1.1 Heat, Temperature, and Pressure
Key Takeaways
- Temperature measures the average kinetic energy of molecules, whereas heat is the actual thermal energy in transit.
- One British Thermal Unit (Btu) is the heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit.
- Sensible heat changes temperature without changing physical state, while latent heat changes state at a constant temperature.
- The latent heat of fusion of ice is 144 Btu/lb, and the latent heat of vaporization of ammonia at atmospheric pressure is 589.3 Btu/lb.
- Absolute pressure (psia) is calculated by adding 14.7 psi to gauge pressure (psig) at standard sea level conditions.
Heat and Temperature: The Foundation of Refrigeration
In industrial ammonia refrigeration, the fundamental goal is the transfer of heat from a space where it is not wanted (such as a cold storage room or process freezer) to a medium where it can be rejected (such as the atmosphere or cooling tower water). To safely and efficiently operate a refrigeration system, a Certified Assistant Refrigeration Operator (CARO) must master the physical concepts of heat, temperature, and pressure, as well as the mathematical relationships that govern them.
Defining Temperature vs. Heat
A common misconception is that heat and temperature are the same. In thermodynamics, they represent distinct physical properties:
- Temperature is a measure of the average kinetic energy of the molecules within a substance. It indicates the intensity or thermal state of the substance (how hot or cold it is) but does not tell us the total amount of heat energy present. In the United States, temperature is typically measured in degrees Fahrenheit (°F), with absolute temperature measured in Rankine (°R).
- Heat is a form of energy in transition, flowing from a substance of higher temperature to a substance of lower temperature due to the temperature difference. It is a quantity of thermal energy, measured in British Thermal Units (Btu) or calories.
An analogy helps clarify this distinction: think of temperature as the height of water in a tank, while heat is the total volume of water in that tank. A small cup of boiling water at 212°F has a high temperature but very little total heat energy. Conversely, a large swimming pool at 80°F has a low temperature but contains vastly more total heat energy because of its large mass.
The British Thermal Unit (Btu)
The standard unit of heat energy in the United States is the British Thermal Unit (Btu).
- Definition: One Btu is defined as the amount of heat energy required to raise the temperature of one pound (1 lb) of pure liquid water by one degree Fahrenheit (1°F), specifically from 59°F to 60°F at a constant pressure of 14.7 psia.
- Refrigeration context: When we remove heat from a space, we measure that heat removal rate in Btus per hour (Btu/hr) or Btus per minute (Btu/min).
Specific Heat Capacity
Not all substances absorb or release heat in the same way. The rate at which a substance changes temperature when heat is added or removed is determined by its specific heat capacity (often abbreviated as specific heat, denoted as $c$).
- Definition: Specific heat capacity is the amount of heat (in Btus) required to change the temperature of one pound of a specific substance by 1°F.
- Reference standard: Liquid water is the reference standard with a specific heat capacity of $1.0\text{ Btu/lb}\cdot^{\circ}\text{F}$.
- Other substances: Ice (solid water) has a specific heat of approximately $0.5\text{ Btu/lb}\cdot^{\circ}\text{F}$. Water vapor (steam) has a specific heat of approximately $0.48\text{ Btu/lb}\cdot^{\circ}\text{F}$. Liquid ammonia (R-717) has a specific heat capacity that varies with temperature but is roughly $1.12\text{ Btu/lb}\cdot^{\circ}\text{F}$ at 0°F, meaning it requires more heat per pound to raise its temperature than water.
Sensible Heat Equation
When heat is added or removed, and it causes a change in temperature but no change in state (e.g., liquid remaining liquid, or vapor remaining vapor), it is called sensible heat. The formula to calculate sensible heat transfer is:
Where:
- $Q$ = Heat transfer (Btu)
- $m$ = Mass of the substance (lbs)
- $c$ = Specific heat capacity ($\text{Btu/lb}\cdot^{\circ}\text{F}$)
- $\Delta T$ = Change in temperature ($T_{\text{final}} - T_{\text{initial}}$ in °F)
Worked Example: How much sensible heat must be removed to cool 500 lbs of liquid water from 75°F to 35°F?
- Identify the values: $m = 500\text{ lbs}$, $c = 1.0\text{ Btu/lb}\cdot^{\circ}\text{F}$, $\Delta T = 75 - 35 = 40^{\circ}\text{F}$.
- Apply the formula: $Q = 500\text{ lbs} \cdot 1.0\text{ Btu/lb}\cdot^{\circ}\text{F} \cdot 40^{\circ}\text{F}$.
- Solve: $Q = 20,000\text{ Btus}$.
A total of 20,000 Btus of heat must be removed to cool the water.
Sensible Heat vs. Latent Heat
Refrigeration systems rely heavily on phase changes (liquid to vapor and vapor to liquid) to move large quantities of heat. This brings us to the distinction between sensible and latent heat:
- Sensible Heat: Heat that causes a change in temperature that can be sensed by a thermometer, but does not change the state of the substance.
- Latent Heat: "Hidden" heat that causes a change of state (phase change) but does not change the temperature of the substance. During a phase change, the temperature remains constant at the saturation point (boiling or freezing point) until the change is complete.
The Two Main Types of Latent Heat:
- Latent Heat of Fusion ($L_f$): The heat required to change a substance from a solid to a liquid, or liquid to solid, at its melting/freezing temperature. For water, the latent heat of fusion is 144 Btu/lb at 32°F.
- Latent Heat of Vaporization ($L_v$): The heat required to change a substance from a liquid to a vapor, or vapor to liquid (condensation), at its boiling temperature. For water at atmospheric pressure, the latent heat of vaporization is 970 Btu/lb at 212°F. For anhydrous ammonia (R-717) at its atmospheric boiling point of -28°F, the latent heat of vaporization is 589.3 Btu/lb. This exceptionally high latent heat of vaporization is why ammonia is such an efficient industrial refrigerant—each pound of ammonia absorbs a vast amount of heat when it boils in the evaporator.
Comparison Table: Sensible vs. Latent Heat
| Characteristic | Sensible Heat | Latent Heat |
|---|---|---|
| Thermometer Reading | Causes a measurable temperature change. | Occurs at a constant temperature. |
| Physical Effect | Speeds up or slows down molecular motion. | Breaks or forms molecular bonds (state change). |
| System Components | Measured in liquid lines, superheated suction lines. | Occurs inside evaporators (boiling) and condensers (condensing). |
| Formulas | $Q = m \cdot c \cdot \Delta T$ | $Q = m \cdot L$ (where $L$ is latent heat of fusion or vaporization). |
| Example in Water | Heating liquid water from 32°F to 212°F. | Melting 32°F ice to 32°F water ($144\text{ Btu/lb}$), or boiling 212°F water to 212°F steam ($970\text{ Btu/lb}$). |
Pressure in Refrigeration Systems
In refrigeration, pressure and temperature are directly linked. The boiling temperature of a refrigerant depends entirely on the pressure exerted upon it. To control the temperature of an evaporator or condenser, we must control its pressure.
Defining Pressure
Pressure ($P$) is defined as force ($F$) exerted per unit of area ($A$):
In the United States, pressure is measured in pounds per square inch (psi).
Gauge vs. Absolute Pressure
Operators must be careful to distinguish between two reference scales for pressure:
- Gauge Pressure (psig): The pressure measured relative to atmospheric pressure. A standard pressure gauge reads 0 psig at sea level, ignoring the pressure exerted by the earth's atmosphere.
- Absolute Pressure (psia): The pressure measured relative to a perfect vacuum. Absolute pressure includes the atmospheric pressure.
At sea level, the earth's atmosphere exerts a pressure of 14.7 psi (equivalent to 29.92 inches of mercury). Therefore:
Pressure below Atmospheric (Vacuum)
When system pressure drops below atmospheric pressure (0 psig), it enters a vacuum. Vacuum is typically measured in inches of mercury vacuum (in. Hg vac) on a gauge.
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A perfect vacuum is approximately $29.92\text{ in. Hg vac}$ or $0\text{ psia}$.
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To convert vacuum to absolute pressure:
For example, a vacuum gauge reading of $10\text{ in. Hg vac}$ is approximately:
.
Why absolute pressure matters: Standard refrigerant tables (pressure-temperature charts) use absolute pressure (psia) or clear gauge markings. Calculations involving thermodynamics, compression ratios, and gas laws must always be performed using absolute pressure (psia) and absolute temperature (Rankine, where $^{\circ}\text{R} = ^{\circ}\text{F} + 460$). If an operator uses gauge pressure instead of absolute pressure in these calculations, the results will be incorrect, potentially leading to incorrect equipment sizing or unsafe operating parameters.
Environmental Factors Affecting Readings
Standard calculations assume sea-level atmospheric pressure of 14.7 psi. However, high-altitude locations have lower atmospheric pressure. For instance, in Denver, Colorado (at 5,280 feet above sea level), the atmospheric pressure is approximately 12.2 psia. At this altitude, a gauge reading of 0 psig corresponds to 12.2 psia, not 14.7 psia. If an operator needs to determine the saturation temperature of ammonia at high altitudes, they must account for the local barometric pressure to convert psig to the correct psia before referring to standard pressure-temperature (P-T) charts. Failing to do so can result in significant temperature errors in low-temperature or vacuum systems.
How much heat energy must be removed to convert 10 pounds of liquid water at 32°F into ice at 32°F?
An ammonia pressure gauge on a low-pressure receiver reads 15.3 psig. If the local atmospheric pressure is 14.7 psi, what is the absolute pressure inside the vessel?
Which of the following describes sensible heat?