Short Answer:

Sensible heat load is the part of the total heat load that causes a change in temperature of the air without changing its moisture content. In simple terms, it is the heat energy that increases or decreases the dry-bulb temperature of the air. This heat can come from sunlight, lighting, electrical equipment, people, or air leakage from outside. Sensible heat load is an important factor in air conditioning design, as it affects the temperature control of a room or building.

The sensible heat load can be either external or internal. External sensible heat load comes from sources like solar radiation through windows, walls, and roofs, while internal sensible heat load is caused by people, lighting, and appliances operating inside the space. Understanding the sensible heat load helps in selecting the right size of cooling equipment to maintain desired indoor temperature and comfort.

Detailed Explanation:

Sensible Heat Load

Sensible heat load refers to the amount of heat energy that must be removed from or added to a space to change the temperature of the air without affecting its moisture content. In other words, it is the part of the total heat load that is associated with temperature changes only, not with humidity or condensation. The term “sensible” is used because this type of heat can be sensed or felt by a thermometer, as it changes the air temperature directly.

In an air conditioning system, the sensible heat load plays a major role in determining the total cooling requirement. Engineers must calculate it accurately to ensure that the system maintains a comfortable temperature efficiently.

1. Nature of Sensible Heat Load

Sensible heat load occurs when there is a difference in temperature between the inside and outside environments, or between warm objects and the surrounding air. This difference causes heat to flow until equilibrium is reached. The more the temperature difference, the greater the heat transfer rate.

For example, if a room has a higher temperature due to sunlight or equipment operation, the cooling system must remove the sensible heat to bring the temperature down to the desired level. Similarly, during winter, the heating system must add sensible heat to increase the indoor temperature.

2. Sources of Sensible Heat Load

The sensible heat load in a building or conditioned space can come from several sources, both external and internal:

(a) External Sources:

• Solar Radiation: Sunlight passing through windows and absorbed by walls, roofs, and floors increases the indoor temperature.
• Conduction through Walls and Roofs: Heat transfers from the warm outside air through building materials into the room. Poor insulation increases this transfer.
• Air Infiltration: Hot air entering the room from gaps, windows, or doors raises the temperature, adding to the sensible heat load.

(b) Internal Sources:

• Occupants: People inside a room release body heat, which increases the air temperature.
• Lighting: Electric lights produce heat as a byproduct of illumination.
• Electrical Equipment and Appliances: Computers, televisions, refrigerators, and machinery generate heat while operating.
• Cooking Equipment: In kitchens and food processing areas, cooking devices add a significant amount of sensible heat.

Each of these sources adds to the total sensible heat load that must be removed by the cooling system to maintain comfort conditions.

3. Measurement of Sensible Heat Load

Sensible heat load is generally measured using the following relationship:

Where:

•  = Sensible heat load (in kJ or kW)
•  = Mass flow rate of air (kg/s)
•  = Specific heat of air (kJ/kg·K)
•  = Temperature difference between supply and return air (°C)

This formula shows that sensible heat load depends on the mass of air, its specific heat, and the change in temperature. The greater the temperature difference, the higher the sensible heat load.

4. Importance in Air Conditioning Design

In designing an air conditioning system, calculating the sensible heat load is essential because it directly affects how much cooling capacity is required to lower the air temperature.

If the sensible heat load is underestimated, the air conditioner may not be able to cool the room effectively. On the other hand, if it is overestimated, the system may be oversized, leading to higher initial costs and energy wastage.

The ratio of sensible heat load to the total heat load (which includes latent heat) is known as the Sensible Heat Factor (SHF). A higher SHF means most of the cooling is used for temperature reduction, while a lower SHF means more energy is used for dehumidification.

5. Control of Sensible Heat Load

To reduce excessive sensible heat load, several design and operational measures can be taken:

• Proper Insulation: Use of insulated walls, roofs, and ceilings to reduce heat gain.
• Reflective Glass and Shading: Using tinted glass, curtains, or external shading devices to minimize solar radiation.
• Efficient Lighting: Replacing incandescent lamps with LEDs to reduce heat generation.
• Regular Maintenance: Ensuring air ducts and filters are clean for better cooling performance.
• Minimized Equipment Operation: Turning off unused electrical appliances to lower internal heat generation.

These steps help control sensible heat gain and improve the overall efficiency of the air conditioning system.

Conclusion:

Sensible heat load is the portion of total heat load responsible for changing the air temperature without affecting its humidity. It results from heat gains due to sunlight, walls, people, lights, and appliances. Understanding and accurately calculating sensible heat load is essential for the proper design of air-conditioning systems to maintain indoor comfort and energy efficiency. Proper control of sensible heat through insulation, ventilation, and energy-efficient equipment ensures an economical and effective cooling process.