A technician is installing a TXV remote sensing bulb on a 1-1/8 inch OD horizontal suction line (a larger diameter line). Per standard industry practice, where should the bulb be mounted for accurate sensing?
- AAt the 12 o'clock position (top of the line)
- BAt the 6 o'clock position (bottom of the line)
- CAt the 4 or 8 o'clock position, avoiding the oil layer at the bottom and vapor stratification at the top
- DOn the liquid line near the condenser outlet
Show answer & explanationAnswer & explanation
Correct answer: C. At the 4 or 8 o'clock position, avoiding the oil layer at the bottom and vapor stratification at the top
On larger suction lines (typically 7/8 inch OD and above), oil tends to pool at the bottom (6 o'clock) while refrigerant vapor can stratify at the top (12 o'clock), both giving inaccurate temperature readings. The 4 or 8 o'clock position avoids both issues and gives the most accurate bulb temperature sensing. Smaller lines can typically use the 12 o'clock position since stratification is less of a concern.
Why the other options are wrong
- A. On larger lines, vapor stratification at the top can cause inaccurate readings; 12 o'clock is preferred only on smaller lines.
- B. Oil pooling at the bottom of larger lines skews temperature readings, making 6 o'clock unreliable.
- D. The TXV bulb senses the suction line temperature, not the liquid line.
TXV Bulb Placement
The TXV remote sensing bulb must be mounted on the suction line at a position that avoids oil pooling and vapor stratification for accurate superheat control.
- Small suction lines (under ~7/8 in): mount at 12 o'clock
- Large suction lines: mount at 4 or 8 o'clock to avoid oil (bottom) and stratification (top)
- Bulb must be insulated and have good thermal contact with the line
Memory trick: Big pipe? Go to the side (4 or 8), not top or bottom