Refrigeration: Compression Ratio, Glide, and System Balance
The Refrigeration domain covers light commercial walk-ins and reach-ins: superheat and subcooling on blended refrigerants, compression ratio as a health indicator, and the coil-to-box relationships that define a properly balanced system.
Blends and glide
- R-404A, R-448A, R-449A, and R-407A are zeotropic, so the pressure-temperature relationship is a range, not a line.
- Measure superheat against the dew-point column and subcooling against the bubble-point column; mixing them up biases every diagnosis.
- Charge blends as liquid from the cylinder; vapor charging fractionates the blend and shifts glide.
Compression ratio and TD
- CR = (discharge psig + 14.7) ÷ (suction psig + 14.7). Ratios above roughly 10:1 stress the compressor; look at condenser cleanliness and suction-side restrictions.
- Evaporator TD = box temperature − saturated suction temperature. Coolers typically run 10–12°F, freezers about 8–10°F; a TD well off design points to airflow, ice, or charge.
Worked example
An R-448A walk-in cooler reads 30 psig suction and 250 psig discharge. CR = 264.7 ÷ 44.7 = 5.9 - healthy. Later the same box reads 15 psig suction and 300 psig discharge: 314.7 ÷ 29.7 = 10.6. That jump is not a compressor failure; it is a dirty or fan-failed condenser plus a starved evaporator, and both must be corrected before any charge adjustment.
Defrost and accessories
- Coolers often use off-cycle or air defrost; freezers use electric or hot-gas defrost terminated on coil temperature, time, or pressure.
- Pump-down with a liquid-line solenoid, EPR valves, and CPR valves change what normal suction pressure looks like - know which are installed before judging readings.
Traps that cost marks
- Reading superheat off the bubble point on a high-glide blend.
- Treating a full sight glass as proof of correct charge; it only proves a solid liquid column.
- Replacing a compressor when the real fault is a restricted filter-drier, shown by a temperature drop across it.