Why do the glass doors at grocery stores become foggy when...

[mk9898]

Why do the glass doors at grocery stores become foggy when opened but are clear when closed?

My idea is that it has to do with the vapor and saturated pressure but I am at a loss to really nail down the physics behind it.

Any insight is appreciated.

 

[kuruman]

You are talking about doors to freezer units I presume. Yes, it has to do with saturated pressure. The % humidity in the store is much higher than the temperature in the freezer can sustain without condensation. When you open the freezer door, the water vapor in the store condenses on the inside of the door that is practically at freezer temperature. When you close the door, eventually the condensation disappears because the air in the freezer is continuously circulated and passed over a coolth exchanger (opposite of a heat exchanger) that condenses the water vapor in the freezer. The condensate is then either drained away or is passed through a heat exchanger that evaporates it and sends it back into the room or vents it to the outside. I am not sure about the details. In a household refrigerator, the condensate is evaporated and vented back into the kitchen (where else?)

 

[mk9898]

Thanks for the write up. I would like to break down the physics a bit more in detail if that's okay. So there is more water vapor in the store than there is in the "freezer". When we open the door conduction occurs from the warmer outside to the colder inside and hence they begin to try to form a temperature equilibrium (attempting to achieve saturated pressure). While the door is open warmer air comes in and makes the inside of the door warmer and hence condenses. Do I have that right?

 

[jrmichler]

Air has water vapor in it. The amount of water vapor can be expressed as absolute humidity, relative humidity, or dew point. The dew point is the highest temperature of a surface where water vapor from the air will condense on the surface.

The inside of a freezer door is cold. When you open the door, the warm humid air from the room contacts the inside of the door. When the door temperature is below the dew point of the room air, water vapor from the room air condenses on the cold surface. When you close the door, the dry air inside evaporates the condensation.

The physics is shown on a psychrometric chart.

 

[russ_watters]

It's the dew point: the temperature of the inner surface is lower than the dew point in the store.

 

[mk9898]

Ah got it. It's the dew point. Thanks!