[DHABHAR.BEHRAM]

Dear Pratheesh - if the compressor is not on, humidity causes the glasses to frost over.

best regards,

Behram Dhabhar

[Aroy]

Most of the AC units in cars are coupled directly to the engine, and can either be connected or disconnected (I have read that some advanced units have variable vane geometry and vary the refrigerant circulated to maintain constant temperature at varying load, but I am not considering those), hence
. They present a constant load to the engine
. At any time, be it at stand still or at 100+, the AC is consuming power. The proportion of power used by AC to that used for propulsion is more at low speeds than at high speeds (Infinite at traffic lights!)
. Like room AC, increasing the temperature, will switch the AC off more often, reducing fuel consumption. Decreasing the temparature will similarly increase the fuel consumption.
. The fan control just varies the fan speed and not AC compressor.

Every electrical device is ultimately powered by your car engine (the alternator that charges the battery or accessories is powered by the engine), hence the AC is in fact burning specious petrol/diesel, and so is the fan, head light, the music system.

In older car AC; say M800; the AC and heater switches were not coupled. So you could be running AC and the heater simultaneously. I hope that in a climate controlled AC there is enough intelligence in the system to prevent this. You have to check it out your self in your car.

[SPIKE ARRESTOR]

Sir, got your point. How does this happen, there must be a mixing provision for this in the heating mode also? Also how does the compressor help? (How is the humidity controlled etc.)

Spike

[Aroy]

Originally Posted by DHABHAR.BEHRAM

Dear Pratheesh - if the compressor is not on, humidity causes the glasses to frost over.

best regards,

Behram Dhabhar

Who will win in this case the AC or the Heater?

[Sutripta]

Originally Posted by SPIKE ARRESTOR

Hi,

^^ I was thinking why the compressor needs to be on when the heater is switched off? Can a set up be made where compressor is not required? Or is there any specific reason for the compressor to come on.

Spike

As mentioned, dehumidification.

In my Gypsy with factory fitted AC, it did not have a heater, but if you moved the lever towards the red zone, it would cut of the compressor. Its all a matter of system design. Seemingly trivial details. Extremely difficult to cater to everyone/ all situations.

Regards
Sutripta

[dhanushs]

Originally Posted by SPIKE ARRESTOR

Sir, got your point. How does this happen, there must be a mixing provision for this in the heating mode also? Also how does the compressor help? (How is the humidity controlled etc.)

AFAIK, the evaporator coil, is where the humidity control takes place. The water vapor in the air is condensed and drained off.

This air, is further passed passed on directly if the user needs only cold air, if hot air requirement is there, the air is diverted (by a diverted valve), so that it takes up some heat from the engine coolant, and further lead into the passenger cabin. Hence the hot air too, is not humid, and also, this makes the AC compressor to be ON always a requirement in design.


P.S: This is just based on experience. Please correct if required.

[oxyzen]

Originally Posted by jalsa777

The Cool/Hot switch controls whether the wir wil be taken only from compressor OR only from heating coil OR mixture of both

Suppose I choose a setting which has the temp selection somewhere in the middle. Now why mix cold air from compressor & hot air from radiator to get normal temperature. This is a waste of energy.

I have heard manual AC also has a thermostat in the evaporator. So in-case you use less air flow (through the evaporator), the evaporator coil will cool too much & trip. Can anyone verify it.

[dhanushs]

Originally Posted by oxyzen

Suppose I choose a setting which has the temp selection somewhere in the middle. Now why mix cold air from compressor & hot air from radiator to get normal temperature. This is a waste of energy.

As mentioned before, de-humidification.

And cold air, is not really from the compressor.

[SPIKE ARRESTOR]

Originally Posted by Sutripta

As mentioned, dehumidification.

In my Gypsy with factory fitted AC, it did not have a heater, but if you moved the lever towards the red zone, it would cut of the compressor. Its all a matter of system design.

Turning the switch to red zone means you need less cooling right (as there is no heater) ? I am asking how dehumdification process happens when there is cold air / hot air requirement.

Originally Posted by dhanushs

AFAIK, the evaporator coil, is where the humidity control takes place. ----

Yes, you are right, all this happens inside the Evaporator unit. I am more concerned of how and why? Strangely, I never gave this a thought anytime.

Spike

[SirAlec]

Originally Posted by mayankjha1806

Increasing or decreasing the speed does not increase or decrease the load on the engine, it only increases or decreases the speed of the Fan. On the other hand increasing or decreasing the temperature does impact the load on the engine as the AC compressor has to work that much harder/realized depending on the temperature setting in the car.

You can feel this while driving and changing the settings and feeling the drag on the car.

I agree to your fan speed quote but little bit of correction, Ac compressor does not work harder. It works on the same speed all the time. The catch is with thermostat and expansion valve. which cuts off the compressor after attaining a particular temperature. and comes back on when the temperature increases.

[dhanushs]

Originally Posted by SPIKE ARRESTOR

I am asking how dehumdification process happens when there is cold air / hot air requirement...

... I am more concerned of how and why? Strangely, I never gave this a thought anytime.

De-humidification in Auto-AC is mainly done due to two reasons.
1. Prevent misting of the windsheild.
2. People usually feel more comfortable and less tired in less humid environment, as it constantly evaporates what we perspire, hence keeping the skin surface cool, clean and non-sticky (a.k.a comfortable).

About how they dehumidify in a laymans language..

The water vapor in the air drawn from the cabin when passed through the evaporator coil condenses to water. This continuous process, gradually reduces the water vapor in the cabin air.

[SPIKE ARRESTOR]

^^This is agreed, my next question was how. How is the humidity maintained at a "comfortable" level. Also are the conditions for "comfort" same in heating and cooling mode?

Spike

[jalsa777]

Originally Posted by SPIKE ARRESTOR

^^This is agreed, my next question was how. How is the humidity maintained at a "comfortable" level. Also are the conditions for "comfort" same in heating and cooling mode?

Spike

There is no constant level.

Some theory:
The amount of water air can hold is dependent on the air's temperature.
The higher the temperature, more water it can hold.

So, when temperature of air is reduced, its ability to hold water vapour also reduces. If the temperature is reduced to such a low level that the holding capacity of air reduces to below the currect level of water vapour, the excess water vapour is converted to water. (Dew point)

Hence, we always see water coming out of ACs. Because air is cooled to below its dew point there.

This is how demudification happens. And it will keep happening untill the dew point of air is low enough.

[dhanushs]

Originally Posted by SPIKE ARRESTOR

How is the humidity maintained at a "comfortable" level. Also are the conditions for "comfort" same in heating and cooling mode?

AFAIK, the De-humidifidation that happens in cars below 10L is just a by-product of the AC, which is used productively.

Also, humidity is inversely proportional to the temperature to which the air is cooled. This.. is controlled by thermostats and/or refrigerant pressure sensors. The cut off value is determined considering a comfortable level.

IMO, this is the only control over humidity. And, yes, the level of humidity ought to be the same in cold and hot air, if the compressor is running.

For more expensive HVAC systems, after the evaporator coil there might be a separate heater or condenser which further aids to control humidity. but, you see.. I'm only speculating, and not sure.

P.S: OT: btw, whats with the signature? Is this peace?

P.P.S: I think AC dehumidification discussion posts need to be moved to sperate thread.

[mayankjha1806]

Originally Posted by SirAlec

I agree to your fan speed quote but little bit of correction, Ac compressor does not work harder. It works on the same speed all the time. The catch is with thermostat and expansion valve. which cuts off the compressor after attaining a particular temperature. and comes back on when the temperature increases.

Thanks, a question.

Wouldn't this be applicable on a Climate control AC only and not a normal one? My understanding is in a normal AC it does not sense the temperature inside and keeps blowing cold air without cutting it off ever. Is my understanding wrong?

As per the original post the question was around normal AC and not climate control AC.