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As an example, my 2004 Honda CRV has air conditioning that is controlled by three controls: on/off button, blower speed knob, temperature knob. The blower speed knob affects the speed of the blower, the temperature just affects a gate to the heater core (not 100% positive my CRV works that way, but my other cars did), and the on/off button turns on/off AC, assuming the blower speed is at least above 0.

Does the AC compressor or expansion valve actually change based on any of those? I imagine the combination of on/off and blower speed engage a clutch in the compressor that can only engage/disengage. So, is there any significant downside to turning the blower on max?

Or, maybe I'm wrong and there are gears or something in the compressor or maybe the expansion valve is electronically controlled.

Thanks!

4

AC load absolutely varies with blower speed.

On most modern vehicles (like your CRV, without an orifice tube) the amount of liquid refrigerant metered into the evaporator is controled by a Thermal Expansion Valve, or "TXV". The TXV takes it's cue from the temperature of the evaporator core.

Here's a great desciption of (stationary) TXV operation

Critical here is the pressure balance operation of the TXV:

TXV Pressure Balance EquationTXV

P1+P4 = P2+P3
P1 = Bulb Pressure (Opening Force)
P2 = Evaporator Pressure (Closing Force)
P3 = Superheat Spring Pressure (Closing Force)
P4 = Liquid Pressure (Opening Force)

enter image description here

Note that the "bulb" (P1) is a sealed system, with a refrigerant inside, but unrelated to the refrigerant in the rest of the system. The bulb is physically buried in the evaporator core, and reads the temperature of the core by thermal conductivity. The pressure inside the bulb rises as the core temperature increases. This pressure cooperates with the head pressure to open the TXV and allow more refrigerant into the evaoprator core.

Now, the temperature of the evaporator core rises proportionately as the temperature and flow of "hot" air across it. It's not completely linear, as the efficiency of any heat exchanger is usually flat only across a narrow range of secondary flow. Fast moving air just doesn't maintain enough "contact time" for heat exchange. Nevertheless, the overall amount of work done by the system is ultimately dictated by the temperature and flow across two heat exchangers, the evaporator and the condensor.

Another useful purpose of the TXV is to prevent evaporator icing, which has a runaway effect if left unchecked. Ice crystals block the airflow, less heat is absorbed into the evaporator, more ice is formed, less airflow ... The TXV bulb will radically drop in pressure as the evap temperature approaches freezing, and cause the TXV to completely shut off liquid refrigerant flow into the evaporator to prevent this condition.

While I agree that most compressors (like your '04 CRV) are basically on/off devices, this does not describe the entire picture of energy, work, and heat conservation. Also, I am NOT suggesting that the TXV is used for cabin temperature control. Other posters have correctly mentioned this is almost always accomplished with the "blend door" mixing in heat with the post evaporator flow.

In fact, some compressors [very expensive German ones] use a variable swash plate that can change the stroke (displacement) of the compressor on the fly. Controlled by the ECU usually, by a multitude of inputs such as engine RPM, AC load, road speed (condensor airflow), emissions, idle, engine load (WOT?), and fuel efficiency goals. In this case, the compressor changes how much "compressinating" it needs based on load control from the ECU. These systems, while uber-trick, probably cost more than their efficiency gains.

In any case:
- High hot evaporator airflow raises evaporator temperature
- Evaporator temperature expands the magic juice in the TXV bulb
- TXV bulb pressure opens liquid refrigerant flow into the evaporator
- Refrigerant expasion in the evaoprator removes heat from the airflow
- (Repeat)

  • i didnt even know a txv valve existed until this answer; thanks! so does the txv valve actually have a way of stopping the compressor in a run-of-the-mill car like mine? dont get me wrong though. this answer was extremely informative, but id love to know more! it basically sounds like this txv valve regulates the two sides of the ac system (in the way i used to think the expansion valve did), but the load on the engine would be basically the same unless it has a way to disengage the compressor, right? – tau Jul 15 '16 at 2:13
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    @tau The TXV has no direct control of the compressor. The compressor will "compressinate" as long as the clutch is engaged. If the TXV is completely closed shop, the high side pressure will rise until the high limit switch breaks open and disengages the clutch. The system is actually quite crude, but nevertheless workable for the last 5 decades. – SteveRacer Jul 15 '16 at 3:01
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From my understanding, the occupants cannot control the actual pressures (and thus temperatures) produced my the AC system, nor can the occupants change the state of the expansion valve.

It is my understanding that auto AC systems (which I have little experience with) work similarly to commercial building HVAC (which I have a bit of experience with). The main similarity being that it is a Boolean systems. On or off, nothing else. I know that the compressor can change because of RPM of the motor, but it will engage/disengage if the pressure created is out of range.

From what I've read, normal expansion valves are only affected by the ratio of internal temp/pressure to external temp/pressure.

That said, the only downside to running your blower at max would be that you may get cold. Technically you do lose some of the motors power to the AC compressor when AC is on, but that is a constant loss, and is quite negligible.

UPDATE

One thing I forgot is the temperature knob or climate control. If this is like residential and commercial systems, it simply sets a target temperature to maintain. So the actual compressor and heater coil gate will be activated or deactivated as is needed to get that temperature. This is usually monitored by an indoor sensor as far as I know. Anyone with more specific knowledge in this aspect is welcome to amend this statement.

Point being, with regard to

Dies AC usage change...

Yes. But in an on/off fashion. Not in terms of strength or degree of cooling. That part is constant.

  • thanks for explaining your experience with commercial hvac systems because i am also interested in and do some residential work. – tau Jul 14 '16 at 22:45
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    No problem! :) I personally think commercial and residential systems are simpler since they're a static system (meaning they don't vary in too much with outdoor temps, line pressure and compressor speeds). But if you understand residential HVAC, it's not a big jump to understanding auto as well. ;) – kyle_engineer Jul 14 '16 at 22:51

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