, R407F, R448A, or R449A, it is recommended to change the elastomeric seals in the system to minimize the possibility of external leaks. This is especially important with an R22 system retrofit. Elastomeric gaskets and O-rings that have been exposed long-term to R22 and mineral oil have a greatly increased risk of leaking after a deep vacuum being applied to the system, followed by exposure to HFC refrigerants and POE oil.
When retrofitting an R404A system, the seals are not exposed to different refrigerant and oil types after the retrofit. However, the existing seals will likely have been in place for several years. Pulling a deep vacuum on the system will still alter the set the seals have taken from being under internal pressure, and cause small amounts of refrigerant the seals have absorbed over time to begin to release from them. While the risk of leaks may be less for an HFC-to-HFC (or HFO) and POE-to-POE retrofit, it is nonetheless best practice to change the elastomeric seals. Minimizing external refrigerant leaks benefits both the system’s owner and the environment.
In a retrofit from R404A, existing liquid side components may be too large, and TEV thermostatic elements are not compatible. Replacement of at least TEVs may be necessary. R404A suction side components may be okay. In a retrofit from R22, both liquid and suction side components are likely okay. Though most critical for an R22 system, any retrofit should include measures to ensure a clean and dry system, and minimize external leaks, including changing of filters, refrigeration oil, and elastomeric seals. Figure 6 summarizes:
HVACR Tech Tip article contributed by John Withouse, senior engineer - Refrigeration, Sporlan Division of Parker Hannifin.
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