Vapor-Compression Cycle Question

Factual Questions
1

I read quite a few online articles on the vapor-compression cycle in order to understand how refrigerators and air conditioners work. I have a few remaining questions:

  1. Does the refrigerant itself (ammonia, Freon, whatever it may be) ever decay? That is, does it ever start to lose its physical properties? Does the substance become less efficient over time?

  2. Why does the refrigerant reject heat while it’s being pumped through the coils? How is this effective at dissipating heat? Why doesn’t it lose pressure in addition to losing heat?

  3. How does an expansion valve turn a liquid into a gas? Does it merely reduce pressure on the liquid, allowing it to become a gas and absorb heat?

  4. How does the “cool” actually get put into the refrigerator? Does a fan blow over the evaporator?

2

  1. No. But leaks sometimes develop, and you’ll lose refrigerant over time.

  2. On the cold side of the system (the low-pressure side), the gas is expanding and absorbing heat from the surrounding coil, which in turn absorbs it from the room air being drawn across it. The hot side (the high-pressure side) is where the gas is again compressed into a liquid, giving up its heat and warming up the surrounding coil. That heat is exhausted away.

  3. The expansion orifice or valve works by constricting the flow through the coil tubing, allowing one side to become compressed and the other to expand, resulting in the hot and cold sides respectively. Different refrigerants and applications will dictate different pressure differentials, and hence differently set expansion valves or orifice diameters.

  4. The “cool” gets into the refrigerator by passing the air inside it across the radiating fins on the evaporator coils, which cools it off by transferring its heat to the refrigerant.

3

Best way to look at the cycle:

  1. The compressor compresses the gas into the condensor coils, where it turns into a hot gas.

  2. The coils get hot, and a fan blowing across the coils cools them (and the gas) down.

  3. The cooled gas turns into a liquid. Yes, technically pressure declines when this happens; it has to. However, the compressor is still working, remember; it just keeps cramming it in, as fast as it can.

  4. The liquid flows through the expansion orifice. The best way to look at this is as a controlled leak. I suspect you have trouble viewing this cycle because it’s not something best envisioned as a series of steps; rather, it’s something that happens continuously. As in step 3 above, pressure declines when the liquid exits through the expansion orifice: hey, it’s a leak, right? But again, remember: the compressor keeps cramming it in.

  5. The liquid flows into the evaporator (a small radiator, like the condensor coils).

  6. A fan blows across the evaporator, heating the liquid.

  7. The liquid evaporates into a gas again, cooling as it does so.

  8. The cooled gas cools the air exiting through the evaporator .

  9. The gas returns to the compressor for another go-round.

  10. The intake to the compressor is like the intake to any vacuum pump: it’s constantly sucking the evaporator dry.

  11. The end result: the compressor’s constant operation continously crams coolant into the hi-pressure (condensor) side faster than it can leak out through the expansion orifice (this keeps the pressures different), and constantly sucks coolant out of the low pressure side (this also keeps the pressures different). The tough part is understanding, “why don’t the pressures equalize?” Because the compressor is keeping the pressures unequal by working harder than the system can allow them to equalize (nothing happens in instantaneously).

  12. When the A/C is shutoff, it generally needs time for the pressures to equalize to start up again. It’s dangerous to start up the compressor while the pressure in the condensor is still high. To avoid this, the compressor has a hi-pressure interlock switch to prevent this from happening.