Defrosting and Demisting

[AastroGuru]

In automobiles having both AC and heater, what mode should be used for defrosting and demisting? AC or heater? Whether AC should be ON while Defrost/Demist? AC is dehumidifying the air. So i hope if fresh air is taken for defrost it should pass through evaporator first reducing its temperature and RH% and then passed through Heater increasing temperature (further decrease in RH) and then passed to the windshield. Is this right? Or only passing through heater will serve the purpose?

[nike123]

Both will serve the purpose for demisting, but with AC switched ON, it is much faster.
Defrosting is faster with AC switched OFF because air is hotter.

[AastroGuru]

Thanks nike123!
I need to acquire some knowledge in RH and Evaporation!! Can u help me!!

[nike123]

Thanks nike123!
I need to acquire some knowledge in RH and Evaporation!! Can u help me!!

You are on right place. I am sure that many members here will help, and many things are already written here, so start by searching this great forum and ask questions.

[AastroGuru]

The manufacturer says that for both defrost and demist AC should be off and Heater should be fully on. Is that right? I still feel AC should be ON in Demist and OFF in Defrost. The HVAC system is of Cooling and Dehumidification. Kindly suggest further what should be followed.

[Brian_UK]

Bear in mind that when defrosting the ambient air temperature is low and the compressor may not run due to the low limit thermostat.

Demisting with AC can only be done normally above 6-7°C ambient.

[AastroGuru]

Thanks a lot Brian_UK!!!

In fresh air mode, the cabin pressure lies some 100-120 Pa higher than OSA pressure. But in recirculation can u tell me what could be the scenario?

Like whether it will be negative below OSA pressure. Or it will be higher pressure but of lesser magnitude say 50 Pascals or something.

[Brian_UK]

In theory there will be no difference between the cabin pressure and the OSA pressure.

The fan obviously generates a differential pressure across itself but only sufficient to overcome the ducting resistances.