Compressors are very effective in turning nearly all consumed electrical energy into heat. At the same time, compressor operating temperatures need to be controlled for both longevity and efficiency, so improper installation of exhaust ductwork can lead to reliability and air quality issues, especially during warmer months. To help you avoid these issues, we’ll discuss some common mistakes and practical solutions:
Common Mistakes in Ductwork Installation
Customers and sometimes installers may undersize the ductwork by matching it to the dimensions of the hot air exhaust opening on the compressor. This is the number one mistake we see. The ductwork’s cross section area should be the size of the compressor cooler, which may be twice the area of the exhaust area. For example, the hot air exhaust opening on a Kaeser 25 hp air cooled screw compressor is 11” X 22”, but the cooler dimensions for that machine are 23” X 22”.
The duct below on the left is exactly sized to the air exhaust grate. By contrast, on the properly done ductwork on the right, you see that the cross section immediately increases after the air exhaust grate. If you could see the cooler on this compressor, you’d see that the cross section of the duct work is more or less equal to the cooler size.
Long duct runs may lead to air stagnation, and too often, the length of elbows and bends is not factored in. For long duct runs or high back pressure situations, install booster fans to maintain airflow.
These two ducts (both undersized by the way) join a common header, but there is nothing to prevent air from one flowing back to the exhaust of the other. This will impede cooling.
Ducting multiple air compressors into a common duct header can cause uneven airflow, leading to backflow and overheating. Use separate ductwork for each air compressor and consider adding louvers to prevent hot air from feeding into units that are not operating. Consult a professional when tying multiple units together.
Sharp angles in ducting impede airflow, especially in undersized ductwork. Larger ducting or sweeping bends as shown below make the difference.
Unintentionally recirculating warm air back into the compressor coolers will increase operating temperature and increase maintenance. It may also decrease efficiency. Ensure proper spacing between the air inlet and exhaust to prevent recirculation.
Lack of thermostatic control can cause overheating in hotter months. Installing automated louvers and fans will help keep things cooler. Of course, you need to periodically check that they are working, and monitoring the compressor room temperature is a great way to identify problems. For systems installed in freezing climate zones, thermostatic controlled systems can effectively mix exhaust air with incoming fresh air to maintain optimal running temperatures and prevent several issues related to freezing.
Small compressors are often starved for cooling air. They get put in closets or sheds with little or no ventilation. In the photo below, note the dirty filter mat getting sucked into the room as the compressor in this closet starves for air. At the very least, install louvers for incoming and outgoing air. In some cases, a fan may be necessary to ensure adequate airflow, and as mentioned above, thermostatically controlled fans are ideal. Monitor the room temperature and add ventilation (larger louvers/fans) as needed. Even a cheap thermometer checked routinely can help avoid costly repairs.
Best practices for ductwork installation
- Consult professionals and manufacturer resources: Work with an experienced HVAC contractor and reference manufacturer-provided data, such as Kaeser's Installation Data Sheets, to ensure proper duct sizing and installation.
- Consult the manufacturer's data: Use the cooling fan flow (CFM) and maximum allowable back pressure (inches water column) for duct sizing.
- Air-cooled screw compressors are generally designed to operate with a target differential of 105°F between compressor room temperature and the internal airend discharge temperature (before the aftercooler). This is often referred to as the ADT and this internal operating temperature information is commonly available on the controls of any major compressor brand to keep an eye on the cooling system. So if the room temperature is 90°F, an internal operating temperature of 195°F is healthy. But if that temperature creeps higher, it could indicate issues with the compressor cooling system or the ductwork (or both).
- Regular maintenance: Schedule regular checks and maintenance for ductwork, fans, and louvers to ensure proper operation.
Key takeaways
Properly sized and installed ductwork is crucial for the efficient operation of air-cooled screw compressors. Miscommunication between customers and contractors often leads to common mistakes. By consulting with HVAC professionals and following manufacturer guidelines, you can ensure reliable compressor performance, prevent overheating, and avoid costly shutdowns. One important note, the ventilation system should also be designed to prevent freezing during colder months. The heat of compression can be used to warm the compressor room, preventing nuisance shutdowns due to cold weather. A well-installed duct system contributes to a reliable and efficient compressed air system, saving time and money in the long run.
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