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 Exhaust Duct Sizing and Installation
Proper exhaust duct sizing and installation are among the most overlooked factors in compressed air system performance, and the consequences of getting it wrong show up quickly in the form of overheating, increased maintenance costs, and premature equipment failure.
Undersizing the Ductwork
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 No. 1 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 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 ductwork is more or less equal to the cooler size.
Long Duct Runs and Back Pressure
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 duct flowing back into the exhaust of the other. This will impede cooling.
Ducting Multiple Compressors into a Common Header
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 and Bends
Sharp angles in ducting impede airflow, especially in undersized ductwork. Larger ducting or sweeping bends, as shown above, make the difference.
Warm Air Recirculation
Unintentionally recirculating warm air back into the compressor coolers will increase operating temperatures and maintenance. It may also decrease efficiency. Ensure proper spacing between the air inlet and exhaust to prevent recirculation.
Lack of Thermostatic Control
A lack of thermostatic control can cause overheating during 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, thermostatically controlled systems can effectively mix exhaust air with incoming fresh air to maintain optimal running temperatures and prevent several issues related to freezing.
Inadequate Ventilation for Small Compressors
Small compressors are often starved for cooling air. They get put in closets or sheds with little or no ventilation. In this photo, 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, routinely checked thermometer can help avoid costly repairs.
Best practices for ductwork installation
Getting compressed air ductwork installation right starts before the first piece of duct goes up, and the compressors that run the longest are almost always the ones backed by good planning and routine follow-through. The best practices below draw on manufacturer specifications and field experience to give your compressed air system the cooling it needs to perform reliably.
- 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 exhaust duct sizing.
- Monitor temperatures: 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 monitor 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 routine inspections and maintenance of compressed air ductwork, fans, and louvers to ensure proper operation.
Key Exhaust Duct Sizing and Installation Takeaways
Proper exhaust duct sizing and installation are 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 generated by compression can be used to warm the compressor room, preventing nuisance shutdowns caused by cold weather. A well-installed duct system contributes to a reliable and efficient compressed air system, saving time and money in the long run.
Get Your Compressed Air Ductwork Right the First Time
Poorly planned compressed air ductwork is one of the fastest ways to turn a reliable system into a costly, recurring problem. Our team has the field experience and manufacturer expertise to assess your compressor room, spec the right exhaust duct sizing, and catch the issues that generic contractors miss. Contact us today to schedule a site evaluation, or use the Kaeser Toolbox to determine the optimization potential of your compressed air system.
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