TXV Charging Curve Quick Reference

TXV Charging Curve Overview

• The chart used is the TXV charging curve; axes: left vertical = refrigerant line lift (ft), bottom axis = total refrigerant line length (ft). Middle shows three curves: lower, middle, upper.
• Rule: if the operating point lands below the curves, you need to add refrigerant; if it lands above the curves, you need to remove refrigerant.

Curve Selection Criteria

• Step 1: Measure the liquid line temperature. Example: $T_{ ext{LL}} = 100^{\\v0}\mathrm{\,\circ F}.$
• Step 2: Determine line length and vertical lift:
  • Total line length: distance from condenser to evaporator.
  • Vertical lift: is the evaporator higher than the condenser? quantify height in feet.
  • If indoor section is above condenser, use the given lift value.
• Step 3: Plot the intersection of (line length, lift) on the chart to choose the curve: lower, middle, or upper.
• Guidance: for long line sets or large vertical lift, use the upper curve to account for pressure drop and require more subcooling.
• Practical purpose: manufacturers design these curves to ensure proper charge and better system performance.

TXV Curve Application: Pressure-Temperature Plot

• After selecting the curve, plot the pressure and temperature on the TXV curve:
  • LLT: $T_{ ext{LL}}$ on the liquid line axis.
  • High-side pressure: $P_{ ext{HS}}$ (psig).
• If the intersection lands on the chosen curve (e.g., middle curve), the charge is considered correct.

Example: R-22 System

• Given: $T<em>{ ext{LL}} = 100^{\circ}\mathrm{F},\quad P</em>{ ext{HS}} = 233\ \mathrm{psig}.$
• The intersection with the middle curve aligns with the middle curve, indicating the charge is appropriate.
• Read the corresponding saturation temperature: on the chart for R-22, around the point corresponding to this pressure, $T_{ ext{sat}} \approx 112^{\circ}\mathrm{F}$.
• Subcooling calculation (per example): $\Delta T<em>{\text{sub}} = T</em>{ ext{sat}} - T_{ ext{LL}} \approx 112^{\circ}\mathrm{F} - 100^{\circ}\mathrm{F} = 12^{\circ}\mathrm{F}.$
• So, using the middle curve yields about $12^{\circ}\mathrm{F}$ of subcooling.

Subcooling and Reading Charts

• Subcooling is the difference between the saturated liquid temperature at the high-side pressure and the actual liquid line temperature after the coil: $\Delta T<em>{\text{sub}} = T</em>{ ext{sat}} - T_{ ext{LL}}.$
• Read and compare against the P-T chart to verify the curve selection and the implied subcooling.
• Important: each manufacturer may format charts differently; the core skills are accurate measurements and applying them to the chart correctly.

Key Takeaways

• Use the line length and lift to choose the appropriate TXV curve (lower/middle/upper).
• If the plotted point is below the curves, add refrigerant; if above, remove refrigerant.
• Verify with the P-T chart: ensure the LLT and HS pressure intersect the correct curve for the expected subcooling.
• Read charts and tables carefully; manufacturer variations require solid measurement and interpretation skills.