Superheat: Measurement and Charging Charts — Quick Reference

Superheat: concept and measurement

• $SH = T<em>{su} - T</em>{sat}(P<em>{su})$, where $T</em>{su}$ is suction line temperature and $T<em>{sat}(P</em>{su})$ is saturation temperature at suction pressure $P_{su}$.
• Measure suction line temp with a digital thermometer and suction pressure with gauges; calculate $SH$ from the difference with the saturation temp.

Using the manufacturer's superheat charging chart

• Chart basis: airflow $= 400\,\mathrm{CFM/ton}$; for a 3-ton system, total airflow $= 3\times 400 = 1200\,\mathrm{CFM}$.
• Assumed humidity: $50\%$ RH.
• Chart layout: left axis is $SH\,(°F)$, bottom axis is outdoor temperature $\,(°F)$; lines correspond to indoor dry-bulb temperatures.
• Goal: compare measured $SH$ to the chart-predicted $SH$ under current conditions to determine if unit is undercharged, overcharged, or correct.

Example calculation (manufacturer chart)

• Suction pressure $P<em>{su} = 65\,\mathrm{psig}$ → $T</em>{sat} \approx 37\text{–}38^\circ\mathrm{F}$.
• Measured suction temp $T_{su} = 70^\circ\mathrm{F}$; $SH = 70 - 38 \approx 32^\circ\mathrm{F}$.
• Outdoor temp $T<em>{out} = 90^\circ\mathrm{F}$; indoor dry-bulb $T</em>{in} = 80^\circ\mathrm{F}$; intersection on the chart corresponds to target $SH \approx 12^\circ\mathrm{F}$.
• If actual $SH$ is higher than the target (e.g., 32°F), the evaporator is not absorbing enough heat early in the coil; add refrigerant and recheck until $SH \approx 12^\circ\mathrm{F}$.

Alternative chart: fixed orifice superheat chart

• Generic fixed-orifice chart used when manufacturer data is incomplete; often stored on a phone or in a toolkit.
• This chart uses actual return-air wet-bulb temperature instead of humidity assumption.
• Example: return wet-bulb $T<em>{WB} = 67^\circ\mathrm{F}$; outdoor $T</em>{out} = 90^\circ\mathrm{F}$; intersection between the $10-14^\circ\mathrm{F}$ band yields $SH \approx 12^\circ\mathrm{F}$.
• Conclusion: both charts converge on the same target $SH$; use to determine whether to add or remove refrigerant.

Practical charging procedure

• After each small refrigerant adjustment, re-measure suction pressure/temperature and recalculate $SH$.
• Repeat until measured $SH$ matches the chart target under the current operating conditions.

Summary

• Determine $SH$ from measured $T<em>{su}$ and $T</em>{sat}(P_{su})$.
• Use manufacturer chart (400 CFM/ton, 50% RH) to find target $SH$ under given outdoor/indoor conditions.
• If actual $SH$ is too high, add refrigerant and recheck; if too low, recover refrigerant and recheck (under the expected target).
• If manufacturer data is unavailable, use the fixed-orifice chart with return wet-bulb temperature to obtain the same target $SH$.