HVAC Systems and Operation Theory Stage 1

Pressure and Flow is controlled by

A metering device provides a restriction to contain the high pressure side and promote the pressure drop of the refrigerant.

high pressure relief valve

A pressure relief valve is located on the rear compressor head or compressor case. Under certain conditions, discharge refrigerant pressure exceeds the designed limit. For example, this may occur when other high pressure controlling devices have not controlled maximum pressures. This could cause catastrophic compressor system damage. To prevent damage, the pressure relief valve opens and releases pressure.

Basic Refrigeration Cycle

A/C system creates these special conditions by using pressure and heat transfer to control the changing states of liquid and vapor.

Cycling Clutch Orifice Tube

All Cycling Clutch Orifice Tube (CCOT) refrigerant systems use fixed displacement compressors and an orifice tube. CCOT systems cycle the compressor on and off to maintain the refrigeration cycle within predetermined limits. These systems also include a condenser, accumulator, evaporator, and various control components.

Condenser

Any obstruction that reduces or blocks airflow through the condenser will affect the efficiency of the condenser and the overall A/C refrigeration system performance. If air is diverted around the condenser because of missing seals or damage to the air dam, the A/C refrigeration system performance will also be reduced.

Scroll-Type Compressor Operation - Part 2

As the compressor shaft rotates and drives the moveable scroll, refrigerant vapor is forced against the fixed scroll toward the center axis. The space between the two scrolls decreases toward the center, increasing refrigerant pressure while heating the vapor. The high pressure refrigerant exits the compressor through the outlet port, located at the center of the fixed scroll.

R-1234yf Refrigerant Properties (Part 2)

At vapor pressures above 40ºC (104ºF), the R-1234yf trends slightly lower than R-134a. This property results in lower high side pressures being normal but producing a less efficient refrigeration cycle. In order to improve efficiency, an integrated heat exchanger is utilized to raise high side temperatures. The integrated heat exchanger is incorporated into the lines and transfers heat into the refrigerant vapor traveling into the suction side of the compressor. The integrated heat exchanger also lowers the temperature of the liquid refrigerant entering the thermostatic expansion valve.

Heat energy of an object is measured using the

British Thermal Units (BTU) measurement. One BTU is the amount of energy needed to raise the temperature of 1 pound of water 1° Fahrenheit (F) at sea level. Describes heat transfer

pressure sensor

Certain applications use a pressure sensor located on the high side of the refrigeration system. It is a three-wire transducer that is an engine control module input for the compressor clutch, cooling fans, idle speed control, and low and high pressure protection.

R-1234yf Refrigerant Properties (Part 1)

Comparison of the "Refrigerant Temperature to Pressure Relationship" data in the table shows that the vapor pressure of R-1234yf is slightly different than that of R-134a, at both low temperatures and at high temperatures.

Piston-Type Compressor Operation

Compressors differ in their design and displacement; however, in basic piston-type compressor operation, when the air conditioning system is activated, current travels through the compressor's coil. The magnetic field draws the clutch plate against the clutch's pulley, locking the pulley and clutch plate together and turning the compressor shaft. As the compressor shaft rotates, the swash plate rotates with it. Because it is attached to the shaft at an angle, the swash plate rotation moves the pistons back and forth. As the pistons move away from the intake port, low pressure refrigerant vapor is drawn into the compressor through the inlet valve. When the pistons move towards the intake port, it closes and the refrigerant pressurizes and exits the compressor through the outlet valve and port as a high pressure vapor.

Condenser Concerns

Condenser concerns are caused by leaks, refrigerant flow restriction, or restricted airflow across the condenser. Condenser flow restrictions may result in excessive compressor discharge pressure. Partial restriction may cause ice, frost, or a temperature change to occur immediately after the restriction. During normal condenser operation, the outlet pipe is slightly cooler than the inlet pipe. Restricting airflow across the condenser results in high-discharge pressure and poor passenger compartment cooling.

Factors that effect heat load include the

Coolant fan, condenser, blower motor, humidity, and Sun Load

three main configurations of refrigeration systems

Cycling Clutch Orifice Tube, Variable Displacement Orifice Tube, Variable Displacement Thermostatic Expansion Valve

TXV Failure

Depending on the failure mode, TXV failure is indicated by high- or low-discharge pressure and insufficient evaporator cooling. TXVs may fail due to clogging of the inlet screen caused by internal compressor failure, refrigerant contamination, corrosion, or loose desiccant material. TXVs may also stick in the inlet open or outlet open positions.

Evaporator Operation

During air conditioning system operation, liquid refrigerant flows from the TXV or orifice into the evaporator, creating a cool evaporator surface. As the passenger compartment air stream flows over the evaporator fins, the air transfers its heat to the cooler surfaces of the fins and into the refrigerant. When the liquid refrigerant absorbs enough heat, it changes states to a low pressure refrigerant vapor.

low pressure cycling switch

During the A/C refrigeration cycle, fixed displacement compressors start and stop automatically through control of the compressor clutch. This electromagnetic clutch is energized and de-energized by a pressure sensitive electric switch mounted in the low side of the refrigeration system. Cycling generally occurs between 137.9 and 275.8 kilopascals/20 and 40 pounds per square inch (psi), which effectively eliminates freezing temperatures within the evaporator when heat load/demand is low. The low pressure cycling switch is often located on the accumulator, although this is not always the case. The switch is electrically connected in series with the compressor clutch coil. The low pressure cycling switch is typically mounted on a Schrader valve low-side fitting. A Schrader valve is a spring-loaded valve that allows removal and installation without complete evacuation of the refrigeration system. Always consult Service Information (SI), to determine if a Schrader valve is utilized on the device being serviced.

Coolant Fan

Engine coolant fans are an important part of the air conditioning system. In addition to causing engine overheating, a nonfunctional cooling fan hinders the heat transfer processes taking place at the condenser and radiator. Insufficient heat transfer at the condenser causes the compressor to work too hard at compressing the refrigerant vapor. This, in turn, causes compressor head pressure, or high-side pressure, to rise to unacceptable levels.

Evaporator thermistor

Evaporator thermistors monitor evaporator temperatures. The sensor is typically connected to the A/C control system and is an input to request or deny control of the compressor clutch. When the evaporator temperature drops below approximately 1°C (34°F), the compressor shuts off, preventing the formation of frost and ice on the evaporator fins.

POE Oil

General Motors Two-Mode Hybrid and electric air conditioning systems use POE oil due to its electrical insulating properties. POE oil prevents loss of isolation, Diagnostic Trouble Codes (DTCs), and possible no-crank concerns.

PAG Oil

General Motors non-Hybrid R-134a systems use PAG oil. R-1234yf utilizes its own specific oil as well. When working on any refrigeration system, ensure there is no cross-contamination of refrigerant or oil

Internal Sensing Thermal Expansion Valve

General Motors refrigeration systems can utilize both internal and external sensing TXVs. Other than the location of the sensing device, both TXVs control the amount of refrigerant flowing in and out of the evaporator to maximize cooling and prevent evaporator freeze-up. The refrigerant in the sensing device provides pressure to the back side of the diaphragm. Refrigerant pressure on the outlet side of the evaporator and the superheat spring regulate the size of the orifice. The inlet and outlet of the TXV are physically connected, so when the inlet is closing, the outlet is opening and vice-versa. This promotes maximum heat transfer within the evaporator.

Electromagnetic Clutch

HVAC systems use an electromagnetic clutch to engage or disengage the compressor. An electromagnetic clutch connects and disconnects the compressor from the drive pulley. The clutch consists of a clutch hub, a pulley and rotor assembly, and a clutch field coil assembly. The clutch hub connects the drive pulley to the compressor shaft when the field coil is energized. The compressor clutch is either turned on or off according to the pressure on the low side of the A/C system.

Heat

Heat is a form of energy. The heat energy from the sun or any other source can cause a transfer of energy into a substance to increase its energy, and therefore the amount of heat

Condenser Action

High-side pressures are equally important in the refrigerant cycle in order to release the heat captured by the evaporator. The compressor builds pressure and increases the temperature of the refrigerant vapor prior to entering the condenser. The building of pressure raises refrigerant temperature to a much higher level than ambient air. Because heat travels from areas of higher to lower heat, energy is released by the refrigerant to the ambient air. This allows the refrigerant to change state as it releases latent heat. If pressure is too low, the refrigerant may not be able to release the heat, causing poor A/C performance. Too high of a pressure/temperature within the high side typically is due to inadequate cooling system operation or refrigerant contamination.

Condenser Operation

Hot, high pressure vapor enters the top of the condenser from the inlet. The hot vapor passes downward through the condenser coils, transferring heat to the coils and fins. The fins then transfer the heat into the air passing over them. The condenser is placed in front of the radiator, allowing it to receive maximum airflow from vehicle movement and coolant fan operation. As this heat transfer occurs, the hot, high pressure refrigerant vapor changes into a high-temperature, high pressure liquid. The high-temperature liquid travels from the outlet at the bottom of the condenser.

Humidity

Humidity is a measure of the water vapor present in air. When water vapor condenses on the evaporator, the heat of vaporization of the water is absorbed by the cooler evaporator surface. This heat is then absorbed by the refrigerant in the evaporator. This reduces the amount of heat that can be removed from the air. Therefore, on a humid day, the air is not cooled as efficiently as it is on a dry day.

Pressure Bleed Ports

In addition to a central discharge port, the fixed scroll has pressure bleed ports. A pressure-activated control valve, mounted in the channel plate directly below the fixed scroll, diverts some high pressure vapor from the bleed ports back to the suction side when heat load is low.

Compressor Oil Level

It is recommended that compressor oil level not be checked routinely, but it should be checked when there is evidence of a major loss of system oil. This may be caused by a broken refrigerant hose, hose fitting leak, a large compressor seal leak, or collision damage to system components.

Orifice Tube Failure

Orifice tube failure is indicated by high discharge pressure and insufficient evaporator cooling. Orifice tube failure is generally caused by a restriction, such as a clogged orifice tube inlet screen due to compressor particles, contamination, corrosion particles, or accumulator desiccant failure.

General Motors uses two types of oils

Polyalkylene Glycol (PAG) oil and Polyolester (POE) oil

Refrigeration Cycle

Refrigerant evaporates as it absorbs heat and condenses as it releases heat. As a result, it changes from a liquid to a vapor and back to a liquid as it flows through the A/C system.

Refrigeration System

Refrigeration systems are used to transfer heat from the air entering the passenger compartment to the refrigerant. The refrigerant is then cycled through the system, and heat is released into the ambient air.

The term used to describe the heat that promotes a change in temperature of a substance is referred to as

Sensible Heat

Sun Load

Sun load is the intensity of the long-wave heat rays from the sun. Ambient temperatures, together with the type and color of the interior and exterior materials, affect heat load, and thus, the efficiency of the A/C system.

temperature sensors

Temperature sensors are used by certain applications for enhanced control of compressor operation. For instance, if the engine coolant temperature is too high or the ambient air temperature is too low, the compressor clutch operation is inhibited.

Thermal Expansion Valve

The TXV is installed at the evaporator inlet and outlet fittings. The valve converts high pressure liquid refrigerant from the condenser to a low pressure liquid refrigerant by forcing it through a controlled port varying in size, based on the heat load/demand placed on the evaporator. When the heat load increases or decreases, the TXV supplies the correct quantity of refrigerant to the evaporator for maximum heat transfer.

Variable Displacement Thermostatic Expansion Valve

The Variable Displacement Thermostatic Expansion Valve (VDTXV) system uses a variable displacement compressor with a TXV. The compressor clutch remains engaged whenever the A/C mode is selected. The compressor varies its output and the TXV varies its metering to maintain the refrigeration cycle within predetermined limits. In addition to a compressor and TXV, VDTXV refrigerant systems typically include a condenser, receiver and dehydrator (receiver/dryer), and evaporator.

Accumulator Functions

The accumulator receives refrigerant vapor, as well as any remaining liquid refrigerant from the evaporator not used during the superheat phase of the refrigeration cycle. The accumulator separates the vapor from the liquid and oil, and then releases the vapor to the compressor. The accumulator tank houses a desiccant that acts as a refrigerant oil drying agent by absorbing moisture that enters the system. An oil bleed hole located near the bottom of the accumulator provides a path for the oil to return to the compressor. Lastly, it filters the refrigerant as it circulates throughout the system.

Air Distribution System

The air distribution subsystem of the HVAC assembly is a "blended air-mix" system that directs conditioned air through the outlets

Blower Motor

The blower motor affects evaporator heat transfer in the same way that the coolant fan affects condenser heat transfer. The blower motor speed controls airflow speed, which determines the volume of air flowing over the evaporator.

A/C Refrigerant Compressor

The compressor acts as the pump of the system, keeping refrigerant circulating during air conditioning operation. It draws in low pressure refrigerant vapor and compresses it into hot, high pressure vapor, which supports the release of latent heat from the condenser.

Scroll-Type Compressor Operation - Part 1

The compressor has a fixed displacement, but can control temperature by diverting some high pressure vapor back to the suction side during low heat and load conditions. This compressor has a different design. Instead of pistons and a swash or wobble plate, it contains two scrolls. One scroll is fixed and the other is moveable.

Condenser

The condenser releases heat out of the vapor coming out of the compressor, condensing the refrigerant into liquid.

Dual Stage Orifice Valve

The dual stage orifice valve consists of a solenoid driven plunger, which provides two possible sizes of restriction: energized position, 1.57 millimeters (mm) / 0.062 inches (in) de-energized position, 2.03 mm (0.080 in) In warm to hot ambient conditions, the valve allows the compressor to stay on by reducing compressor head pressure. The valve may also reduce compressor cycling under mild ambient conditions.

Engine Cooling System

The engine cooling system transfers heat from the internal combustion engine and supplies heat to the heater core.

Evaporator

The evaporator absorbs heat from the passenger compartment air into the refrigerant, vaporizing the refrigerant.

Evaporator

The evaporator cools and dehumidifies the air stream that enters the passenger compartment. The evaporator consists of aluminum fins that rapidly transfer heat to refrigerant-carrying coils.

Fixed displacement compressor

The fixed displacement compressor, also known as the cycling clutch compressor, contains pistons driven by a fixed position swash plate that rotates with the compressor shaft. Fixed displacement compressors meet A/C demand by cycling on and off according to the pressure of the low side of the refrigeration system.

Behavior of the Refrigerant Cycle

The heat transfer efficiency of automotive A/C systems is greatly affected by heat load, which is the amount of heat that must be absorbed by the refrigerant

What controls Compressor operation

The low pressure cycling switch, high pressure relief valve, Evaporator thermistor, pressure sensor, and temperature sensors

Metering Device

The metering device lowers the pressure of the liquid, thereby lowering the temperature of the refrigerant.

Air Dehumidification

The moisture or humidity present in the air stream condenses on the cool surfaces of the evaporator and drains off as water. A drain tube in the bottom of the evaporator housing leads the water outside the vehicle. This dehumidification of the air adds to passenger comfort.

R-134a Refrigerant Cycle

The normal boiling point of R-134a is -29.8°C (-21.6°F). It is a vapor at any temperature over -29.8°C (-21.6°F) in a non-pressurized system. Within the refrigerant cycle, R-134a refrigerant circulates and changes temperature through forced changes in pressures.R-134a continuously evaporates and condenses as it moves through the high and low pressure sides of the refrigeration system. Heat transfer (absorbing and releasing heat) occurs as a result of the continuous evaporation and condensation process.

Orifice Tube

The orifice tube provides a fixed restriction to the high pressure refrigerant in the liquid line. Filter screens protect both the inlet and outlet sides of the orifice from contamination. The orifice tube regulates the flow of refrigerant into the evaporator and is located in the refrigerant line between the condenser outlet and the evaporator inlet. The refrigerant flow rate is determined by the diameter of the orifice.

Receiver and Dehydrator Functions

The receiver and dehydrator acts as a storage tank, receiving liquid refrigerant from the condenser and holding it until it is required by the evaporator. It also acts as a protection agent for the refrigeration system. The portion of the receiver and dehydrator that contains desiccant absorbs moisture from the refrigerant, thereby protecting system components from harmful moisture. The receiver and dehydrator filters the refrigerant, thus preventing contaminants from damaging the TXV.

Receiver and Dehydrator

The receiver and dehydrator, previously called the receiver-drier, is located on the high pressure side of the refrigeration system and may be mounted on or next to the condenser. It may also be located anywhere before the TXV. The primary function of the receiver and dehydrator is to deliver clean, moisture-free refrigerant liquid and oil to the TXV. In this way, it helps protect the TXV from damage that may occur from foreign material or particles entering its intricate passages. The receiver and dehydrator, because of its location and operating characteristics, is used with variable TXV-type metering device systems.

Refrigerant

The refrigerant absorbs and releases heat as it changes state. Refrigerant also carries lubricant which extends compressor life.

Evaporator Action

The refrigerant flow is restricted prior to entering the evaporator. The restriction lowers pressure/temperature and forces the refrigerant to boil at a lower temperature. Because heat travels from areas of high to low heat, energy is absorbed by the refrigerant. In effect, this allows the refrigerant to change states as it absorbs latent heat from the air as it passes over the evaporator. If the pressure is too low, the refrigerant temperature may drop below 0°C (32°F), causing evaporator core "freeze-up". Too high of a pressure/temperature within the evaporator will cause reduced A/C performance as there is little capacity to transfer heat to the refrigerant.

Variable Displacement Compressor

This variable displacement compressor contains pistons and utilizes a wobble plate instead of a swash plate. The wobble plate's angle to the compressor shaft is variable, which allows piston travel to be altered between a long and short stroke. Compressor piston stroke length is controlled by the position of a bellows-actuated control valve that dictates pressure in the crankcase of the compressor.

Variable Displacement Orifice Tube

Variable Displacement Orifice Tube (VDOT) refrigerant systems use variable displacement compressors and an orifice tube. Either the variable displacement piston or scroll compressors are used. The compressor clutch remains engaged whenever the A/C mode is selected. The compressor varies its output to maintain the refrigeration cycle within predetermined limits. In addition to a compressor and orifice tube, VDOT refrigerant systems typically include a condenser, accumulator, and evaporator.

Inoperative Evaporators

When an evaporator is inoperative, it results in poor refrigeration performance or a low volume of cool air. This symptom can be due to dirt on the evaporator tubes and fins, a cracked case, or a leaking seal. Also, if the evaporator surfaces are too cold, the moisture collecting on the fins does not drain off as water. Instead, it freezes and restricts the air from the blower motor. Cracks or damage to the evaporator core result in a low refrigerant charge, as well as oil collecting on the exterior of the core.

A/C refrigeration systems operate by using

a "closed system" where heat is both transferred into and transferred out of the refrigerant.

Cooling systems include the

air intake condenser radiator heater core fan(s) seals and shroud

Air distribution systems are composed of these components

blower evaporator heater core temperature and air doors/valves air ducts control system

The compressor

builds pressure to concentrate the heat of the refrigerant vapor.

The condenser

consists of refrigerant tubing mounted in a series of thin cooling fins. Various tubing arrangements and refrigerant flow paths allow for maximum heat transfer within the available engine compartment space. The condenser liquefies the hot, high-pressure refrigerant vapor it receives from the compressor by transferring heat and condensing the vapor.

refrigerant systems use these components

evaporator accumulator/dryer compressor condenser restriction

(Metering Device) orifice tubes

fixed metering devices have openings of fixed diameters for metering refrigerant flow

The R-1234yf refrigerant system operates similarly to R-134a systems but

is NOT compatible with other refrigeration systems. This new refrigerant utilizes a different lubricant, which is also not compatible with other systems.

A bellows-actuated control valve

is located on the rear of the compressor. The control valve senses the suction (low-side) pressure to control crankcase pressure. This regulated pressure within the crankcase controls movement of the wobble plate and its angle, which determines piston stroke and the resulting compressor displacement.

Temperature

is the intensity level of heat

The accumulator

located near the evaporator outlet, acts as a storage container receiving liquid refrigerant, vapor, and oil from the evaporator. The primary function is to separate the vapor from the liquid and oil, then release the vapor to the compressor. In this way, it helps protect the compressor from damage that could occur from attempting to compress liquid. The accumulator, because of its location and operating characteristics, is used together with a fixed, orifice-type of refrigerant metering device.

Compressors rely on

oil-saturated refrigerant to lubricate their internal moving parts; therefore, it is imperative that the proper oil level, viscosity, and purity be maintained for correct system operation.

Rear A/C Systems

operate in conjunction with the front system, not independently. These systems receive high pressure liquid refrigerant from the main front A/C system. Connected in parallel, the refrigerant is metered into the rear evaporator as a low pressure liquid by a rear TXV. After absorbing heat in the rear evaporator, vaporized refrigerant is returned to the front refrigerant system.

The accumulator

receives vapor and some liquid carried in the refrigerant and oil coming from the evaporator. The accumulator then separates the vapor from the liquid, and then releases the vapor to the compressor.

Compressor lubrication begins when

refrigerant enters the compressor. Some oil droplets separate from the refrigerant and fall into the crankcase, lubricating the moving parts. The oil is then picked up by the exiting refrigerant and continues through the system. Some oil always remains in all refrigeration system components.

HVAC System includes

refrigeration engine cooling and air distribution systems

When A/C demand is high

suction pressure is above the control point. At this time, the compressor control valve maintains a pressure bleed from the crankcase to suction. With no crankcase-to-suction pressure differential, the wobble plate moves to the maximum angle, and the compressor operates at maximum displacement. During low A/C demand, suction pressure is low. This results in a high regulated pressure within the crankcase, moving the wobble plate to a minimum angle and reduced displacement.

Compressing a gas or vapor increases its

temperature

Latent heat is

the amount of energy needed to change an object from one state to another without changing the temperature of the matter.

(Metering Device) Thermostatic expansion valves

vary refrigerant flow based on evaporator outlet temperatures.