Short Answer:

The by-pass factor in cooling coils is the ratio of the amount of air that passes through the coil without coming into full contact with the cooling surface to the total air passing through the coil. In simple terms, it shows how much air “escapes” cooling. A lower by-pass factor means better cooling efficiency, as more air gets cooled to the desired temperature.

By-pass factor depends on factors like coil design, air velocity, fin spacing, and surface area. It helps engineers evaluate the performance of air-conditioning coils and decide how effectively the system removes heat and moisture from the air.

Detailed Explanation:

By-pass Factor in Cooling Coils

The by-pass factor is an important term used in air conditioning and refrigeration systems to describe the efficiency of heat exchange in cooling or heating coils. It refers to the portion of air that does not come into effective contact with the coil surface during the heat exchange process and therefore is not completely cooled or heated to the coil surface temperature.

When air passes through a cooling coil, ideally, all the air should be cooled to the coil surface temperature. However, in real-life systems, some part of the air escapes effective contact due to the flow pattern, coil geometry, and speed of the air. This portion of air that “bypasses” the cooling process determines the by-pass factor (BPF).

The by-pass factor indicates the inefficiency of the coil. A smaller value of BPF represents a more efficient coil because it means most of the air gets cooled, while a higher BPF indicates poor performance as more air remains uncooled.

Definition and Formula

Mathematically, the by-pass factor can be defined as:

Where,

•  = Temperature of air entering the cooling coil
•  = Temperature of air leaving the cooling coil
•  = Temperature of coil surface

From this formula, it is clear that the by-pass factor shows how close the leaving air temperature is to the coil surface temperature. If the air leaving the coil is very close to the coil temperature, then the BPF will be small, indicating better cooling performance.

The complement of the by-pass factor is known as the contact factor (CF), which indicates the fraction of air that effectively contacts and exchanges heat with the coil.

Hence, a higher contact factor represents better heat transfer efficiency.

Physical Meaning of By-pass Factor

In practical terms, when warm and humid air passes over a cooling coil, not all air molecules come into perfect contact with the cold surface of the coil. Some air streams pass between coil fins or through regions of higher velocity where the temperature drop is smaller.

The by-pass factor quantifies this behavior.

• If BPF = 0, it means perfect heat exchange, and all air is cooled to the coil surface temperature.
• If BPF = 1, it means no air is cooled, which is practically impossible.

Therefore, in real systems, the by-pass factor is always between 0 and 1. Typical values range from 0.05 to 0.25 depending on design and air speed.

Factors Affecting By-pass Factor

1. Coil Design:
   • The number of rows of tubes and the spacing between fins affect air contact. More rows mean better heat transfer and a lower by-pass factor.
2. Air Velocity:
   • Higher air velocity reduces contact time between air and coil, increasing the by-pass factor.
   • Lower velocity increases contact time, allowing more air to cool effectively.
3. Fin Surface Area:
   • Increasing the fin surface area enhances heat transfer by providing more surface for air contact, reducing the by-pass factor.
4. Coil Material and Cleanliness:
   • Materials with higher thermal conductivity (like copper or aluminum) transfer heat efficiently, lowering the BPF.
   • Dirty or clogged coils reduce effective contact and increase the BPF.
5. Air Distribution:
   • Uniform air distribution across the coil helps achieve consistent cooling and reduces the by-pass factor.

Significance of By-pass Factor

The by-pass factor is essential for the design and performance evaluation of air conditioning and refrigeration systems. It helps in:

1. Evaluating Cooling Coil Efficiency:
   • Engineers use BPF to measure how efficiently the coil cools the air. Lower values indicate better coil performance.
2. Determining Air Leaving Temperature:
   • It helps calculate the temperature and humidity of air leaving the coil, which is important for indoor comfort and system balance.
3. Selecting Equipment:
   • By understanding the by-pass factor, engineers can select coils that meet the required cooling performance with minimal energy waste.
4. Energy Conservation:
   • A low by-pass factor means better energy utilization and reduced cooling load on compressors, leading to energy savings.
5. System Maintenance:
   • Monitoring the by-pass factor can help identify performance degradation due to dust, corrosion, or improper air flow, allowing timely maintenance.

Practical Example

Suppose the air enters the cooling coil at 30°C and leaves at 15°C, while the coil surface temperature is 10°C.

This means that 25% of the air bypassed the cooling process and did not come into full contact with the coil surface. Hence, the coil has a by-pass factor of 0.25 and a contact factor of 0.75, meaning 75% of the air effectively participated in cooling.

Conclusion:

The by-pass factor in cooling coils is a measure of how effectively a coil cools the air passing through it. It represents the fraction of air that does not get cooled to the coil surface temperature. A lower by-pass factor indicates a more efficient coil and better system performance. Understanding and controlling the by-pass factor helps improve air-conditioning efficiency, ensures uniform air distribution, and reduces energy consumption in HVAC systems.