Reused TEV After R-448A Condensing Unit Replacement
A supermarket service company replaces a failed 1.5 HP R-404A condensing unit serving an 8 ft x 10 ft x 8 ft walk-in cooler with a new R-448A unit. The new unit is rated 10,500 Btu/h at 25°F evaporator and 120°F condensing; the old unit was 9,000 Btu/h at the same conditions. The existing evaporator has a 1-ton TEV with an external equalizer, a 5/8 in. suction line, and a 3/8 in. liquid line. The line run is 40 ft with 12 ft of lift. The installer reused the TEV and line set, charged by making the suction line sweat, and the box pulls down from 55°F to 38°F but never reaches 35°F. Suction pressure is 28 psig, suction-line temperature at the evaporator outlet is 55°F, liquid-line temperature is 95°F, and condensing pressure corresponds to 115°F. The sight glass is clear. The compressor runs continuously, and the space temperature stabilizes at 39°F.
1. Calculate the subcooling and estimate the evaporator superheat. What do these two readings together tell you about the charge and the TEV feed?
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Subcooling = condensing saturation temperature - liquid-line temperature = 115°F - 95°F = 20°F. That is above the usual 8-12°F target and, with a clear sight glass, argues against an undercharge. Evaporator superheat = suction-line temperature - saturated suction temperature. At 28 psig the R-448A saturation temperature is well below 40°F, so 55°F at the outlet is roughly 30°F or more of superheat, far above the 8-12°F target for this cooler.
The combination is the key: high superheat means the evaporator is starved, while high subcooling and a clear glass mean liquid is backing up upstream. A starved evaporator with liquid available points to a TEV/feed problem or a restriction, not simply low charge. Check the TEV inlet for flash gas and pressure drop before condemning the charge.
2. The installer says the new unit is larger, so the reused 1-ton TEV is fine. Evaluate that judgment, including refrigerant and line-size effects.
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The 'larger unit means the old TEV is fine' judgment is not supported. Nominal capacity is close, but a TEV must match the refrigerant, evaporator capacity, and pressure drop. R-448A is a zeotropic blend with different pressure-temperature behavior and glide than R-404A; a power element charged for R-404A may not control superheat correctly. The 3/8 in. liquid line on a 40 ft run with 12 ft of lift is also marginal at 10,500 Btu/h; excessive liquid-line pressure drop can flash gas before the TEV. The 5/8 in. suction riser needs proper trapping and sizing for oil return at minimum load. The correct action is to verify TEV selection and line sizing against the manufacturer's tables, not assume the reused parts are compatible.
3. List the next diagnostic checks and the corrective action you would take before condemning the condensing unit.
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Before condemning the condensing unit: verify evaporator airflow and coil condition; measure TEV inlet pressure and temperature to detect flash gas; inspect the inlet screen, bulb charge, bulb location, and external equalizer; compare TEV capacity and refrigerant charge with the new unit. If the TEV is wrong or damaged, replace the power element or valve with one rated for R-448A and the actual evaporator capacity. Then set superheat to the manufacturer's target (typically 8-12°F), confirm subcooling, and recheck box temperature. A condensing unit that is running continuously with a starved evaporator is not proven bad.