Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 1 — Fundamental Concepts
1.1 HVAC/R Terms
Matter, substance, element, atom, and molecule — the foundational vocabulary
of materials science as applied to refrigeration, air conditioning, and HVAC/R work.
In HVAC/R work, technicians handle air, water, metals, refrigerants, and gases
every day. To understand these materials properly it is important to know the
precise meaning of five foundational terms: matter, substance, element, atom,
and molecule.
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Matter
Anything that has mass and takes up space. In HVAC/R this includes
refrigerant inside a system, air in a duct, water in a hydronic pipe,
copper tubing, and compressor oil. If it can be weighed and it occupies
space, it is matter.
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Substance
A type of matter with uniform composition and specific properties —
the same material throughout. Pure R-134a, pure copper tubing, pure water
in a closed system, and oxygen gas are all substances. Dirty compressor oil
is not, because it contains oil mixed with contaminants.
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Element
A pure substance made of only one kind of atom — it cannot be broken
down into simpler substances by ordinary chemical means. Copper in
refrigerant tubing, aluminium in evaporator fins, iron in steel
components, and oxygen in air are all elements.
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Atom
The smallest particle of an element that retains the properties of that
element — the basic building block of all materials. A copper pipe contains
a huge number of copper atoms joined together in solid metal.
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Molecule
Two or more atoms chemically joined together — the atoms can be the
same or different kinds. O2 (oxygen), N2 (nitrogen),
H2O (water), and CO2 (carbon dioxide) are all
molecules encountered in HVAC/R work.
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How to Connect the Terms
Matter is anything with mass that takes up space.
A substance is a pure, uniform kind of matter.
An element is a substance made of only one kind of atom.
An atom is the smallest unit of an element.
A molecule is a group of two or more atoms chemically joined together.
Example — Copper Refrigerant Tubing
The tubing is matter — it has mass and takes up space
Pure copper is a substance
Copper is an element
The tubing is built from millions of copper atoms
Example — Condensate Water
Water is matter
Pure water is a substance
Water is not an element — it contains more than one kind of atom
Water is made of H2O molecules (two hydrogen atoms + one oxygen atom)
1.1.2 — Air, Moisture, and the Dew Point
At atmospheric pressure, the amount of water vapour that air can hold depends
entirely on its temperature. Using grains as the unit of measurement
(1 grain = 1/7000 of a pound), the saturation values at three reference
temperatures are:
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5 °C
Air can hold 38 grains of water per pound of dry air
at 100% relative humidity.
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12 °C
Air can hold 61 grains of water per pound of dry air
at 100% relative humidity.
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21 °C
Air can hold 110 grains of water per pound of dry air
at 100% relative humidity.
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The Warmer the Air, the More Moisture It Can Hold
Moving from 5 °C to 21 °C — a swing of just 16 degrees —
nearly triples the air's moisture-holding capacity (from 38 to 110 grains).
This single principle explains condensation, dew, and many of the moisture
problems HVAC/R technicians are called to solve.
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Temperature and Moisture Capacity
Think of air as an invisible sponge. A warm sponge is soft and expanded,
able to absorb far more liquid than a cold, stiff one. At 21 °C a
parcel of air can comfortably carry 110 grains of moisture without releasing
any — the air is simply not yet "full." As that parcel cools, its
capacity shrinks.
When the air cools to 12 °C, its maximum capacity drops to 61 grains.
If it was carrying more than 61 grains, it cannot hold the excess — the moisture must go somewhere.
That "somewhere" is condensation: water vapour converts back into liquid water and deposits on any available surface.
This is not a chemical reaction — it is purely a physical limit imposed by temperature.
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How Morning Dew Forms
On a clear summer evening, air near ground level might be at 21 °C
and carrying 90 grains of moisture — not yet at 100% RH, but close.
As the sun sets, radiative cooling begins and the ground loses heat quickly. The thin layer of air resting directly on the ground cools with it.
By midnight that air layer may have cooled to 12 °C. Its maximum capacity is now only 61 grains — but it is still carrying 90 grains.
The 29 excess grains (90 − 61) are forced out of the vapour phase and condense as tiny liquid droplets on grass, leaves, car hoods, and surfaces.
If cooling continues toward 5 °C (capacity: 38 grains), even more moisture is wrung out. Surfaces are visibly wet by morning — even though it never rained.
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The Dew Point — Definition
Key Term
The dew point is the specific temperature at which a given
parcel of air reaches 100% relative humidity — the exact point at which
it can no longer hold all its moisture and condensation begins. It is not a
property of temperature alone; it depends on how much moisture the air is
already carrying.
Example: Air at 21 °C carrying 61 grains has a dew point of 12 °C — the temperature at which those 61 grains fill the air to capacity.
Cool below 12 °C and dew forms. Keep above 12 °C and the moisture stays invisible as vapour.
A high dew point means moisture-laden air — only a small temperature drop will trigger condensation.
A low dew point means dry air — it must cool significantly before any moisture is released.
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Why the Dew Point Matters in HVAC/R
Controlling the dew point is critical in building science and refrigeration
work. Condensation inside walls, on cooling coils, or on windows signals a
moisture problem that can lead to mould, corrosion, and structural damage. When
the surface temperature of any component drops below the dew point of the
surrounding air, condensation will form — regardless of intent.
1.1.3 — States of Matter
Matter commonly exists in three states relevant to HVAC/R: solid, liquid, and
gas (or vapour). The refrigeration cycle depends entirely on controlled changes
between the liquid and vapour states to move heat from one location to another.
Refrigerants are selected and designed to boil and condense at useful temperatures
and pressures.
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Solid
Definite shape and definite volume. Strong molecular attraction holds
atoms tightly together with minimal motion. Examples: copper pipe,
compressor housing, steel brackets.
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Liquid
Definite volume but takes the shape of its container. Moderate molecular
mobility allows flow. Examples: liquid refrigerant in the liquid line,
water in a hydronic system, compressor oil.
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Gas / Vapour
No definite shape or volume. Molecules are far apart and in rapid motion.
Examples: refrigerant vapour in the suction line, air in a duct,
nitrogen used for pressure testing.
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Critical Point
The critical point of a refrigerant is the combination of critical
temperature and critical pressure beyond which
the liquid and vapour phases are indistinguishable. Above the critical
temperature, the substance cannot be condensed into a liquid regardless
of how much pressure is applied.
On a pressure–enthalpy (P–h) diagram, the critical point sits at the very top of the saturation dome.
CO2 (R-744) critical point: 31 °C / 88 °F and 73.8 bar / 1,070 psi — one of the lowest critical temperatures of any common refrigerant.
In transcritical CO2 systems (such as supermarket refrigeration), the compressor pushes refrigerant into a supercritical state where there is no phase change. A gas cooler rejects heat rather than a condenser.
This requires components rated for pressures exceeding 100 bar (1,450 psi) — standard manifold sets and fittings are not suitable.
Operation above the critical point eliminates traditional condensation and fundamentally changes system design, efficiency calculations, and service procedures.
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Bubble Point
The bubble point is the temperature at which the first bubble of
vapour forms in a liquid mixture at a given pressure. In
refrigeration, this term is primarily used with zeotropic and
near-azeotropic refrigerant blends such as R-407C and R-404A.
At a specified pressure, the bubble point represents the saturated liquid temperature — below this temperature the fluid is fully liquid; at the bubble point it begins to boil.
On a pressure–temperature (P–T) chart for blends with temperature glide, the bubble point curve is used to determine the temperature of the liquid refrigerant.
Technicians use the bubble point to calculate subcooling in blended refrigerant systems.
It is critical to know whether the P–T chart value represents bubble point or dew point when charging and diagnosing systems using blended refrigerants.
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Dew Point of Refrigerants
For refrigerant blends, the dew point is the saturated
vapour temperature at a given pressure — the point at which the first
drop of liquid would form from a vapour mixture.
On a P–T chart, the dew point is used to determine the temperature of the vapour for superheat calculations.
For pure (single-component) refrigerants, bubble point and dew point are the same — there is no temperature glide.
For blended refrigerants with glide (e.g., R-407C), the bubble point and dew point are different temperatures at the same pressure. Always confirm which value applies before making a diagnosis.
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Bubble Point vs. Dew Point — A Practical Rule
When working with blended refrigerants: use the bubble point
to calculate subcooling (you are measuring liquid), and use the
dew point to calculate superheat (you are measuring vapour).
Mixing these up leads to incorrect charge diagnoses. Always verify which
value the P–T chart or refrigerant manufacturer provides.
