Page Outline

               First Law of thermo Dynamics                                                        Second Law of thermodynamics

              Ohms Law                                               Boyles Law

                             Daltons Law                                                                                       Saturation

                           Subcooling                                                                                          Superheat

                       Measuring Heat:                                                          Refrigerant Blends

                                    Energy                                                                             Temperature Conversions

First Law of thermo Dynamics  

 Energy can be changed from one form to another but it can not be created or destroyed. The total amount of energy and matter in the universe remains constant, merely changing from one form to another. The first law of thermodynamics (conservation) states that energy that energy is always conserved, it cannot be created or destroyed, in escense, energy changes from one form to another.

Second Law of thermodynamics

The second law of thermodynamics states that "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state."

 Ohms Law

 

. V = Voltage     I = Current    R = Resistance.

    V = I x R               R = V / I         I = V / R 

 Boyles Law

 The relationship between temperature and pressures is called Bolyles Law. Gases have properties which we can observe with our senses, including the gas pressures, temperatures,mass and the volume, which contains the gas.

Daltons Law  The sum of all the pressures equals the sum of all the pressures of all parts

Saturation:

Saturation is the point where a change of state in a substance is taking place. For water at sea level, the boiling temperature is 212 degrees F. Therefore, we say the saturation (boiling temperature) is 212 degrees. As soon as the temperature of the steam is heated above it’s “saturation” temperature, it has been superheated. Refrigerant that has boiled (turned into a vapor) at 40 degrees has a saturation temperature of 40 degrees. If the refrigerant vapor is heated to 41 degrees it is no longer saturated, it is then superheated by 1 degree. Remember, only a gas or vapor can be superheated. Superheat is any temperature of a gas or vapor above it’s saturation temperature.


Subcooling:

Only liquids and solids can be subcooled. Subcooling is any temperature of a liquid or solid below it’s saturation temperature. Let’s use water as an example again. Liquid water at sea level has a saturation (boiling) temperature of 212 degrees F. If we were to add heat to the saturated water it would first boil away with no change in temperature (remember latent heat?) and then become superheated if still more heat were added to the vapor (steam) after it had all turned to a vapor.

Instead of boiling our 212 degree water by adding heat, we shall remove heat from the 212 degree water. As heat is removed from the liquid water it’s temperature will drop below it’s boiling (saturation) temperature. Water at 211 degrees has been subcooled by one degree F. If the temperature of the water is decreased to 180 degrees the water has been subcooled from 212 degrees to 180 degrees. That is, it has been subcooled by 32 degrees. When you drink 180 degree coffee, you are drinking a subcooled liquid!

 

Superheat:                                                                                                                                                                    Only a gas or vapor can be superheated. Let’s use water as an example as we explain these terms.Water at sea level boils at 212 degrees F. When heated to 212 degrees F the molecules which make up water are moving at a high enough speed that they overcome the air pressure above the water. As additional heat is added to liquid water at 212 degrees, the water begins to boil. As water boils it is changing state from a liquid to a gas. During the boiling process the temperature remains (212 degrees F). There is no change in temperature during a change of state. This phenomenon is true for all substances as they change state no matter how much heat is added. As long as the water is still boiling and not all the water has completely changed to a gas (steam) the temperature remains at 212 degrees F. This heat of boiling is called latent heat. The word “latent” is a Latin word for “hidden”. The heat added to the water is hidden from the thermometer since the temperature remains unchanged during the boiling process.
Any increase in temperature of the steam above it’s boiling point (212 degrees) is called “superheat”. Steam at 213 degrees F is superheated by one degree F.

Superheat is then any temperature of a gas above the boiling point for that liquid. When a refrigerant liquid boils at a low temperature of 40 degrees in a cooling coil and then the refrigerant gas increases in temperature superheat has been added. If this refrigerant changed from a liquid to a gas or vapor at 40 degrees and then the refrigerant vapor increased in temperature to 50 degrees F, then it has been superheated by 10 degrees.

We commonly think of boiling as always being accomplished by a liquid when it is hot. This is because we are familiar with boiling water. However, air conditioning and refrigeration systems use liquids (refrigerants) with much lower boiling points. Lowering the pressure over a liquid lowers the boiling point. There is less pressure above the liquid to overcome. That is why water at the top of a mountain may boil at 190 degrees (depending upon the altitude) rather than at 212 degrees F. By controlling the pressure over a liquid, we can control the boiling temperature. That is why a service technician monitors the pressures in an air conditioning system. The technician is actually monitoring the pressures and temperatures where the refrigerant is changing state                      

Sensible Heat & Latent Heat                                                                                                                                                             Sensible heat is heat that can be measured by a thermometer. Anytime heat is added or removed from a substance and a temperature change occurs, a sensible heat change has taken place. Since both Superheat and Subcooling are changes in temperature, they are both sensible heat processes.

When an air conditioning system cools air sensible heat has been removed. In fact, since the air is a gas or vapor and is heated far above it’s boiling (saturation) point, it is superheated air. Yes, you are breathing superheated air as the air is hundreds of degrees above the temperature at which the gases which make up air would condense back into liquid form.

Superheated does not necessarily mean hot. And, subcooled does not necessarily mean cold. Superheat and Subcooling are determined by the boiling temperature of the substance and unlike water many substances have low boiling temperatures.

Recalling that latent heat is the heat which is added to a liquid to cause it to change from a liquid to a gas (boiling) without a change in temperature. Cooling the gas removes it’s superheat. When all the superheat is removed from a gas, the gas will condense back into a liquid. The heat removed from a saturated gas to allow it to condense back into a liquid is once again latent or hidden heat and is not a sensible heat process. That is, during the process of changing from a gas to a liquid it occurs at a constant temperature therefore a thermometer will not detect any temperature change. That is latent heat.

Air contains water vapor or moisture. Humid air is not comfortable. Too much humidity (moisture) in air is uncomfortable. As air containing too much moisture passes over a properly designed, installed and operating air conditioning system, the air is cooled by the air conditioning coil (evaporator) located at the indoor blower section. If the air containing the moisture is cooled to the condensing temperature (dew point) of the moisture in the air, some of the moisture will condense and deposit on the coil and fins of the cooling coil. Since the water vapor is changing from a gas or vapor to a liquid, this is a latent heat process. The condensed water should run off the coil and be drained away.

A properly operating air conditioning system both cools (a sensible heat process) and dehumidifies (a latent heat process) the air. For example, given a 3-ton residential air conditioning system, a percentage of the total capacity of the system is utilized to cool the air while the remaining percentage of the total capacity is used to dehumidify the air. Properly controlling both the temperature (sensible heat) and the humidity (latent heat) will provide the optimum comfort for the occupants.

  Measuring Heat:

Latent heat cannot be directly measured as we can sensible heat. In order to properly adjust, troubleshoot and repair air conditioning equipment it is necessary that we understand heat and how to measure heat.

Superheat and Subcooling are both sensible heats and therefore can be measured with a thermometer. Superheat and Subcooling are also temperature differentials. That is, each is a number of degrees a gas or liquid are above or below their saturation temperatures. It is essential that a service technician be able to accurately measure these differentials and diagnose system operation from them

 Refrigerant Blends

Azeotrope a blend that behaves like a single component refrigerant. When a blend forms an azeotrope it displays unique and unexpected properties.

Zeotrope a blend that behaves like a mixture of the individual components. Zeotropes have predictable properties based on combinations of the pure components’ properties.                                                  (Generally speaking blends with less than 5ºF glide are considered near-azeotropes).

Fraction is the change in composition of a blend because one (or more) of the components is lost or removed faster than the other(s).The greater the difference between the pressures of the starting components will cause a greater difference in the vapor composition compared to liquid. This will worsen the effect of fractionation on that blend.

In order to avoid charging the wrong composition and fractionating the remaining blend, zeotropic blends must be removed from the cylinder as a liquid. This can be done by turning the cylinder over so the valve is on the bottom, or forcing the product through a diptube to the valve.

Liquid charging does not mean that liquid refrigerant should be pushed into the suction line of the system and it be allowed to slug the compressor. After the initial charge into the high side of a system, the compressor can be started and charging can be completed by flashing the refrigerant from liquid to vapor in the charging hose or across specially designed valves. Any method which allows the refrigerant to go to vapor before it hits the compressor should work. Generally the refrigerant needs to be added slowly at this point.

                                                                                ENERGY IS THE ABILITY TO DO WORK                                                                  There are all sorts of different ways that energy can transfer between different objects. Often, people group these into three broad classes which give a pretty good idea of the main types.

1. Conduction: Hot atoms or molecules or electrons rattle around more than cold ones. As they bump into each other, the hot ones tend to transfer energy to the cold ones. So heat flows from hot regions to cold ones.

2. Convection. In liquids and gases, usually the density depends on temperature. Usually the hotter stuff is less dense. Gravity makes denser stuff sink and cooler stuff rise. If there’s something hot (like a stove burner) below something cold (like a pot of water) this convection helps carry that hot stuff up into the cold regions.

3. Radiation. The hotter something is, the more electromagnetic radiation it gives off. Very hot things, like the sun or a glowing burner, give off radiation that we can even see (light). So the net flow of radiation energy is from the hot regions to the cold ones. The key pattern here is that even though there are diverse mechanisms, they all carry energy from the hot regions to the cold ones.

                                                                     Energy can be divided into two forms:                                                   Kinetic energy The word "kinetic" is derived from the Greek word meaning to move, and the word "energy" is the ability to move. Thus, "kinetic energy" is the energy of motion --it's ability to do work

Potential  energy is stored energy.

 There are different units that can be used to measure energy and these units can be converted from one to another. Some units that you have heard of include calories and kilowatt hours. There are other measurements more commonly used in science, such as British thermal units or joules.

Power is the rate at which energy is used or work is done. Some units of measure for power include horsepower, watts, kilowatts, or megawatts


Temperature conversion formulas

Convert Farenheit to Celcius: Celcius = (5/9) * (Farenheit - 32)
Convert Celcius to Farenheit: Farenheit = (9/5) * Celcius + 32
Convert Celcius to kelvin: Kelvin = Celcius + 273
Convert kelvin to Celcius: Celcius = kelvin - 273


Set Points    Fahrenheit    Celsius    Kelvin
water boils       212           100         373
water freezes    32              0           273
absolute zero    -460 no more heat 
no more heat    -273 no more heat

Superheat & Subcooling web pg

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