An air-conditioner is a generic term for a unit which maintains air within a given space at a comfortable temperature and humidity. To achieve this, an air-conditioning unit must have a heater, cooler, moisture controller and a ventilator.
The principle of an HVAC system:
● heater – adding heat by transferring it;
● cooler – removing heat by transferring it;
● humidity – removing or adding moisture;
● purification – by filtration;
● ventilation – air movement through the vehicle.
The HVAC system creates a comfort zone for the occupants which can be adjusted within a range. Not all humans desire exactly the same environment, variations occur due to different continents, countries, cultures, gender, age or simply due to the type/amount of clothing being worn when inside the vehicle. The system must provide a way of controlling the climate inside the vehicle which is generally referred to as ‘climate control’.
Heat
Heat is a basic form of kinetic energy which cannot be destroyed; it can only be converted to or from other forms of energy. In accordance with scientific laws all heat will travel from a hot to a cold surface until the temperatures have equalised. The rate at which heat will travel is dependent upon the difference in temperature between a hot (more energetic molecular movement) and cold (less energetic molecular movement) area. The SI unit for heat energy is the joule. Other units include calories and BTU (British Thermal Units). The effects of heat energy are measured using temperature.
Figure 1.38
Heat intensity
The SI unit for heat intensity is the kelvin, the derived unit is Celsius or Fahrenheit.The Kelvin scale is a theoretical scale based on the laws of thermodynamics using absolute zero as the start of its scale instead of zero as used with °C. Scientists state that a temperature called ‘absolute zero’ is the point at which all heat is removed from an object or substance (a complete absence of molecular movement). The intensity of heat can be measured using a thermometer. This only gives the heat intensity of a substance and not the heat quantity.
Sensible heat
Heat which causes a change in temperature is called sensible heat. As previously stated it can
be ‘sensed’ using a thermometer or pyrometer. Theory tells us that by adding heat to a liquid
such as water there will be a proportional increase in its temperature which can be measured
on a thermometer scale, e.g.:
The amount of heat required to raise the temperature of 1 kg of water by 1°C is 4.2 kJ.
For example:
420 kJ of heat must be applied to 1 kg of water at 0°C to bring it up to the boiling point of
100°C.
Conversely, the same amount of heat must be removed from the boiling water to cool it down
to freezing point.
Alternatively:
1 BTU heat quantity changes the temperature of 1 lb water by 1°F
Specific heat capacity
Different substances absorb different amounts of heat to cause the same increase in temperature. Specific heat capacity is used to measure the amount of heat required to cause a change
in the temperature. The basic unit for specific heat capacity is the joule per kilogram kelvin
(J/kg K). Different materials have different specific heat capacity values.
Latent heat and a change of state
Latent heat (hidden heat) is the heat energy required to change the state of a substance without changing its temperature. Heat can have a direct effect on substances when they change
state. Evaporation is the term used when enough heat is absorbed by a substance to cause it to
change into a vapour. Condensation is when enough heat is removed from a vapour causing it
to change to a liquid. When a change of state occurs a great deal of energy is either absorbed
or released. This is called the latent heat of vaporisation and the latent heat of condensation.
Example:
2260 kg of latent heat is absorbed when 2 kg of water at 100°C changes state from a liquid to
a vapour.
970 BTU of latent heat is absorbed when 1 lb of water at 212°F changes state from a liquid to
a vapour.
This will occur without any change in the thermometer reading.
If ice is heated it will reach 0°C (32°F) then start to melt.This means you will have liquid and a solid existing together. The more heat you add the more the ice will melt with no increase in temperature until no ice exists, then the water will start to increase in temperature (sensible heat). When water reaches 100°C (212°F), any more heat energy added will result in some or all the liquid changing into a vapour. The amount of vapour produced depends on the heat energy available and the pressure above it.
When a substance changes state it can absorb hundreds of times more energy than when it is just increasing in heat intensity.This latent heat is the common process used to transfer heat from the interior of the vehicle to the exterior.The key to successful cooling is to get the liquid to the point where it wants to change state (evaporate) at the right point within the A/C system. To do this we manipulate the system pressure.
The function of the heating system is to increase or reduce heat input to the inside of the vehicle. A typical automotive combustion engine is only about 30% efficient, which means only 30% of the fuel delivered to the combustion engine is converted into usable energy. Some of this energy is transferred to the cooling system which is where the heating system will obtain its heat source for the inside of the vehicle.The rest is lost through exhaust gases and radiation. Heat can be transferred using one or a combination of three processes – conduction, convection or radiation.
Conduction, convection and radiation
Conduction
Figure 1.40 illustrates the direct transmission of heat by conduction within a substance, for example, if heat is applied to one end of a steel bar, the other end will eventually increase in
Figure 1.39 1.40
temprature. Some materials are excellent conductors of heat – aluminium, copper – and other
materials act more like insulators – polymers (plastics).
Convection
Convection is heat movement through a medium like a liquid in a saucepan (Fig. 1.41). Convection is a continuous movement of the medium and heat. The medium, liquid or gas, moves
and releases heat to the surrounding areas. When heating occurs in a liquid or gas, expansion
occurs and parts of the substance become lighter than other parts which contain less heat.
Natural convection currents occur in any substance which is not heated evenly.
Radiation
Heat can travel through heat rays and pass from one location to another without warming the air
they travel through (Fig. 1.42). An example is ultraviolet radiation which travels from the sun.
Radiant heat can travel from a warmer object like the sun to a cooler object like the earth’s surface.The surface colour and texture affect the heat emitted and absorbed. Colour is not as important as texture; dark rough surfaces make better heat collectors than light smooth surfaces.
Figure 1.41 1.42
From the engine to the interior
Convection occurs when material, such as an engine, passes heat to the cooling system of the vehicle. As the potential energy of the fuel is converted to mechanical and heat energy by the engine combustion process, the heat of the engine must be removed. The liquid in the cooling system is pumped through the engine, and the convection process transfers engine heat to the liquid. The cooling system liquid then takes this heated coolant to the radiator.The metal heat exchanger uses the conduction process to remove the heat from the liquid coolant and to the exchanger fins.The radiator fins then pass the heat of the radiator to the passing air flow through the heat exchanger.
Enthalpy
Enthalpy is the measure of the usable energy content of a substance. When a liquid increases
in temperature it also increases in enthalpy. When the liquid changes into a vapour through
Table 1.1
latent heat of evaporation it does not increase in temperature but does increase in enthalpy
because the energy within the substance increases.
Pressure
Pressure is defined as the force exerted on a unit area by a solid, liquid or gas.The SI unit used
to indicate pressure is the pascal (Pa). Other units of pressure are pounds per square inch (psi),
kilograms of force per centimetre squared (kgf/cm2), atmospheres (atm) and millimetres of
mercury (mmHg). Atmospheric pressure at sea level is 101.325 kPa (14.6 psi). This is generally
shown on a gauge as zero (gauge pressure) unless it is a gauge which measures atmospheric
pressure. This means that absolute pressure is atmospheric pressure plus gauge pressure.
Previously it was discussed that water changes state at 100°C. This is if the liquid is at sea
level under atmospheric pressure. If you reduce the pressure on the liquid by moving it above
sea level or apply a vacuum to it then the boiling point is lowered. If you increase the pressure
applied to the liquid by moving it below sea level or specifically applying a pressure on it, then
the boiling point is raised.
Table 1.1 shows that if a deep vacuum (1 bar vacuum) is created in a closed system like an
A/C system water will boil at 10°C (50°F). This enables technicians to remove moisture from
the A/C system using a vacuum pump (refer to Chapter 5).The chart also presents the importance of creating a deep enough vacuum under cold ambient conditions.
A pressurised liquid
Consider Figure 1.43. If we place a liquid that will readily evaporate at atmospheric pressure
inside the box but close the tap, the pressure will build up and increase the boiling point of the
liquid (e.g. at a pressure of 5 bar water only boils at 152°C). When the tap is open the pressure
will suddenly decrease and the liquid will readily evaporate lowering the temperature (by
absorbing the heat inside the box)
Critical temperature and pressure
There is a critical temperature which is the maximum point at which a gas can be condensed
and a liquid can be vaporised by raising the pressure. Refrigerants R134a and R12 have critical
Fiure 1.43
pressures and temperatures that dictate the maximum pressure/temperature they can be subjected to. If an air-conditioning system operates below the critical points of its refrigerant then
they are termed subcritical systems. If they exceed their critical pressure/temperatures then
they are termed transcritical systems.
Refrigerants
Refrigerants are the working fluids of the A/C system. An ideal refrigerant would have the
following properties:
1. Zero ozone depleting potential and zero global warming potential.
2. Low boiling point.
3. High critical pressure and temperature point.
4. Miscible with oil and remain chemically stable.
5. Non-toxic, non-flammable.
6. Non-corrosive to metal, rubber, plastics.
7. Cheap to produce, use and dispose.
Refrigerant CFC12 – dichlorodifluoromethane
R12 is a CFC (Chloro Fluoro Carbon).The refrigerant consists of chlorine, fluorine and carbon,
and has the chemical symbol CCL2F2. It was used for many years from the early development of
A/C systems up to the mid-1990s when it was progressively phased out leading to a total ban on
1 January 2001 due to its properties which deplete the ozone and contribute to global warming
(see Chapter 6, section 6.1).A benefit of R12, when it was originally designed, was its ability to
withstand high pressures and temperatures (critical temperature and pressure point) without
deteriorating compared to other refrigerants that were around at that time. R12 mixes well with
mineral oil which circulates around an A/C system. It is non-toxic in small quantities although
it does displace oxygen and is odourless in concentrations of less than 20%. R12 can also be
clean/recycled.You must not burn/heat R12 to a high temperature (300°C) with a naked flame
because a chemical reaction takes place and phosgene gas is produced. A lethal concentration
of phosgene is 0.004% per volume.
R12 properties:
1. It is miscible with mineral oils.
2. It does not attack metals or rubber.
3. It is not explosive.
4. It is odourless (in concentrations of less than 20%).
5. It is not toxic (except in contact with naked flames or hot surfaces).
6. It readily absorbs moisture.
7. It is an environmentally harmful CFC gas (containing chlorine which destroys the
atmospheric ozone layer).
8. It is heavier than air when gaseous, hence the danger of suffocation.
Refrigerant HFC134a – tetrafluoroethane
R134a is a known substitute for R12. R134a is an HFC (Hydro Fluoro Carbon). The refrigerant consists of hydrogen, fluorine and carbon, and its chemical symbol is CH2FCF3. Because the refrigerant has no chlorine it does not deplete the ozone. R134a is non-toxic, non-corrosive and does contribute to global warming; it is not miscible with mineral oil so synthetic oil, called PAG (Poly Alkaline Glycol), was developed. PAG oil is hygroscopic and absorbs moisture rapidly which means when in use you must ensure that the container is resealed as quickly as possible. R134a cannot be mixed with R12 and is not quite as efficient at high pressures and temperatures. R134a can be cleaned and recycled.
R12 and R134a have different size molecules, the R12 molecule is larger. This means the quantity of refrigerant required for an R134a system is higher than an R12. This also requires the flexible hoses and seals including oil to be replaced with compatible R134a components if a conversion from R12 to R134a is required (see Chapter 5). R134a contributes to global warming and will eventually be replaced, certainly within Europe, with another cooling medium which is reported to be less harmful to the environment. R134a is not a drop-in replacement for R12 A/C systems. A number of modifications are required to the system components to allow an R12 system to use R134a as a cooling medium (see Chapter 5).
Properties of R134a:
1. It is only miscible with synthetic polyalkylglycol (PAG) lubricants, not with
2. mineral oils.
3. It does not attack metals.
4. It attacks certain plastics, so only use special seals suitable for R134a.
5. It is explosive.
6. It is odourless.
7. It is not toxic in low concentrations.
8. It readily absorbs moisture.
9. It is inflammable.
10. It is heavier than air when gaseous, hence the danger of suffocation near the ground.
Refrigerant blends
The use of alternative refrigerants (Table 1.2) to R134a and R12 are not accepted by Original
Equipment Manufacturer’s (OEM’s) standards. Manufacturers only use the approved R134a refrigerant in vehicle A/C systems. The US has a range of Snap Approved Refrigerants which can be used as ‘alternatives’ to R12. If blends are used then separate approved service units and accessories must be used to avoid contamination. Strict procedures must be adhered to with regard to record keeping, barrier hose fitment, high pressure release device replacement,
Table 1.2
service connectors fitment, strict labelling requirements, oil replacement and possible seal replacement. Blends currently cannot be recycled using an approved service machine. This means refrigerant has to be sent back to the supplier to be recycled. Blends are compounds made of other refrigerants, R22, R134a, etc. They are either azeotrope or zeotrope. Azeotrope has a single boiling point while zeotrope are a boiling point range. Zeotrope blends’ boiling point range starts when the lighter elements start to boil and ends when the heavier elements boil off. If a leak occurs the blends’ lighter elements will vaporise and escape leaving the heavier elements. This is called fractionising, which changes the characteristics of the blend.
Note – R134a is the only refrigerant that should be used as a replacement for R12.
Retrofitting must be carried out in accordance with SAE requirements (see legislation).
Substitutes
Substitutes are reviewed on the basis of ozone depletion potential, global warming potential, toxicity, flammability, and exposure potential. Lists of acceptable and unacceptable substitutes are updated by the EPA (Environmental Protection Agency) several times each year.
Refrigerant service connectors
To prevent accidental mixing of the refrigerants the SAE (Society of Automotive Engineers) developed guidelines for different service valve connectors for R12 and R134a (Figs 1.44 and 1.45). If refrigerants are mixed severe damage will occur to the system. R12 uses threaded connections and R134a quick release couplings.
Figure 1.44 1.45 1.46
The service connector forms a valve with the valve core screwed inside it. This valve allows the disconnection of a pressure gauge,A/C machine or control switches/sensors to be removed without draining the system.These are called Schrader type spindle valves (Fig. 1.48).They are similar in design to tyre valves. The needle is held in the closed position by spring force.
Before the coupling is fitted the shut-off valve (blue) must be closed ensuring the valve is not open. Once connected the valve is opened to obtain a reading on the service unit. Blue is allocated to the low pressure side or suction side and red is allocated to the high pressure side or discharge side.
Figure 1.47 shows a threaded low pressure R12 connector with ball valve preventing refrigerant loss.
Figure:1.47 1.48 1.49
The protective cap prevents the valve getting dirty and also provides an additional seal when
the system is working. The protective caps must be screwed on again after the system is filled.
The service connector valves must seal completely. To check, apply a few drops of compressor
oil to the needle. If bubbles are formed, the valve is leaking and the valve core must be renewed.
Tools are available to remove Schrader valves without draining the system, providing there
is sufficient space.
Hose material
A/C systems are designed to use as little flexible hoses as possible due to leakage. Aluminium extruded tubing is generally used with the exception of the compressor which uses flexible hoses because it is connected to the engine. Modern hoses for R134a use an inner lining of nylon due to the size of the R134a molecule and to reduce moisture ingression. Covering the nylon is an external tubing of Neoprene. Polyester braid is used as reinforcement with a final covering of PVC (Polyvinyl Chloride). R12 hose is constructed in the same way but without the nylon lining due to the larger size molecule. Electric compressors may remove the need for
Table 1.3
any flexible hoses. The government and environmental groups are placing increased pressure
on zero leak rates.
OEM will always advise you to use the correct refrigerant for the correct system. Deviation
from the original set-up should only occur if you are converting an R12 system to an R134a. If
converting from R12 to R134a then hoses must be replaced with the curent type.
The pressure/temperature relationship of R134a
The graph in Figure 1.50 shows the pressure/temperature curve for refrigerant R134a.
The graph shows the refrigerant to be in a gaseous/vapour state above the curve and a liquid below the curve. The curve represents the boiling point of the refrigerant under varying pressure and temperature relationships.
1. The refrigerant is in a gaseous/vapour state and if the temperature is kept constant and the
pressure is increased then the refrigerant will condense into a liquid.
2. If the pressure is kept constant and the temperature is reduced then the refrigerant can be
condensed into a liquid.
3. If the temperature is kept constant and the pressure is reduced then the refrigerant will
evaporate into liquid/vapour.
4. If the pressure is kept constant and the temperature is increased then the refrigerant will
evaporate into liquid/vapour.
The A/C system is designed to manipulate these relationships to enable the refrigerant to
transfer heat from the cabin space.
Comfort – humidity
Humidity is the term used to describe the wetness or dryness of air.The air around us contains
a percentage of water vapour. Humans perspire (sweat) which evaporates and cools the surface
of the skin by convection. If humans are in hot humid conditions then this makes us feel sweaty,
Figure 1.50
uncomfortable, anxious and can induce stress.A fan to force air over the occupant of a vehicle
can improve the evaporation rate and improve comfort.
There are generally two ways to measure humidity, relative humidity and absolute humidity (Fig. 1.51). Relative humidity is the most common measurement and tells us how much water vapour by weight the air actually contains compared to how much it could contain at that given temperature. As an example, if the relative humidity is 50%, the air could hold twice as much vapour as it does at that given temperature. The amount of vapour that the air can hold changes with its temperature. If the air warms up then it can hold more vapour which would reduce the relative humidity because it could hold more vapour than what it was actually carrying. If the air cools then its relative humidity reduces because it can now hold less.
Absolute humidity is the amount (by weight) of vapour that the air contains compared with the amount of dry air. When air becomes saturated with water and then cools the relative humidity will eventually (depending on the rate of cooling) become 100%.The temperature is called the ‘dew point’ of air for the absolute humidity. If the air cools any further then the vapour it contains will condense.
The significance of this information is the control of the humidity within the A/C system. Humidity is controlled by the surface area of the evaporator and the volume and flow of air travelling through it.The cold surface of the evaporator causes the moisture in the air to condense and cover the surface in water droplets. This reduces the moisture content thus drying the air and improving comfort. Relative humidity for comfort levels is generally about 60%.
Dry bulb temperature
This is the temperature indicated by an ordinary thermometer used to measure air temperature.
Wet bulb temperature
In a wet bulb thermometer the heat sensitive bulb of a glass tube thermometer is wrapped in a gauze, one end is suspended in a water container to allow the water to be drawn upwards by capillary action and moisten the bulb. The moisture robs a percentage of the heat surrounding the bulb which is dependent upon how easily the water can evaporate. The temperature that is registered is referred to as the ‘wet bulb temperature’. Some equipment suppliers sell as an accessory a probe with a wet sock which attaches to a multi-meter allowing the electronic
Figure 1.51
measurement of temperature.The sock can be removed to obtain the dry bulb temperature.As discussed, relative humidity is measured by comparing the wet bulb temperature against the dry bulb temperature. Instruments that contain both measuring devices are called psychrometers.
As an example – in Figure 1.52 the graph shows the wet bulb temperature 19.5°C (follow diagonal line), dry bulb temperature 25°C (horizontal line) and relative humidity 60% (point where both intersect).
After measuring the dry and wet bulb temperatures of the air entering the evaporator and the air exiting the centre vent inside the vehicle, the graph can be used to calculate the relative humidity and compare the two results ensuring good evaporator performance. Example, relative humidity of the ambient air entering the HVAC unit – 70%, relative humidity of the air exiting the centre vents inside the vehicle – 50%.
Humidity sensor
See Chapter 3, section 3.2.
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