Liquid-to-Suction Heat Exchanger: Net Capacity vs. Discharge Temperature
A 1.5-ton R-404A freezer unit operates at a 20°F evaporator and 110°F condensing with a thermostatic expansion valve. The owner asks whether adding a liquid-to-suction heat exchanger will increase capacity enough to pull the box down faster. Before the change, site data show an evaporator outlet enthalpy of 93.0 Btu/lb, liquid entering the TEV at 58.0 Btu/lb, compressor suction specific volume of 1.60 ft³/lb, and 10°F superheat at the evaporator outlet. Predicted after the heat exchanger: evaporator outlet remains 93.0 Btu/lb, liquid to the TEV is 55.0 Btu/lb, compressor suction is 95.5 Btu/lb, and specific volume is 1.72 ft³/lb. Compressor displacement is 13.7 ft³/min at the operating point. The manufacturer’s maximum discharge temperature is 250°F, and measured discharge before the change was 225°F. The compressor is not unloaded, and the TEV holds 10°F evaporator superheat.
1. Calculate evaporator capacity before and after the liquid-to-suction heat exchanger, and explain why the per-pound refrigerating effect changes by more than the capacity does.
Read the worked answer
Baseline
- Mass flow = (13.7 ft3/min x 60 min/h) / 1.60 ft3/lb = 513.8 lb/h.
- Refrigerating effect at the evaporator = 93.0 - 58.0 = 35.0 Btu/lb.
- Capacity = 513.8 x 35.0 = 17,983 Btu/h.
After the heat exchanger
- Mass flow = (13.7 x 60) / 1.72 = 477.9 lb/h.
- Evaporator refrigerating effect = 93.0 - 55.0 = 38.0 Btu/lb. The 95.5 Btu/lb value is at the compressor inlet; the extra 2.5 Btu/lb was picked up in the heat exchanger, not in the evaporator, so using 95.5 - 55.0 = 40.5 would double-count it.
- Capacity = 477.9 x 38.0 = 18,160 Btu/h.
Net gain = 177 Btu/h, about 1.0%. The per-pound evaporator effect rises 8.6%, but mass flow falls 7.0% because the suction vapor is warmer and has a higher specific volume. The heat exchanger moves heat internally; it does not add evaporator load.
2. Estimate the discharge-temperature effect and recommend whether to keep the heat exchanger. What checks or adjustments would you require?
Read the worked answer
Suction temperature at the compressor would rise from about 30°F (20°F evaporator plus 10°F superheat) toward 40°F. At a compression ratio near 4.5, that typically adds roughly 15-25°F to discharge temperature, putting the 225°F baseline at about 240-250°F. The 250°F limit would be reached or exceeded, while capacity gain is only about 1%. That is a poor trade.
Before keeping the heat exchanger, verify:
- Evaporator superheat is still 10°F at the bulb; the heat exchanger raises compressor inlet temperature but should not change evaporator superheat.
- The compressor is approved for the higher suction temperature and for R-404A/POE.
- Discharge temperature is measured at the compressor discharge, not at the condenser inlet.
Recommendation: do not keep it for a 1% capacity gain unless the manufacturer approves the higher discharge temperature and the system needs suction-line protection. A smaller heat exchanger or none is preferable; do not compensate by lowering evaporator superheat below 10°F, because that risks floodback.
3. How would you verify the actual capacity gain in the field, and why is suction pressure alone misleading?
Read the worked answer
Use a calibrated load or a timed box pull-down with a known product load, or measure evaporator airflow and air enthalpy change; compare before and after at the same box temperature, ambient, and suction superheat. Suction pressure alone is misleading: it reflects evaporator saturation temperature, not mass flow or capacity. A slightly lower suction pressure can come from a restriction or a colder coil, not more capacity. Hold the same entering-air conditions and measure the actual heat removed.