The purpose of this article is to provide a simple explanation of these terms
for those who desire a concise understanding as well as a review for those who
understand the terms but want to review them. An understanding of these terms
and the concepts related to them is essential to understanding the air
conditioning and refrigeration mechanical – refrigerant cycle as well as being
necessary to troubleshooting cycle problems.
Superheat:
Most materials can exist in three forms, solids, liquids and gases. Water is a
common example. Water can exist as a solid (ice), a liquid, or a gas or vapor
(steam). Only a gas or vapor (these are interchangeable terms), 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 the water boils it is
changing state from a liquid to a gas. In addition, during the boiling process
the temperature remains the same (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 means that a thermometer placed in
boiling water will remain at 212 degrees throughout the boiling process even
though heat is added to cause the water to boil. 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.
After all the water has changed to a gas or vapor (steam), then the addition of
still more heat to the vaporized water or steam will cause the temperature of
the steam to increase above it’s boiling temperature of 212 degrees. 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 an
air conditioning 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. If a liquid refrigerant boils at -10 degrees and is then
warmed up to zero degrees, it is then a superheated gas containing 10 degrees of
superheat. Heating that same refrigerant gas to +10 degrees means that it now
has been superheated by 20 degrees.
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 air conditioners refrigerant is changing state.
Saturation is simply the term used to describe 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.
Sub cooling:
Sub cooling is now easy to understand. 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 sub cooled by 32 degrees. When you drink 180
degree coffee, you are drinking a subcooled liquid!
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, let’s
go to the next step. When a gas or vapor is above it’s boiling point it is said
to be superheated. 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, that is where central air comes in
. 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 air 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. Sizing of the air conditioning system is the
most important part of removing moisture. If your air conditioning contractors
over size the AC system it will short cycle and not remove the humidity. The
airconditioner coil has to have air moving over the air coil for it to properly
remove water. AC over sizing will leave the basement damp and your home cold and
uncomfortable.
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 in an AC system..
Superheat and Sub cooling for central split ACs 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.for you air conditioning system to operate as designed.
A high quality, accurate electronic thermometer capable of measuring temperature
differentials is almost an essential tool for the technician and highly useful
to the interested homeowner. An example of such an instrument is Bacharach’s
“Dual Channel. Model TH3000, digital thermometer” with data hold and max hold
functions.
Another very useful digital thermometer capable of performing additional air
quality test functions is Bacharach’s “Comfort Check” 100 & 200 model Indoor Air
Quality Analyzers. Essential for anyone interested in investigating or
monitoring indoor air quality.
We service and and install all types of AC Air conditioners and air
conditioning central and Ductless refrigeration systems in the following areas.
If you are a home owner in Mississauga Oakville Toronto Burlington and
Etobicoke you now know what every AC tech should know.
Air conditioners
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