Refrigeration – Automatic control – Diverse – cascade or compound refrigeration-producing system
Reexamination Certificate
1999-04-28
2001-05-22
Doerrler, William (Department: 3744)
Refrigeration
Automatic control
Diverse, cascade or compound refrigeration-producing system
C417S004000, C236S0010EA
Reexamination Certificate
active
06233954
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the art of fluid compression. More specifically, the present invention relates to methods for controlling the operation of an air compression system having a plurality of rotary screw compressors.
2. Brief Description of the Prior Art
Compression systems having a plurality of individual compressors are generally sequenced so as to improve the overall efficiency of the compression system and minimize premature wearing of the individual compressors. Sequencing has traditionally been accomplished using external devices or controllers. The earliest sequencing devices were electromechanical devices that controlled compressors with a series of staggered pressure bands.
More recent sequencing devices include host controllers with electronic microprocessors for loading and unloading compressors over a single pressure band. These electronic sequencers are also capable of automatically changing the sequencing order of the compressors. Generally, the host controller receives a pressure reading from a supply line pressure sensor and compares the actual pressure reading to a set point pressure range or pressure bandwidth, to determine whether it is necessary to load, unload and/or modulate any of the compressors in the system.
In such conventional compressor systems, one of the compressors is designated the lead compressor for the system and one or more of the compressors are designated the lag compressors for the system. During operation of the conventional compressor system, the supply line pressure fluctuates primarily because of changes in the demand for compressed fluid. As a result, from time to time it is necessary to modulate one or more of the compressors in order to maintain the required system pressure.
FIG. 1
shows a prior art compression system including an external device for sequencing operation of two or more compressors. The compression system
10
includes a host controller
12
, a lead compressor
14
, a lag compressor
16
, and a supply line pressure sensor
18
that is flow connected to compressed fluid supply line
20
. In the compression system shown in
FIG. 1
, the demand for compressed air requires that the lead compressor
14
be continuously operated in a fully loaded state, and the lag compressor
16
is modulated between unloaded and fully loaded operation.
During operation of the compression system
10
, the host controller
12
decides when to modulate the lag compressor
16
based upon the pressure level sensed by the supply line pressure sensor
18
. The host controller compares the actual sensed supply line pressure with the predetermined set point pressure range stored in the memory of the host controller. When the supply line pressure is outside the acceptable supply line pressure range and is greater than the acceptable set point supply line pressure, the host controller sends a signal to the lag compressor
16
and unloads the compressor until the actual supply line pressure is within the acceptable supply line pressure range. The lead compressor
14
remains fully loaded at all times.
When the actual supply line pressure is less than the acceptable supply line pressure, the host controller
12
transmits a load signal to the lag compressor
16
to fully load the lag compressor, and the lag compressor
16
runs fully loaded until the actual supply line pressure is within the acceptable operating range. When the lag compressor is fully loaded, the lead compressor remains fully loaded. The host controller
12
continuously cycles the compressors between loaded and unloaded modes during operation of the multiple compressor system
10
, thereby minimizing wear of the compressors.
Adding a host controller or external device for controlling operation of a plurality of compressors adds significant cost and complexity to an air compression system. Thus, it is apparent that it would be advantageous to provide a method for controlling the operation of a compression system that sequences air compressors over a single pressure band without using an expensive host controller or external device. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
SUMMARY OF THE INVENTION
In accordance with one preferred embodiment of the present invention, a fluid compression system includes a plurality of compressors, each compressor having a local controller for controlling operation thereof and a sensor for sensing the pressure of compressed fluid discharged therefrom. The method includes the steps of establishing a set point pressure threshold for loading and unloading each compressor, and assigning a ranking to each compressor for identifying a highest ranked compressor and a lowest ranked compressor. In preferred embodiments, the highest ranked compressor initiates all commands for controlling all of the lower ranked compressors in the compression system. The method also includes the step of commencing a loading subroutine including loading the highest ranked unloaded compressor, setting a load delay timer, sensing the pressure of the compressed fluid discharged from the highest ranked compressor, comparing the sensed discharge pressure of the highest ranked compressor to the set point pressure threshold established for the highest ranked compressor, and transmitting a load command from the controller of the highest ranked compressor to the controller of the next highest ranked unloaded compressor if the sensed discharge pressure of the highest ranked compressor remains less than or equal to the set point pressure threshold and if the load delay timer has expired (i.e. equals zero (0)). The loading subroutine is repeated until the discharge pressure of the highest ranked compressor is greater than the set point pressure threshold established therefor.
In certain preferred embodiments, the local controller of the highest ranked compressor commences an unloading routine if the discharge pressure of the highest ranked compressor is greater than the set point pressure threshold of the highest ranked compressor. The unloading subroutine includes the steps of unloading the lowest ranked loaded compressor and setting an unload delay timer. After the lowest ranked loaded compressor has been unloaded, the pressure of the compressed fluid discharged from the highest ranked compressor is sensed and the sensed discharge pressure of the highest ranked compressor is compared to the set point pressure threshold established for the highest ranked compressor. An unload command is then transmitted from the controller of the highest ranked compressor to the controller of the lowest ranked loaded compressor if the sensed discharge pressure of the highest ranked compressor remains greater than the set point pressure threshold established for the highest ranked compressor, and the unload delay timer has expired (i.e. equals zero (0)). The unloading subroutine is repeated until the sensed discharge pressure of the highest ranked compressor is less than or equal to the set point pressure threshold established for the highest compressor. The highest ranked compressor is preferably designated as the lead compressor for the compression system for a set time period. After expiration of the set time period, a lead transfer command may be transmitted from the highest ranked compressor to the next highest ranked compressor, wherein the highest ranked compressor is reassigned as the lowest ranked compressor and the second highest ranked compressor is reassigned as the highest ranked compressor.
Each compressor preferably has a unique identifier or address assigned thereto. During operation, the highest ranked compressor sends out a series of commands to all of the lower ranked compressors. Each command has a unique identifier so that each lower ranked compressor only responds to a command received from the highest ranked compressor that bears the unique identifier therefor. In preferred embodiments, each command is transmitted in a sequential order from the high
Connell Marty L.
Mehaffey James D.
Doerrler William
Drake Malik N.
Ingersoll-Rand Company
Michael & Best & Friedrich LLP
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