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Message: [QUOTE="Lethalthreat7.3, post: 690668, member: 10651"] Useage of the a.c compressor happens more than when it is set to "a.c". The a.c is used to dehumidify when the vehicle is put in defrost. Try turning the defrost on and you will hear the clutch cycle on/off. Horsepower usage is dependent on load and can range from approx 2hp-20hp. Load is dependent on compressor displacement, cab temp and outside air temp. Refrigerant absorbs the cab interior heat and is rejected by the condenser. A.C does NOT blow cold air. The refrigerant flowing through the evaporator absorbs the heat from the air inside the cab and is carried outside of the cab and rejected. An A.C system works like this: The compressor compresses refrigerant vapor. Compressing makes heat so the refrigerant is a hot gas leaving the compressor in route to the condenser. As air flows through the condenser the refrigerant condenses from a HOT VAPOR to a HOT LIQUID. The hot liquid refrigerant travels from the condenser to the accumulator. The accumulator is a reservoir for the hot liquid to accumulate and assure that a full liquid is available to allow the change of state to a COLD LIQUID. The refrigerant's change of state occurs when the HOT LIQUID is forced through the metering device, capillary tube or txv(thermo-expansion valve). The COLD LIQUID fills and flows through the evaporator and absorbs the heat in the cab. As heat is absorbed in the evaporator it causes the refrigerant to change state to a COLD VAPOR. The last coil pass is where the COLD LIQUID TO COLD VAPOR state change occurs and is critical on the proper efficiency and a.c operation. Too little refrigerant and the evap will not be flooded, poor cooling(absorption) in the cab as well as to the compressor. Too much refrigerant and "slugging" will occur, you can't compress liquid. I needed to explain the cycle so that the "load" can be understood. When the cab is say 100*, O.A is 90* and the vehicle is just idling or has minimal airflow pressures can be over 300+psig discharge/48+psig return. Hp usage can be close to 20. With the cab at say 70*, O.A still at 90* but cruising at say 75 the pressures can be 225/35 and a much shorter cycle/on time and longer "off" time. The compressor cycles on/off via the low pressure switch, lower load=shorter cycle. Not only is the cycle shorter but the load requires less hp because the pressures are lower. The hp required can be as low as 10% at approx 2hp. Rejecting the heat is extremely critical. Not only does the efficiency drop but pressures obviously rise and component life is shortened from bent fins and bugs in the condenser. The condenser is first in the barrage of objects blowing through the grill, then the intercooler and radiator. You very rarely if ever can see a temperature rise in the vehicles coolant but install a set of gauges on the a.c and you can see over a 50+psig rise on the discharge, load=fuel. The #1 reason for A.C failures in residential and commercial systems is HEAT!( Yes bad power(brown-outs, surges and single phasing) is #2). Most residential compressor failures(if the system is initially installed correctly) are caused by heat due to the condenser being plugged full of grass, leaves and dirt. Not only can you help prevent component failures but you can save up to approx 25% on your utility bill by cleaning the condenser out every time you mow the grass. You can save 3% to 7% for every degree you lower the thermostat. Keeping the fins straight and debris/bugs out of the condenser does the same for a vehicles a.c efficiency. Running down the road at highway speeds with the windows down does cause a large aerodynamic drag and does cost more in fuel mileage. If you want to know approx hp needed to operate your various components, a.c, alternator, ect..., manufacturers will usually list the wattage required to power the component. Divide the wattage required by 745.7 to obtain the hp requirement. Example for alternator. Amps x Volts = Watts. 100a x 14.2v = 1419.999w =1.902hp. So 1420 watts requires approx 2hp. This is however a formula and pulley size, bearing wear, belts, tensioner and anything else that causes unforeseen drag can change this. Personally, 2hp or even 5hp does not make a difference on a large diesel motor and is very minimal on smaller ones. Gas engines are different when your little chipmunks are screaming to give you the 150hp they are supposedly rated for using the large hill and tail wind. Lol. One of my boy's econo-cars almost feels like a tiny nos shot when the a.c is shutoff under wot. Turn it back on and I swear a parachute was released. LOL! [/QUOTE]