Theory of Heat and Refrigeration
THEORY OF HEAT AND REFRIGERATION
BASIC PRINCIPLE OF AIR CONDITIONING
The basic principle at work in a climate control system is heat transfer. An automotive A/C system takes heat inside the passenger compartment and transfers it outside.
In an A/C system, heat is transferred using a refrigerant. The refrigerant absorbs heat from air entering the passenger compartment, carries the heat outside the compartment, releases the heat, and then re-enters the compartment to begin the cycle again.
An A/C system does not "add cold" to air - it removes some of the heat from it. Some heat is always present, but the less heat the air contains, the cooler it feels.
TEMPERATURE
The amount of heat energy present is measured as the temperature. There are two different temperature scales, Fahrenheit and Celsius.
BTUs AND CALORIES
Heat is measured in British Thermal Units (BTUs) and calories.
- BTU - amount of heat energy required to raise one pound of water one degree Fahrenheit.
- Calorie - amount of heat energy required to raise one gram of water one degree Celsius.
HEAT TRANSFER
An air conditioning system's efficiency is based on how well it moves heat. Heat always travels from warm 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 cooler surrounding air. The heat from the surrounding air moves to the cooler soda, until a balance is reached.
TEMPERATURE AND STATE CHANGES
At sea level, water freezes at 32°F (0°C) and boils at 212°F (100°C). These are the temperatures at which water changes state.
When a liquid boils (changes to a gas), it absorbs heat. When a gas condenses (changes back to a liquid), it gives off heat.
Water requires one BTU of heat per pound to rise one degree Fahrenheit. If you place one pound of water at 32°F in a container over a flame, its temperature rises 1°F for each BTU of heat the water absorbs from the flame. Once the water has reached a temperature of 212°F it has absorbed 180 BTUs of heat.
As the flame continues to heat the water, it boils, changing from a liquid to a gas, and it continues to boil until all of it has changed to a gas.
If this gas is collected in a container and checked with a thermometer; it would also have a temperature of 212°F. The temperature has not risen further; but the flame has applied an additional 970 BTUs of heat. The heat is absorbed by the liquid as it boils. It is "hidden" in the water vapor.
If the vapor contacted cool air, the heat would flow into the cooler air as the vapor condensed back into water. This hidden heat is called the latent (hidden) heat of vaporization."
Water has a latent heat of vaporization of 970 BTUs. This means one pound of water at 212°F will absorb 970 BTUs of heat when it boils and becomes a vapor. In the same way, the vapor will give off 970 BTUs of heat when it condenses back to water.
LATENT HEAT OF VAPORIZATION
In other words, water acts like a "heat sponge": it soaks up a small amount of heat (180 BTUs per pound) when its temperature rises from 32°F to 212°F and it soaks up a large amount of heat (970 BTUs per pound) when it changes from a liquid to a gas.
EVAPORATION
Evaporation is one of the basic principles by which a refrigeration system works. In evaporation, liquid changes to a vapor. Adding heat causes a liquid to evaporate.
CONDENSATION
Condensation is the reverse of evaporation. In condensation, a vapor changes to a liquid. Removing heat causes a vapor to condense to a liquid.
The task of an air conditioning system is to absorb a large amount of heat, move it away from the passenger compartment, and exhaust it. When the refrigerant in the A/C system evaporates, it absorbs a large amount of heat from the air entering the passenger compartment. As the refrigerant vapor is pumped outside the passenger compartment, it transports this heat with it. When the refrigerant condenses back into a liquid, this heat is released.
THE EFFECTS OF PRESSURE ON BOILING POINTS
- As the pressure on a liquid is increased, the boiling point rises.
- As the pressure on a liquid is decreased, the boiling point drops.
At sea level, where the atmospheric pressure is 14.7 psi, the boiling point of water is 212°F (100°C). At any point higher than sea level, the atmospheric pressure is lower and so is the boiling point. In Denver, Colorado (elevation 5,300 feet), water boils at only 206°F (97°C).
Atmospheric pressure is approximately 14.7 psi (absolute) at sea level, and somewhat lower at higher elevations. At sea level, the entire weight of a "column" of air approximately 600 miles high, presses down on everything. At higher elevations, the column of air is shorter and the air is thinner, so the pressure is lower.
Of course, you don't notice the 14.7 psi pressing in on everything, and air pressure gages are calibrated to read 0 psi at atmospheric pressure. But this atmospheric pressure exists, and you can feel its effects, particularly at higher elevations; for example, if you exercise vigorously, at a high elevation, you become winded more quickly.
In an air conditioning system, the pressure in the evaporator is low, so that all the refrigerant vaporizes. The pressure in the condenser is high, so that all the refrigerant readily changes state to a liquid.
PRESSURE/TEMPERATURE RELATIONSHIP
Raising the pressure of a vapor raises its temperature; lowering the pressure decreases its temperature.
In an air conditioning system, a compressor is used to increase the pressure of the refrigerant; this raises its temperature. The refrigerant vapor entering the condenser is hot.
In BMW air conditioning systems, an expansion valve is used to lower the pressure of the refrigerant; the refrigerant in the evaporator is cold,
Automotive A/C systems are designed to operate at pressures that keep the refrigerant at the optimum temperature for taking heat out of the passenger compartment.
THE "COMFORT ZONE"
- Temperature / 70°to 80°F (21°to 27°C).
- Humidity / 45% to 50% (at 70°to 80°F).
- Movement of air around the body (the more air; the cooler the body feels).
AIR CONDITIONING AND COMFORT
The purpose of an A/C system is to make the driver and passengers comfortable. An A/C system achieves this by cooling the air temperature inside the passenger compartment and removing moisture (humidity), dust, and pollen particles.
By removing moisture and lowering the humidity, an A/C system can achieve passenger comfort at higher temperatures. The reason for this is that the human body cools itself by allowing moisture on the skin to evaporate.
The relative humidity governs how quickly evaporation occurs:
- High relative humidity = low evaporation rate.
- Low relative humidity = high evaporation rate.
When the A/C system removes moisture from the air, the relative humidity in the passenger compartment decreases. By reducing the relative humidity, the A/C system increases the rate at which the moisture on passengers skin will evaporate.