Why is Total Air Pressure Constant in the Study of Properties of Moist Air?

[ajay jadaun]

Why we assume total air pressure
(dry air +water vapour) to be constant during study of properties of moist air even when it is being heated or cooled how it is possible that pressure is constant , on what basis it is assumed?

I think locally there must be variation in total pressure also due to effect of heating , cooling ,humidification and dehumidification

Please someone clarify this in detail .

 

[Chestermiller]

Why we assume total air pressure
(dry air +water vapour) to be constant during study of properties of moist air even when it is being heated or cooled how it is possible that pressure is constant , on what basis it is assumed?

I think locally there must be variation in total pressure also due to effect of heating , cooling ,humidification and dehumidification

Please someone clarify this in detail .

Are you referring to a closed system?

 

[ajay jadaun]

@Chestermiller

Basically i am referring to open system as generally almost all thermodyanamic interaction to air is more or less in open system .
Kindly clarify further

 

[Chestermiller]

Why do you feel that the pressure is not constant, if the system is in open contact with the atmosphere?

 

[ajay jadaun]

@Chestermiller
There are two cases for my doubt
1. Since air is being heated or cooled , let's say by air conditioner coil in a room ,i feel locally there must be change in pressure .
2.For large centralised air conditioning systems air flows through a long duct isn't it a case of pressurised air i.e air is not in direct contact of atmosphere

 

[Chestermiller]

@Chestermiller
There are two cases for my doubt
1. Since air is being heated or cooled , let's say by air conditioner coil in a room ,i feel locally there must be change in pressure .
2.For large centralised air conditioning systems air flows through a long duct isn't it a case of pressurised air i.e air is not in direct contact of atmosphere

Flowing through a duct, there are going to be pressure variations. Heating and cooling in contact with the atmosphere, there are going to be small pressure variations, but, these can't be very large since pressure variations travel with the speed of sound, so they are rapidly compensated.

 

[ajay jadaun]

Flowing through a duct, there are going to be pressure variations. Heating and cooling in contact with the atmosphere, there are going to be small pressure variations, but, these can't be very large since pressure variations travel with the speed of sound, so they are rapidly compensated.

Thanks i got it