Mastering the Bypass and Contact Factor in Psychrometry | GATE Application of Thermodynamics

Summary

This lecture explains the concepts of bypass factor and contact factor in heating and cooling coils. It details how air interacts with a coil, either bypassing it or coming into direct contact, and the resulting temperature changes. In heating coils, cold air absorbs heat but doesn't reach the coil's full temperature due to resistances like convection and radiation. In cooling coils, hot air loses heat but doesn’t cool to the temperature of the coil. The bypass and contact factors are crucial for calculating the efficiency of these processes, with their sum always equaling 1.

Takeaways

• 😀 The bypass factor and contact factor are essential concepts in heat transfer, particularly when air flows over heating or cooling coils.
• 😀 For heating coils, cold air (T1) flows over a coil that heats it to a higher temperature (T2), with some air molecules coming into direct contact with the coil, while others only receive heat through convection and radiation.
• 😀 The psychometric chart for heating coils shows a temperature increase from T1 (cold air) to T2 (heated air), with the coil's temperature being higher than T2.
• 😀 Bypass factor (BPF) is calculated as the ratio of the temperature difference between the coil and the air at the outlet (Tcoil - T2) to the total temperature difference (Tcoil - T1).
• 😀 Contact factor (CF) represents the portion of air that directly comes into contact with the coil and is calculated by dividing the temperature difference between T2 and T1 by the total temperature difference.
• 😀 The sum of bypass factor (BPF) and contact factor (CF) is always equal to 1, showing the balance between the two processes of heat transfer.
• 😀 For cooling coils, hot air (T1) flows over a cooler coil, losing heat and cooling to a lower temperature (T2). The coil's temperature is lower than the hot air, and the air cools to a temperature higher than the coil's temperature.
• 😀 In cooling coils, the bypass factor is the ratio of the temperature difference between the air outlet (T2) and the coil (Tcoil) to the total temperature difference (T1 - Tcoil).
• 😀 The contact factor in cooling is calculated as the ratio of the temperature difference between T1 and T2 to the total temperature difference (T1 - Tcoil).
• 😀 The same rule applies for cooling coils: the sum of the bypass factor and contact factor is always equal to 1, ensuring the proper balance of heat transfer.

Q & A

What is the concept of bypass factor in heating coils?

The bypass factor in heating coils refers to the temperature difference between the coil temperature (Tcoil) and the air temperature after passing through the coil (T2), divided by the total temperature difference between the coil temperature and the initial air temperature (T1). It represents the portion of air that does not fully heat up and bypasses the coil's heating effect.

How is the contact factor defined in the context of heating coils?

The contact factor is the temperature difference between the air's temperature after passing through the coil (T2) and the initial air temperature (T1), divided by the total temperature difference between the coil temperature (Tcoil) and the initial air temperature (T1). It represents the portion of air that directly interacts with the coil and reaches a higher temperature.

What is the relationship between bypass factor and contact factor?

The sum of the bypass factor (BPF) and the contact factor (CF) always equals 1. This relationship indicates that the air is either bypassing the coil or coming into direct contact with it, but the total heat transfer effect is distributed between these two factors.

What happens to the air temperature as it passes over a heating coil?

As air passes over a heating coil, it gains heat through convection and radiation. The temperature of the air increases but does not reach the temperature of the coil due to resistance in the heat transfer process. Therefore, the air exits with a temperature lower than the coil's temperature.

How does resistance affect the heat transfer in a heating coil?

Resistance in heat transfer, such as convection and radiation resistance, prevents the air from reaching the coil's temperature. This results in the air exiting the coil at a temperature lower than the coil temperature, which leads to the concept of bypass and contact factors.

How can the bypass factor be mathematically expressed?

The bypass factor (BPF) is calculated using the formula: BPF = (Tcoil - T2) / (Tcoil - T1), where Tcoil is the temperature of the coil, T2 is the air temperature after passing through the coil, and T1 is the initial air temperature.

What is the role of the psychometric chart in understanding the bypass and contact factors?

The psychometric chart visually represents the sensible heating or cooling process. It helps identify the temperature changes of the air as it passes over a coil, showing the points of contact (from T1 to T2) and bypass (from T2 to Tcoil), which correspond to the contact and bypass factors.

In the case of cooling coils, what is the primary goal of the system?

In the case of cooling coils, the primary goal is to cool the air. The hot air (T1) is cooled down to a lower temperature (T2) after passing over the coil. The air cannot reach the coil temperature due to heat transfer resistance, leading to the calculation of bypass and contact factors.

How is the bypass factor calculated for cooling coils?

For cooling coils, the bypass factor (BPF) is calculated as: BPF = (T2 - Tcoil) / (T1 - Tcoil), where Tcoil is the temperature of the cooling coil, T1 is the initial air temperature, and T2 is the air temperature after cooling.

Why is it important to understand the bypass and contact factors in HVAC systems?

Understanding the bypass and contact factors is crucial in optimizing HVAC systems because they help determine how effectively a coil heats or cools the air. This understanding aids in designing more efficient systems, reducing energy consumption, and improving thermal comfort in controlled environments.