Presentation on theme: "1 Basics of Heating and Air Conditioning Heat energy Heat measurement Heat movement States of matter Latent and sensible heat Boiling points Pressures-"— Presentation transcript:
1 Basics of Heating and Air Conditioning Heat energy Heat measurement Heat movement States of matter Latent and sensible heat Boiling points Pressures- gauge and absolute Refrigerants
2 Heat Is Energy Heat, like other forms of energy, cannot be created nor destroyed. However, we have the ability to move it from one place to another. Moving heat is more efficient than using energy to produce heat.
3 Heat Transfer Mobile heating, ventilation, and air conditioning, MVAC systems are concerned with moving heat to or from the interior of a vehicle. Heat transfer involves moving something that we can feel but cannot see. Understanding the principles of heat transfer helps a technician understand what happens within an A/C system.
4 Heat Measurements, Temperature We are all familiar with temperature. This is a measurement of heat intensity. It is measured using a thermometer. Two different scales are commonly used: Celsuis (C) which is used in most of the World and Fahrenheit (F) which is commonly used in the United States.
5 Heat Measurements, Quantity Heat quantity is also measured using two scales: Calories (c) by most of the World and British Thermal Units (BTU) in the United States. One BTU will increase the temperature of one pound of water by one degree F. One c will increase the temperature of one g of water by one degree C. Burning a large wooden match produces about 1 BTU of heat.
6 Intensity and Quantity A MHVAC technician is concerned with heat intensity when he or she is testing or repairing a HVAC system to make sure it is operating correctly. A vehicle design engineer is concerned with heat quantity while designing a system to make sure it is the correct size to provide adequate heat transfer. It must move enough heat to keep the interior of a vehicle warm in cold climates and cool in very hot areas.
7 Heat Measurement Humidity: the amount of water vapor suspended in the air Warm air can hold much more water vapor than cold air Humid cold air feels much colder than cold dry air Humid hot air slows down evaporation of perspiration, which makes the air feel hotter
8 Heat Movement In this example, heat within the engine will move from the very hot combustion chamber to the cooler coolant. After the coolant is moved to the radiator, the heat will move to the cooler air passing through it. Heat always travels from Hot to Cold.
9 Heat Transfer Rate Heat transfer rate is determined by the temperature difference between the hot and cold items. A minor difference (top) will have a slow rate. A large difference will cause a much faster rate of heat exchange. Animation: Heat TransferHeat Transfer
10 Heat Movement 3 ways heat can move: Conduction Convection Radiation
11 Heat Conduction Heat transfer through a material, from one molecule to the next is called conduction. The heat entering this metal rod is flowing to the ends.
12 Convection When we move heat by circulating a medium like air or water, we call the process: convection. The coolant circulating in the cooling system transfers heat from the engine to the radiator and the warm or cool air circulating through the vehicle are both examples of convection.
13 Radiation The heat transfer from the Sun to the Earth is an example of radiation. Heat is transferred, but it does not warm the medium (space) through which it passes. Radiant heat will warm the interior and exterior of a vehicle that is parked in the sun.
14 Adding Heat When we add heat, we will warm up the liquid, in this case it is water. We are converting the energy in the fuel to fire and transferring this heat to the water. We would have to remove heat if we wanted to cool the water.
15 States of Matter Water is a molecule formed from one hydrogen atom and two oxygen atoms. So is ice and steam. Ice is normally a solid at cold temperatures, and steam is the high temperature, gas form of water. They are all the same molecule, but they have a tighter or looser molecular bond between the atoms. It takes energy to change this bond. Solid: Ice Below 32 F Liquid: Water Water: H 2 O Gas: Steam, Above 212 F Water: H 2 O
16 Sensible and Latent Heat Sensible heat causes a change in temperature Latent heat causes a change of state but no change in temperature
17 Sensible and Latent Heat If we add heat to water, the temperature will increase, and this is called sensible heat. Sensible heat causes a change in temperature. If we add heat to ice at 0 o C or to water at 100 o C, the temperature will not increase. The added heat will melt some of the ice or boil some of the water. This is called latent heat. Latent heat causes a change in state but not a change in temperature. This heat energy changes the molecular bond within the molecule.
18 Latent Heat of Evaporation & Condensation A rather large amount of heat, 979 BTUs, is required to change one pound of water into one pound of steam with no temperature increase. This change of state is used as the refrigerant boils when it enters the evaporator and condenses when it passes through the vehicle’s condenser.
19 Latent Heat of Vaporization It takes 180 Btu’s to raise the temperature of 1 lb. of water from 32 deg. to 212 deg. It takes 970 Btu’s to boil or vaporize 1 lb. of water at 212 deg.
20 Latent Heat of Vaporization In an automotive A/C system, the refrigerant changes state and absorbs heat in the evaporator and releases heat as it changes state again in the condenser Evaporator: liquid changing to gas Condenser: gas changing to liquid
21 Boiling Points Boiling points can be increased or decreased by raising or lowering the pressure on the liquid Increased pressure=increased boiling point Decreased pressure=decreased boiling point
22 Boiling Points and Pressure The boiling point will increase if we raise the pressure on a liquid. In this case, the boiling point will increase about 3 o F for each psi. A 10 pound pressure will increase the boiling point about 3 X 10 or 30 o F. The condensing point of a vapor is the same as the boiling point. Boiling Point = 242 Degrees F Atmospheric Pressure Boiling Point = 212 Degrees F 10 psi
23 Boiling Points Why are A/C systems evacuated prior to recharging?
24 Boiling Points At 29” of vacuum, the boiling point of water is 76 deg. F
25 Boiling Points The condensing point of a gas is the same as the boiling point Heat is removed from a gas at 212 deg. to cause the gas to condense to a liquid Raising the pressure of a gas allows the gas to condense at temperatures above the normal boiling point
26 Boiling Points Critical temperature: the maximum point at which a gas can be liquefied by raising the pressure Critical pressure: the pressure that is necessary to liquefy a gas at critical temperature
27 Boiling Points Superheat: the temperature of a vapor above its boiling point Saturated vapor: a liquid and a gas inside a closed chamber
28 Pressures Pressure: a certain amount of force exerted on an area Pressure at sea level is 14.7 psi Vacuum: pressures below atmospheric pressure Vacuum is measured on a gauge calibrated in inches of mercury 29.92”Hg. is a perfect vacuum and represents no pressure (0 psi)
29 Pressures A deep or high vacuum is close to a perfect vacuum and measured in microns of mercury Gauge pressures: gauges with zero points at atmospheric pressure (psig) Absolute pressures: gauges with zero points at an absolute vacuum (psia)
30 Refrigerants The working fluid of an A/C system First developed by DuPont under the brand name of Refrigerants must have a very low boiling point- below 32 deg. Refrigerants must be able to mix with oil and remain stable
31 Refrigerants Two main types in automotive applications R-12: CFC-12 R-134a: HFC-134a R-12 contains chlorine molecules which are thought to be harmful to the environment R-134a is very similar to R-12 but does not contain chlorine
32 Refrigerants R-134a weighs about 90% of R-12 R-134a is not compatible with mineral oil R-134a systems use polyalkaline glycol (PAG) oils R-12 systems converted to R-134a can use ester oil as a lubricant
33 Refrigerants R-134a is not compatible with desiccants used in R-12 systems Refrigerants must not be mixed Some refrigerant blends contain flammable chemicals and cannot be recycled in a shop Abnormally high operating pressures can be caused by blend refrigerants
34 Refrigerant R-134a has a boiling point of –15 o F. When it is released into the vehicle’s evaporator it will boil immediately and absorb heat. R-134a will absorb almost 78 BTUs of heat for each pound that boils.
35 Refrigerant R134a has a critical temperature of 214 deg. F and a critical pressure of 589 psi This is the upper limits of the high pressure side of an A/C system using R134a
36 Refrigerant Oils R-12 systems use mineral oil R-134a systems use PAG oils Viscosities are chosen by manufacturer R-12 systems converted to R-134a use ester oil