Cooling effect of evaporation occurs even if the surroundings are colder?

[John Constantine]

TL;DR Summary

Does the temperature reduction effect of evaporation always decrease the ambient temperature?

I was studying the principles of air conditioning and came across some questions. In an air conditioner, the refrigerant absorbs heat while evaporating indoors, thereby lowering the temperature, and releases heat while condensing outdoors, thereby raising the temperature. According to the second law of thermodynamics, heat can only move from a hotter place to a cooler place. When a substance absorbs heat energy from its surroundings and evaporates, it lowers the temperature of the surroundings, and the absorbed heat energy is used for the phase change.

Imagine a sealed room with a temperature of 10°C, and you spray water at 30°C in that room. Since water can evaporate at temperatures below 100°C if there is a vapor pressure and partial pressure difference in the atmosphere, it will absorb heat energy. If the water absorbs heat from the air in the room and evaporates, it seems to violate the second law of thermodynamics because it appears to absorb heat from a cooler place to a warmer place. In this case, will the room temperature decrease due to the heat absorption of evaporation, or will it increase due to thermal equilibrium?

In scenarios like this, if the refrigerant evaporates when the room temperature is lower, or if the refrigerant condenses when the external temperature is higher, how does the heat transfer occur?

 

[russ_watters]

Internal evaporation is adiabatic, so it does not violate the 2nd law. The energy content of the room does not change. You are simply exchanging latent heat of vaporization with the sensible heat of the air and water temperatures.

 

[Lnewqban]

.. In scenarios like this, if the refrigerant evaporates when the room temperature is lower, or if the refrigerant condenses when the external temperature is higher, how does the heat transfer occur?

Electricity and mechanical work are used to go against nature by forcing a flow of thermal energy from a cold source (room) to a hot one (atmosphere).

Without a working compressor and an expansion device, the refrigerant will naturally transfer heat from hotter outdoors to cooler indoors, just like a roof or an external wall do.

The air conditioning works on the principle of keeping the mean temperature of the working fluid (flowing liquid refrigerant) colder than the dry bulb temperature of the indoor air in the evaporator.

In that way, all the heat and humidity that penetrate the membrane of the room, and the ones that are generated inside, are somehow forced to flow through that evaporator (via recirculating fan).

Once that warm and humid mass of air hits the thin metal fins and walls containing the flowing liquid refrigerant, the thermal energy of the former naturally flows into the later, making it boil, process which carries that energy out of the room.

Simultaneously, some of the contained humidity in the cooled air naturally condensates and is carried away.

The heat of compression is added to that energy, which makes the temperature of the refrigerant greater than the mean temperature of the thin metal fins and walls containing the flowing refrigerant in the condenser.

Either cooler air or water, which is forced through those metal fins and tubes, will carry that heat into the atmosphere one way or another, by inducing condensation of the hotter refrigerant vapor.

The thermal energy of the hotter refrigerant vapor naturally flows into the chilled water or cooler air, making it condensate, process which carries that energy out of the condenser.

 

[John Constantine]

Internal evaporation is adiabatic, so it does not violate the 2nd law. The energy content of the room does not change. You are simply exchanging latent heat of vaporization with the sensible heat of the air and water temperatures.

Even if the temperature at which the refrigerant evaporates is higher than the temperature of the room, does the refrigerant still absorb heat from the relatively cooler room?

 

[John Constantine]

Thank you for your response. While I am not an expert in the process of air conditioning cooling a room, I have a basic understanding of it. What I am curious about is whether the temperature of the refrigerant evaporating indoors must necessarily be lower than the indoor air temperature, or whether the temperature of the refrigerant condensing outdoors must necessarily be higher than the outdoor temperature. If evaporation or condensation always makes the surrounding environment cooler or hotter, I am curious whether it is necessary to create a temperature difference between the refrigerant and the surrounding environment. (Of course, a lower temperature might be more efficient due to the transfer of sensible heat...)

 

[russ_watters]

Even if the temperature at which the refrigerant evaporates is higher than the temperature of the room, does the refrigerant still absorb heat from the relatively cooler room?

Yes, because the heat of vaporization of the water is much greater than the sensible heat of the water. The warmer water reduces the cooling effect of evapoation, but only a little.

 

[John Constantine]

Yes, because the heat of vaporization of the water is much greater than the sensible heat of the water. The warmer water reduces the cooling effect of evapoation, but only a little.

How is it possible for heat to move from a cold place to a hot place? Is this possible during a phase change? Isn't it necessary to obtain heat from a heat source that is always higher than the object undergoing the phase change?

 

[russ_watters]

How is it possible for heat to move from a cold place to a hot place?

The water cools (heat transfers from the water to the air) at first and then the steady state evaporation takes over and the water temperature is below the air temperature and heated by the air.

 

[John Constantine]

The water cools (heat transfers from the water to the air) at first and then the steady state evaporation takes over and the water temperature is below the air temperature and heated by the air.

If a sealed space is in thermal equilibrium, how can evaporation occur? Since the process of evaporation would require the absorption of heat energy from a heat source with a relatively higher temperature than the substance attempting to evaporate, wouldn't it be impossible for evaporation to occur if the surrounding temperature is uniform and there's no heat energy to absorb from the environment?

 

[Lnewqban]

What I am curious about is whether the temperature of the refrigerant evaporating indoors must necessarily be lower than the indoor air temperature, or whether the temperature of the refrigerant condensing outdoors must necessarily be higher than the outdoor temperature.

Yes and yes.
Unless a temperature difference is achieved, heat transfer will not happen.

Direct expansion coils have an average superficial temperature of 45°F, while chilled water evaporators have around 55°F.
Both are trying to cool air flow that is returning from the room at around 75°F.

Please, see:
https://en.wikipedia.org/wiki/Heat_transfer

https://theengineeringmindset.com/hvac-heat-exchangers-explained/

If evaporation or condensation always makes the surrounding environment cooler or hotter, I am curious whether it is necessary to create a temperature difference between the refrigerant and the surrounding environment. (Of course, a lower temperature might be more efficient due to the transfer of sensible heat...)

Both processes are hungry for thermal energy.
Although, both happen at constant temperature, an intense flow of energy happens.

Please, see:
https://en.wikipedia.org/wiki/Evaporation

https://en.wikipedia.org/wiki/Condensation

https://en.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle

 

[russ_watters]

If a sealed space is in thermal equilibrium, how can evaporation occur? Since the process of evaporation would require the absorption of heat energy from a heat source with a relatively higher temperature than the substance attempting to evaporate, wouldn't it be impossible for evaporation to occur if the surrounding temperature is uniform and there's no heat energy to absorb from the environment?

The water and air are not at equilibrium with each other. The water, after its initial cooling, is cooler than the air, so heat transfers from the air to the water.