The compressor raises the refrigerant’s pressure until its boiling point sits above the temperature of the air or water that must take the heat away, and the work it needs is set by the temperature lift between the cold space and that warmer sink. The ideal limit on cooling per unit of work is the cold temperature in kelvin divided by the lift: 277.15 ÷ 31 = 8.9 for a 4 °C box in 35 °C air, and every extra degree of lift costs more work for the same cooling.
Part of Refrigeration
Step one of the cycle: pressurising the vapour, why that pressure is set by two temperatures, the ideal limit on work, and what to measure on a running machine.
The Required Pressure Is Set By Two Temperatures, Not By Choice
A refrigerant vapour condenses at a temperature that rises with its pressure. To reject heat to air at 35 °C the vapour must condense above 35 °C, so the compressor must lift it to the pressure at which the fluid boils above that temperature. The evaporator needs the opposite: a pressure low enough that the fluid boils below the cold-box temperature.
So the compressor’s job is a pressure ratio, high side over low side, fixed by the condensing and evaporating temperatures and by the fluid’s vapour-pressure curve. The designer chooses the temperatures; the fluid dictates the pressures. That is why Working Fluids comes before any machine drawing, and why a fluid that needs a very high pressure at ambient temperature, as the carrier says of carbon dioxide, makes the whole machine heavier.
Compression also heats the vapour. The corpus gives the ideal adiabatic rule in Gas Laws in Practice: the outlet temperature is T1 × r^k, with k = (γ − 1) ÷ γ. For air in that article, a pressure ratio of 4 gives 162 °C and a ratio of 8 gives 258 °C, from 20 °C air. A refrigerant has a different exponent, so those numbers show the principle, not the discharge temperature of a refrigeration machine: measure that on the machine itself. The discharge line is hot enough to burn.
The Ideal Limit On Cooling Per Unit Of Work Is Set By Absolute Temperatures
For a cycle between a cold temperature Tc and a warm one Th, both in kelvin, the best any refrigerator can do is a cooling effect per unit of work of Tc ÷ (Th − Tc). This follows from the reversible (Carnot) limit, a standard thermodynamic result recalled here, not derived. It is the refrigerator’s counterpart of the engine limit in Engine Theory.
Worked Example: Lift, Not Cold, Is What Costs
All temperatures here are assumptions chosen to show the trend.
Box at 4 °C in 35 °C air, ideal.
- Tc = 4 + 273.15 = 277.15 K. Th = 35 + 273.15 = 308.15 K. Lift = 308.15 − 277.15 = 31 K.
- Limit = 277.15 ÷ 31 = 8.94, about 8.9.
Freezer at −18 °C in the same air, ideal.
- Tc = −18 + 273.15 = 255.15 K. Lift = 308.15 − 255.15 = 53 K.
- Limit = 255.15 ÷ 53 = 4.81.
Colder by 22 K, and the ideal cooling per unit of work nearly halves.
Real heat exchangers need a temperature gap. The evaporator must run colder than the box and the condenser hotter than the air, or no heat flows. Assume 10 K at each end for the 4 °C box.
- Evaporating at 4 − 10 = −6 °C, which is 267.15 K. Condensing at 35 + 10 = 45 °C, which is 318.15 K.
- Lift = 318.15 − 267.15 = 51 K. Limit = 267.15 ÷ 51 = 5.24.
So poor heat exchange, not the thermodynamics of the fluid, cut the limit from 8.94 to 5.24. For an assumed heat load of 100 W, the least possible shaft power is 100 ÷ 5.24 = 19 W. A real machine needs more; the fraction of the ideal that a rebuilt machine achieves is not sourced here and must be measured. The refrigeration carrier quotes 50 to 100 W of mechanical power for a small box of 0.2 m³; it gives no heat load for that box, so the two figures cannot be reconciled from this page.
The practical lessons are two. Lower the lift: put the condenser in the coolest air or water available and keep it clean. Do not run colder than the food needs: each step down in storage temperature raises the work.
What To Measure On A Running Machine
| Reading | What it tells you |
|---|---|
| Suction pressure | The evaporating temperature the fluid is working at |
| Discharge pressure | The condensing temperature; high means a dirty or starved condenser |
| Discharge line temperature | Whether the compression is as hot as expected; much hotter means trouble |
| Condenser outlet temperature | Whether the vapour fully condensed |
The gauge is the corpus Pressure Gauge; use one rated above the highest pressure the circuit can reach. Convert pressure to temperature with the fluid’s own property table, not with a table for another fluid.
Common Mistakes
- Chasing a colder box than the food needs. The lift grows and the work with it.
- Putting the condenser in a hot enclosure. The air that cools it is already warm, so the lift is larger.
- Letting liquid reach the compressor. A liquid is nearly incompressible, as the corpus says of water in Pressure Testing, so a slug of it in the cylinder breaks valves, rods or the head. The suction vapour must be fully vaporised.
- Treating compressed air figures as refrigerant figures. The exponent differs.
Safety
A compressor discharges hot, high-pressure vapour and is a moving-machinery hazard
The discharge line and head can burn on contact, and the corpus air example reaches 258 °C at a ratio of 8. Guard belts, flywheels and couplings. A relief device must protect the discharge side against a blocked condenser or a shut valve, and it must be one the operator cannot defeat; see Pressure Vessels. Never run a compressor against a closed discharge valve.
What’s Next
- Learn next: Condensation — the heat exchanger that receives this hot vapour
- Combine with: Compressor Types — the machines that do the squeezing