The original question:
"does the A/C in the car use more energy when the fan is turned up higher?"
Assumptions:
The A/C System includes the compressor (and compressor motor), evaporator, all conduits of flow and the fan/motor that forces air across the evaporator.
Now let's begin
"the fan speed u set the AC on does not affect how much power the AC system consume's. " ~jolancer
Patently false, assuming you include the fan motor in the A/C system. Even if you do not, it affects the rate of heat transfer into the evaporator which consequently results in higher power consumption (which I will address in the next section).
As the fan speed increases, the torque exerted by the motor must increase (air resistance on the fan is proporitonal to the velocity^2 (linear) of the fan blades. This resistence must be countered by additional torque produced by the motor. This torque comes from - you guessed it - electrical power! As torque and RPM increase (they must increase together for the fan to go faster), the power consumed by the motor also increases. Any electrical engineering text will have the equations for a particular motor type (though they all follow this basic principle) so you can calculate the voltage and current yourself. Using Power = Voltage * Current, it can be shown that the power (power = rate of energy usage) will increase as the fan turns faster. To be precise, the fan turns faster because you supply it with more energy through a potentiometer connected to the fan controls, but this does not change the equations nor the behavior.
" actually if u think about it, the amound of heat transfered to the evaperator prolly doesnt afect it at all.
once heat is transfered to the low side, pressure risess, so the refrigerant is just that much closer to the psi that it needs to be compressed to.
so instead of having long intervals between cycle time, and long cycles... u would end up with shorter cycle times with short cycles. so there's litteraly no change at all."~jolancer
Actually if you think about it correctly, the amount of heat transferred to the evaporator does affect it directly.
For a refrigeration cycle, specifically the power cycle of the compressor (Ref. Fundamentals of Engineering Thermodynamics, 3rd ed.)
Work = Energy_out - Energy_in
Operating at steady-state, and using standard refrigeration cycle models, the inlet and outlet temperature of the compressor are approximately the same. As the temperature and pressure of the 'low side' increases, the work necessary to compress it increases as well. While the higher pressure on the low side *does* help the compressor in a sense, the additional energy required due to the higher temperature of the refrigerant being compressed is greater, still. The net result is actully more energy required as the temperature and pressure of the refrigerant from the evaporatior increases. Basically, energy_out grows faster than energy_in as temperature and pressure increase for a given compressor, thus more work is required.
"1. the amound of electrical current draw from a fan doesn't increas load on the engine, the alternator puts out more then enough to power anything and more around the car, and never usess more or less torque.
2. there are 2 things that for only god knows why, the ppl who look like thy know what there talking about, do not understand... A) the high and low side of an AC system work in relation to each other. B) contents under pressure works the oposite of atmospheric pressure... high atmospheric pressure is Cold... High line pressure is hot.
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HVAC quote- "In the case of a TXV, as the load on the evaporator increases, the valve opens to allow more liquid refrigerant to pass into the evaporator. In the case of the CCOT (orifice tube), the compressor cycles less as the load increases." [<--u ever heard of 'spam' with 'incorrect information']
ZeroNine8 quote- "Just because the compressor is not running any more or less often doesn't mean it's not doing more or less work when it is running. It takes less energy to compress colder refrigerant than it does to compress warmer refrigerant in the same compressor." [<--- try this then... suck air out of a jar that has so little air in it that its packing a vacume, then blow what little amount of air u got from it into a different jar that already has a lot of air, and see how easy that is to compress]" ~jolancer
I will address these in order
1) the question was not whether it increases load on the engine, the question was does the A/C use more power, and it does. The engine may indeed allocate more power than is used, but that is irrelevant to the power consumption of the A/C unit.
2) A) This is fine, though it is sufficiently vague to be meaningless B) This is actually 100% untrue. Everything follows the same basic laws, and the equations that model fluid behavior at atmospheric pressure are able to model fluid behavior at the pressures/temperatures that exist within an automotive A/C system just as well. The temperature variations you describe are not the work of pressure alone, there are many other factors (such as directly inputting heat into the A/C return system) that account for these phenomena. High atmospheric pressure (whatever that means) is cold because you are not heating it up through an evaporator, not because it is at high pressure.
HVAC quote: Your interpretation of this quote is wrong, you appear to confuse "...the compressor cycles less as the load increases" with something along the lines of "the compressor does less work as the load increases". By 'cycles less', they simply mean that it stays on more often and doesn't turn off to save energy.
Air & jar analogy: I'll do my best to understand your point - basically you seem to be saying that it is easier to compress less matter (little bit of air from vacuum jar) than it is to compress more matter (more air from non-vacuum jar). I agree completely, though fail to see where you were intending to go with it.
note- THIS IS MY "LAST" Post in this thread for me, blab all u wont... ~jolancer
I would recommend this, you look foolish enough as it is.
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