Refrigerant Pressure-Temperature Relationships - Quick Notes

Saturated Refrigerant and the P–T Relationship

  • Saturated refrigerant: mixture of liquid and vapor in a closed system; the liquid/vapor mix follows the pressure–temperature relationship shown on the P–T chart.
  • Saturated means not superheated or subcooled; three states: saturated, superheated, subcooled.
  • Heat in the system changes pressure and temperature according to the P–T chart.

Using the Pressure–Temperature Chart

  • The P–T chart links temperature and pressure for the refrigerant.
  • If you know the suction (low-side) pressure, you can determine the evaporating temperature: P<em>evap⇒T</em>evapP<em>{evap} \Rightarrow T</em>{evap}
  • If you know the head (high-side) pressure, you can determine the condensing temperature: P<em>head⇒T</em>condP<em>{head} \Rightarrow T</em>{cond}
  • Equivalently: T<em>cond↔P</em>headT<em>{cond} \leftrightarrow P</em>{head} and T<em>evap↔P</em>evapT<em>{evap} \leftrightarrow P</em>{evap}

High‑Side Examples (R‑22 and R‑410A)

  • For R‑22: head pressure P<em>h=202 psig⇒T</em>cond=102∘FP<em>h = 202\text{ psig} \Rightarrow T</em>{cond} = 102^{\circ}F
  • For R‑410A: head pressure P<em>h=376 psig⇒T</em>cond=112∘FP<em>h = 376\text{ psig} \Rightarrow T</em>{cond} = 112^{\circ}F
  • At a coil temperature T=120∘FT = 120^{\circ}F, the corresponding pressures are: P<em>R22≈260 psig,  P</em>R410A≈418 psigP<em>{R22} \approx 260\text{ psig}, \; P</em>{R410A} \approx 418\text{ psig}
  • Common condensing temperature range for comfort cooling: ≈100∘F to 130∘F\approx 100^{\circ}F \text{ to } 130^{\circ}F
  • High‑side pressure ranges (typical): R‑22 roughly 2×102 to 3×102 psig2\times 10^2 \text{ to } 3\times 10^2\text{ psig}; R‑410A roughly 3×102 to 5×102 psig3\times 10^2 \text{ to } 5\times 10^2\text{ psig}

Low‑Side Examples (Suction Pressure)

  • R‑22 suction P<em>evap=50 psig⇒T</em>evap≈26∘FP<em>{evap} = 50\text{ psig} \Rightarrow T</em>{evap} \approx 26^{\circ}F
  • R‑410A suction P<em>evap=110 psig⇒T</em>evap≈36∘FP<em>{evap} = 110\text{ psig} \Rightarrow T</em>{evap} \approx 36^{\circ}F
  • If T<em>evap=44∘FT<em>{evap} = 44^{\circ}F, then: P</em>evap,R22≈75 psig,  Pevap,R410A≈128 psigP</em>{evap,\text{R22}} \approx 75\text{ psig}, \; P_{evap,\text{R410A}} \approx 128\text{ psig}
  • For the same evaporator temperature, R‑410A operates at higher pressures than R‑22.

Temperature, Heat, and Phase Change Basics

  • Temperature describes the intensity of heat; measured in degrees on a temperature scale.
  • Quantity of heat: the amount of heat transferred or contained; measured in BTUs.
  • Sensible heat: heat that can be sensed with a thermometer; causes a temperature change.
  • Latent heat: hidden heat; causes a phase change without a temperature change.

Sensible vs Latent Heat

  • Sensible heat: added or removed heat changes temperature.
  • Latent heat: during a phase change, temperature remains constant.

Phase Change Examples

  • Ice melting at 32° F: solid to liquid; temperature stays at 32° F during melting.
  • Boiling water at 212° F: liquid to steam; temperature stays at 212° F during boiling.

Endnote

  • The P–T chart ties temperatures to pressures for each refrigerant; saturated states follow the chart; high-side vs low-side determine condensing vs evaporating temperatures; different refrigerants have different pressure ranges but operate at related temperatures.