Compressor Efficiency And Modern Deicers

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.

SYSTEM BACKGROUND

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:

  • Pressure
  • Flow

COMPRESSOR TECHNOLOGIES

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.

This latter characteristic can be better explained by reviewing test data or performance maps of typical centrifugal and positive displacement-type machines. Figure 1 depicts performance mapping of an available positive displacement-type machine, in this instance, a rotary screw compressor. Figure 2 shows performance mapping of a centrifugal-type compressor.

Although multivariate data are displayed in these maps – and they may appear quite complicated at first blush – the mapped performance differs substantially between these two technologies.

For the screw compressor, constant speed operating lines appear almost vertical. Very small changes in flow rate result in large changes in pressure while operating at constant speed. Maintaining appropriate discharge pressure and, hence, nozzle velocity relies on precisely defined downstream flow geometry.

As a result, variable speed control becomes necessary to “trim” operation to the precisely correct discharge pressure.

For the centrifugal machine, the characteristic is the exact opposite. (See Figure 2.) Constant speed operating lines are seen as nearly horizontal. Very large changes in flow have little affect on discharge pressure. So a single compressor can accommodate a wide range of flow with relative stability of discharge pressure while operating at constant speed. Impeller speed is the knob for controlling discharge pressure alone, and there is little concern over precisely matching the downstream flow geometry. Also, a system can be enhanced to attain higher flow operation as long as sufficient power is available without the need to change the compressor component.

POWER GAME

Other differences can be seen when comparing overall package size and weight and power consumption. The latter is directly influenced by compression efficiency, a by-product of compressor design and technology choice. The performance maps of Figures 1 and 2 depict efficiency performance of the screw and centrifugal units respectively. Although these data are peculiar to specific products, they may be taken as representative for our purposes.

Clearly, the centrifugal unit exhibits generally higher operating efficiency – 79 percent peak vs. 65 percent peak for the screw product. But does a 15-point efficiency difference really matter? What impact does this have on overall system requirements and operating costs? Moreover, at what efficiency is the compressor operating at the particular flow and pressure point of interest?

Fortunately, efficiency differences and their overall affect on power and, hence, operating costs can be readily evaluated via the polytropic compression equations. Table 1 summarizes such a study with performance taken at the 1.9 PR and 100 PPM air flow operating conditions. For these calculations, inlet air is taken at 20 degrees F and 14.50 psia, typical of conditions during deicing operations.

Contrasted are three compressor technologies including the Figure 1 and Figure 2 screw and centrifugal devices, and a third representing a rotary lobe roots device typically found in a wide variety of industrial applications. Also, even though a given device or technology may perform at a certain peak efficiency level, what is more important is the efficiency at the specific pressure and air flow operating point of interest.

For example, the Figure 1 twin-screw device attains a peak 65 percent efficiency, but is only 54 percent efficient at the 1.9 PR – 100 PPM flow point of interest. The Figure 2 compressor is closer – 76 percent operating efficiency against a best of 79 percent.

This reveals the importance of close scrutiny of mapped performance of any given compressor technology or device, and whether it is truly optimum for the specific air supply needs. This raises the challenge to develop compressor stages that are actually tuned so that peak efficiency performance is attained specifically at the desired pressure and flow operating condition. This assures the absolute minimum power and, hence, the lowest operating cost condition for the forced air deicing equipment.

From the Table 1 summary, compressor operating efficiency considerably impacts overall power requirements; power is directly related to fuel consumption and, hence, operating costs. Further, power generators on board the deicing vehicle must be upsized to accommodate a more power hungry, inefficient compressor. Since most modern deicing vehicles favor a hydraulically driven compressor drive arrangement, this implies the use of a larger, more costly power source, pump and hydraulic motor. Clearly, compressor efficiency must be at the top of the list for choosing a potential air source solution, if overall system downsizing, operating cost and emissions reductions are at all relevant.

MODERN DEICERS

For the reasons discussed, the mechanically driven centrifugal compressor technology has been the choice of deicing vehicle manufacturers for several years now.

With what appears to be a growing trend and global initiative toward energy footprint, emissions and operating cost reductions, GSE manufacturers have also adopted a trend to develop equipment that can accomplish more with less, i.e., do a better job while reducing cost burden to their customers, all while maintaining an edge in a very competitive global market.

For forced air deicers, increased airflow is desired since this enables improved blow-off performance at greater distances due to the increased jet core to surface area ratio. Such performance is desired to better accommodate deicing needs of large aircraft such as the Airbus A380.

As a result of these challenging objectives, new compressor designs promise to move the forced air deicer performance and efficiency state-of-the-art forward. These next generation products offer increased air flow of 140 PPM (1,830 CFM) courtesy of new generation, ultra-efficient compressor stages.

Such a system with this level of efficiency performance incurs only modest power increases; the 1.9 PR at 140 PPM flow operating point is attained at 81 percent overall efficiency, resulting in 96 horsepower of compressor power at the typical 20 degrees F @ 14.5 psia inlet conditions. (See Figure 3.)

As many deicing vehicles in service today are already equipped with this level of compressor power rating, upgrading to these newer high performance compressors can be accomplished with little other modification to the existing vehicle system. The peak efficiency of this device is attained at the exact operating point of interest!

About the Author: Robert B. Anderson, MSME, is senior technologist at the AirPower® Group of companies, which includes the Vortech Engineering and Vortron product brands, where he has worked since 2001.

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