Patent Application
20130039781
This application relates generally to a method to optimize the operation of a compressor which delivers pneumatic power in order to maximize performance and maintaining proper stability margin for both steady state and transient operation.
An APU is a small gas turbine engine that provides a number of functions associated with an aircraft other than propulsion, particularly before the main aircraft engines are started. Some APU configurations include a shaft-mounted load compressor which delivers pneumatic power to aircraft accessories.
Load compressor bleed air is delivered to aircraft systems through a bleed control valve. Some of the air may be utilized as cabin air before the main gas turbine engines are started, and the load compressor may also supply air to the main gas turbine engines at start-up.
Normally, the APU-supplied accessories have a wide range of compressed air requirements, which the APU bleed control valve has to accommodate. If the air demand drops below certain limits an unstable operation on the compressor, called “surge” may occur.
To maintain the compressor flow above its surge level an anti-surge valve is used. The engine is programmed with anti-surge logic to modulate the anti-surge valve to divert a portion of the compressor flow to the atmosphere and therefore to maintain the compressed air flow above minimum limits to avoid surge.
The compressor minimum air flow limits are set taking into account numerous parameters including operating conditions, compressor inlet guided vanes position, bleed demands, etc. Another important aspect which is considered is the operation during transient maneuvers. During a fast reduction of the air flow requirement, the compressor minimum air flow could drop momentarily below the minimum limit and cause surge.
Thus, in order to prevent such situations and maintain a proper operation of the compressor, the minimum bleed flow limit has to be raised. A higher minimum bleed flow limit reduces in certain operating conditions the APU performance, increasing the engine temperature and fuel consumption and, in the same time, reducing the bleed air quality, lower bleed pressure and flow.
A load compressor includes an air inlet and an outlet, a bleed valve and an anti-surge valve positioned on the outlet. The bleed valve serves to selectively pass air to a downstream use while the anti-surge valve dumps it to atmosphere. A control for the compressor and the surge control valve is also included. The control is provided with a variable indicative of the operation of the load compressor, the variable being indicative of movement to transient operation of the compressor. The control is provided with a low surge control limit for steady state operation, and a higher surge control limit The low and higher surge control limits are utilized to control the opening of the surge control valve.
An aircraft air supply system and a method are also disclosed.
These and other features of the present invention can be best understood from the following specification and drawings, of which the following is a brief description.
A controller 29 controls compressor 24 and the valve 28. When the surge control valve is open, it directs a portion of the compressed air to a dump 30 where the compressed air is returned to atmosphere. Other air is directed into line 34 and to use 32. So-called “fast-acting” valves may be utilized for surge control valve 28.
The valve 28 is a valve which can divert a portion of the flow to the dump line 30, and another portion to line 34. When a surge limit is exceeded, as will be explained below, the control 29 controls the valve 28 to divert at least a portion of the air to the dump line 30.
The prior art control of a phenomenon known as surge margin can be described by the operating conditions shown in
To provide the most efficient performance at steady state operation, a lower surge control limit such as Z is desirable.
On the other hand, this limit may not be acceptable for fast transient operations, such as startup of a main gas turbine engine on an aircraft receiving APU 22/122.
The present invention provides a control logic which anticipates a movement to transient operation, and a need for the higher limit.
Some variable is observed and a change in this variable would be indicative of a movement towards surge. Since the illustrated variable LDC DPP is proportional to flow (and inversely proportional to pressure), a drop in the LDC DPP, or a negative derivative or slope would be indicative of the movement towards surge.
Thus, as shown in
Looking at the change in the variable provides very prompt response such that the system will operate on the higher transient surge control limit extremely quickly once a transient condition has begun.
In one embodiment, a threshold for the change in LDC DPP may be if the rate of change is below −2% per second, for example. In some embodiments the threshold limit may be function of engine speed, inlet guide vane position or other operating condition. Further, the logic may require that there be consecutive readings below this threshold. For example, readings could be taken every 20 millisecond, and only when a particular number of consecutive readings, three for example, are below this threshold would trigger the surge control limit be changed from the steady state low value to the high transient value. Of course, the numeric values are purely examples. Any other rate of change, timing between readings, or number of consecutive readings can be utilized.
The system will return to the lower steady state limit once the amount of change is reduced such that a new steady state condition appears to have been reached.
Switching from a lower surge control limit to a higher surge control limit works slightly different depending where the compressor operating point was prior to the occurrence of the fast reduction of flow requirement:
a. If the compressor operating point is above limit Y (e.g., point T1), the anti-surge control logic may overcome the time lag of anti-surge valve control and operation by commencing the valve opening before the steady state limit is reached.
b. If the compressor operating is below limit Y (e.g., point T3) the anti-surge control logic will detect a high delta between the compressor operating point and the transient limit (limit Y). The control can generate a higher opening command to accelerate the valve opening. As an example, the signal to the valve may be increased proportionally based upon the magnitude of the error.
By allowing a lower surge control limit (e.g., limit Z) under certain operating conditions, APU performance may be improved and fuel consumption reduced. While a load compressor operating with an APU is disclosed, the features of the application may be beneficial in other types of compressors.
Although embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
