Twenty years ago, very little attention was paid to the efficiency of dry bulk blowers. Today, efficiency is becoming a significant issue when selecting a truck-mounted dry bulk blower. Using a more efficient blower to off-load dry bulk trailers reduces fuel consumption and results in operational cost savings.

Although all the machines used are typically called “blowers” there are actually two types of machines: positive displacement (pd) blowers and compressors. A pd blower has lobe impellers that trap a fixed amount of air and transport it from the blower suction port to the discharge port; there is no internal compression.

A compressor takes a fixed amount of air in the suction port and compresses it internally.

The amount of internal compression can vary dramatically between different types of compressors depending upon the duty. For example, a shop air compressor will have a high compression ratio to allow it to operate at 150 psi. For trucks it is common to see screw compressors and vane compressors in operation.

Screw compressors are designed to have no internal contact between the rotors. In contrast, vane compressors have sliding vanes that remain in contact. The sliding vanes require periodic replacement, thus making them typically more expensive to operate than a screw compressor.

When the compression ratio is sized correctly screw compressors can show considerable improvements in efficiency over pd blowers or other low-compression ratio screw compressors.

An example of a high-efficiency screw compressor is the 25-psi Tuthill T650. This machine has a higher compression ratio than typical machines used on trucks in North America but is low enough to make it efficient at the operational pressure of a dry bulk tank. Throughout the cfm range the T650 consumes approximately 25% less energy.

When tank and receiving sites allow, higher-pressure blowers such as the T650 can reduce off-load times. In empirical tests the T650 was able to achieve discharge rates of more than one ton per minute.

Most dry bulk blowers are used with separate inlet air filters that are mounted on the outside of the frame rail. With the use of additional emissions control equipment the frame rail space may not be available for mounting blower inlet air filters. The Tuthill T650 blower is different because the air filter is mounted directly to the inlet of the machine, leaving the outside of the frame rail unused.

In empirical testing the Tuthill T650 when used with the direct-mount muffler is typically 2-3 dBA quieter than competitive dry bulk blowers. This means that operating two Tuthill T650 compressors would sound the same as operating one blower that was 3 dBA louder.

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