Refrigerant Pressure-Temperature (P-T) Chart

What You Need to Know

A refrigerant pressure-temperature (P-T) chart is a lookup tool that ties saturation pressure to saturation temperature for a specific refrigerant. You use it to translate the pressure you measure on a running system into the temperature at which that refrigerant would be boiling/condensing at that pressure.

The core idea (why P-T charts work)

For a pure refrigerant (and for blends at their bubble/dew points), there’s a one-to-one relationship between saturation pressure and saturation temperature:

• At saturation, refrigerant can exist as a mixture of liquid and vapor.
• At that same saturation condition, pressure determines saturation temperature (and vice versa).

Critical reminder: A P-T chart is only valid for saturated refrigerant. If the refrigerant is superheated vapor or subcooled liquid, pressure alone does not uniquely determine temperature.

What you use it for (high-yield field/exam uses)

• Convert suction pressure $\rightarrow$ **evaporator saturation temperature** (aka **evaporating temp**, $T_{evap,sat}$).
• Convert discharge/high-side pressure $\rightarrow$ **condenser saturation temperature** (aka **condensing temp**, $T_{cond,sat}$).
• Calculate superheat (evaporator/suction vapor quality check).
• Calculate subcooling (condenser/liquid quality check).
• Interpret system behavior: restrictions, over/undercharge symptoms, airflow/waterflow issues.

Pure vs blended refrigerants (the exam trap)

• Single-component / azeotropic refrigerants: one saturation temperature for each pressure.
• Zeotropic blends: at a given pressure there are two saturation temperatures:
  • Bubble point: saturated liquid begins to boil.
  • Dew point: saturated vapor begins to condense.

For blends, you must know whether to use bubble or dew based on what you’re calculating (subcooling vs superheat).

Step-by-Step Breakdown

A) Using a P-T chart to get saturation temperature from pressure

1. Identify the refrigerant (from unit nameplate or cylinder label). Do not guess.
2. Measure pressure at the correct service port:
   • Low side for evaporator/evaporating temperature.
   • High side for condenser/condensing temperature.
3. Confirm units match the chart (common: $\text{psig}$, $\text{kPa}$, $\text{bar}$).
4. Look up the measured pressure on the chart.
5. Read the corresponding saturation temperature:
   • For pure refrigerants: read the single $T_{sat}$.
   • For blends: read dew or bubble as appropriate.

B) Calculating superheat (using P-T chart)

You’re checking how much hotter the vapor is than its saturation temperature at the same pressure.

1. Measure suction pressure (as close to evaporator outlet as practical).
2. Use the P-T chart to find evaporator dew saturation temperature $T_{sat,dew}$ at that suction pressure.
3. Measure actual suction line temperature $T_{suction,actual}$ with a clamp thermocouple (good contact + insulation).
4. Compute superheat:

$\text{Superheat} = T_{suction,actual} - T_{sat,dew}$

Decision point:

• If refrigerant is a blend, use dew for superheat.
• If it’s a pure/azeotropic refrigerant, dew/bubble are the same (single $T_{sat}$).

C) Calculating subcooling (using P-T chart)

You’re checking how much cooler the liquid is than its saturation temperature at the same pressure.

1. Measure liquid (high-side) pressure at/near condenser outlet or liquid line service port.
2. Use the P-T chart to find condenser bubble saturation temperature $T_{sat,bubble}$ at that pressure.
3. Measure actual liquid line temperature $T_{liquid,actual}$.
4. Compute subcooling:

$\text{Subcooling} = T_{sat,bubble} - T_{liquid,actual}$

Decision point:

• If refrigerant is a blend, use bubble for subcooling.

D) If your gauge reads vacuum (below 0 psig)

Low-side readings during dehydration or very low-temp operation may be in vacuum.

• Convert properly before using a chart that expects absolute pressure.

At sea level (approximate):

$P_{abs} = P_{gauge} + P_{atm}$

where $P_{atm} \approx 14.7\,psi$.

If using inches of mercury vacuum $\text{inHg}$:

$P_{abs}(psi) \approx 14.696 - 0.491\,\times\,(\text{inHg vacuum})$

Many HVAC P-T charts are printed in $\text{psig}$ and already include negative pressures or vacuum scales—use the chart’s scale that matches your instrument.

Key Formulas, Rules & Facts

Definitions you must have locked in

• Saturation temperature $T_{sat}$: the boiling/condensing temperature at a given saturation pressure.
• Evaporating temperature: $T_{evap,sat}$ from suction pressure on the P-T chart.
• Condensing temperature: $T_{cond,sat}$ from high-side pressure on the P-T chart.
• Superheat: vapor temperature above $T_{sat}$ at the same pressure.
• Subcooling: liquid temperature below $T_{sat}$ at the same pressure.
• Bubble point: saturated liquid temperature (start of boiling) for blends.
• Dew point: saturated vapor temperature (start of condensing) for blends.
• Temperature glide (blends):

$\text{Glide} = T_{dew} - T_{bubble}$

High-yield formula table

Which column do you use? (blend rule)

What a P-T chart is NOT

• It is not a superheat/subcooling chart by itself—you must measure line temperature too.
• It does not account for pressure drop between your gauge port and the actual evaporator/condenser location.
• It does not diagnose non-condensables/moisture directly—those show up as weird pressure-temp behavior.

Examples & Applications

(These are “process” examples—on an exam you’ll be given pressures/temps or a P-T table to read values.)

Example 1: Find evaporating saturation temperature from suction pressure

Given: A system uses a specific refrigerant. You measure suction pressure $P_{suction}$.

Steps:

1. Go to that refrigerant’s P-T chart.
2. Find $P_{suction}$ on the pressure axis.
3. Read $T_{evap,sat}$ (for blends, read dew if asked for vapor saturation).

Key insight: $T_{evap,sat}$ is the boiling temp in the evaporator only if your pressure reading represents evaporator saturation pressure (watch pressure drop).

Example 2: Calculate superheat

Given:

• Refrigerant is a blend.
• Suction pressure measured: $P_{suction}$.
• Suction line temperature measured at the outlet: $T_{suction,actual}$.

Setup:

1. From P-T chart at $P_{suction}$, find $T_{sat,dew}$.
2. Compute:

$\text{SH} = T_{suction,actual} - T_{sat,dew}$

Key insight: Using bubble here is wrong for blends and will skew your diagnosis.

Example 3: Calculate subcooling

Given:

• Liquid pressure at condenser outlet: $P_{liquid}$.
• Liquid line temperature: $T_{liquid,actual}$.

Setup:

1. From P-T chart at $P_{liquid}$, find $T_{sat,bubble}$.
2. Compute:

$\text{SC} = T_{sat,bubble} - T_{liquid,actual}$

Key insight: Subcooling confirms you have solid liquid feeding the metering device (not flashing).

Example 4: Spot when a P-T chart is being misused

Scenario: You measure suction pressure at the compressor, and you also measure suction line temperature there. Superheat seems extremely high.

What might be happening:

• Your pressure is influenced by pressure drop (evaporator outlet vs compressor inlet).
• Your temperature is heated by warm suction line routing.

Correct approach: Measure pressure/temperature as close as possible to the evaporator outlet (or apply a pressure-drop correction if given).

Common Mistakes & Traps

1. Using the wrong refrigerant on the chart

   • What goes wrong: You read $T_{sat}$ from a different refrigerant’s P-T relationship.
   • Why it’s wrong: Each refrigerant has its own saturation curve.
   • Fix: Verify refrigerant type first (nameplate, service records, cylinder).

2. Mixing up dew vs bubble for blends

   • What goes wrong: You use bubble for superheat or dew for subcooling.
   • Why it’s wrong: Vapor calculations reference dew; liquid calculations reference bubble.
   • Fix: Memorize: Dew = vapor, Bubble = liquid.

3. Assuming pressure alone gives temperature in superheated/subcooled regions

   • What goes wrong: You treat any measured pressure as if the refrigerant is saturated.
   • Why it’s wrong: Off-saturation, many temperatures can exist at the same pressure.
   • Fix: Use P-T charts for saturation temperature only, then compare with actual line temperature to determine SH/SC.

4. Forgetting gauge vs absolute pressure differences

   • What goes wrong: You use a chart/spec in $\text{psia}$ with a $\text{psig}$ gauge (or vice versa).
   • Why it’s wrong: They differ by atmospheric pressure.
   • Fix: Convert with:

$P_{abs} = P_{gauge} + P_{atm}$

5. Ignoring pressure drop between measurement point and coil

   • What goes wrong: You compute $T_{sat}$ from compressor suction pressure and call it “evaporator saturation temperature.”
   • Why it’s wrong: Pressure drop means evaporator pressure is higher than compressor suction (or vice versa depending on location), shifting $T_{sat}$.
   • Fix: Measure as close to coil outlet/inlet as possible; if an exam gives pressure drop, account for it.

6. Reading the chart at the wrong units/scale

   • What goes wrong: Confusing $\text{kPa}$ with $\text{bar}$, or misreading vacuum scales.
   • Why it’s wrong: Small scale mistakes produce large $T_{sat}$ errors.
   • Fix: Match instrument units to chart; convert before lookup.

7. Bad temperature measurement technique

   • What goes wrong: Loose clamp, radiant heat, no insulation leads to wrong $T_{actual}$.
   • Why it’s wrong: SH/SC are small differences—measurement error dominates.
   • Fix: Firm clamp on clean copper, use thermal paste if allowed, insulate sensor from ambient air.

8. Assuming near-azeotropes always have zero glide

   • What goes wrong: Treating all blends like pure refrigerants.
   • Why it’s wrong: Many blends have non-zero glide; charts may show dew/bubble separately.
   • Fix: If the chart provides dew/bubble, use them correctly—even if glide is small.

Memory Aids & Quick Tricks

Quick visual: Superheat points right (higher temp) above saturation; Subcooling points left (lower temp) below saturation.

Quick Review Checklist

• You can define saturation and explain why P-T charts map $P \leftrightarrow T_{sat}$.
• You always confirm the refrigerant type before using a P-T chart.
• You know blends may require dew and bubble temperatures.
• You can compute:
  • $\text{SH} = T_{suction,actual} - T_{sat,dew}$
  • $\text{SC} = T_{sat,bubble} - T_{liquid,actual}$
• You check units and don’t mix $\text{psig}$ with $\text{psia}$.
• You remember pressure readings may be skewed by pressure drop from coil to gauge port.
• You trust your SH/SC only if your temperature clamp technique is solid.
• You can explain when a P-T chart is not applicable (superheated/subcooled conditions without SH/SC calculation).

You’ve got this—if you can consistently pick the right saturation temperature (dew vs bubble) and compute SH/SC cleanly, most P-T chart questions become automatic.