HVAC School - For Techs, By Techs
Oversized Evap Coils? - Short #237
· Summarized
Key takeaways
- Consult the manufacturer's expanded performance data to determine the appropriate coil size for your HVAC system.
- The saturated suction temperature is influenced more by heat entering the coil than by coil size.
- Optimizing airflow rates is essential for effective cooling and dehumidification, particularly with oversized coils.
- Turbulent flow is preferable for heat transfer efficiency; avoid laminar flow conditions.
- Regular testing and monitoring are critical for successful HVAC system performance, particularly when working with larger coils.
Summary
Understanding Oversized Evaporator Coils
In this episode, Bryan dives into a listener's question regarding oversized evaporator coils, particularly when coils are half a ton or a full ton larger than the condenser. He emphasizes the importance of consulting the expanded performance data from manufacturers to determine the correct coil size for the system. Despite common assumptions, the size of a coil does not directly influence the saturated suction temperature; instead, this temperature is more significantly affected by the amount of heat entering the evaporator coil and how effectively that heat is being absorbed.
Saturated Suction Temperature Explained
Bryan clarifies that the saturated suction temperature (SST), also referred to as evaporator temperature, is not contingent upon the size of the coil. In a controlled steady-state scenario, the SST is influenced primarily by the heat entering the coil. Thus, larger coils with greater surface areas can enhance temperature absorption at lower airflow speeds, impacting both cooling efficiency and dehumidification capabilities, provided that these systems receive adequate airflow.
The Role of Coil Size and Airflow Dynamics
The size of the coil is crucial for accommodating specific airflows and refrigerant rates. While a larger coil can facilitate better dehumidification due to its increased surface area and lower bypass factors, it also presents challenges such as a longer time to reach cooling temperatures and the necessity of careful superheat settings. Bryan stresses that the relationship between airflow and coil size matters significantly for achieving desired indoor climate objectives.
Turbulent vs. Laminar Flow in Coil Efficiency
Attention is drawn to the dynamics of airflow over the evaporator coils, specifically the detrimental effects of laminar flow when airflow velocities drop below certain levels. Turbulent flow, in contrast, enhances heat transfer efficiency. Bryan discusses the importance of testing systems to ensure optimal performance and hints at future explorations into how airflow dynamics affect heating and cooling processes.
Best Practices for HVAC Installations
Bryan concludes the episode by urging listeners to conduct thorough testing and commissioning in HVAC setups. Correctly adjusted airflow rates, as indicated by manufacturer specifications in expanded performance data, are paramount, particularly when utilizing oversized evaporator coils. Awareness of both sensible and latent heat transfers during system operation is vital to achieving desired HVAC performance.
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Show notes
In this short podcast episode, Bryan answers a listener-submitted question about oversized evaporator coils, particularly when they're half a ton or a full ton larger than the condenser.
The expanded performance data contains the information that'll tell you whether the coil is right for the system. However, when it comes to the specific issue of oversizing, there is a common misconception that coil size affects the saturated suction temperature. The truth is that they're not related; you could fully insulate different-sized coils and move the same amount of refrigerant, but they will behave similarly.
The saturated suction temperature (suction pressure, evap temperature) is driven by the heat entering the system. Coils are designed to handle refrigerant flow rates and airflows, so having a lower airflow over a large coil allows for greater cooling due to the bigger surface area, lower bypass factor, and dwell time. When more heat enters the evaporator coil, the saturated suction temperature increases, but some of that heat enters as latent heat and helps with dehumidification. However, a larger coil also takes longer to get colder, and the superheat settings become more important.
You have to be careful with lower face velocities across the evaporator coil, as laminar flow isn't as good at transferring heat as turbulent flow. That is one of the limitations of using a larger coil with lower airflow. If the expanded performance data has a rated match for a larger coil, then that same data will also contain information about the required airflow. Testing in and testing out is also important.
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