One HVACR reader: Condensers: Subcooling

Subcooling:

Definition: Cooling liquid refrigerant below its condensing or saturation temperature.
Example:
- Standard System:
  - 95°F ambient, 125°F condensing temperature (30°F split).
  - 115°F liquid line temperature at condenser outlet = 10°F subcooling.
- Importance: Prevents flash gas in the liquid line due to pressure drops.
- Efficiency: For every 1°F of subcooling, system efficiency increases by about 0.5%.

Conditions:

Example:
- Ambient: 90°F
- System: R-22 with a high side pressure of 260 psig (120°F condensing temperature).
- Liquid Line Temperature: 120°F (zero degrees subcooling).
- Pressure Drop: A 30-foot vertical liquid line causes a 15 psig drop.
- Impact: Results in a condensing temperature of 115°F, causing liquid to flash and improper feeding to the metering device.

Low Ambient Conditions:

Definition: Air entering the condenser is below 60°F, more common in commercial systems.
Impact: Low head pressure leads to metering problems and starves the evaporator.
Minimum Head Pressure: Essential to ensure proper refrigerant metering.
Problem: Head pressure below 90°F condensing temperature.

Methods to Control Head Pressure:

Fan Cycle Control:
- Operation: A pressure switch controls the condenser fan based on discharge pressure.
- Process:
  - Fan off when head pressure falls.
  - Fan on when head pressure rises.
- Issue: Can result in rapid fan cycling.
Condenser Flooding:
- Operation: Uses a head pressure regulating valve to restrict liquid flow and back up refrigerant in the condenser.
- Process:
  - Liquid backs up in the condenser, raising head pressure.
  - Hot gas bypassed from discharge line into the top of the liquid receiver increases liquid refrigerant pressure.
- Example:
  - Above 60°F Ambient: Full flow through the valve, liquid refrigerant fed to the liquid receiver and onto the liquid line.
  - Below 60°F Ambient: Valve restricts flow, floods the condenser, and bypasses hot gas to increase liquid line pressure.

One HVACR reader