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Air Conditioning TheoryThe Four Major Functions Of An A/C systemTo be effective, the dash air conditioner must control four (4) conditions within the vehicleinterior:It must cool the airIt must purify the airIt must circulate the airIt must dehumidify the airThese functions are essential if passenger comfort is to be maintained when the ambienttemperature and humidity are high.By performing these functions, the air conditioner maintains the body comfort of the d r i ve ra n d f ro n t passenger.ASHRAE has developed a comfort range that is the standard in HVAC commercial, residentialand automotive industries. In the summer, the comfort range is between 73⁰F (22.5⁰C) dry bulb(db) temperature and 79.5% relative humidity (rh) up to 81⁰F (27⁰C) db and 19.8% rh. In winter,it is between 67.1⁰F (19.5⁰C) db and 86.5% rh to 76⁰F (24.5⁰C) db and 23% rh.

Air Conditioning TheoryA/C System Basic OperationAir from either the interior of the vehicle (Recirculation Air Inlet Mode) or from the exterior of thevehicle (Fresh Air Inlet Mode) is sucked into the HVAC unit by an air flow moving device called ablower assembly.The air flow passes through a heat absorbing coil called an evaporator located within the HVACunit.The evaporator then transfers the heat from the air to a cool fluid medium called R134arefrigerant which is encapsulated within a plumbing network.The heated refrigerant is transferred by a pump or compressor into the engine compartmentwhere it then rejects this heat to exterior air flow traveling through a heat rejection coil called acondenser.This is a continuous process that occurs whenever the compressor is operating.

Air Conditioning TheoryHeat MeasurementFirst we need to understand thefundamentals of Heat.The amount of heat energy present in the air andrefrigerant is measured as the temperature.There are two different temperature scales mostcommonly used, Fahrenheit and Celsius.Heat is measured in British Thermal Units(BTU’s) and Calories.BTU – amount of heat energy required to raiseone pound of water one degree Fahrenheit.Calorie – amount of heat energy required toraise one gram of water one degree Celsius.Three Types Of HeatSensible Heat: When the heat that is applied to a substance merely raises its temperature, butdoes not change its physical state. It is the heat which, added to or subtracted from a substance,produces the changes in temperature indicated on a thermometer. (i.e. It is the heat that youfeel or sense)Latent Heat: The heat released or absorbed by a substance when it changes its physical stateto another with no change in temperature. (i.e. ice to liquid and liquid to vapor)There are two forms of latent heat:latent heat of fusion in the conversion of a liquid to a solid, or vice versa (i.e. The FreezingPoint – For water it is 32 F or 0 C).latent heat of vaporization in the conversion of a liquid to a vapor, or vice versa. (i.e. TheBoiling Point – For water it is 212 F or 100 C).The latent heat of vaporization phenomenon is the founding principle in refrigeration and airconditioning. It is known as THE COOLING EFFECT!

Air Conditioning TheoryUnderstanding Heat TransferHow does heat get inside a vehicle?When a car is driven or parked in the sun, heat enters the vehicle from many sources.These sources include:- Ambient air- Sunlight- Engine heat- Road heat- Transmission- Exhaust heatAll of these and other miscellaneous heat sources, increase the air temperature within thevehicle.In a high ambient temperature situation, (e.g. on a 99 ⁰F or 37⁰C day), the interior of avehicle left standing in the sun with windows closed could reach 150 - 158 ⁰F or 65-70⁰C!SunloadSunloadSunloadEngine HeatExhaust HeatRoad HeatExhaust Heat

Air Conditioning TheoryUnderstanding Heat Transfer (Continued)How does heat transfer work in an A/C system?Three processes of heat transfer:An air conditioning system’s efficiency is based onhow it moves heat.Heat Transfer is the method by which heat flows.Heat always travels from warm material to cold.The reverse is never true.For example, if a hot cup of coffee is left standing.it will cool off, while a cold soda will get warm.The heat from the warm coffee moves to the coolersurrounding air (i.e. condenser’s heat rejection).The heat from the surrounding air moves to thecooler soda, until a balance is reached (i.e.evaporator’s heat absorption).1. Radiation: Heat moves from a heat source to an objectby means of heat rays.For example, you feel heat from a fireplace, eventhough air is traveling past you and going up thechimney. You are warmed by radiated heat. (i.e.Engine compartment heat, body of the vehicle exposedto the sun, etc.)2. Convection: Heat flows in a stream of air or liquid thatis hotter than what it flows over, around, or through.For example, a blow dryer generates a stream ofheated air to dry hair. The hair is heated by convection.(i.e. The inlet air of the HVAC unit scrubbing thealuminum fins of the evaporator or vice versa with thecondenser)3. Conduction: Heat travels along a material.For example, if a spoon is left in a pot of boiling water,the spoon handle will get hot, even though the handle isoutside the pot. Heat is conducted along the spoonhandle. (i.e. The heat in the coil’s fins passing to therefrigerant passages & into the refrigerant)

Air Conditioning TheoryThe Added Value Of Latent Heat Transfer 6 TIMES MORE HEATTRANSFERRED THAN SENSIBLEHEAT.R134a refrigerant is about one tenth less in latent heat value but is used instead of waterbecause it boils at temperatures below the freezing point of water and at a higher pressure thanatmospheric pressure.It has the needed characteristics to boil at a low temperature and is able to change its statereadily from liquid to vapor, and vice versa.The A/C system creates the situation were the refrigerant is either evaporating orcondensing to provide the most efficient means of heat transfer.

Air Conditioning TheoryHeat Transfer Diagram 6 Times more heattransferred thansensible heat.Sensible Heat which isnot very efficient methodof heat transfer

Air Conditioning TheoryTemperature/Pressure RelationshipHow does the AC system create this phenomenon?Pressure / temperature relationship: As thepressure on a liquid is increased, the boiling pointrises. As the pressure on a liquid is decreased, theboiling point drops.In an air conditioning system, the refrigerant iscontained in a closed loop plumbing system that canbe pressurized.The pressure in the evaporator is low, so that all therefrigerant vaporizes. The pressure in thecondenser is high, so the refrigerant readilychanges state to a liquid.In an air conditioning system, a compressor is usedto increase the pressure of the refrigerant; thisraises its temperature. The refrigerant vaporentering the condenser is hot.In this air conditioning system, an expansion valveis used to lower the pressure of the refrigerant; therefrigerant in the evaporator is cold.Automotive A/C Systems are designed to operate atpressures that keep the refrigerant at the optimumtemperature for taking heat out of the passengercompartment.

Air Conditioning TheoryTypical A/C Operating ConditionsA/C system typically operates in a 50 Fto 110 F environmental range.In those given conditions the A/Coperating pressures range from 5 to 30psig on the evaporator side and 75 to325 psig on the condenser side.This correlates in refrigeranttemperature to -2 F to 35 F on theevaporator side and 73 F to 166 F onthe condenser side.In 110 F environment, the air flowingthrough the evaporator is 75 F hotterthan the refrigerant and it boils andbecomes a vapor.In 110 F environment, the air flowingthrough the condenser is 56 F coolerthan the refrigerant and it condensesand becomes a liquid.This picture shows the direction of refrigerant and engine coolant flow in the system. The air conditionerevaporator coil and condenser, and the heater core, are the main points of heat transfer.

Air Conditioning TheoryAir Conditioning System1. The compressor sucks in &compresses the cool R134a refrigerantgas, causing it to become hot, highpressure gas.2. This hot gas runs through thecondenser & dissipates its heat into itscooling air flow and condenses into aliquid.3. The high pressure liquid enters thereceiver/drier for storage and moistureremoval.4. The liquid is drawn off the bottom of thereceiver/drier and runs through apressure dividing, fixed size orifice holein the thermostatic expansion valve.5. A bulb containing R134a liquidrefrigerant controls the flow ofrefrigerant by using a diaphragm topush down or retract a pin. The pinpushes downward onto small, metalball plugging the orifice allowing liquidrefrigerant to enter the evaporator. Theball is cradled by an upward springforce to counter the pin’s downwardforce. Together, these forces willmodulate the refrigerant flow throughthe TXV.6. The low pressure liquid refrigeranttravels through the coil and evaporatesthus becoming cold, low pressure gaswhich absorbs heat from the hot airflowing through the coil.7. A small amount of lightweight oil ismixed in with the refrigerant to lubricatethe compressor.

Air Conditioning TheorySuper HeatAt a certain point in the evaporator the R134a refrigerant is completely vaporized, after that pointany additional heat absorbed by the R134a vapor is described as SUPER HEAT.The value of this SUPER HEAT is the temperature difference above the point at whichR134a liquid changes to a vapor. A proper Super Heat value is the insurance that vaporizedrefrigerant will enter the vapor compressor instead of liquid (i.e. Slugging).The thermal expansion valve (TXV) super heat setting is established at the factory for particularapplications. Ensure when a valve is replaced that it is of the type suited to the R134a A/Csystem.Saturation temperature The temperature at which refrigerant in liquid form changes to avapor at a given pressure. Saturation temperatures values can be derived from a R134aTemperature/Pressure Chart.Actual temperature The temperature of refrigerant at the evaporator outlet.Example - Calculation for Super HeatActual Refrigerant Temperature 40⁰FminusSaturated Refrigerant Temperature 29⁰FSuper Heat 11⁰F25 Psi40⁰FOutlet VaporTemperature25 Psi29⁰FSaturated LiquidTemperatureAir Flow

Air Conditioning TheoryR134a Temperature / Pressure ChartThe numbers above represent the boiling points for R134a

Air Conditioning TheorySub CoolingAt a certain point in the condenser the R134a refrigerant is completely condensed, after that pointany additional heat released by the R134a liquid is described as SUB COOLING.The value of this SUB COOLING is the temperature difference below the point at whichR134a vapor changes to a liquid. A proper Sub Cooling value is the insurance that liquidrefrigerant will enter the thermostatic expansion valve instead of vapor.Saturation temperature The temperature at which refrigerant in vapor form changes to a liquidat a given pressure. Saturation temperatures values can be derived from a R134aTemperature/Pressure Chart.Actual temperature The temperature of refrigerant at the condenser outlet.Example - Calculation for Sub CoolingSaturated Refrigerant Temperature 138⁰FminusOutlet Refrigerant Temperature 125⁰FSub Cooling 13⁰F225 Psi125⁰FOutlet LiquidTemperature225 Psi138⁰FSaturated VaporTemperature

Air Conditioning Theory Air Conditioning System 1. The compressor sucks in & compresses the cool R134a refrigerant gas, causing it to become hot, high pressure gas. 2. This hot gas runs through the condenser & dissipates its heat into its cooling air flow and condenses into a liquid. 3. The high pressure liquid enters the