The industry standard for virtually all major deicing vehicle manufacturers worldwide is forced air deicing. This article attempts to provide an introduction to blowers or compressors used on current state-of-the-art products and various system design variables that influence overall performance.
Various types of compressor component technologies are available as possible candidates to provide the typical 12-13 psig air pressure required. A universal specification is 100 pounds-per-minute air flow at this pressure. The design of the discharge nozzle at the business end accomplishes this – more on that in a bit. To reveal what may make a sensible compressor technology choice, you need to understand a little more about this high performance blow-off system’s design.
Typically, air velocities at the nozzle should discharge at close to unity mach, say M=0.97, while also maintaining subsonic operation. Such a velocity equals about 67,000 feet per minute or 761 miles per hour. Various vehicle manufacturers typically advertise the latter number, and it’s widely accepted as the best possible operating condition to attain the most effective “blow-off.”
At standard sea-level conditions, this velocity dictates a nozzle operating pressure ratio of approximately 1.83:1; meaning, the absolute discharge pressure divided by the absolute inlet pressure equals 1.83.
We’ve limited our discussion to nozzle operating pressures. Required compressor discharge pressures will be somewhat higher due to the minor losses experienced between the compressor discharge and the nozzle, owing to the required piping, fittings and other apparatus in between. These losses must be added to the required nozzle pressure. In other words, any given compressor unit must provide additional pressure margin – up to even 2.0 pressure ratio performance depending on the overall system’s execution.
The last major system operating variable focuses on the nozzle design itself – the key to regulating the entire system’s operation. Modern forced air deicers employ a patented nozzle design that is essentially isentropic or “loss-less” in operation.
Nonetheless, the isentropic operating assumption allows for convenience in system design by allowing direct use of polytropic process equations. Do this and you’ll discover that the throat diameter of the converging nozzle is the single parameter that controls the system’s overall flow rate! Given this, the nozzle operating pressure ratio is the knob for determining discharge velocity.
We now have at hand, as the overall system’s requirements dictate, two highly relevant compressor operating parameters:
Clearly, the typical deicer pressure and flow requirements will require robust compressor technology operating at considerable power levels – on the order of 100 horsepower or more. Choices include various types of positive displacement devices such as roots and twin-screw designs and the more commonly seen turbomachinery-based centrifugal designs.
Although both basic types can produce the required pressure and air flow, their differences could not be more dramatic. Positive displacement devices are generally found to be considerably larger, heavier packages operating at relatively slow shaft speeds. On the other hand, the turbomachinery-based counterpart technology is generally small, lighter in weight and fast. In some cases, very fast as impeller shaft speeds of 50,000 RPM or more are often required to attain the desired pressures.
Factors for selecting any of these technologies include package size and weight, operating efficiency or power consumption at a particular operating point, noise, durability and cost. Other more subtle factors include the general performance characteristic and stability at a given operating point.
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