1.1 Thermodynamics: Sensible/Latent Heat & Pressure-Temperature Relationship

The science of refrigeration is firmly rooted in thermodynamics—the branch of physics that deals with heat, work, and temperature, and their relation to energy, radiation, and the physical properties of matter. To fully grasp the vapour-compression cycle and successfully pass the C&G 2079 Category I exam, a solid understanding of these fundamental principles is strictly required.

Heat Transfer and Energy

In refrigeration, our primary objective is to move heat from an area where it is not wanted (e.g., inside a cold room or air-conditioned office) to a place where it makes no difference (e.g., the outside atmosphere). Heat always flows naturally from a warmer substance to a cooler one. To reverse this natural flow and move heat from a cooler space to a warmer space, we must expend energy—this is the fundamental purpose of the refrigeration cycle.

Refrigerants are the working fluids that carry this heat. As they move through the system, they absorb and release heat energy, undergoing specific physical changes. These changes are defined by two distinct types of heat: sensible heat and latent heat.

Sensible Heat

Sensible heat is the heat energy added to or removed from a substance that results in a measurable change in temperature, without any change in its physical state. If you place a thermometer in a glass of water and heat it on a stove, you will see the temperature rise. Because this temperature change can be 'sensed' or measured by a thermometer, it is called sensible heat.

For example, when a liquid refrigerant at 20°C is cooled down to 10°C, sensible heat has been removed. The substance remains a liquid throughout this process. In the refrigeration cycle, sensible heat changes occur when we superheat a vapour or sub-cool a liquid. While sensible heat is important, it accounts for a relatively small portion of the total heat transferred in a refrigeration system compared to latent heat.

Latent Heat

Latent heat is the heat energy absorbed or released by a substance during a change of state (or phase change), occurring entirely without a change in temperature. The word 'latent' means hidden. When a substance undergoes a phase change, the heat energy goes into breaking or forming the molecular bonds of the substance rather than raising its temperature.

There are two primary forms of latent heat relevant to refrigeration:

1. Latent Heat of Vaporisation: The amount of heat energy required to change a liquid into a vapour (gas) at a constant temperature. In a refrigeration system, this occurs in the evaporator. As the liquid refrigerant boils, it absorbs a massive amount of latent heat from the surrounding space, producing the cooling effect.
2. Latent Heat of Condensation: The amount of heat energy released when a vapour changes back into a liquid at a constant temperature. This takes place in the condenser. The high-pressure, high-temperature refrigerant vapour releases its stored latent heat to the outside air or cooling water, returning to a liquid state.

The sheer volume of heat energy transferred during a phase change is significantly greater than during sensible temperature changes. This is the core secret of the vapour-compression cycle: by deliberately forcing refrigerants to change state, we can move vast quantities of heat efficiently.

The Pressure-Temperature Relationship

One of the most vital concepts in refrigeration is the relationship between pressure and the boiling point of a fluid. For any given pure substance, there is a strict, predictable correlation between the pressure applied to it and the temperature at which it boils (evaporates) or condenses.

• Increasing Pressure Raises the Boiling Point: If you increase the pressure on a liquid, it will require a higher temperature to boil.
• Decreasing Pressure Lowers the Boiling Point: Conversely, if you reduce the pressure, the liquid will boil at a lower temperature.

Water, for example, boils at 100°C at standard atmospheric pressure (1.013 bar). However, at the top of Mount Everest where atmospheric pressure is much lower, water boils at roughly 68°C. If we pressurise water inside a sealed boiler, it might not boil until 150°C.

In refrigeration, we manipulate this relationship to our advantage. We use a compressor to raise the pressure of the refrigerant vapour. By increasing the pressure, we drastically increase its saturation temperature (the temperature at which it condenses). This allows the refrigerant to reject its latent heat of condensation into the outdoor ambient air, even on a hot summer day. Conversely, by using an expansion device to drop the pressure of the liquid refrigerant, we lower its saturation temperature so that it can boil and absorb heat from a cold room.

Saturation

When a refrigerant is in the exact process of changing state—either boiling from a liquid to a vapour or condensing from a vapour to a liquid—it is said to be saturated. A saturated mixture contains both liquid and vapour existing together at equilibrium.

The specific temperature at which this phase change occurs for a given pressure is called the Saturation Temperature, and the corresponding pressure is the Saturation Pressure. As long as both liquid and vapour are present, the pressure and temperature remain locked together according to the refrigerant's specific Pressure-Temperature (P-T) curve. This means that if you know the pressure of a saturated pure refrigerant, you instantly know its exact temperature, and vice versa. Engineers use P-T charts or digital manifolds to read these values daily.

Zeotropic Blends and Temperature Glide

Historically, the industry used pure fluids (like R134a) or azeotropic blends, which behave exactly like pure fluids. They evaporate and condense at a single, constant temperature for a given pressure.

However, environmental regulations (like the F-Gas Regulation) have led to the widespread adoption of zeotropic blends (the 400-series refrigerants, such as R407C, R448A, and R449A). These are mixtures of different refrigerants with differing boiling points. Because the individual components of the blend boil and condense at different temperatures, zeotropic blends do not have a single saturation temperature at a given pressure.

Instead, they experience Temperature Glide. Temperature glide is the difference in temperature between the start and end of the phase change process at a constant pressure. As a zeotropic blend boils in an evaporator, the most volatile component (the one with the lowest boiling point) boils off first. As the process continues, the remaining liquid becomes richer in the less volatile components, causing the boiling temperature of the remaining mixture to gradually rise.

To manage this mathematically and practically, two new reference points are used for zeotropic blends:

1. Bubble Point: The temperature at which a saturated liquid begins to boil (the very first bubble of vapour appears). We use the Bubble Point temperature when calculating sub-cooling at the condenser outlet.
2. Dew Point: The temperature at which a saturated vapour begins to condense (the very first drop of dew/liquid appears). We use the Dew Point temperature when calculating superheat at the evaporator outlet.

Failure to use the correct point (Bubble or Dew) when setting up a system with a zeotropic blend will result in incorrect superheat and sub-cooling values, potentially leading to severe compressor damage or poor system efficiency. Understanding glide is a major focus of the C&G 2079 Category I assessment.