I don’t fully understand this explanation . . . the evaporator coil (cold side) in AC should be designed to operate at a relatively constant temperature, typically ~40 degrees, not a differential from the outside temperature. Typical room A/C units allow for a setting down to 60 degrees, so on your explanation they would cool the coil to 25 degrees, which of course would lead to frosting/freezing. When I set a unit cooler, it means it runs more often - if the intake air is 90 degrees, the air will have to pass over the coils more frequently than if the air is 80 degrees in order to reach my set point of, say, 70 degrees. But that’s why AC runs more when the inside is hotter (or when it’s hotter out and more heat is being added to the inside of the house.
Andrew, great question! In fact, as you lower the temperature setpoint and drive down the cellar temperature, the coil temperature will drop too - leading to frost and removal of humidity.
For more information, take a look at the chart attached, which shows the balance point between the compressor capacity and the evaporator capacity at different entering air temperatures. The system operates where the lines intersect. As you can see, the saturated suction temperature is not constant - it balances out with the compressor operating curve based on the entering air temperature of the evaporator coil.
There are four distinct areas of design for cooling systems.
- A/C which is designed for latent (humidity) and sensible (temperature) cooling,
- High temp refrigeration which is designed for sensible cooling with minimal frosting (ie wine cellars)
- Medium temp refrigeration for sensible cooling with some frosting,
- Low temp refrigeration for sensible cooling with constant frosting.
Compressors and evaporator coils have different design features for each of these applications, and each given system is optimized for only one of these application areas.
Let me know if you have any additional questions!
Interesting stuff ben…
One more question for your experts:
When an AC starts to lose refrigerant, why is freezing more likely?
TTT
Refrigerant has a pressure temperature relationship as the pressure goes down (as in a leaky or undercharged system) so does the temperature. Ie. a coil in a freezer runs at a lower pressure than a coil in a refrigerator. Once the compressor no longer has enough gas to compress all capacity is lost.
PV=nRT
Paul,
Here’s the technical reply from our engineer department:
When refrigerant leaks out there is not enough refrigerant to feed a solid column of liquid to the thermal expansion valve (or other metering device) and it becomes a mixture of vapor and liquid. The vapor binds the orifice inside the expansion valve and does not allow as much flow through it, the effect is the same as if the valve closes down. This valve regulates the pressure change from the high pressure side to the low pressure side, so the more it is closed the lower the pressure becomes on the low side. Pressure and temperature move together, so a lower suction pressure means a lower coil temperature, and it can reach a point where it is below freezing. A symptom of this is that the bottom of the coil ices up because the limited amount of liquid let through stays at the bottom of the coil and evaporates, and the remaining vapor is superheated through the top of the coil.
Let me know if you have additional questions.
