Sensible heat is the heat that is transferred to or from a substance without causing a change in phase. It is the type of heat that we feel when we touch something that is hot or cold.
Latent heat is the heat that is transferred to or from a substance during a phase change, such as from a liquid to a gas or from a gas to a solid. It is the heat that is required to change the state of a substance without changing its temperature.
Total heat is the sum of sensible heat and latent heat. It is the total amount of heat that is transferred to or from a substance.
Equations
Sensible heat:
Sensible heat is the type of heat that we can feel and measure with a thermometer. It is the energy required to change the temperature of a substance without changing its phase (e.g., from solid to liquid or liquid to gas).
$$H_S = 1.08 \times CFM \times \Delta T$$Latent heat:
Latent heat is the energy required to change the phase of a substance (e.g., from solid to liquid or liquid to gas). It does not cause a change in temperature.
$$H_L = 0.68 \times CFM \times \Delta W_{GR}$$Total heat:
Total heat is the sum of sensible heat and latent heat.
$$H_T = H_S + H_L$$where:
- HS is the sensible heat (Btu/hr)
- HL is the latent heat (Btu/hr)
- HT is the total heat (Btu/hr)
- CFM is the air flow rate (cubic feet per minute)
- ΔT is the temperature difference (°F)
- ΔWGR is the humidity ratio difference (grains H2O/lb. DA)
Example
An air conditioner is removing 10,000 Btu/hr of total heat from a room. The air flow rate is 1000 CFM and the temperature difference is 20°F. The humidity ratio difference is 0.005 grains H2O/lb. DA.
Sensible heat:
$$H_S = 1.08 \times 1000 \times 20 = 21,600 Btu/hr$$Latent heat:
$$H_L = 0.68 \times 1000 \times 0.005 = 3.4 Btu/hr$$Total heat:
$$H_T = H_S + H_L = 21,600 + 3.4 = 21,603.4 Btu/hr$$U-Value and Area
The U-value of a material is a measure of its thermal resistance. The lower the U-value, the better the insulation.
The area of a surface is a measure of its size.
Equation
$$H = U \times A \times \Delta T$$where:
- H is the heat transfer rate (Btu/hr)
- U is the U-value (Btu/hr. ft². °F)
- A is the area (ft²)
- ΔT is the temperature difference (°F)
Example
A wall has a U-value of 0.25 Btu/hr.ft². °F and an area of 100 ft². The temperature difference between the inside and outside of the wall is 20°F.
Heat transfer rate:
$$H = 0.25 \times 100 \times 20 = 500 Btu/hr$$Sensible Heat Ratio (SHR)
The sensible heat ratio (SHR) is the ratio of sensible heat to total heat. It is a measure of how much of the total heat is sensible heat.
Equation
$$SHR = \frac{H_S}{H_T} = \frac{H_S}{H_S + H_L}$$Example
In the previous example, the sensible heat is 21,600 Btu/hr and the total heat is 21,603.4 Btu/hr. Therefore, the SHR is:
$$SHR = \frac{21,600}{21,603.4} = 0.999$$Conclusion
Sensible, latent, and total heat are important concepts in HVAC. By understanding these concepts, you can better design and operate HVAC systems.
FREQUENTLY ASKED QUESTIONS
The specific heat capacity of a substance determines how much heat energy is required to change its temperature by a given amount. Substances with high specific heat capacities, such as water, require more heat energy to change their temperature than substances with low specific heat capacities, such as air. Therefore, when designing HVAC systems, it’s essential to consider the specific heat capacity of the substances involved in heat transfer to ensure efficient sensible heat transfer.
A common example of latent heat transfer in an HVAC system is the dehumidification process in an air conditioning system. When moist air passes over a cooling coil, the latent heat of vaporization is transferred from the air to the coil, causing the water vapor to condense into liquid water. This process reduces the humidity of the air and removes heat from the space, making it an essential component of air conditioning systems.
Total heat is calculated by summing the sensible heat and latent heat transferred in an HVAC system. The sensible heat can be calculated using the specific heat capacity of the substance and the temperature change, while the latent heat can be calculated using the latent heat of vaporization or fusion and the mass of the substance undergoing a phase change. For example, in a cooling coil, the total heat transfer can be calculated by adding the sensible heat transfer due to the temperature change of the air and the latent heat transfer due to the condensation of water vapor.
The units of measurement for sensible, latent, and total heat are typically measured in joules (J) or British thermal units (BTU). The specific heat capacity of a substance is typically measured in joules per kilogram per kelvin (J/kg·K) or BTU per pound per degree Fahrenheit (BTU/lb·°F). The latent heat of vaporization or fusion is typically measured in joules per kilogram (J/kg) or BTU per pound (BTU/lb).
The humidity of the air has a significant impact on latent heat transfer in an HVAC system. When the air is humid, there is more moisture available to condense on the cooling coil, resulting in a greater amount of latent heat transfer. Conversely, when the air is dry, there is less moisture available to condense, resulting in less latent heat transfer. Therefore, it’s essential to consider the humidity of the air when designing HVAC systems to ensure efficient latent heat transfer.
Total heat transfer is an essential concept in various HVAC applications, including air conditioning systems, heat pumps, and refrigeration systems. It’s used to calculate the total cooling or heating capacity of a system, which is critical for selecting the appropriate equipment size and designing efficient systems. Additionally, total heat transfer is used to analyze the performance of HVAC systems and identify opportunities for energy savings and optimization.
